SMITHSONIAN CONTRIBUTIONS TO BOTANY NUMBER 13 
Swollen-Thorn Acacias of 
Central America 
Daniel H. Janzen 
SMITHSONIAN INSTITUTION PRESS 
City of Washington 
1974 
ABSTRACT 
Janzen, Daniel H. Swollen-Thorn Acacias of Central America. Smithsonian 
Contributions to Botany, number 13, 131 pages, 119 figures, 1974.-This nomen- 
clatural, taxonomic, and ecological treatment of 1 1 Central American obligate 
ant-acacias (Acacia allenii, A. chiapensis, A. collinsii, A. cookii, A. cornigera, 
A. gentlei, A. globulifera, A hindsii, A nzayana, A. melanoceras, and A. sphaero- 
cephala) and one quasi-obligate ant-acacia (Acacia ruddiae) is based on ex- 
tensive field study from 1963 to 1972 and on herbarium specimens where of use. 
7 he population boundaries of all species are mapped and described with respect 
to ecological parameters. Morphological variation, details of the interaction with 
the ants, and acacia reproductive biology are presented for most species. 
OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded 
in the Institution?s annual report, Srnithsonian Year. SI PRESS NUMBER 4817. SERIES COVER DESIGN: 
Leaf clearing from the Katsura tree Cercidiphyllurn japonicum Siebold and Zuccarini. 
Library of Congress Cataloging in Publication Data 
Janzen, Daniel H: 
Swollen-thorn acacias of Central America. (Smithsonian contributions to botany, no. 13) 
Bibliography: p. 
1. Acacia. 2. Ants-Central America. 3. Botany-Central America-Ecology. I. Title. 11. 
QKl.S27 no. 13. [QK495.L52] 581?.08s [583?.321] 73-4401 
Series: Smithsonian Institution. Smithsonian contributions to botany, no. 13. 
For sale by the Superintendent of Documents, US. Government Printing O5ce 
Washington, D.C. 20402 . Price $2.35 (paper cover) 
Contents 
Page 
Introduction ................................................... 1 
General Biology ................................................. 2 
General Comments on Swollen-Thorn Acacia Taxonomy .............. 6 
Materials, Methods, and Terminology ............................... 14 
General Morphological Traits .................................. 15 
Taxonomic Information .............. ....................... 25 
A . Key to Mature Central American Swollen-Thorn Acacias, 
Based on All Traits ........................................... 28 
B . Key to Mature Central American Swollen-Thorn Acacias, 
Based on Vegetative Traits .................................. 29 
Acacia cookii Safford ................. ................... 29 
Acacia ruddiae, new species ..................................... 34 
Acacia melanoceras Buerling ..................................... 42 
Acacia allenii, new species .......................................... 53 
Acacia chiapensis Safford ....................................... 61 
Acacia globulifera Safford ........................................ 67 
Acacia sphaerocephala Schlectendal and Chamisso ..................... 73 
Acacia cornigera (L.) Willdenow .................................. 80 
Acacia mayana Lundell ........................................... 93 
Acacia gentlei Standley ....................................... 95 
Acacia collinsii Safford ......................................... 99 
Acacia hindsii Bentham ........................................ 113 
Literature Cited ................................................. 130 

Swollen-Thorn Acacias of 
Central America 
Daniel H. Janzen 
Introduction 
The swollen-thorn acacia group (Acacia species) 
is one of the two most diverse and widespread of 
the neotropical myrmecophytes. These acacias are 
the characteristic ant-plants in the drier lowlands 
(?tierra caliente?) of Central America, just as the 
genus Cecropia (Aloraceae) is the most conspicuous 
and diverse myrmecophyte of wetter habitats in 
Central and South America. 
This monograph on the taxonomy and natural 
history of neotropical swollen-thorn acacias is of- 
fered at this time for several reasons. (1) The 
neotropical swollen-thorn acacias do not appear to 
constitute a tight phjletic unit, yet they share many 
adaptive ecological and morphological traits. They 
are thus outstanding examples of evolutionary con- 
vergence. The most noteworthy of the traits they 
have in common is that they are protected from 
herbivores by colonies of ants in the genus Pseudo- 
myrmex, rather than by chemical traits as are most 
plants (Janzen, 1966a, 1967b; Rehr et al., 1973). (2) 
The ants can be readily removed, just as though 
one were able to easily remove the alkaloids and 
cyanogenic glycosides from the leaves of ordinary 
acacias such as Acacia farnesiana. This ease of ex- 
perimentally disarming the plant means that the 
protective role of the obligate acacia-ants has been 
easy to study and provides an excellent system for 
understanding the coevolution of complex interac- 
tions between higher organisms. For further and 
more detailed comparative studies among the swol- 
Daniel H. Janzen, Department of Zoology, University of Mich- 
igan, Ann Arbor, Michigan 48105. 
len-thorn acacias, a stable nomenclature is essential 
at this time. (3) Detailed data on populations of na- 
tive wild Central American plants are noticeably 
absent from the literature, A better concept is 
needed of the population structures of a wide va- 
riety of tropical plants of different life forms and in 
different habitats in order to generate specific hy- 
potheses that can lead to an understanding of com- 
plex tropical communities. (4) The lowland 
deciduous forests of Central America have nearly 
disappeared. The population structures of the spe- 
cies remaining in these habitats are already greatly 
altered. The swollen-thorn acacias are clearly in this 
category. I would like to summarize the ecological 
behavior of swollen-thorn acacias before their pat- 
terns are so far altered that we can no longer recog 
nize the conditions that produced the genotypes we 
find in today?s cattle pastures, roadsides, and 
abandoned fields. Most wild plants in high-yield 
agricultural areas must be studied under conditions 
not unlike studying elephants and deer in a zoo. 
It is hoped that the incompleteness of the data 
and hypotheses offered here will encourage field 
biologists to look harder at why plants do what they 
do in natural habitats as well as in those badly 
altered by agriculture. 
ACKNOWLEDGMENTS.-~ am greatly indebted to 
Dr. Velva E. Rudd of the National Museum of 
Natural History for her very competent assistance 
and advice throughout this study. Her patience 
with the bumblings of an entomologist in the world 
of taxonomic botany immensely facilitated the un- 
raveling of the nomenclatorial spaghetti surround- 
ing the swollen-thorn acacias. Personnel in various 
1 
SMITHSONIAX CONTRIBUTIOXS TO BOTANY 
herbaria have kindly allowed me to examine types 
and at times to borrow specimens. 
A special acknowledgment is due Karen Janzen, 
who participated in the laborious task of recording 
field data and voucher specimens. 
This study was supported by National Science 
Foundation Grant GB-5206, and is in part a by- 
product of NSF Grants GB-91, 1428, 7805, and 
7819. Substantial portions of this study were com- 
pleted while I was serving in Costa Rica as a faculty 
member for the Organization for Tropical Studies 
in Costa Rica (1965-1971). The paper is Contribu- 
tion No. 1360 from the Department of Entomology, 
The University of Kansas, Lawrence, Kansas. 
General Biology 
The 12 species of swollen-thorn acacias in aggre- 
gate range from the Tropic of Cancer on both 
coasts of Mexico only as far south as northwestern 
Colombia. They are generally found at elevations 
below 1200 m (Figure 1). One Argentinian acacia, 
Acacia cauen (Molina) Molina, has been suggested 
as a swollen-thorn acacia (Fiebrig, 1909; Chodat 
and Vischer, 1920); however, Wheeler (1 942), 
Havrylenko (in litt.), and Sliillink (in litt.) have 
convinced me that the ants living in the thorns 
were only fortuitous invaders. My own unpublished 
observations of A. cazlen in Chile give no hint of 
myrmecophytism with this plant. Greenhouse A. 
cazlen (from Argentinian seeds) shows thorn and 
leaf development like that of normal acacias and 
therefore will not be discussed further. The African 
swollen-thorn acacias, or ?whistle-thorn? acacias 
(e.g., A. seyal, A. drepanolobium, A. formicarum, 
A. bussei), appear to have a mutualistic interaction 
with ants of the pantropical myrmicine genus 
Crematogaster (Hocking, 1970; Monod and 
Schmitt, 1968; Bequaert, 1922) . This is a clear case 
of mild convergence on the Central American sys- 
tem and is excluded from generalizations made in 
this paper. This restricted view should not, how- 
ever, be taken to suggest that hypotheses based on 
the much better understood Central American 
system are irrelevant to the African system. 
The pantropical and subtropical mimosoid le- 
gume genus Acacia contains at least 750 species, 
though the number may be reduced substantially if 
the synonymy found in this study is representative 
of the remainder of the genus. Plants of the genus 
Acacia are primarily in areas with less than 3 me- 
ters of annual rainfall and with a well-developed 
dry season of 3-11 months duration. Throughout 
Africa and the neotropics, those species with foliar 
nectaries (probably all species) have a loose associ- 
ation with ants. Many genera of arboricolous ants 
(e.g., Crematogaster, Pseudomyrmex, Camponotus) 
visit these nectaries and in the process probably in- 
crease the intensity of their foraging activity on 
these plants compared with adjacent plants lacking 
nectaries. This, accompanied by similar benefits 
from parasitic Hymenoptera attracted to the nec- 
taries, has probably been the selective force leading 
to the evolution of these nectaries (cf. Janzen, 
1966a; Bequaert, 1922). 
In a few cases, however, the ant and acacia inter- 
action has evolved to a much higher level of com- 
plexity, that of obligatory mutualism (Janzen, 
1966a). For background information, a brief resume 
of the interaction in Central America is given in 
the following paragraphs. More details can be ob- 
tained from Janzen?s (1967b) lengthy description of 
Acacia cornigera and Pseudomyrmex ferruginea in 
lowland eastern Mexico and in peripheral papers 
on ants and acacias (Janzen, 1967a, 1969a, 1969b, 
1973; Beulig and Janzen, 1969). 
A colony of obligate acacia-ants lives in the swol- 
len, stipular thorns of one to several individual 
shrubs or small trees that have been given the sub- 
generic common name of ?swollen-thorn acacia? 
(Janzen, 1967a). New World obligate acacia-ants 
are a behaviorally distinct group (at least 13 species: 
P. belti, P. ferruginea, P. nigrocincta, P. venefica, 
P. satanica, etc.-Janzen, MS) within the exclusively 
New World, and mostly tropical, genus Pseudo- 
myrmex (150-200 species). Most species of Pseudo- 
myrmex have small colonies in dead hollow stems 
above the ground in habitats ranging from brushy 
pastures to well-developed forest. The exceptions, 
besides obligate acacia-ants, are those with an obli- 
gate mutualistic interaction with the neotropical 
trees Trip 1 ar is (Pol ygonaceae) and Tach iga lia 
(Leguminosae). I have never found any of the many 
species of obligate acacia-ants as a viable colony 
living elsewhere than in a swollen-thorn acacia. 
Those rare cases where swollen-thorn acacia pop- 
lations are unoccupied by obligate acacia-ants are 
discussed in later parts of this paper. 
New World swollen-thorn acacias differ morpho- 
logically and behaviorally from other acacias in 
XUMBER 13 3 
FIGURE 1 .-General distribution of swollen-thorn acacias in Central America, based on herbarium 
records, field observations, and inferences from general vegetation. Contemporary distributions 
are usually continuous throughout the areas in black, owing to human destruction of the pri- 
mary forest that previously isolated the swollen-thorn acacia subpopulations along river edges 
and in other kinds of disturbed vegetation. 
that they have: (1) large and partly hollow stipular vested by the ants-these are the protein, lipid, and 
thorns that the ants clean out through an entrance major carbohydrate sources of the ant larvae; (3) 
hole cut in one thorn tip while the thorn is still greatly enlarged foliar (petiolar) nectaries that are 
green and relatively soft; (2) modified leaflet tips continually secreting the nectar that constitutes the 
called Beltian bodies (see Rickson, 1968, 1969, 1971, primary carbohydrate and moisture source of the 
for micromorphological details), which are har- adult ants; (4) year round leaf production and 
4 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
retention by most individuals, yielding a relatively 
continuous food source for the ants; and (5) lost 
most of the chemical and structural traits that pro- 
tect other acacias from most herbivores (cf. Rehr 
et al., 1973). Swollen-thorn acacias differ ecologically 
from other acacias in that in natural habitats they 
cannot survive to reproductive maturity without 
the patrolling activities of a large colony of obligate 
acacia-ants. The ants remove herbivores (primary 
insects) and intrusive vegetation by biting and 
stinging the invading pait of the plant (as do their 
relatives, Pachysinza, living in Barteria trees in 
Africa-Janzen, 1972). By analogy, the ants sub- 
stitute for the chemical and physical abilities of 
most acacias to deal with their predators and com- 
petitors. The one exception to the above generaliza- 
tions, A. rziddiae, will be discussed in its own species 
account. 
The species of obligate acacia-ants are not specific 
to a swollen-thorn acacia species, but rather to its 
life form. For example, A. collinsii has at least eight 
species of obligate acacia-ants living in it over its 
range from Mexico to Colombia, and all of these 
ants also live in other swollen-thorn acacias. In 
nature, one ant colony may be found occupying 
two adjacent individuals of two different swollen- 
thorn acacia species. The only species of swollen- 
thorn acacia that are occupied by only one species 
of ant are A. melanoceras (occupied by Pseudo- 
inyrmex sntanica) and A. allenii (occupied by 
Pseudonzy~mex ferruginea) . They appear to be this 
way because the climate or resources will not sup- 
port other ant species, rather than because the 
plants are unsuitable for other obligate acacia-ants. 
Both of these species of acacias will support other 
obligate acacia-ants in the greenhouse. 
Swollen-thorn acacias grow in natural and man- 
made disturbed vegetation-both primary and 
secondary succession. A. cornigera, A. hindsii, and 
A. collinsii have very successfully invaded brushy 
pastures, roadsides, and fallow fields, and may con- 
stitute a major part of the standing biomass in 
some l-5-year-old fallow regeneration. On the other 
hand, A. allenii, A. melanoceras, and A. ruddiae 
are apparently in danger of extinction because they 
do not maintain a breeding population except in 
very lightly disturbed, or undisturbed, primary rain 
forest. 
In contrast to most other acacias, swollen-thorn 
acacias originally inhabited naturally disturbed 
sites in forests-deciduous to evergreen-with 2-5 
m annual rainfall. Other acacia species usually live 
in drier sites. The ability to compete with rapidly 
growing vegetation in wetter habitats is the direct 
result of the behavior of the occupant acacia-ants. 
In pruning vines out of the swollen-thorn acacia 
crown and in killing intruding branches from 
neighboring trees, they place the acacia in a highly 
insolated microhabitat (this is, incidentally, a form 
of allelopathy by the acacia-Janzen, 1969a). Sec- 
ond, since the growing points of the acacia are very 
heavily patrolled (protected) by ants, they lack the 
usual structural toughness associated with protec- 
tion from herbivorous insects that is found in other 
acacia shoot tips. This tenderness is associated with 
a swollen-thorn acacia rate of vertical height incre- 
ment high enough to make the acacia an effective 
competitor with other plants in wet sites. Ironically, 
this high growth rate would probably not be possi- 
ble in drier sites, and in drier sites the acacia would 
not be able to support the ant colony owing to loss 
of leaves during the dry season. From the wettest to 
the driest sites the species are: melanoceras, allenii, 
ruddine, cookii, gentlei, chiapensis, globulifera, 
hindsii, mayana, sphaeyocephala, cornigera, and 
collinsii. This progression also ranges from those 
with the smallest geographic ranges to the largest, 
except for A. mayana and A. sphaerocephala, which 
have minute ranges. 
It is noteworthy that some swollen-thorn acacias 
have the largest leaves of any acacia species. This 
may be useful in the cloudy weather which is more 
probable in the wetter sites than drier habitats; it 
is also ?allowed? by the generally reduced problem 
of foliar water loss in wetter habitats. In the driest 
swollen-thorn acacia habitats, many of the larger 
leaves are dropped during the dry season, and then 
axillary clusters of very small leaves provide nectar 
and Beltian bodies for the ant colony. Very few 
swollen-thorns and their associated large leaves are 
produced during the dry season. 
Even though they occupy wetter habitats than 
other acacias in general, swollen-thorn acacias dis- 
appear when annual rainfall exceeds about 4 m 
(e.g., the Puerto Viejo region of northeastern Costa 
Rica) and the dry seaeon is less than two months? 
duration. The reason for this probably centers on 
the decreasing number of sunny hours per day as 
rainfall increases. When days are overcast in rain- 
forest areas, the amount of light reaching small 
NUMBER 13 
5 
shrubs on the forest floor is very low, and it may 
be that the swollen-thorn acacia has been unable to 
convert its primitively heliophile physiology to that 
of total shade tolerance. 
Their upper elevational limit is about 800 to 
1200 m, depending on the moisture and tempera- 
ture conditions at upper levels. The wetter and 
colder an elevational transect, the quicker the 
swollen-thorn acacias disappear from the vegetation 
with increasing altitude. Cold weather does not 
kill the ants on the acacias but restricts patrolling 
activities to the point where the acacia is either 
covered over by the rising vegetation, defoliated 
severely, or both. All species are found at sea level 
in at least some part of their range; their upper 
elevation limits vary widely, with A. globulifera, 
A. sphaerocephala, and A. cornigera attaining the 
greatest heights. 
In contemporary communities (disturbed by 
man), swollen-thorn acacias reproduce very com- 
monly by stump and root suckers and only rarely 
by seedlings. In naturally disturbed sites (where 
clearing of the vegetation is not periodically re- 
peated), however, the rare seedlings are the usual 
means of population recruitment. Though a large 
seed crop is produced annually, most seeds are 
killed by bruchid beetles, against which the ants 
offer no defense (Janzen, 196713, 1969~). There is a 
very high mortality rate of seedlings until the trees 
become occupied by an ant colony. Sucker shoots 
(often from lateral roots) are often near a large 
obligate acacia-ant colony, which simply incorpo- 
rates the sucker into the colony?s territory; this is 
especially noticeable with the obligate acacia-ant 
species that have multiple queens and thus un- 
limited colony size (e.g., Pseudomyrmex venefica in 
A. hindsii in western Mexico north of the Rio 
Balsas basin; Janzen, 1973). An unoccupied seedling 
must wait 9 months or longer for a founding queen 
ant to grow a young colony and by then the seed- 
!ing is often well below the surrounding vegetation 
and therefore at a severe competitive disadvantage. 
Sucker shoots produce their first seed crop about 2 
years earlier than do seedlings. Adults of either 
seedling or sucker shoot origin generally live 15-35 
years. 
Swollen-thorn acacias flower during the dry sea- 
son, as do so many other deciduous forest woody 
plants (Janzen, 1967c), and are pollinated by rela- 
tively unspecific pollen-collecting bees. In the 
greenhouse, swollen-thorn acacias never set fruit 
though they flower copiously. I inEer from this that 
they are obligatory outcrossers. The bees are 
occasionally attacked by a worker ant, but these 
bees are persistent in getting at the flowers, and 
the ant?s aggressiveness probably does little more 
than maximize cross-pollination by forcing bees 
to visit several trees. The small pods remain on 
the receptacle until the following dry season, 
growing large and maturing several months be- 
fore the next crop of flowers. There may be 
mature pods on the tree with the flowers, but these 
are from the previous year except in the species in 
very wet sites, such as A. allenii, A. ruddiae, and 
A. melanoceras. The seeds are distributed by birds 
that either eat the pod entirely (only A. cornigera 
and A. sphaerocephala) or eat the cluster of seeds 
imbedded in a sweet moist pulp that is yellow or 
white pulp. Swollen-thorn acacias appear to be the 
only neotropical acacias with bird dispersed seeds 
(Janzen, 1969b). If it were not for bird dispersal, 
there would be virtually no survival of seeds be- 
cause the bruchid beetles will kill virtually every 
seed that remains on the tree (Janzen, 1969c, 1971). 
Swollen-thorn acacias have a representative 
flowering phenology. At any given site, nearly all 
mature plants bear open inflorescences for 4-8 
weeks at about the same time each year. This 
flowering period may start anytime during the first 
2 months of the dry season, depending on the spe- 
cies and the area. For a given species and area the 
timing is the same from year to year. A single 
species may show quite extreme variation from site 
to site in both the absolute dates of flowering and 
the dates relative to onset of wet and dry season. In 
general, however, at least 80 percent of the sub- 
populations of swollen-thorn acacias flower during 
the dry season. 
In addition to the major peak of flowering shown 
by each species at each site, there are almost invari- 
ably some individual acacias that are out of phase 
with the remainder of their population. These are 
usually damaged, growing on locally peculiar drain- 
age types (creek bottom or hill crest) or senescent. 
They are often helpful in taxonomically identifying 
the population, but are very misleading in under- 
standing the morphology, ecology, and behavior of 
the population. Botanists have a tendency to collect 
these flowering or fruiting plants in preference to 
sterile individuals. This leads to erroneous records 
6 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
of flowering times. These individuals often set little 
or no seed, which gives a false impression of seed- 
crop size. Those seeds that are produced are some- 
times only mildly attacked by bruchids, giving the 
false impression that seed mortality is trivial for the 
population (see Janzen, 1968, for a discussion of 
this point among plants in general). These plants 
also may bear a reduced number of inflorescences 
per axil and inflorescence-bearing branch, and these 
branches are often sparser and smaller than on 
healthy acacias. 
Swollen-thorn acacias have very thoroughly oc- 
cupied most of the Central American habitat that 
looks as though it should support them; however, 
they have not yet spread into the semiarid areas of 
South America that are presently occupied by the 
species of plants that co-inhabit their Central 
American habitats. Their absence from the decidu- 
ous and semideciduous forests of Venezuela, Brazil, 
and Colombia is particularly striking. It seems rea- 
sonable to expect that these areas are not inimical 
to swollen-thorn acacias, but rather that they simply 
have not yet gotten there. A. collinsii in Colombia, 
however, must surely be a seed source for the re- 
mainder of the country. If swollen-thorn acacias 
were not such an economic pest in brushy Central 
American pastures, the obvious transplantation ex- 
periment would be well worthwhile. 
Swollen-thorn acacias have not been generally 
successful at colonizing islands. They occur on off- 
shore islands such as those in the Gulfo de Nicoya, 
Costa Rica, in the Gulfo de Fonseca, Honduras, and 
off the eastern coast of Yucatan, Mexico. The only 
far island they have reached is Isla Providencia, 
Colombia, in the Caribbean (about 125 miles off 
the coast of Nicaragua). A. collinsii is here occupied 
by one or two species of obligate acacia-ants. In 
colonizing islands, the acacia must already be there 
when the founding ant queen arrives; however, the 
acacia probably has little chance of surviving to 
reproduction unless occupied by an ant colony. It 
will take a high rate of simultaneous colonization 
by both ants and acacias for establishment. A. 
cornigera, introduced as an ornamental into Cuba 
(and Florida) without ants, has not escaped to form 
a noticeable population. While this may be in part 
due to a lack of pollinators, the absence of obligate 
acacia-ants may also contribute. 
Since agricultural man came on the scene (at 
least since 1200-900 B.c.-Coe et al., 1967), the 
population structure of most species of swollen- 
thorn acacia has probably been grossly altered. The 
majority of species have greatly increased their local 
density, absolute population sizes, and thoroughness 
of geographic coverage by invading fields, pastures, 
and roadsides which now connect the natural dis- 
turbance sites that occur along rivers, streams, and 
lakes. This means that the populations now ob- 
served are operating with a genotype evolved in 
slightly to grossly different habitats, and the local 
barriers to gene flow (climax forest on hills and 
ridges) have in great part been removed. 
Present-day populations of swollen-thorn acacias 
show distinct clinal variation in traits such as leaflet 
size, thorn size and shape, life form, seed-pod 
morphology, and Beltian body traits. Rather than 
being solely the result of extreme sensitivity to 
small linear environmental changes, these may be 
the consequence of the recently established genetic 
continuity between previously isolated populations 
on different drainage systems. 
Humans have probably not severely altered the 
gross geographic range of swollen-thorn acacias by 
their activities. They have, however, served as 
efficient long-range-dispersal agents. Fruits of A. 
cornigera are eaten by Central American indians 
and, with modern bus transport being common- 
place, seedlings are sometimes found near bus stops 
several hundred miles from a known acacia popula- 
tion. Further, cattle are often trucked long distances 
in Central America, and cattle eat the pods of A. 
cornigera and others. There are pockets of A. 
cornigera around cattle corrals scattered through 
the northern part of the Yucatan Peninsula; the 
natural vegetation at these sites is normally occu- 
pied by A. collinsii (though A. cornigera appears 
native to local wet sites in Yucatan). 
General Comments on Swollen-Thorn Acacia 
Taxonomy 
A close examination of the taxonomic status of 
swollen-thorn acacias is warranted by the fact that 
(1) as many as 25 percent of the swollen-thorn 
acacias in some herbaria are incorrectly determined, 
(2) there are at least 32 synonyms for the 10 previ- 
ously described species, (3) swollen-thorn acacias 
have never been treated with monographic thor- 
oughness (though there are short discussions in 
Safford, 1910, 1914, 1915, 1922; Rudd, 1964; 
NUMBER 13 
7 
Schenck, 1913; Janzen, 1967b), and (4) their spe- 
cies-level taxonomy has become so confused that it 
is currently nearly impossible to generate a detailed 
discussion of the mutualistic interaction in a com- 
parative and evolutionary framework. 
Examination of the distributions of the type 10- 
calities on the range maps of each species of swollen- 
thorn acacia leads one to suspect what is shown by 
examination of the types, namely that most syno- 
nyms are the result of the taxonomist possessing a 
herbarium sheet from two widely separated portions 
of a species population but having no information 
on the clinal changes across the connecting part of 
the population. Further taxonomic difficulties were 
generated by the greatly differing thorns that may 
be found on a single plant. There is no evidence 
that we are dealing with many sibling species but 
rather that there is extreme geographic variation of 
several conspicuous vegetative traits. Finally, the 
more conservative (reliable) vegetative traits of 
swollen-thorn acacias have not generally been used 
as taxonomic characters. 
The early parts of this study emphasized the 
location and delimitation of populations of swollen- 
thorn acacias in Central America. Once these pop- 
ulations were understood geographically and 
morphologically, then the type specimens of swol- 
len-thorn acacias were associated with them in the 
herbaria. Since virtually all the variation encoun- 
tered in this study was clinal, and since the clines 
showed little or no concordance with the subpopula- 
tions of a given species (being contiguous almost 
without exception), there is no basis for the designa- 
tion of infraspecific Latin names. Fortunately, the 
breaks between species are clearly delimited by 
many morphological, behavioral, and ecological 
traits in the numerous areas of sympatry. 
The species discussed here are clearly morpho- 
species, but all circumstantial and ecological evi- 
dence indicates that they are independent 
evolutionary units that do not normally exchange 
genetic information. The various species grow side 
by side over much of their ranges with almost no 
obvious hybridization, though ant-acacias may in- 
trogress with nonant-acacias on occasion (e.g., A. 
hindsii X A. pennatula (Schlect. and Cham.) Benth. 
and A. hindsii X A. cochliacantha Humb. and 
Bonpl. in western Mexico). Those species that have 
no range overlap with other swollen-thorn acacias 
are presumed to be distinct species because of the 
P 
BRACTEOLE 
PULVINUS 
FIGURE 2.-Swollen-thorn acacia terminology. 
many morphological and ecological differences be- 
tween them and other acacia populations. 
The total morphological variation within the 
populations of some of the widely distributed spe- 
cies (e.g., A. cornigera, A. collinsii, A. hindsii) is 
much greater than that within pairs of closely 
related species (e.g., A. allenii and A. melnnoceras, 
A. nzayana and A. cornigera). This, plus the im- 
possibility of designating what might be called a 
?species-level? trait, prevents a uniform criterion 
for the designation of ?species.? 
Swollen-thorn acacias are readily determined to 
species in the field, either in vegetative or sexual 
phases. In fact, it is easier in the field than from 
herbarium specimens. Because of this, only oc- 
casional voucher specimens were collected in this 
study. Since swollen-thorn acacias are generally 
recognizable even from a fast-moving car, continu- 
ous distribution records along roads were often 
obtained in this study and are plotted on the 
distribution maps as continuous data rather than as 
?spot localities? as are usually taken from her- 
barium specimens. In most parts of Central 
America, the total absence of swollen-thorn acacias 
for more than 5 km along a road or watercourse 
means that they are absent for reasons other than 
persistent attempts by humans to remove them. 
Their absence within areas of secondary vegetation 
of a few hectares in extent may be due to nonhuman 
FIGURE 3.-Representative leaves subtended by type A swollen thorns: A, B, Acacia cookii; c, A. 
hindsii; D, A. gentlei; E, A. ruddiae; F, A. globulifera; G, A. rnelanoceras; H, A. collinsii; I, A. 
cornigera. (All photographs taken to same scale; the leaf of A. cookii in upper left-hand corner 
is 40 cm long.) 
FIGURE 4.-Representative leaves subtended by type A swollen thorns: A, Acacia allenzi; B, A. 
maynna; c, A. cliiapensis; D, A. sphaevocephala; E, A. chiapetzsis. (Scale as in Figure 3.) 
10 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
TABLE 1.-Meristic traits of two representative fully developed leaves subtended by type A thorns on 
lateral branches of occupied adult swollen-thorn acacias (mean for large samples are not presented since 
this would be a study in itself; these two leaf exemplars are judged to be representative by my own obser- 
vations) 
- - - 
Total Total Total Length X X X Total Number of Number of 
Species and thorn length 
(mm) 
allenii (55+56) . . . . . . . , 
allenii (50 + 49) , , . , , . , , 
hindsii (41 +42) . . . . . . . . 
hindsii (36+38) . , . . , , . . 
cookii (51 + 48) . . . . . . , , 
cookii (67 + 71) , , , , , , , . 
mayana (51 +48) . . . , , . . 
collinsii (46 + 48) . . . . . . . 
collinsii (42+43) , . . . . , . . 
gentlei (48 + 48) . . . . . . , 
gentlei (38+39) . . . . . . . . . 
globulifera (43+44) . . , . . 
globulifera (39+33) . . . . . 
cornigera (45 +47) , , , . . , . 
cornigera (43 + 42) . . . . . . . 
ruddiae (no thorn) . . . . . . 
ruddiae (no thorn) . . . . . . 
chiapensis (48+43) , . . . , . 
chiapensis (79+75) , , . , . , 
sphaerocephala (63+63) 
sphaerocephala (54+52) , 
melanoceras (36+34) . . . . 
melanoceras (32 + 45) , , . . 
- 
leaf number number longest length length width pinnules nectaries nectaries 
ength pinnae pinnules pinna pinnae pinnules pinnules on longest on rachis on petiole 
(mm) (mm) (mm) (mm) (mm) pinna 
383 84 1170 98 70.7 10.53 3.08 47 19 4 
40 1 
178 
150 
404 
367 
225 
101 
101 
100 
106 
252 
228 
110 
81 
205 
258 
288 
190 
155 
149 
255 
237 
38 
42 
42 
96 
58 
20 
20 
22 
14 
16 
48 
52 
20 
12 
38 
74 
74 
42 
26 
24 
48 
46 
1?630 
1924 
1912 
12242 
7013 
1049 
588 
632 
784 
988 
4007 
4080 
848 
536 
3200 
8180 
7644 
2955 
952 
1072 
2134 
1592 
121 
47 
42 
69 
87 
100 
42 
47 
65 
58 
62 
62 
54 
55 
67 
62 
63 
48 
54 
53 
49 
35 
81.1 
36.3 
35.6 
61.5 
77.5 
72.5 
33.6 
36.6 
55.0 
52.1 
49.2 
46.9 
43.2 
50.5 
61.4 
47.6 
46.7 
39.2 
36.7 
46.1 
38.9 
28.8 
10.30 2.94 63 
4.46 1.58 56 
4.17 1.26 52 
4.04 0.60 140 
6.82 1 .oo 136 
10.90 1.93 66 
6.39 1.89 34 
8.18 2.22 38 
8.45 1.98 62 
8.53 1.93 64 
5.51 1 .oo 89 
5.14 1.00 96 
7.09 1.61 52 
7.26 1.82 48 
5.77 1.18 88 
2.87 0.80 144 
2.83 0.77 130 
4.46 1 .oo 88 
6.34 1 .go 46 
6.28 1.76 52 
4.90 1.12 54 
5.46 1.10 42 
28 
21 
21 
54 
28 
9 
1 
0 
4 
8 
14 
1 
4 
20 
37 
35 
0 
0 
0 
25 
24 
a 
?3 
0 
0 
0 
1 
1 
4 
1 
2 
4 
2 
1 
1 
2 
0 
4 
14 
1 
I 
18 
21 
8 
factors, but more likely is due to repeated cutting 
immediately followed by burning. Relative changes 
in density are especially susceptible to human ac- 
tivity. It is therefore very difficult to determine 
what climates and/or habitats are ?best? for 
swollen-thorn acacias. Cutting, burning, and graz- 
ing patterns up to at least 20 years previous may 
grossly influence both density and presence of 
swollen-thorn acacias. There are even cases of heavy 
insecticide use, where the ants, and apparently most 
other insects, have been killed and the unoccupied 
acacias are relatively healthy (e.g., A. hindsii in the 
Leon area of western Nicaragua, along cotton field 
edges). 
Some of the herbarium specimens collected during 
this study have been deposited in the National 
Museum of Natural History of the Smithsonian 
Institution (US), Gray Herbarium (Harvard) (GH), 
New York Botanical Garden (NY), University of 
California at Berkeley (UC), Missouri Botanical 
Garden (Mo), Field Museum of Natural History (F), 
University of Michigan (MICH), and University of 
Mexico (MEXA). The majority have been deposited 
as intact series at the Field Museum. 
Despite their spectacular association with ants, 
swollen-thorn acacias have been poorly collected 
and generally receive little attention in regional 
floras. They are not famous for medicinal uses, 
which is not surprising since the ants are their 
?chemical? defense system. Collectors consistently 
avoid them (or more properly, avoid their ants). 
Those parts of the plants that are collected tend to 
be old foliage and branches, which are poorly 
patrolled by the ants and often unrepresentative of 
the foliage of the plant as a whole. The branches 
with well-developed swollen thorns, especially those 
with very large thorns on short laterals, are almost 
entirely missing from herbaria, presumably because 
they do not press easily and usually add a large 
number of ants to the plant press. This produces 
the illusion, when examining herbarium material, 
that the thorns of swollen-thorn acacias are gen- 
NUMBER 13 
11 
FIGURE 5.--Shoot tip from end of an elongating healthy 
branch: A, Acacia coo& frorn the Cardenas, Tabasco, Mexico, 
region; B, A. allenii from the Rincon, Osa Peninsula, Punta- 
renas Province, Costa Rica, region. 
erally small, monomorphic, and without much 
geographic or intraplant variation. 
Wheeler (1942) reviewed the generally undistin- 
guished presence of swollen-thorn acacias in the 
literature prior to 1930. Acacia cornigera was the 
first to be described (Linnaeus, 1753) as Mimosa 
cornigera, but since it was based on a European 
greenhouse specimen, the ant interaction was not 
discussed at that time. The last species to be de- 
scribed in the current literature, A. mayana in 
1937, is the rarest and most restricted of the swollen- 
thorn acacias. The two species described in the 
present paper (A. allenii, A. ruddiae) had been 
collected earlier, but the herbarium specimens were 
misidentified. 
Herbarium specimens are often misleading in re- 
spect to the ant-acacia interaction. There is gen- 
erally no way of knowing if the specimen is from a 
heavily occupied acacia, one that has recently lost 
its ant colony, or one that was never occupied. The 
presence of ant-entrance holes in the thorns are not 
reliable indicators of occupation because founding 
queens may have made them and because, once 
made, they last as long as the thorn (2 or more 
years). Occupied acacias (those with an ant colony 
large enough to patrol the tree) have fully devel- 
oped lateral branches, short lateral branches with 
very large thorns, inflorescence-bearing branches, 
etc. Unoccupied ones have stunted branches with 
short internodes, smaller leaves, fewer inflorescences 
at times only partial damage and shading; directly, 
they are not a product of the ants. 
From 60 to 95 percent of the mature swollen- 
thorn acacias of each species examined in this study 
were occupied by obligate acacia-ants. The excep- 
tions are A. ruddiae, which is never occupied by 
obligate acacia-ants, and A. sphaerocephala. Where 
the latter species grows on the dunes south of Vera- 
cruz, Veracruz, Mexico, it is unoccupied within 
range of heavy wind and salt spray on leading 
dunes, but tenanted by Crernatogaster. Those plants 
tenanted by other ants (they often use the hollow 
thorns for nest sites as they would use hollow 
branches on other trees) are unoccupied from the 
plant?s viewpoint. The statements about vegetative 
characters in this paper are drawn from healthy 
and occupied acacias. Numerically, the majority of 
any swollen-thorn acacia population in repeatedly 
disturbed sites is made up of unoccupied seedlings 
and sucker shoots, but these are almost never no- 
ticed or collected because they are less than 50 cm 
tall and below the general weed canopy (mature 
acacias are usually emergent). These small plants 
are often much more difficult to identify to species 
than are the adults, owing to their convergence in 
leaf and thorn characteristics. 
The swollen-thorn acacias did not escape from 
Britton and Rose?s (1928) attempts to split the genus 
Acacia into more manageable subunits, The genera 
Tauroceras and Myrmecodendron were proposed 
but are not retained in the present study because 
they represent only one of the many ways the 12 
species could be arbitrarily grouped, and thus add 
little to the understanding or ease of manipulation 
of the morphological affinities within the swollen- 
thorn acacias. 
Perhaps of more importance in subgeneric taxon- 
omy, the evolutionary affinities of the 12 species 
are not clear in most cases, and their speciation 
may have involved some very interesting complica- 
tions. Despite their possession in common of many 
unique vegetative traits-Beltian bodies, swollen 
thorns, etc.-the swollen-thorn acacias appear to be 
a polyphyletic group within the genus Acacia. Pairs 
or triplets of species appear to be more similar in 
the majority of their traits to acacia species not 
12 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE &-Variation in nectary morphology on mature A. globulifera plants (and see Figure 
7c): A, maximal number of petiolar nectaries, as occasionally found on old trees in tufts of 
leaves not subtended by swollen thorns (Honduras, site 26); B, maximal condensation of the 
petiolar nectary area, as characterizes leaves from type A thorns at high (cold) and dry areas 
(Honduras, site 28) ; c, maximal attenuation of the petiolar nectary area, as characterizes leaves 
from type A thorns on rapidly growing 2-4-year-old trees in riparian (wet) sites (Honduras, 
site 26). 
Acacia collinsii sometimes has nectaries similar to Figure B, and A. gentlei sometimes has 
nectaries similar to Figure C (and see Figure 7). 
involved in mutualism with ants than they are to 
other swollen-thorn acacias. Even the designation 
of subgenera based on these small groups seems to 
have little usefulness (e.g., Tauroceras would con- 
tain A. cornigera, A. sphaerocephala, and A. 
may an a). 
The possibility of a polyphyletic origin for 
swollen-thorn acacias is especially noticeable if only 
the reproductive parts are compared. Three expla- 
nations are available. The first, and least likely in 
view of dissimilarity of sexual reproductive parts, 
is that a single swollen-thorn acacia evolved, and 
by classical speciation patterns split into the 12 
modern populations (species) discussed in this pa- 
per. This carries the implausible implication that 
floral and fruit morphology are as evolutionarily 
unconservative as the vegetative parts of the plant, 
and that the similarity of the flowers and fruits of 
swollen-thorn acacias to those of nonant-acacias is 
simple convergence. The second possibility is that 
each cluster of similar swollen-thorn acacias had an 
independently evolved, swollen-thorn acacia paren- 
tal species. Here, the vegetative similarity between 
the clusters would be convergence selected for by 
the requirements of the mutualistic interaction. 
This also seems unlikely since it requires that a 
complex interaction must have arisen several times 
independently. The third hypothesis, and the one 
that I favor, is a combination of the two above by 
the mechanism of introgression. This would yield 
a reticulate phylogeny. Starting with one species of 
swollen-thorn acacia, new acacia species become 
part of the system by introgression of the set of ant- 
acacia traits into their genotype. The outcome is 
two species of swollen-thorn acacia, one of which 
(the donor) is probably not much changed while 
the other may change its ecology considerably since 
it now possesses a very different, and in some ways 
more effective, set of defenses against herbivores 
and competitors. 
NUMBER I3 I3 
FIGURE 7.-A, Representative petiolar nectary on a leaf subtended by a type A thorn of A. col- 
linsii from the moister end of the species? range; the number of nectaries may range from 1-4 
and there never are rachis nectaries (Mexico, site 15). B, Representative petiolar and rachis 
nectary of A. gentlei on a leaf subtended by a type A thorn (Guatemala, site 21). c, Representa- 
tive petiolar and rachis nectary of A. globulifera on a leaf subtended by a type A thorn (Guate- 
mala, site 13). 
14 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE %-A, Representative petiolar and rachis nectaries on leaf from type A thorn of A. hind- 
sii; the number and size of rachis nectaries is variable (Mexico, site 17). B, Representative 
petiolar nectary of A. cornigera; on rare occasions there are two petiolar nectaries and a rachis 
nectary (Mexico, site 6). c, Representative petiolar and rachis nectaries on leaf from type A 
thorn of A. ruddiae; nectaries vary greatly in height (Guatemala, site 8). 
This type of ?speciation? (note that there is no 
net increase in the number of species of Acacia) 
will almost certainly result in a range extension for 
the ant-acacia interaction, and perhaps lead to con- 
tact with, and subsequent incorporation of, other 
nonant-acacia species. It appears to be occurring 
now between A. hindsii and A. pennatula (see later 
discussion of hybrids). This process, coupled with 
the usual kind of speciation, could easily lead to 
the present 12 species. 
Materials, Methods, and Terminology 
A bit of jargon has evolved in the discussion of 
swollen-thorn acacias and needs brief review here 
(also see Janzen, 196713). An ?occupied? swollen- 
thorn acacia is one that has an ant colony large 
enough to patrol its surface with sufficient intensity 
such that the acacia will gain or remain in moderate 
to excellent health. An unoccupied acacia may 
therefore have founding queen ants in its thorns or 
have a large colony of an unprotective ant species 
tenanting its thorns. The ?basal circle? is that 
circular patch of bare ground directly under a 
swollen-thorn acacia that results from the ants 
attacking seedlings and sucker shoots beneath the 
acacia. A ?multiple-queen? ant colony is of one of 
the species that has egg-producing queens in many 
of the tree?s thorns, in contrast to having only one 
NUMBER 13 
15 
queen per colony. Multiple-queen ant colonies tend 
to be associated with swollen-thorn acacias that 
naturally occur as dense thickets, such as A. hindsii 
in western Mexico. 
The stipular thorns of Neotropical swollen-thorn 
acacias are not galls in any sense of the word. Inci- 
dentally, despite the caution displayed by Hocking 
(1970), the enlarged thorns of African acacias are 
not galls either, as shown by their full development 
in North American greenhouses. ??Type A? thorns 
are those more ordinary-appearing thorns to be 
found on long, lateral branches and on the central 
axis; ?type B? thorns are the large and often con- 
voluted thorns found on greatly shortened lateral 
branches. See Figure 2 for other morphological 
characteristics. 
For precise localities, the lists under each species 
should be referred to, rather than to the distribu- 
tion maps. Localities are listed under the state or 
province containing a reference town. The pro- 
jected ranges on the distribution maps are just 
that; they are based on my intuition and the gen- 
eral ranges of other plants from the same habitats. 
They are probably overall correct in general, but 
cannot be expected to be accurate in detail. The 
key collection localities are given in the distribution 
figures as sites or as continuous distributions along 
roads (solid broad lines). Where my own distribu- 
tion records overlap those obtained from herbarium 
specimens, to avoid redundancy I usually have not 
given the herbarium data. The site numbers are 
unique only within a species; for example, site 16 
for A. cornigera is not the same locality as site 16 for 
A. hindsii. While some data are given as to the 
reproductive condition of the plants on the dates of 
observations or collection (fl = inflorescences, gp = 
immature fruits, mp = mature fruits), little reli- 
ance should be placed on these data in determining 
sexual timing owing to the collectors? habit of 
seeking out plants in flower and fruit but out of 
phase with the remainder of the population. 
All scales shown in photographs are in milli- 
meters unless the caption indicates otherwise. 
In the terminology of this paper, ?isotypes? are 
collections made from the same plant at the same 
time, while the ?holotype? is only that herbarium 
sheet with the type on it. ?NC? is used to signify a 
plant examined in the field but not collected. While 
Safford?s descriptions sometimes were not in Latin, 
they were published before 1 January 1935 and are 
still recognized as valid in accordance with Article 
36 of the International Code of Botanical Nomen- 
clature (1972). 
General Morphological Traits 
The leaves of swollen-thorn acacias differ from 
those of other Central American acacias primarily 
in being larger overall and having larger leaflets; 
in short, the leaves extend far past the tips of the 
swollen type A thorns and can be easily removed 
from the tree without contacting the thorns. On 
any given tree the leaves vary strongly in size as a 
function of season (smaller leaves being produced 
during the dry season) and part of the tree (smaller 
leaves being subtended by type B thorns). However, 
if only the leaves produced during the rainy season 
and subtended by fully developed type A thorns 
on lateral branches from healthy trees are consid- 
ered (Figures 3 and 4, Table 1) , there are diagnos- 
tic interspecific differences in leaf and leaflet size 
and shape. Not surprisingly, those species from the 
driest habitats have the smallest leaves (e.g., A. 
collinsii, A. cornigera, A. hindsii). There may be 
considerable variation within a species over its 
range on the parameter of leaf size. In Figure 3, a 
leaf from A. cookii at the northern and dry end of 
its range in the Cardenas, Tabasco, Mexico, area 
(Figure 3~) may be compared with a leaf from the 
wet end of the range in the Puerto Barrios, Guate- 
mala region (Figure 3~). One species, A. chiupensis, 
shows strong variation at a given site as well, as 
earlier discussed for the Temascal, Oaxaca, Mexico, 
area (Janzen, 1967b); this variation (Figures 4c 
and 4E) is probably an expression of introgression 
of Acacia macracantha and A. cornigera genes into 
the A. chiupensis population in this area (and see 
Rehr et al., 1973). 
The shoot tips on elongating branches of swollen- 
thorn acacias are unprotected physically (as well 
as chemically) when compared with other plants. 
The shoot tip of A. allenii in Figure 5~ is repre- 
sentative of swollen-thorn acacias in being free of 
hairs or defensively sticky compounds and in having 
the newly expanding leaves quite separate and ex- 
posed. They are extremely brittle and easily broken, 
reminding one of the inner shoot of a celery plant. 
The shoot tips of A. cookii (Figure 5A) and A. 
ruddiae are more like those of nonant-acacias, with 
the new leaves clasped tightly around each other 
16 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 9.-Ontogenetic development of nectaries of A. collinsii: A, characteristic seedling leaf 
produced at about the 20th node and often not subtended by swollen thorns; B, leaf subtended 
by type A thorn on juvenile but well occupied plant; c, leaf subtended by type A thorn on 
relatively short lateral branch in crown of mature acacia. 
and covered with a dense pile of short stiff hairs. 
In the northern part of its range, A. cookii is some- 
times unoccupied, and in this area the difference is 
more pronounced than in the Puerto Barrios, 
Guatemala, region where the shoot tips appear 
more like those of a regular swollen-thorn acacia. 
The extrafloral nectaries on the petiole 
and rachis of swollen-thorn acacia leaves are 
one of the most useful and reliable characters for 
species-level determinations. While extrafloral nec- 
tary morphology may vary strongly in detail from 
area to area for a few species (the most extreme 
case being A. globulzfeia, Figure 6), for the ma- 
jority of species there is little geographic variation 
and at any given site the morphology is quite con- 
stant; it may be the only way to distinguish between 
young plants of species that are easily separated as 
adults on the basis of thorn shape. Since the entire 
function of the extrafloral nectaries appears to be 
physiological-excreting sugar for the ant colony- 
they are not under strong pressure for adaptive ex- 
ternal morphologies; much as with the thorns, I 
suspect that numerous different kinds of external 
morphology may be equally functional and that the 
interspecific variation is primarily due to inter- 
specific differences in the adaptive peaks from which 
the different swollen-thorn acacia genotypes were 
derived. The extrafloral nectaries in Figures 7 and 
8 and in Table 1 are representative of the gamut 
of interspecific variation. As a seedling grows, there 
is a progression of extrafloral nectary development 
(young sucker shoots from cut stumps tend to have 
NUMBER 13 17 
FIGURE ~O.-A, Cross section of a 5- 
year-old main trunk of A. hindsii 
(Mexico, site 17). 8, Conspicuous 
lenticels on 6-month-old branch of 
A. ruddiae (Costa Rica, site 5). 
FIGURE 1 1.-Representative bark topography of 2-5-year-old main trunks of swollen-thorn aca- 
cias: A, Acacia cornigera (Mexico, site 26) ; B, A. gentlei (Guatemala, site 11); c, A. chiupensis 
(Mexico, site 9) ; D, A. hindsii (Mexico, site 15) ; E, A. ruddiae (Costa Rica, site 5) ; F, A. al- 
lenii (Costa Rica, site 5). 
18 
Species 
mayana , , . . , . . , . . . 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
Knobby Thorn 
projection shape in 
Thorn on inner cross 
surface External thorn wall section General thorn 
contour pubescence at base near base configuration 
smooth c+ none present elliptical flat V that is bent downward 
slight 
ridges 
smooth 
smooth 
with 2 sharp 
elongate 
projections 
absent round to 
slightly 
elliptical 
slightly in larger thorns; 
never recurved around stem 
gentlei . . . . . . . . none flat V, slightly distorted and bent 
irregularily in the plane of the 
V; thorn may be closely appressed 
to stem longitudinally and partly 
bent around it 
,... 
,... 
..., 
.... 
globulifera . . , . 
melanoceras . . . 
cookii . . . . . , . . 
absent round flat V, slightly to strongly curved 
downward; never recurved 
around stem 
flat V, slightly curved downward 
in larger thorns 
heavily warped U, asymmetrical 
with one side or both twisted 
around the stem partly or fully; 
asymmetry highly regular in 
pattern 
flat V, tendency for one thorn half 
to be larger than the other; never 
recurved about stem 
none 
smoothed 
longitudinal 
ridges 
smooth 
smooth c 
slight ridges 
sometimes 
smooth 
smooth 
smooth 
none 
heavy 
present round to 
elliptical 
absent round 
ruddiae very light absent round to 
slight 
ellipse 
round chiapensis .... 
.... 
,... 
none absent flat V that may be bent slightly 
downward in very large type B 
thorns 
flat V; never recurved around the 
stem 
flat V or U, bent upward or down- 
ward to various degrees; type B 
thorns often heavily recurved 
around stem, at right angles to it 
sphaerocephala 
cornigera . . . . . 
none 
none 
absent 
absent 
round 
round to 
slightly 
elliptical 
collinsii none usually 
present 
smooth 
smoothed 
longitudinal 
ridges 
smooth to 
faint 
smoothed 
longitudinal 
ridges 
round to 
elliptical 
to very 
elongate 
and thin 
elliptical 
round to 
flat V that may be very open and 
curve around the stem at various 
angles 
allenii . . . . , 
hindsii . . . . 
flat V; never recurved around the 
stem 
none present 
none absent elliptical to 
very elongate 
and thin 
flat V; never recurved around the 
stem 
NUMBER 13 
on adult 
plant over 
entire range 
brown, nearly 
black; trace 
of red on 
green thorns 
dark brown, 
nearly black 
to ivory 
white; bicolor 
beige + ivory, 
brown+ ivory 
ivory; darker 
thorns with 
reddish cast 
ivory to beige 
to black 
very dark 
brown to 
very dark 
brown, nearly 
black; no 
trace of red 
on green 
thorns 
red to brown 
to very dark 
brown 
19 
foliar nectaries and other leaf traits essentially 
identical at the qualitative level to those of the 
adult crown). A characteristic progression is dis- 
played in Figure 9; in general, with increasing age 
of the plant, the number of nectaries increases, their 
size increases, the distance between the first pair of 
pinnae and the thorn decreases, and nectar flow 
increases. 
Swollen-thorn acacias are more ?treelike? than 
other Central American acacias in that they tend 
to have more of the growth concentrated in the 
central axis (see figures in Janzen, 1967b). The 
wood is harder than that of most fast-growing 
tropical trees and generally displays obvious annual 
growth rings (Figure 10~); the slower wood produc- 
tion coincides with the dry season and, in areas 
with almost no dry season, the rings are much more 
difficult to see if they are present at all. The tree 
bark (Figure 11; Table 5) is thin and often green- 
ish; presumably considerable photosynthesis occurs 
just under the bark. There are well-developed 
lenticels on young branches (Figure ~OB), and many 
of these persist on trunks 2-6 years old (e.g., Figures 
The swollen thorns on adult acacias are very 
distinctive at the species (and sometimes local- 
population) level, but the differences are difficult 
to quantify and incorporate in a dichotomous key. 
Tables 2 and 3 attempt to organize some of these 
differences, but for a good feeling for the differences 
the reader must also compare the thorn photographs 
associated with each species account. Thorn color 
is perhaps one of the most useful diagnostic species- 
level traits at a given site, but varies strongly with 
location and the plant?s history. Damage to green 
thorns may produce premature darkening of the 
llA, C, E, F). 
TABLE 3.- Color of swollen thorns for seasoned and undamaged types A and B thorns from living 
branches on occupied adult swollen-thorn acacias 
1 Thorn color Usual color Color at any Color among 
Species 
allenii . , , . . . . . 
cornigera 
collinsii 
hindsii 
rnelanoceras . . 
gentlei 
.. 
change from one site 
50 cm to among 
adult plant adult plants 
none no variation 
observed 
beige to dark beige+brown; 
brown; white brown+very 
to ivory dark brown; 
ivory+ brown 
beige to beige + brown; 
brown beige + ivory 
beige to beige + black; 
black; white ivory+ beige; 
to ivory; brown + black 
beige to 
brown 
none no variation 
observed 
brown to red + dark 
reddish; brown 
light brown 
to dark brown 
seedlings Color clines 
over entire where obvious 
range 
as adult plants in full sun have slightly 
browner thorns than those in full 
shade 
white to 
brown, usually 
beige 
ivory in dry and hot lowlands to very 
dark brown in wet upper elevations; 
ivory in dry lowlands to brown in 
wet lowlands 
white to none obvious 
beige to 
brown 
ivory to beige, lightest thorns in driest sites but black 
thorns there also; black thorns most 
common at low elevations, light at 
high elevations 
usually brown 
as adult none obvious 
reddish brown none obvious 
to brown 
20 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
TABLE 3.- Color of swollen thorns for seasoned and undamaged types A and B thorns from living 
branches on occupied adult swollen-thorn acacias-continued 
Species 
globulifera . . . . 
chiapensis 
ruddiae 
mayana , . . . . . . 
sphaerocephala . 
cookii 
Thorn color Usual color Color at any Color among 
on adult change from one site seedlings Color clines 
plant over 50 cm to among over entire where obvious 
entire range adult plant adult plants range 
brown to bicolor brown + brown to bicolor brown+ none obvious except that wet sites 
olive to ivory; brown black to bi- ivory; brown 
bicolor brown to white; color brown+ with white 
ivory and white to black ivory thorn tips 
ivory+ black 
very dark gray 
reddish cast 
to green thorns 
black; reddish 
cast to green 
thorns 
brown (seedling 
unknown) 
lark brown to none 
dark brown to none 
very dark none 
lark brown to none 
ivory: bicolor 
brown + beige, 
black or very 
dark brown + 
ivory; yellow 
dark red 
lark brown to none 
no variation dark brown to 
observed very dark 
gray 
no variation dark brown to 
observed black 
no variation as adult? 
observed (only (seedling 
one plant unknown) 
known from 
each site) 
brown to usually cream 
ivory; bicolor to beige 
dark brown + 
ivory to yellow 
dark brown to as adult 
dark red 
appear to have the darkest thorns 
none obvious 
none obvious 
none obvious 
darkest in wettest sites, most strongly 
bicolor inland and solid color near 
coast 
none obvious 
thorn wall. The death and drying (seasoning) 
of the thorn wall proceeds from tip to base, often 
resulting in temporarily bicolor or tricolor thorns; 
in some areas, however, all thorns are bicolor irre- 
spective of age. The thorns of all species turn lighter 
with age and bleaching by the sun. There is likely to 
be some adaptive significance to thorn color in the 
context of optimizing the internal thorn microclim- 
ate for the ants. In addition to detailed swollen- 
thorn morphology, the placement of the thorns on 
the branches varies from species to species; when 
looking down the long axis of a branch, the thorns 
either radiate out in all directions or lie in two 
parallel planes, one on each side of the branch. This 
appears to be a by-product of leaf placement, rather 
than being adaptive in the context of the thorns 
themselves. In general, the species of dry regions 
tend to have small leaves-and thorns-radiating 
outward from all sides of branches that are oiten 
more vertical than horizontal (e.g., A. collinsii), 
while those of wetter areas often have the leaves in 
a more or less horizontal plane from more hori- 
zontal branches. There are conspicuous differences 
among the species as to the proportion of leaves 
lacking swollen thorns (as in Figure 10~) but quanti- 
tative data are not available on this parameter. This 
parameter is also greatly confused by strong vari- 
ation in swollen-thorn production as a function of 
tree age and health; in general, seedlings and senes- 
cent trees produce the fewest swollen thorns, and 
leaves produced during the dry season often lack 
swollen thorns. Though not discussed here, there is 
moderate interspecific and intraspecific variation in 
thickness of the thorn wall, thorn-wall hardness, 
longitudinal fiber orientation in the thorn wall, 
insulating quality of the thorn wall, point of attach- 
ment of the leaf to the thorn, ease of thorn removal, 
and rates of natural thorn shedding. 
Healthy swollen-thorn acacias produce large num- 
bers of bright yellow inflorescences during a short 
part of the dry season. They are either elongate 
cylindrical (Figure 12) or spherical (Figure 13) and 
NUMBER 19 21 
FIGURE 12.--Swollen-thorn acacias with elongate inflorescences: A, Acacia cornigera (Honduras, 
site 63) : B, A. collinsii (Mexico, site 15) : C, A hindsii (Mexico, site 16). 
composed of many individual florets (Table 4). 
Pollinators-pollen-gathering bees-generally move 
around over the surface, scraping off pollen with 
their legs; there is no evidence of nectar production. 
Those inflorescences that are pollinated are re- 
tained with their very small and immature pods 
through the following rainy season, but only a small 
fraction of the inflorescences are retained. Presuma- 
bly the plant produces a great number of inflores- 
cences to attract pollinators, and then sheds what 
it does not have sufficient energy to mature. A. 
cornigera and A. collinsii have produced large 
quantities of flowers in greenhouses at the Uni- 
versity of Kansas and the University of Chicago. 
Swollen-thorn acacias differ from all other Cen- 
tral American acacias (and probably from the re- 
mainder of the genus) in having bird-dispersed seeds 
(Janzen, 1969~). The seeds are enclosed in a sweet 
white to yellow pulp or aril, and in all species ex- 
cept the A. cornigera, A. sphaerocephala, and A. 
mayana group the pod opens to expose the pulp 
when ripe (Figure 14; Table 5). Presumably this 
22 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 13.-Swollen-thom acacias with spherical inflorescences: A, Acaciu globulifera, florets 
unopened; two immature pods on receptacle on far right (Honduras, site 28); B, A. melanoceras 
(Canal Zone, site 6); c, A. allenii (Costa Rica, site 2). 
NUMBER 13 23 
Species 
collinsii . . . , , . . . 
Distance Range Infl. In$ 
Znfl. Znfl. Range Range Number from sexual peraxil bearing 
General max. max. inp. Peduncle peduncle florets bracteole branchlet of axils per 
shapeof diam. length length length length this toinfl. length sexual sexual 
inflorescence (mm) (mm) (mm) (mm) (mm) infl. base (mm) (cm) branchlet branchlet 
elongate 4 17 14-20 12 11-13 227 10 5-14 3-6 5-8 
cylinder 
mayana , . , . . . . . 
gentlei . . . . , . . 
globulifera . . . . . 
melanocerac . , . . 
cookii . . . , . . . . . 
terminally 
tapered 
cylinder 
cylinder 
elongate 
globular 
globular 
globular 
ruddiae . . , . . . . . 
chiapensis . . . . . 
sphaerocephala . 
hindsii 
cornigera , , . . . . 
. . . . . . . . 
allenii . . . . . . . . . 
3.5 
4.5 
3.5 
5.0 
4.5 
4.0 
3.8 
4.0 
4.0 
5.5 
5.5 
? 
5 
5 
9 
6.5 
6 
6 
6.5 
6 
7 
7 
6 
6 
8 
9 
5 
5 
globular 
globular 
nearly 
globular 
cylinder 
cylinder 
tapered 
tylinder 
elongate 
slightly 
globular 
17.5 
I, 
26 22-26 
23 20-25 
30 25-34 
46 40-50 
23 20-24 
21 
28 26-28 
29 
5 4-5 
5 11 
? ? 
5 45 
5 
9 9-10 
6.5 6-7 
6 5.5-6.5 
6 
6 
11 10-14 
10 
27 25-36 
22 15-23 
n 
w 
I, 
I1 
6.5 5.5-7.0 
I, 
I, 
26 
28 
I1 
I1 
5 4-5 
5 
It 
10 
16 
8 
3 
10 
14 
8 
14 
15 
6 
12 
12 
3 
6 
37 
8 
9 
9 
18 
20 
12 
12 
20 
20 
5 
6 
3 
7 
I, 
14-18 
13-17 
1-4 
5-12 
8-12 
10-15 
6-12 
10-12 
3-6 
26-38 
8-18 
6-10 
16-20 
8-12 
A 
,, 
I1 
I, 
I1 
I1 
,I 
1422 
1-10 
11 
I1 
3-6 
,I 
222 
301 
283 
1003 
1408 
533 
534 
494 
519 
69 
124 
? 
54 
117 
537 
105 
198 
182 
124 
120 
340 
319 
153 
142 
1112 
992 
65 
91 
8 
12 
5.5 
.2 
2 
8 
5 
8 
10 
6 
12 
12 
1 
1 
18 
4 
3 
3 
5 
3 
10 
10 
13 
13 
4 
5 
2 
3 
I, 
2-3 
4-13 
13 
5-15 
2-14 
6-9 
1-9 
8-22 
10-20 
85-1 80 
10-23 
4-12 
15-30 
II 
I, 
,I 
I, 
I1 
11 
2-18 
,I 
10-23 
I, 
5-20 
r, 
3-15 
I1 
I, 
2-3 
3-5 
1-2 
1 
1 
1 
2-5 
It 
w 
n 
2-5 
3-5 
37-86 
25-46 
n 
8-8 
,I 
35-60 
34 
,I 
n 
5-7 
1-4 
I, 
I1 
20-40 
I, 
I, 
5-6 
4-8 
20 
21 
4-15 
4-10 
41 1 
10-20 
29-36 
16-20 
5-18 
2-4 
10-15 
2-12 
I1 
I, 
,, 
II 
0 
I1 
II 
8-1 3 
11 
4-1 3 
I1 
2-10 
,I 
difference reflects the ease with which birds can 
enter the crown and area beneath an occupied swol- 
len-thorn acacia, as contrasted to mammals which 
are much more bothered by the ants. The pods of 
swollen-thorn acacias are very diagnostic at the 
species level except within the A. cornigera group. 
A. collinsii displays the most extreme geographic 
variation in pod morphology of all the species, rang- 
ing from very short and linear pods with thick 
walls in the drier parts of Panama to much longer, 
curved, and thin walls in Mexico and Guatemala. 
All swollen-thorn acacias have very hard seeds (ex- 
cept A. allenii and A. melanoceras) which pass 
through the intestinal tract unharmed and are scari- 
fied in the process. Swollen-thorn acacias are sub- 
ject to heavy seed predation by bruchid beetles 
(e.g., Janzen, 1969c) except for those species in 
very wet habitats-A. melanoceras and A. allenii 
and perhaps A. ruddiae. The general pattern is that 
the eggs are laid on the full sized and nearly mature 
pod, the larvae bore in through the pod wall and 
into a seed, and one larva matures inside each seed. 
Upon emerging from the pod, the new adult beetle 
will oviposit on pods that remain, resulting in de- 
struction of that portion of the seed crop that has 
not been removed by dispersal agents (Janzen, 
1969c.) Undamaged and scarified seeds yield almost 
100 percent germination if taken from truly mature 
pods. 
With considerable experience in studying the 
populations in the field, all species of swollen-thorn 
acacias can be correctly identified from specimens 
24 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
TABLE 5.-Reproductive and other traits of swollen-thorn acacias recorded from occupied adult trees 
Species 
mayana , . . . . . . 
gentlei . . . . . 
glo bulifera 
melanoceras 
cookii , . . . . . . . . 
ruddiae . . , . . , . 
chiapensis . , . . , 
sphaerocephala . 
hindsii . . 
cornigera , . . . . 
allenii . . . . . . . . . 
collinsii . . . . . . . . 
Estimated Maximum Maximum Life Bark 
age at first Pod valve Pod Color of height basal form color 
seed repro- color when dehiscion Aril mature of adult diameter of and 
iuction (yrs) mature behavior color seed (m) (4 adult texture 
4-6 maroon indehiscent yellow dark 45 10 single smooth 
6 
5 
8 
a 
10+ 
6-7 
4 
6 
4 
7 
4 
brown 
black dehiscent pale olive 
along both yellow green 
margins 
but cen- 
trally 
together 
along one 
margin 
and 
strongly 
curled 
black dehiscent yellow 
greenish dehiscent white 
black along one 
margin 
olive 
green 
black 
black ? 
greenish ? 
black 
trunk gray - 
tree green 
trunk rust red 
tree 
14 12-25 single rough 
8 12 
dark dehiscent white 
brown along both 
margins 
maroon indehiscent yellow 
yellow 
red 
brown 
with 
waxy 
bloom 
maroon 
yellow 
red 
greenish 
black 
dark 
brown 
12 15 
? ? 20 30 
? 7 18 50 
dehiscent yellow 
along one 
margin 
yellow 
dehiscent white 
along both 
margins 
along both 
margins 
dehiscent yellow 
greenish 
beige 
black 
dark 
brown 
black 
black 
olive 
drab to 
black 
single olive drab 
trunk to reddish 
tree 
single or green to 
multiple light 
trunk tree brown 
single dark 
trunk brown 
tree 
single smooth gray- 
trunk green with 
tree longitudinal 
whitish 
cracks 
5 20 single rough 
trunk rust 
tree red 
1-4 15 single trunk smooth 
tree to gray- 
multiple green 
trunk shrub smooth 
la 35 single trunk gray- 
tree to green 
multiple 
trunk tree 
6-18 20 single trunk smooth gray- 
tree green to 
rough 
25 25 single trunk brown 
tree green to 
light brown, 
6 15 single trunk rough 
tree silvery gray 
to brown 
NUMBER 13 
25 
species accounts. Table 6 presents the degrees of 
sympatry and gives some feeling for the amount of 
caution required to identify the species on collec- 
tion locality only; A. melanoceras and A. allenii are 
the only species that are never sympatric with other 
swollen-thorn acacias. 
While hybrids of swollen-thorn acacias with other 
acacias (A. globulifera, A. hindsii, A. collinsii, and 
A. chiapensis are the offenders in this case) may be 
difficult to place to species, swollen-thorn acacias 
apparently hybridize only very rarely (though un- 
derstandably they might be difficult to notice); I 
have seen only one, and that was a single plant of 
A. cornigera x A. sphaerocephala on the dunes 
south of the city of Veracruz, Mexico. 
FIGURE 14.-~, Newly dehisced fruits of A. collinsii, with the 
yellow pulp enclosing the seeds and hanging down beneath 
the valves on thin fibers; birds pick these off in flight (Costa 
Rica, site 71), B, Opened pod valves of A. hindsii; the pulp 
and seeds have been removed by birds (Mexico, site 17). 
from adult trees even if the localities are unknown. 
Individual variation owing to local ecological fac- 
tors, however, can make this process confusing, and 
knowledge of the collection site often aids tremen- 
dously in quick and accurate identification; the 
field work in this study was extensive and intensive 
enough so that the reader should be extremely 
suspicious if his identification leads him to decide 
that he has located a population of a given species 
far from the ranges listed under the individual 
Taxonomic Information 
While working with a population of swollen- 
thorn acacias, one should have no difficulty in 
identifying them with key A. Key B is provided 
for use in conjunction with key A, when flowers and 
fruit are not available. On occasion there may be 
some difficulty in identifying old and very incom- 
plete herbarium material with these keys, but that 
is not likely to be a significant problem inasmuch 
as the value of species-level determinations for such 
material is highly questionable anyway. When a 
population of swollen-thorn acacias is encountered 
during the rainy season, sexual material may be 
difficult to locate. If one searches for damaged trees, 
however, often an occasional inflorescence-bearing 
branch may be located. Since the immature infruc- 
tescences are retained on the tree through the rainy 
season, careful search for these usually succeeds in 
locating sexual material. It is worth noting that the 
inflorescence receptacle generally retains its shape 
while bearing immature or mature fruits (e.g., 
Figure 112), allowing easy use of the first couplet of 
key A even when fresh inflorescences are not avail- 
able. The physical locations of the type specimens 
of swollen-thorn acacias are given in Table 7, but 
these are generally of little use in identification. 
Vernacular names are of little use in correct 
identification of swollen-thorn acacias, inasmuch as 
they tend to be lumped under ?cornizuelo? or some 
similar word at a given site (Table S), in contrast 
to the other acacias which generally carry names 
such as ?guisache.? 
26 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
Name 
collinsii . . . . . . . 
sphaerocephala 
TABLE 6.-Sympatry of swollen-thorn acacias (+ indicates that they may be 
found growing within 200 m of each other in some part of the range and that 
the populations overlap) 
Species 1 COT. sph. may. glo. coo. hin. col. mel. all. rud. gen. chi. 
Type location 
and sheet number Synonyms Location of synonym types 
US 692159* yucatanensis B (presumed destroyed) 
NY (cotype)* GH (?type collection?)* 
US 571618 (?type COIL?)* 
F 417619 (paratype)? 
F 417618 (paratype)? 
US 943274 (potential 
lectotype) * 
GH (cotype)* 
K (same as US 943274)t 
US 677927 (isotype)* 
US 764673 (photo of type)* 
nelsonii US 399366 (type)? 
costaricensis B (presumed destroyed) 
penonomensis NY (type)* 
panamensis B (presumed destroyed) 
(presumed destroyed) ueracruzensis (presumed destroyed) 
US 764680 (frag. and photo)* US 764683 (frag. and photo)* 
US 764684 (frag.)? 
US 692164 (isotype). 
NY (isotype)* 
dolichocephala F 189552 (type)* 
cornigera 
sphaerocephala 
mayana 
globulifera 
cookii 
hindsii 
collinsii 
melanoceras 
allenii 
ruddiae 
gentlei 
chiapensis 
+ 
++ 
+o+ 
+oo+ 
+ooo+ 
+oo+o+ 
+oo+o++ 
ooooooo+ 
oooooooo+ 
oooo+oooo+ 
+ooo++ooo++ 
+oo++ooooo++ 
mayana . . . . . . . 
cornigera . , . . . . 
US 1688106 (isotype)* 
GH (isotype)* 
MICH (type)* 
NY (isotype). 
BM (no number) 
US 2396019 (frag. and 
photQ)* 
spadicigera US 764687 (frag. and photo)* 
cu bensis GH (type)* 
US 764633 (frag. and photo)* 
MO 16066, 46866 (type)? 
US 577752 (?type toll.")* 
GH (?type collection?)* 
NY (isotype)* 
nicoyensis B (presumed destroyed) 
K (cotyfie)t 
hernandexii US 692170 (type)* 
NUMBER 13 
Acacia species 
mayana . . . . . . . . . . . . . 
globulifera . . . . . . . . . . 
cornigera , . . . . . . . . . , 
27 
Vernacular Geographic region 
cruzetillo Campeche, Yucatan, Mexico 
cachetoro 
ishcanal 
cornezuelo blanco Yucatan, Mexico 
cachetoro 
ishcanal 
cornizuelo lowland eastern and southern 
cornezuelo Mexico, generally throughout 
Coban, Alta Vera Paz, Guatemala 
Coban, Alta Vera Paz, Guatemala 
Central America 
TABLE 7.-Location of type specimens of swollen-thorn acacias-continued 
Name 
hindsii . . , . . . , , . 
cookii 
globulifera . . . . . 
chiapensis 
gentlei . . . . . . . . 
melanocerm . . . 
allenii . . . . . , . . . 
ruddii . . . . . , , . . . 
Name 
gladiata . . . . . . . , 
hirtipes 
standleyi 
oaxacanum 
Examined in 
.- - 
Type location 
and sheet number Synonyms Location of synonym types 
GH (isotype)* 
K (no number)? 
US 574387 (type)? 
US 764658 (paratype)* 
F 58707 (type)? 
F 36458 (isotype?)* 
US 1267507 (isotype?)* 
GH (istotype?). 
NY (isotype). 
US 692157 (type)? 
F 464247 (photo)* 
F 1000801 (type)* 
furcella 
interjecta 
rossiana 
campecheana 
turgida 
bursaria 
tepicana 
sinaloensis 
bucerophora 
donnelliana 
NY (isotyie)* 
US 692166 (type)* 
unknown 
MO (no number)* 
US (photo). 
B (presumed destroyed) 
(photo only, in Wheeler, 
B (presumed destroyed) 
GH (cotype)* 
US 637234 (type)* 
US 636818 (type)* 
NY (isotype)* 
US 764696 (frag. & photo)* 
F 659304 (?type?)* 
GH (?type?)* 
US 943282 (type)? 
1942) 
K (cotype)t 
locality unknown multiglandulosa B (presumed destroyed) 
US (photo, no. 1281)* 
us (type)* 
us (type)* 
Type location 
US 764660 (frag. & photo)* 
MO 46838 (type)* Bonpl. ex Willd. 
US 764659 (frag. & photo)* 
US 716563 (?paratype?). 
US 694036 (type)* A. hindsii x A. pennatula Benth. 
US 637251 (type)* 
US 1208317 (type)* 
iis study; numbers refer to herbarium sheet numbers. 
and sheet numbers Hybrids 
A. hindsii x A. cochliacantha Humb. and 
A. hindsii x A. pennatula or A. macracantha 
A. hindsii x A. pennatula 
t Information provided by Sir George Taylor (in litt.). 
28 
Acacia species 
....... 
gentlei .............. 
collinsii ............ 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
Vernacular Geographic region 
cuernitos Veracruz, Oaxaca, Mexico 
ishcanal blanco El Salvador 
cuernos de tor0 
ishcanal Pacific lowlands of Guatemala 
ixcanal 
carnizuelo Central Honduras 
torito Salina Cruz, Oaxaca, Mexico 
cornezuelo Michoacan, Guerrero, Mexico 
gara terrera Sinaloa, Nayarit, Mexico 
jarretadera 
cockspur British Honduras 
red cockspur 
cockspur Isla Providencia, Colombia 
crowsfoot British Honduras 
San Luis Potosi, Veracruz, Chiapas, 
Tabasco, Mexico 
TABLE 8.-Vernacular names of swollen-thorn acacias in Central America as 
hindsii 
I cachitos Pacific coast of Panama 
melanoceras ........ 
sphaerocephala ..... 
cookii ............. 
1 cockspur Guatemala 
subin 
cornezuelo 
cuerni tos 
cachitos 
cornezuelo 
guascanal 
Yucatan Peninsula, Mexico 
Chiapas, Yucatan, Mexico, 
Porto Bello, Panama 
Panama 
Veracruz, Mexico 
Guatemala 
Colombia 
A. Key to Mature Central American Swollen-Thorn Acacias, Based on 
All Traits 
1. Inflorescence distinctly globular ................................................... 2 
Inflorescence very elongate or at least distinctly longer than broad ................... 3 
2. Leaves from type A thorns (on lateral branches) over 30 cm long ..... 
Leaves from type A thorns (on lateral branches) less than 25 cm long . 
3. Pinnules less than 1 mm wide ...................................................... 6 
4. Petiole deeply grooved, young thorns pubescent, nectaries b ised above surface of 
Petiole slightly grooved, young thorns glabrous, nectaries str outstanding conical, 
Pinnules 2 or more mm wide .............................. ................. 7 
petiole and elongate, occupied by obligate acacia-ants ..................... .I. A. cookii 
Petiole with many nectaries (8-30) .................................. 3. A. melanoceras 
Petiole with few nectaries (0-6) ........................................... .4. A. allenii 
very soft .............. ......................................... .5. A. chiupensis 
shaped .................................................. 8 
not occupied by obligate acacia-ants ...................... . . , . . , . .2. A. ruddiae 
5. 
6. Petiole with many nectaries (6-25) closely packed and often not in a straight line, wood 
Petiole with few and widel aced nectaries (1-5) in a straight line ..... .6. A. globulifera 
7. Inflorescence peduncle very thick, at least half as wide as inflorescence, nectaries trough 
Inflorescence peduncle very less than one-fourth as wide as the inflorescence, nec- 
8. Inflorescence 34 times as long as broad; secondary venation clearly visible on leaflets. ... 9 
Inflorescence one and one-half times as long as broad, approaching a globular form; 
.................. 
taries round .......................................... 
secondary venation on leaflets not visible ........................ 7. A. sphaerocefhzla 
9. Swollen-thorns round in cross section ................................... .8. A. cm'gma 
Swollen-thorns triangular in cross section ................................. .9. A. mayam 
10. Inflorescence approaching globular form, but nearly twice as long as wide ................ 
................................. ..Hybrid of A. hi&$ with other Acacia species 
Inflorescence very elongate, 3-8 times as long as broad ................................ 11 
NUMBER 13 
29 
11. Seed pods less than 8 cm long, valves curling heavily upon opening ................... 12 
Seed pods more than 12 cm long, curling only slightly upon opening ....... .lo. A. gentlei 
12. Petiolar nectaries dome shaped in lateral view with 1 mm or greater diameter, usually 
3-4 very close together in widened part of petiole and none on rachis .... .11. A. collinn'i 
Petiolar nectaries cylindrical in lateral view with less than 1 mm diameter, usually 1 or 
more on the petiole and one at junction of each pinna with rachis ....... .12. A. hiMi 
B. Key to Mature Central American Swollen-Thorn Acacias, Based on 
Vegetative Traits 
1. Petiolar nectary trough shaped, much longer than broad (Figure 8~) .................. 
2. 
Type A thorns circular or elliptical in cross section .................................. 3 
3. Secondary venation on leaflets clearly visible .......................... 8. A. codgera 
Secondary venation on leaflets not visible ........................ .7. A. sphaerocephala 
4. Type A thorns very heavily flattened, nectaries scattered on petiole and rachis, and small 
enough so as not to cause heavy swelling of petiole or rachis (Figure 8A) ,12. A. hindn'i 
Type A thorns circular or elliptical in cross section, nectaries various ................. 5 
5. Nectaries restricted to petiole and broad dome shaped (Figures 7A, 9) ... .11. A. collinsii 
Nectaries other than above ......................................................... 6 
6. From type A thorns on lateral branches, leaves 30 cm or more long . . , . 
From type A thorns on lateral branches, leaves less than 25 cm long . . 
7. Pinnules less than 1 mm wide ...................................... 
Pinnules more than 2 mm wide , , . . .................................... 9 
8. Petiole deeply grooved, young thorns (and sometimes old thorns) , nectaries 
elongate and low (Figures 18~, 18~)~ obligate acaciarant entrance holes in thorns near 
Petiole only slightly grooved, young thorns glabrous, nectaries conical and well raised 
above petiole and rachis (Figure 8c), never with obligate acacia-ant entrance holes in 
thorns near apex .............................................. , 2. A. ruddiae 
Petiole with few nectaries (0-6) (from Costa Rica) ......................... 4. A. allmii 
10. Petiole with many nectaries (6-25) closely packed and often not in a straight line 
Petiole with 1 to few nectaries; if more than one, then linearly arranged . . , , . , , , , , , . 11 
11. Extrafloral nectaries as in Figures 6 and 7c, a1 always more than one on the petiole 
........................................... ................... 6. A. globulifera 
Extrafloral nectaries as in Figure 78, almost never more than one on the petiole, bark and 
thorns deep red or mahogany in color, type A thorns very slender (as little as 3 mm di- 
2 
Petiolar nectaries circular (domes, cups, conical) . . , , . . ....................... 4 
Type A thorns triangular in cross section ................................. .9. A. mayana 
apex .................................................. ........... 1. A, cookii 
9. Petiole with many nectaries (8-30) (from Panama and Canal Zone) . , 
......................... ............ ................. 5. A. chiapensis 
amrter at base) 
1. Acacia cookii Safford 
Acacia cookii Safford, 1910:677. 
Acacia bucerophora B. L. Robinson, 1913:502. 
Mymecodendron cookii (Safford) Britton and Rose, 1928:93. 
Acacia cookii is easily distinguished from all other 
swollen-thorn acacias by its tomentose, long and 
thin thorns that bend back around the stem and are 
often bilaterally asymmetrical (Figures 15, 16). A. 
collinsii type A thorns may also wrap around the 
branch that bears them, but they are very short and 
thick. Only the type B thorns of A. cornigera wrap 
around the branch that bears them. A. ruddiae 
thorns may be tomentose when green, but they are 
glabrous upon maturing. 
10. A. gentlei 
The type specimens for both A. cookii and A. 
bucerophora are representative of specimens from 
the southern part of the population (Figure 17). 
The branch designated as the type of A. cookii 
(from site 1) lacks swollen thorns and was probably 
taken from the end of a long and old reproductive 
branch such as that in Figure 15~. Since the in- 
florescences on the type are still in bud, the pe- 
duncles are much shorter than they are when in 
flower (e.g., Figures I~B, 16~). The leaves of the type 
specimen are not from the inflorescence-bearing 
branch; they are of the form normally subtended 
by large type A thorns (cf. ends of branch in Figure 
18) and were probably broken from the branches in 
30 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 15.-Lateral branches from mature A. cookii (Guatemala, site 4): A, sterile branch with 
fully developed thorns characteristic of the southern part of the acacia?s range; B, reproductive 
branch with axillary inflorescences characteristic of the southern part of the acacia?s range. 
order to avoid pressing the ant-filled thorns. The 
type specimen of A. bucerophora (site 3) differs in 
no significant way from that of A. cookii. The single 
thorn of the specimen is old and rotted and there- 
fore not the dark brown that is characteristic of 
most thorns on a tree of A. cookii. The single small 
seed pod is immature and shrunken, giving the 
illusion of being thin, while in fact A. cookii pods 
are over 1 cm wide and very long (Figure 19). 
The northern and southern parts of the A. cookii 
population differ on a number of morphological 
traits; however, these differences appear to be the 
result of comparing the two ends of a cline, since 
the intermediate elevation wet-forest habitat of A. 
cookii is continuous from the northern to southern 
part of the range. This hypothesis can only be veri- 
fied by collections from the area between the broken 
distribution in Figure 17. Some of the more out- 
standing changes in A. cookii in progressing from 
the northern to the southern part of the range are: 
(1) shortening of the type A thorns (cf. Figures 15~ 
vs. 16~); (2) shortening of the sexual branchlets (cf. 
Figures 15~ vs. 16~); (3) increased number of in- 
florescences per inflorescence-bearing axil; (4) in- 
creased number of Beltian bodies per mature leaf; 
and (5) increased percentage of the acacia plants 
being occupied. 
KEY COLLECTION LOCALITIES 
1. 
2. 
3. 
4. 
5. 
6. 
Type of A. cookii: Finca Trece Aguas near Secanquim, 
Depto. Aka Verapaz, Guatemala (300 m) . 111-8-1907, 
G.P. Goll, no. 102 (US 692154), fl. 
Paratype of A. cookii: Secanquim, Aka Verapaz, Guate- 
mala (300 m) . Vl-4-1905, O.F. Cook, no. 4. 
Type of A. bucerophora: about Toledo, 4 mi from the 
coast and 5 mi from Punta Gorda, Belize Prov., British 
Honduras (sea level), 111-29-1907, ME. Peck, no 632. 
2.8 mi W Puerto Matias de Galvez (on road to Escobas) , 
Depto. Izabal, Guatemala (10 m). VIII-10-1967, DHJ, 
no. 1524, gp. 
9.8 mi W Cardenas on Mexico 180, Tabasco, Mexico 
(10 m). VI-17-1966, DHJ, no 515. 
From Km 56 sign post, 9.8 mi N turn off Tapilula to 10.8 
NUMBER 13 31 
FIGURE 16.-Lateral branches from mature A. cookii: A, sterile branch with fully developed 
thorns characteristic of the northern part of the acacia?s range (Mexico, site 7); B, reproductive 
branch with long sexual branchlets characteristic of the northern part of the acacia?s range 
(Mexico, site 6). 
mi S Pichucalco on Villa Hermosa-Tuxtla Gutierrez 
highway, Chiapas, Mexico (240 to 380 m) . VII-10-1966, 
DHJ, nos. 530, 537, fl. 
7. N side of Chontalpa, 26 mi S Cardenas, Tabasco, Mexico 
(80 m). VI-17-1966, DHJ, nos. 548, 550. 
8. Along river, Lancetilla Valley, near Tela, Atlantida, Hon- 
duras (5 m) . XII-19-1927 to 111-27-1928, P.C. Standley, 
no. 54627. 
9. Rd. from Secanquim to Sepacuite, Depto. Alta Verapaz. 
Guatemala. IV-1-1902, O.F. Cook and R.F. Griggs, no. 
374 (US 408103). 
10. Sierra del Mico, between Los Amates and Lake Izabal, 
Depto. Izabal, Guatemala (650 m) . 11-27-1908, W.A. Kel- 
Zerman, no. 7169 (US 2442365, US 2442347), fl. 
11. Between Virginia and Lago Izabal, Montana del Mico, 
Depto. Izabal, Guatemala (50-100 m) . IV41940, J.A. 
Steyermark, no. 38758 (F 1056559). 
12. British Honduras. 1944 (F 1403119). 
13. Big Rock, British Honduras (65 m) . XII-16-1932, W.A. 
Schipp, no. 1072 (F 713932). 
14. Maskall, British Honduras? 1-1934, P.H. Gentle, no. 1121 
(F 739552). 
NATURAL HISTORY 
Acacia cookii is a narrowly distributed species 
that has its highest densities at intermediate eleva- 
tion sites (200 to 400 m) with very mild dry seasons 
(e.g., sites 1,2,6). At site 6, this habitat was slightly 
higher than that of A. cornigera and A. chiapensis, 
where all three occur along the east-facing foothills 
of the Sierras in western Tabasco, Mexico. Occa- 
sional adults of A. cookii were found at lower and 
drier sites (sites 5,7) in the northern part of the 
range, but their sympatry with A. cornigera and 
A. chiapensis may have been the result of seed dis- 
persal by road-building machinery. In the southern 
part of the range, isolated plants (e.g., site 4) gen- 
erally grow in naturally disturbed sites along rivers 
in very wet forests. They are sympatric with A. 
gentlei at these sites (as at other Guatemalan sites 
such as sites 10 and ll), and there is no evidence 
32 
SMITHSONlAN CONTRIBUTIONS TO BOTANY 
FIGURE 17.-General postulated distribution (stippled area; type localities indicated by arrows). 
collection site of A. cookii A collection site of A. rnayana 
that they are unnatural populations. A. cookii dis- 
appears when the wet forest is cut in areas like site 
4; this local extinction is probably due to the same 
causes as cited for A. allenii and A. melanoceras. In 
the northern part of the range, where the dry season 
is more severe, A. cookii appears to be able to persist 
in the face of repeated wilting and occasional burn- 
ing. It was very common as a roadside and pasture 
tree shrub at site 6. 
The hard seeds of A. cookii are attacked by un- 
identified bruchids to the same degree as those of 
swollen-thorn acacias of drier areas. The seeds are 
dispersed by birds; the pods dehisce incompletely 
and the dark seeds are buried in a white, sweet 
pulp. The seed crops are not unusually large, but 
the large number of inflorescences per axil is 
matched only by the axils of A. chiapensis (Figure 
20~). In the southern part of the range, where the 
inflorescence-bearing branch is reduced to a bump 
of inflorescences per axil may be as high as 64 for a 
FIGURE 18.-Crown of mature 5-m-tall A. cookii growing 
in the thorn (Figure 15B)9 an average number 
along creek in rain forest (Guatemala, site 4). 
NUMBER 13 
FIGURE 19.-Acuciu cookii (Guatemala): A, rachis nectaries on leaf siibtended by type A thorn 
(site 4); B, petiolar nectaries of leaf; c, mature seed pods (site 1, US 692155). 
large sample (Figures ~OB, 22). In the northern part 
of the range, where the sexual branchlets are long, 
a sample of 10 axils on a representative branch had 
a mean of 32 inflorescences per axil (16, 29, 29, 29, 
31, 31, 35, 37, 38, 42). 
The pattern of development of A. cookii from 
seedling to adult is unknown, but I have no reason 
to suspect it differs in any substantial way from that 
of other wet-forest, swollen-thorn acacias such as A. 
allenii. At site 4, the mature trees had a relatively 
dense crown (Figure IS) perched on top of a single 
and straight thin trunk. Based on growth rings, in 
wet forest the trees die of senescence when about 
15 to 25 years old. In slightly drier sites (definitely 
site 6 and probably sites 1 and 2) the old trees may 
FIGURE 20.-Axillary inflorescences: A, immature pods of A. 
attain the largest size of any swollen-thorn acacia, 
chiupensis (Mexico, site 9); 8, bud scars (with two in- 
frutescence peduncles projecting to 12:OO and 2:OO o'clock) except perhaps '* in Mexico and 
of A. cookii (Guatemala, site 4). A. ruddiae in the Costa Rican mountains. Growing 
34 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
in old landslide scars at site 6, there were trees of 
A. cookii up to 30 m tall, up to 20 m across the 
crown, and with a DBH as much as 40 to 50 cm. 
The ant colonies were not examined in these trees, 
but I suspect that they were either a multiple-queen 
species or they restricted their patrolling to the 
newer foliage. While the crowns of these acacias 
were very large, the actual amount of green foliage 
was small compared to what it would be in another 
species of legume of equal size during the rainy 
season. 
It is possible that the herbivore pressure is some- 
what reduced at this site and/or that A. cookii in 
this area has better chemical defenses than do most 
swollen-thorn acacias. I suspect this because road- 
side clumps of poorly occupied A. cookii at site 6 
had foliage with very little herbivore damage. 1 
favor this hypothesis because the A. cookii growing 
at site 5 was in a lowland roadside pasture where 
unoccupied A. cornigera was being heavily defoli- 
ated by insects that normally feed on other species 
of plants. This A. cookii was, however, completely 
unoccupied and almost in perfect condition. At 
least 5 years old, the plant was 4 m tall and had a 
crown similar in foliage density to the one in Figure 
18. About one out of every four shoot tips had been 
- 
160 
Y 
I 
X 
t-1 
u 
2 
W 
cl 
eaten off by insects, which is about the frequency 
usually observed during the rainy season on non- 
ant-acacias and occupied swollen-thorn acacias. 
These comments should not be construed to indi- 
cate that A. cookii does not have a mutualistic asso- 
ciation with its ants. At site 6 there was ample 
defoliation of unoccupied sucker shoots, in contrast 
to occupied ones. This indicates that the results of 
removing the ants would have had an effect on A. 
cookii much like that with A. allenii, but not as 
dramatic as with ,4. cornigera or A. collinsii. 
It is possible that A. cookii is still in the process 
of coevolving a tighter mutualism with its occupant 
ants-Pseudomyrmex ferruginea in the northern 
part of the range, and P. ferruginea plus an un- 
identified species in the southern part. Its petiolar 
nectaries (Figure 19~) are morphologically poorly 
developed-their sugar production rate may be 
high, however-though the nectaries on the rachis 
may compensate for this (Figure 19~). On some 
individuals at site 6 there were Beltian bodies on 
only the basal 10 to 30 pinnules, and even at the 
sites where the healthiest ant colonies occur (e.g., 
site 4), Beltian bodies are produced on only about 
half the pinnules. The swollen thorns on A. cookii 
in the northern part of the range are very thin- 
walled-birds could probably break into them. with 
ease-and they harden very soon after attaining 
full size, which allows the ant only a short time to 
cut an entrance hole. The very narrow thorns have 
a volume (Figure 21) representative of that of other 
swollen-thorn acacias because of their extreme 
length; however, these narrow thorns probably 
make very cramped living quarters for the ants. The 
largest thorns to be found (Figure 21) had a mean 
volume of only 1.64 cc, which is representative only 
of A. chiupensis, A. globulifera, A. gentlei, and A. 
ruddiae. 
2. Acacia ruddiae, new species 
t-1 100 11 
lo0 200 3 00 
TOTAL THORN VOLUME ( cc ) 
FIGURE !?I.-Distribution of 131.3 cc of thorn volume among 
19 very large type B thorns from one adult A. cookii (Guate- 
mala, site 4). While the largest to be found, these thorns had 
a maximum diameter in cross section of 5-7 mm. 
Arbol usque ad 20 m alta; ramuli novelli 
puberuli, glabrescentes; stipulae spinescentes inter- 
durn magnae, teretisculae, usque ad 10 cm longae, 
9 mm latae; folia circiter 15-40 cm longa; nectaria 
sessilia cupulata ad juncturas pinnarum omnium et 
0-2 in petiolis; pinnae (12-) 25-40-jugae, 4-5 cm 
longae; pinnulae 65-75-jugae, lineariae, 2-4 mm 
longae, 0.5-1.2 mm latae, acutae vel obtusae, 
NUMBER 13 
v) 
W 
W 
V 
v) 
30 
el 
IL 
w 
I 
3 
z 
m 
123456 
A NUMBER OF PODS PER INFLORESCENCE 
u) 
cl 
x 
< 
IL 
0 
U 
I 
1 35 7 9 11 
B NUMBER OF INFLORESCENCES WITH 
PODS PER AXIL 
FIGURE 22.-Acacia cookii (Guatemala, site 4): A, distribution of 113 seed pods among 63 in- 
fructescences; B, distribution of the 63 infructescences among their 10 axils. There were 642 
inflorescences produced by these 10 axils. 
puberulae; flores aurei in capitulo globoso; involu- 
cellum annulare in medio pedunculo; legumina 
(immatura) atrobrunnea, puberula, granulata gla- 
brescentes, oblonga, 5 cm longa, 7 mm lata, com- 
planata, obtusa, stipite 5 mm longo; legumina 
matura probabiliter usque ad 6-8 cm longa. 
TYPE.-?. H. Janzen 828, Costa Rica, Limon 
Province, ferry landing, Moin River, Moin, 13 Feb- 
ruary 1965 (holotype US 2639554; isotypes CAS, F, 
GH, MEXU, MICH, MO, NY, UC). 
Type tree 20 m tall with 35 cm diameter trunk 
1 m above ground; gray-green bipinnate leaves 
mostly 15 to 40 cm long with 50 to 80 pinnae per 
leaf, pinnae 40 to 50 mm long with 130 to 150 pin- 
nules per pinnae, pinnules 2.5 to 3.0 mm long and 
0.75 to 1.0 mm wide (up to 9,000 pinnules per leaf); 
occasional swollen-thorns on less than 10 percent of 
the nodes and very frequently only one side of the 
thorn swollen, thorns not occupied by obligate 
acacia-ants but by a miscellany of other ant species; 
no food bodies (Be1 tian bodies) on pinnules; single 
nectary at junction of each pinna with rachis, 0 to 
2 nectaries on petiole, nectaries cylindrical up to 1 
mm tall; yellow inflorescences spherical and up to 
6 mm diameter, peduncle up to 10 mm long and 
0.50 to 0.75 mm thick; immature fruits flat and 
slightly curved, probably 6 to 8 cm long at maturity. 
Acacia ruddiae is distinctive by never being occu- 
pied by obligate acacia-ants, lacking swollen thorns 
on most branches except the bases of some lateral 
branches, having better than 5,000 very small leaflets 
per leaf, growing in rain forest, becoming a very 
large tree, and by its range (Figure 23). It also 
differs from other Central American members of 
the genus Acacia by the same traits; it is undoubt- 
edly distinguishable from the remainder of the 
genus by fine details of flower, fruit, and leaf struc- 
ture, but it is impossible to know these differences 
at the present?state of Acacia taxonomy. 
The tree from which the type specimens (13 
sheets) were collected was at least 30 years old, and 
perhaps much older, and was still standing in 1967. 
It was about 80 m south (west side of the road) of 
the south beach of the ferry landing on the Moin 
River and 200 m from the ocean beach, at Moin, 
Costa Rica (site 1). The branches were cut from 
high in the tree and were representative of the 1-2- 
year-old branches in the canopy. Although the speci- 
mens were collected during the dry season, the area 
was still subjected to frequent rains and the tree was 
only partly deciduous. It was at the end of its flow- 
ering season, and the full sized but immature green 
pods were from the same flower crop, rather than 
from the previous year?s flowers as is usual with 
acacias of drier habitats. As is usual with the canopy 
branches from A. ruddiae this size, there are few 
swollen thorns on the branches. The tree from 
which the type specimens were collected was ex- 
ceptional in two respects: First, most of the A. 
ruddiae population in this area has long since been 
destroyed by housing development. Second, the tree 
grew in a relatively open site and thus had strong 
36 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE Pt.-General postulated distribution (heavily stippled area) and specific collection sites 
of A. ruddiae (all on the Caribbean side of Ccntral America). General postulated distribution 
(lightly stippled area) and specific collection sites of A. allenii (all on the Pacific side of Central 
America). 
NUMBER 13 87 
development of lateral branches; most A. ruddiae 
are involved in intense intertree competition and 
thus are tall and slender. 
This acacia is named in honor of Dr. Velva E. 
Rudd whose unfailing assistance and encourage- 
ment has greatly aided in bringing this study to 
completion, and whose taxonomic work with Legu- 
minosae will always be a tremendous assistance to 
field biologists. 
Acacia ruddiae is included with the swollen-thorn 
FIGURE 24.-Acacia ruddiae: A, 
very young seedling; note nod- 
ules on roots (Costa Rica, site 
5); B, young sapling (Costa Rica, 
site 5). 
acacias, despite the fact that it is not occupied by 
obligate acacia-ants, because it appears to represent 
an intermediate step in one of several possible evo- 
lutionary pathways to the evolution of swollen- 
thorn acacias. Its ecology is therefore of special 
significance in this study. A. ruddiae is the only 
Central American acacia, aside from hybrids of 
swollen-thorn acacias with nonant-acacias, that ap- 
pears to represent an intermediate step in the evo- 
lution of a New World swollen-thorn acacias. 
38 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
KEY COLLECTION LOCALITIES 
1. TYPE.-Ferry landing, Moin River, Moin, Limon Prov., 
Costa Rica. 11-13-1965, DHJ, no. 828, fl. gp. 
2. Pandora, Limon Prov., Costa Rica. VIII-26-1963, C.D. 
Michner, nc. 
3. 16 mi N Vara Blanca on road from Heredia to Puerto 
Viejo (Sarapiqui Dist.), Heredia Prov., Costa Rica (320 
m) . V-1-1965, DHJ, no. 819, fl. 
4. 1 km W La Virgin, Sarapiqui Dist., Heredia Prov., Costa 
Rica (80 m) , IX-5-1971, DHJ, nc. 
5. 1 km W La Lola (on Limon-Siquirres railroad), Limon 
Prov., Costa Rica. VIII-19-1963, DHJ, nos. 1868, 1872, 
1873, 1874, 1875, 1876, 1877, 1878, 1881. 
6. 11 mi N Florencia, San Carlo Dist., Alajuela Prov., Costa 
Rica. VII-14-1963, DHJ, no. 1883, 1885, 1866. 
7. Along Changuinola River, Changuinola Valley, Bocas del 
Tor0 Prov., Panama. 11-29-1924, V. C. Dunlap, no. 462 
(F 709739, US 1405662) (listed incorrectly as ?A. cookii? 
in Woodson and Schery), fl. 
8. Lancetilla valley near Tela, Depto. Atlantida, Honduras. 
VI-6-1927 to 111-20-1928, P. C. Standley, no. 54627 (US 
1408382). 
9. 1.9 mi W Puerto Matias de Galvez (on road to Escobas), 
Izabal, Guatemala. VIII-10-1967, DHJ, no. 1582. 
10. 27.4 mi S Puerto Barrios, Izabal, Guatemala. VIII-8-1967, 
DHJ, nc. 
NATURAL HISTORY 
Acacia ruddiae is one of the most peculiar Central 
American members of the genus. It lives in very 
wet forest habitats, which is distinctly unlike other 
acacias, yet does not have the obligate acacia-ants 
which generally allow survival of swollen-thorn 
acacias in wet habitats (Janzen, 1966a). 
In Costa Rica, where the only large populations 
of A. ruddiae have been encountered, the natural 
habitat of this tree is in primary succession along 
lowland rivers through rain forest (sites 4, 5), swamp 
edges (site 5), and in large, old landslide scars on 
the sides of very steep gorges (site 3). These areas 
receive 4 to 5 m of rain a year and have a poorly 
developed dry season of 1 to 2 months with occa- 
sional rains. Some of the sites occupied by A. mela- 
noceras and A. allenii receive as much rainfall, but 
have a more noticeable dry season. Judging from the 
paucity of collections, A. ruddiae apparently also 
exists as a very rare tree in the rain forest that ex- 
tends in a narrow strip up along the Caribbean 
coast to the Puerto Barrios area of Guatemala (sites 
8 and 9), but there is no reason to believe that it 
does not become locally common in these forests 
just as it does in Costa Rica. 
Acacia ruddiae is by no means generally dis- 
tributed throughout the rain forest of the Costa 
Rican Caribbean lowlands. At site 4 it was common 
in 1- to 30-year-old primary succession on gravel 
river banks and grew to a 25-m-tall tree with 30 
cm DBH; it had not, however, extended its popula- 
tion out into the surrounding old cacao groves and 
pastures. At site 5 there was a large population 
along about a mile of old river bank, and occasional 
large saplings in the immediately adjacent parts of 
a banana plantation and on swamp edges in pri- 
mary forest. Many of the swamp trees were old and 
scattered widely, in contrast with the many adjacent 
and even-aged A. ruddiae along the river. Extensive 
search throughout many other swamps and dis- 
turbed forest habitats within ten miles located no 
other A. ruddiae. At site 3, A. ruddiae occurs only 
FIGURE 25.-Acacia ruddiae (Costa Rica): A, representative flowering branch (site 1); B, naturally 
split type A thorn (site 6). 
NUMBER 13 
39 
FIGURE 26.-Representative branches of A. ruddiae: A, without swollen thorns from the crown 
of a large adult (Guatemala, site 9); B, from main trunk of large tree (Costa Rica, site 5). 
in a small portion of the total landslide area in this 
strongly dissected topography. It has not spread 
along the road into other disturbed habitats from 
this site. While detailed weather data are not avail- 
able for site 3, it may be slightly drier-hence better 
insolated-than other disturbed sites slightly above 
or below it in elevation. This is indicated by the 
high success of grasses in early stages of succession 
here, in contrast with dicot regeneration at casually 
cleared sites a few miles farther up or down the 
road. 
Seedlings growing in deep shade resemble those 
of A. melanoceras and A. allenii in that the leaves 
are arranged in a whorl so that in vertical view, 
they do not overlap. When growing on bare gravel 
bars, the seedlings are more bushy and asymmetrical 
in appearance (Figure 24~). The nodules on the 
rootlets of A. ruddiae seedlings are more noticeable 
than on any other swollen-thorn acacia (Figure 
24~). 
The seedlings differ from those of all other 
swollen-thorn acacias in lacking swollen thorns. The 
first swollen thorns do not appear until the plant 
is 1 to 3 m tall and has several hundred leaves. The 
nectaries (Figure 8c), however, are produced as 
soon as on any swollen-thorn acacia. The first ones 
appear on the rachis just below the point where the 
pinnae join the rachis, and petiolar nectaries begin 
to appear on the 25th to about the 50th leaf. There 
are no Beltian bodies on A. ruddiae, and thus the 
only contact that the seedlings may have with ants 
is when occasional worker ants visit the nectaries 
and forage on the leaf surfaces. In this sense the 
young A. ruddiae are no different from nonant- 
acacias, except that the nectaries are larger than on 
nonant-acacias and may therefore produce more 
nectar. 
On saplings, the first swollen thorns are scattered 
along the main trunk and lateral branches. They 
are very thin-walled, hollow, easily crushed, and 
40 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 27.-Acacin ruddiae (Guatemala, site 9): A, type B thorns on short lateral branches from 
main trunk; B, representative crown section of 4-m-tall sapling; c, well developed type A thorns 
from the crown in Figure B. 
occasionally only half of the stipule is enlarged. 
They often split at the base upon drying (Figure 
Z~B), and ants, especially Crematogaster, often use 
this as an entrance hole. As the tree grows larger, 
some of the lateral branches have no thorns at all 
(Figure %A), some branches have large swollen 
thorns at the base and tufts of leaves at the ends 
(Figures 27~, 27~), and some branches are very short 
and bear thorns best classified as type B (Figures 
27~, c). The A. ruddiae in the Puerto Barrios area 
(sites 8 and 9) had most of its swollen thorns on 
branches such as those in Figures 26~ and 27, while 
in Costa Rica the branches were usually very short 
with many thorns (as in Figure 27~) or very long 
with just a few scattered thorns (Figure 28~). The 
lower trunk of a large, old A. ruddiae is covered 
with short branches with many thorns, making the 
tree very difficult to climb. 
The crowns of A. ruddiae are more diffuse than 
any other swollen-thorn acacia and therefore look 
more like most mimosaceous legumes than do 
swollen-thorn acacias. Looking up through a crown 
(e.g., Figure 28~), one can often see large amounts 
of sky; only the most densely leafed branches (e.g., 
Figure 5 1~) look like typical swollen-thorn acacia 
branches. This is the case despite the fact that the 
crowns of A. ruddiae are usually fully insolated; 
while the young seedlings appear to grow fairly 
well in heavy shade, the mature trees almost always 
have canopy level or emergent crowns, suggesting 
that A. ruddiae probably cannot mature in heavy or 
broken shade. 
NUMBER 13 
41 
FIGURE 28.-Acacia ruddiae 
(Costa Rica, site 6): A, lateral 
branches in crown of adult; 
B, looking up through crown 
of young adult. 
The trees of A. ruddiae do not bear flowers until 
they are in the 8 to 15 m height and 15 to 30 cm 
DBH class. Small clusters of yellow and bee-polli- 
nated inflorescences (Figure 25~) are scattered 
through the crown and not concentrated on main 
sexual branches as is common for swollen-thorn 
acacias. The seed pods appear to mature during the 
2 to 4 months immediately following flowering, as 
with A. melanoceras and A. allenii, rather than 
waiting until the following dry season as with dry- 
land, swollen-thorn acacias. 
Adults of A. ruddiae show no more obvious insect 
damage than do other acacias not occupied by 
obligate acacia-ants. Since some have no ants living 
on them at all-though some worker ants forage 
in the foliage for nectar and probably insects-this 
immunity is likely due to the usual kinds of chemi- 
cal defenses encountered in ordinary acacias. Shoot 
tips of A. ruddiae are slow growing in comparison 
with more usual swollen-thorn acacias. Associated 
with this they are very tough, compact, bitter in 
taste, and covered with dense hair (Figure 5A). 
They strongly resemble those of A. macracantha 
and A. farnesiana. 
By virtue of the very well developed foliar nec- 
taries and enlarged thorns, A. ruddiae has far more 
ants foraging on its leaf surfaces than ordinary 
nonant-acacias. None of these ants are obligate 
acacia-ants, but rather those species-Pseudo- 
myrmex, Crematogaster, Camponotus-that live 
throughout the general forest canopy and that 
forage for dead, moribund, or live insects on foliage 
in general. While queens of obligate acacia-ants 
may occasionally be dispersed into habitats with A. 
ruddiae (definitely the case at sites 9 and 10 in 
Guatemala), I have never seen one of the character- 
istic type of entrance holes made by these queens 
in about 2,500 swollen-thorns examined carefully 
on 16 individual trees of A. ruddiae. 
To examine the use made of A. ruddiae thorns 
by ants, a representative 3 m tall, fully insolated 
sapling growing in river-edge vegetation at site 4 
was completely dissected. The acacia had 798 leaves 
on 51 branches; far less than 20 percent of the large 
leaves produced by the acacia were subtended by 
swollen thorns. Of the 371 thorns, 302 were, or had 
been, tenanted by some insect (on a ?typical? 
swollen-thorn acacia, all thorns would have been). 
42 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
Nineteen percent of the thorns had split naturally 
at the base, and all of these thorns had been 
entered by insects. Of all the thorns that had been 
entered by insects, by any means, 20 percent had 
parts of ant colonies in them or queens attempting 
to start a colony. Of the remaining thorns that had 
been entered, 95 percent contained microlepidop- 
terous larvae, weevil larvae, and Orthoptera eggs. 
When these insects emerge, the acacia thorns are 
very likely to be occupied by ants. Spiders and 
other insects tenanted the remaining 5 percent of 
the thorns lacking ants. The ants represented at 
least 7 species in 5 genera-Solenopsis, Cremato- 
gaster, Camponotus, Pseudomyrmex, Paracrypto- 
cerus. While a detailed census is not available, 
cursory examination of A. ruddiae thorns in large 
old trees shows almost every thorn to have a part 
of an ant colony in it. It is extremely likely that 
the continual foraging of these ant species on the 
foliage occasionally results in less herbivore damage 
to the tree than if the ants were not present. 
If a mutant strain of A. ruddiae provided any 
type of solid food to the ants, the probability is 
very high that it would quickly lead to the evolu- 
tion of a strongly mutualistic interaction. It is im- 
possible to know if A. ruddiae represents the 
gradual degeneration of a once intense mutualism 
or is on its way to more mutualistic interaction. 
The latter is more likely, since the former would 
require a strong relaxation of selection pressure by 
herbivores, such as might occur if a swollen-thorn 
acacia were to immigrate to a small oceanic island. 
The evolution of a type of Beltian body may be 
starting to occur with A. ruddiae; some of the 
plants at site 4 have clear tips on the pinnules, 
extending into the leaf about 0.25 to 0.5 mm. If the 
ants were to start to chew on this for moisture or 
nutrients during the time of year when insect prey 
is scarcest, and therefore maintain larger colony 
sizes than on plants lacking this modified pinnule 
tip, the evolution of a food body might occur. This 
is likely to be the way that typical Beltian bodies 
first appeared on acacias. Plants with a larger year- 
round population of ants would probably be less 
damaged by the periodic small outbreaks of host- 
specific insects that probably occur on many tropical 
trees, and certainly occur with nonant-acacias. 
Interestingly, the species of ant that began this 
behavior would be likely to concentrate more of its 
foraging on the acacia and less off, and be likely to 
tenant an increasingly greater proportion of the 
thorns on the acacia. This could occur either 
through faster colony growth than could occur with 
ant species totally dependent on insect prey for 
growth, or through the ants moving into thorns left 
untenanted during periodic population lows in the 
other species of ants. Once the entire ant colony 
becomes concentrated in the acacia, it has a vested 
interest in the health of the acacia and coevolution 
of the interaction should occur at an ever increasing 
rate (Janzen, 1966a). As will be discussed further 
in respect to hybrids, this acacia appears to be a 
plant that would have greatly increased fitness by 
the gaining of the genetic information necessary 
for the production of Beltian bodies. This could 
well happen through introgression with a wet-forest 
acacia such as A. melanoceras at the southern end 
of the range of A. ruddiae, or A. gentlei or A. 
cookii, and the northern end of the range of A. 
ruddine. 
3. Acacia melanoceras Buerling 
Acacia melanoceras Buerling, 1854:362. 
Acacia multiglandulosa Schenck, 1913:362. 
Myrmecodendron melanoceras (Buerling) Britton and Rose, 
1928:93. 
Acacia melanoceras is easily recognized since it is 
the only swollen-thorn acacia in the wet lowlands 
of central Panama and the Canal Zone (Figure 29). 
Where its distribution is tangent with that of A. 
collinsii, about halfway through the Canal Zone, 
A. melanoceras is easily distinguished from that 
dry-forest species since A. melanoceras has a very 
large number of nectaries on the leaf petiole, 
longitudinally ridged swollen thorns, spherical in- 
florescences, and huge leaves, 
1 have been unable to locate the type specimen 
of A. melanoceras and it is likely destroyed; how- 
ever, the type specimen of A. multiglandulosa (at 
Berlin, now destroyed, but represented by Field 
Museum photo 1281 ex Berlin) is from the same 
locality as that of A. melanoceras (site 1) and is 
probably described from the same specimen or a 
duplicate specimen of the same collection. The 
description of the type of both A. melanoceras and 
A. multiglandulosa agrees completely with the 
characteristics of the swollen-thorn acacia popula- 
tion whose distribution is plotted in Figure 29. The 
type specimen of A. multiglandulosa is a representa- 
NUMBER 13 
43 
FIGURE 29.-General postulated distribution (stippled area) and specific collections sites of 
A. melanoceras in Panama. 
tive distal end of a fertile branch from an adult 
acacia. Very long (20 cm) inflorescence-bearing 
branchlets such as the one on the type of A. multi- 
glandulosa are commonly seen on old trees growing 
along river edges that have their crowns partly 
shaded by taller trees. The absence of swollen 
thorns on a branch bearing sexual branchlets, as is 
the case with the type specimen, is commonplace 
in old crowns of A. tnelanoceras. In 1915, Safford 
redescribed A. melanoceras, but under the name of 
A. 
1. 
2. 
3. 
4. 
3. 
multiglan dulosa. 
KEY COLLECTION LOCAL~TIES 
Types of A. melanoceras and A, multiglandulosa: Porto- 
bello (Porto Bello), Colon, Panama (sea level). 1824, 
J.G. Billberg, no. 289. 
Head of Gatun Valley, Canal Zone, Panama. 1914, H. 
Pittier, no, 6745 (US 716562, US 716560, US 716561; Saf- 
ford 1915, Wheeler 1942). 
Headwaters Rio Agua Salud, Canal Zone, Panama. 
Wheeler, 111-6-1923 (Wheeler, 1942). 
Marajal, near Colon, Canal Zone, Panama. Wheeler, VII- 
26-1924 (Wheeler, 1942). 
Cerro Jefe, several mi N Cerro Azul, Panama, Panama. 
R.D. Dressier, VIII-28-1967 (pers. comm. plus thorn 
sample). 
6. 
7. 
8. 
9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
400 m upriver (east bank) from the bridge at Rio Pied- 
ras, Colon, Panama. VII-3-1967, DHJ, nos. 1641, 1912, 
1913, 1915, 1916, gp, mp. 
Southwest corner of Zorro Island (in Gatun Lake), 5 km 
NW Nueva Providencia, Canal Zone, Panama. VII-7- 
1967, DHJ, no. 1894, gp. 
10 m post on Al. Ar. Trail, Barro Colorado Island, Gatun 
Lake, Canal Zone, Panama. DHJ, VII-7-1967, nc (I am 
indebted to R.D. Dressler for showing me this plant). 
7 km SW Gatun Locks on road to Escobal, Canal Zone, 
Panama. DHJ, VII-10-1967, nc (I am indebted to M. 
Nauman for showing me these plants). 
France Field, Canal Zone, Panama (sea level). VIII-1924. 
D. Fairchild (US 1405611). 
Between France Field and Catival, Colon Prov., Panama. 
1-9-1924, P.C. Standley, no. 30378 (US 1219256). 
Near Frijoles, Rio Agua Salud, Canal Zone, Panama. 
111-6-1923, C.V. Piper, no. 5853 (US 1167694, US 
1166375). 
Margarita Swamp, Canal Zone, Panama. VI-28-1923, O.F. 
Cook and R.D. Martin, no. 14 (US 1491829). 
Peiia Blanca Bay, Barro Colorado Island, Canal Zone, 
Panama. VII-3-1934, Shattuck, no. 1087 (US 2366760). 
Hills N of Frijoles, Canal Zone, Panama (50 m). XII-19- 
1923, P.C. Standley, no. 27582, (US 1217650). 
Margarita Swamp near France Field, Canal Zone, Panama 
(sea level). 11-1925, J.B. Shropshire (US 1208884, US 
1208883, US 1208885), mp. 
Barro Colorado Island, Canal Zone, Panama. VIII-1927, 
L.A. Kenoyer, no. 373 (US 1319142). 
44 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
NATURAL HISTORY 
Acacia melanoceras is one of the seven species of 
swollen-thorn acacias with small geographic ranges 
(A. allenii, A. mayana, A. ruddiae, A. sphaero- 
cephala, A. cookii, A. gentlei, the others), and like 
these species is restricted to comparatively wet and 
undisturbed forests. In 1928 Standley reported A. 
melanoceras as common in the forests on the At- 
lantic slopes of the Canal Zone; in comparison with 
the more widely distributed dry-land species, it is 
nevertheless very rare. For example, in contem- 
porary forested habitats there appear to be at least 
100 adult plants of A. collinsii on the dry Pacific 
side of Panama for every one adult A. melanoceras 
on the Atlantic side. It is of course impossible to 
know to what degree this difference is due to A. 
collinsii becoming much more common with the 
development of brushy cattle pastures and waste- 
land. One thing may be said with certainty, how- 
ever, and that is that A. melanoceras disappears 
from any habitat subjected to disturbances that are 
any more catastrophic than infrequent logging. 
Since the Atlantic side of Panama, aside from 
banana farming, is infrequently logged, it is likely 
that the contemporary distribution of A. melano- 
ceras is not much different from that of pre-Colom- 
bian times. 
Every reproductively mature A. melanoceras 
found in this study (sites 6 to 9) was in second- 
growth woody vegetation that had grown better 
than 15 years without further disturbance. All sites 
were either areas that had been clear-cut long be- 
fore for logging or farming (e.g., Barro Colorado 
Island) or had been swept clean by a flooding 
river. Seedlings (Figure 30~) were found only in 
the full shade of canopies 10 to 15 m tall, and on 
wet to moderately well-drained soil. A. melanoceras 
generally becomes reproductively mature under, or 
in, jagged breaks in the general canopy; they are 
almost never fully insolated. Senescent canopies are 
often directly under a small hole in the general 
canopy (Figure 31). Neither mature plants nor 
seedlings are common at any site. Two adults per 
acre is a high density for A. melanoceras. Adults 
and young plants almost always occur as single 
FIGURE 30.-Acacia melanoceras: A, seedling from deep forest shade (scale in inches; Panama, 
site 6); B, normal lateral branch from 6-rn-tall plant (Canal Zone, site 9). 
NUMBER 13 45 
FIGURE 31.-Acacia melanoceras (Panama, site 6): A, looking up through crown of 15-m-tall 
mature tree that is still reproducing, but only poorly; B, looking up through senescent crown 
of 12-m-tall tree that has lost its ant colony. 
shoots. The only clump of shoots encountered in 
this study was at site 6 on the edge of an island 
at Gatun Lake. A single tree-based on tree rings, 
at least 27 years old-had produced a large number 
of root suckers and seedlings covering an area about 
20 by 30 m. The cluster was on the north side of a 
hill and situated such that the taller forest trees 
shaded it, yet the opening caused by the lake led to 
day-long indirect insolation. Wheeler (1942) illus- 
trates a small clump at site 4; the plant has the 
appearance of a cluster of stump and root sprouts 
from a large adult that was cut when the swamp 
was cleared of forest. 
Acacia melanoceras appears to be completely 
absent in repeatedly disturbed pasture, field edges, 
roadsides, cleared swamps, etc., in the central and 
northern part of the Canal Zone and adjacent areas 
in Panama; however, it is still present in adjacent 
relatively undisturbed forest, or forest that has 
been allowed to regenerate for a long time. Extinc- 
tion of A. melanoceras following human disturb- 
ance is probably based on the following factors: 
(1) A. melanoceras does not set seed until at least 
8 to 10 years old; I have found heavy seed produc- 
tion only on very healthy old trees that are in 
partial shade of a tall broken forest canopy. Such a 
long time in reaching reproductive maturity is 
likely due to the shady microhabitat occupied by 
this acacia. (2) When adults or subadults are ex- 
posed to direct insolation by clearing the forest, 
they become very shrubby, stunted, and die after 
1 to 3 years. They produce no seed during this time. 
Failure to grow in full sunlight appears to have 
two causes: First, the leaves are probably genetically 
programmed to operate most efficiently at low-light 
density and air temperature, and the opposite 
microclimate is probably damaging. Second, large 
numbers of phytophagous insects in young second- 
ary succession probably find the acacia to be very 
edible. Shoot tips are repeatedly eaten off the verti- 
cally elongating shoots, resulting in a bush life form 
which is probably very inefficient. It is not obvious, 
however, why the occupant Pseudomyrmex satanica, 
which is so effective at protecting A. melanoceras 
46 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE SZ.-Acacia melanoceras: A, reproductive branch with extraordinarily heavy pod set 
(Canal Zone, site 7); B, reproductive branchlets with representative pod set (Panama, site 6); 
c, longitudinal section through immature but full-sized pod (Canal Zone, site 7). 
in the forest, is apparently so ineffective in more 
open sites. It may be that their protective abilities 
per insect attacking are the same in both habitats, 
but that the plant is less able to repair the more 
frequent damage in the open. (3) A. melanoceras 
does not sprout readily from cut stumps. The 
stumps of many young and mature A. melanoceras 
die without sucker-shoot regeneration, in strong 
contrast to the behavior of other swollen-thorn 
acacias. It may be that heavily shaded plants have 
insufficient energy reserves stored in their roots for 
reproduction by sucker shoots. (4) The seed of A. 
melanoceras (Figure 3%) has a soft seed coat and 
apparently needs to germinate immediately. Even 
if a seed is dispersed into early stages of succession, 
it would probably die of desiccation if it arrived 
during the dry season of one of the dry ?spells? 
during the wet season. (5) Being soft-seeded, A. 
melanoceras cannot be dispersed by the same species 
of birds that disperse the hard seeds of species 
living in drier habitats (eg., A. cornigera, A hindsii, 
A. collinsii); these birds (e.g., jays, derby fly-catch- 
ers) pass the hard seeds through their gizzard 
unharmed, while a seed of A. melanoceras would 
surely be digested. Any dispersal of these soft seeds 
must be via birds that regurgitate seeds (e.g., tro- 
gons, cotingas). These birds characteristically live in 
older stages of succession and primary forest and are 
unlikely to disperse A. melanoceras seeds out into 
large tracts of young second growth. (6) Since the 
seedlings of A. melanoceras are normally lound 
only in heavily shaded forest understory, it would 
not be surprising to find that the queens of P. 
satanica do not search in dry or fully insolated 
sites. The young acacia would then be competing 
with many fast-growing plants with a very high 
density of herbivorous insects. It is possible that 
the searching queens of P. ferruginea, coming from 
A. collinsii growing in young second growth, could 
occupy young A. melanoceras. A. collinsii, however, 
has not yet invaded much of the disturbed land 
left when forest suitable to A. melanoceras was 
cleared. This general interaction could perhaps be 
observed if half of an island such as Barro Colorado 
NUMBER 13 47 
Island were clear-cut and turned into brushy pas- 
ture. 
Acacia melanoceras, A. allenii, and A. ruddiae 
live in the wettest sites occupied by the genus in 
Central America. The dry season in its habitats is 
indistinct and the sites generally receive 3.5 to 5.5 
m of rainfall. At sites 6 and 7, the roots of A. 
melanoceras were under water or in mud almost all 
year. The cluster of fruits in Figure 32~ hung down 
to within a few inches of Gatun Lake. The bark of 
A. melanoceras is almost always damp from rain 
and those parts of the acacia not patrolled by ants 
often become covered with lichens and other 
epiphytes. 
That A. melanoceras lives in wet forest that lacks 
other acacia, and for the most part other mimosoid 
legumes, may explain in part why quite healthy 
but unoccupied adults are occasionally encountered 
in the forest. Judging from the holes in the thorns, 
these acacias were at one time occupied by large 
ant colonies, which died for unknown reasons. In 
contrast to the subsequent death of the tree that 
would have occurred were this a dry-land acacia, 
the abandoned acacias appear to live a number of 
years, bear successive seed crops, and are at times 
tenanted by a variety of other ants that normally 
live in hollow twigs in the general canopy. These 
unoccupied acacias suffer herbivore damage, but it 
is not very severe (in contrast to individuals even 
with a strong ant colony in insolated sites a few 
hundred meters away). I suspect that in leaving 
v) 
W 
U 
U 
m 
W 
el 
!A 
z 
6 40 
30 
- 20 
z 10 
B 
3 
z 
behind the younger stages of succession and the 
large number of insects it harbors, especially those 
that normally feed on mimosoid legumes, A. 
melanoceras has moved into a habitat of reduced 
herbivore pressure. Such a strategy is likely to be 
effective until over evolutionary time other mi- 
mosoid legumes make the same move for the same 
reason. 
While A. melanoceras appears to be pollinated 
by small bees-Halictidae, Megachilidae, Ceratina 
in the Apidae-pollination is either very ineffec- 
tual or the plant has little reserves for maturing 
fruits. The seed crops on almost every tree ex- 
amined have been very small, ranging from 2 to 30 
pods; the long flowering branchlets (Figure 32~) 
set only 1 or 2 pods per branchlet and often none 
at all. The 13-seed pod in Figure 32c is represent- 
ative of the number of seeds per pod; A. melano- 
cerns does not show the strong variation in number 
of seeds per pod found in the acacias of drier sites. 
The large pod crop on the acacia at site 7 provides 
several lines of evidence to suggest that the usual 
small seed crop is due to lack of energy reserves 
rather than poor pollination: (1) This tree was 
growing in an exceptionally good site, as is evi- 
denced by the large number of healthy sucker 
shoots, had an immense ant colony, and large 
amounts of lush foliage (as in Figure 30~ and 
contrasted with Figure 31~). There were no other 
adult swollen-thorn acacias within at least 200 m 
of this plant (down the island) and at least 500 m 
123 13 57 
A NUMBER OF PODS PER 6 NUMBER OF INFLORESCENCES WITH 
INFLORESCENCE PODS PER BRANCHLET 
FIGURE BB.-Acacia melanoceras (Canal Zone, site 7): A, distribution of 64 mature pods among 
the 49 infructescences on 10 sexual branchlets from one tree with an extraordinary seed crop; 
B, distribution of the 49 infructescences in Figure A among the 10 sexual branchlets. 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
across the lake. This plant set at least 900 pods in 
large clumps such as that in Figure 32~. ?The im- 
plication is that the tree is self pollinated, or 
else the clump of shoots originated from two seeds. 
(2) The total number of inflorescences produced by 
each sexual branchlet appeared to be no different 
from that of other acacias examined that had pro- 
duced only 0 or 1 pod per sexual branchlet. The 
number of sexual branchlets per major tree branch 
was likewise no different in any obvious way; how- 
ever, the number of inflorescences that produced 
pods, per branchlet, was much higher (Figure 3313) 
than is usually observed with A. melanoceras. Two 
sexual branchlets had 7 pods each on them, and 
none had only one. (3) There were even more than 
one pod set per inflorescence in many cases (Figure 
33~). That the pods were distributed among the 
inflorescences in this manner, rather than being 
uniformly distributed (i.e., retain more inflores- 
cences, each with one fruit), suggests the possibility 
that the plant chooses out-crossed female florets as 
those to mature. This tree had an average of 31 
inflorescence-bearing axils per sexual branchlet 
(=lo). Its total crop of about 18,540 inflorescences 
produced about 960 pods on 735 inflorescences for 
a total seed crop of about 10,560 seeds. There are 
about 10 female florets on each inflorescence, mean- 
ing that only 0.6 percent of the tree?s ovules 
matured. 
Associated with the small seed crop observed on 
most adult A. melanocel-as, it is particularly note- 
worthy that this species is free from predation by 
the bruchid beetles that take such a high toll of 
hard-seeded acacias. This means that virtually every 
FIGURE 34.-Acacia melanoceras (A-C, Canal Zone, site 7): A, type B thorns on short lateral 
branches from the main trunk of a mature tree; B, immature type B thorns on newly develop- 
ing short lateral branch from same trunk; ants are Pseudomyrmex satanica; c, maximally de- 
veloped cluster of type B thorns on lateral branch of same tree; D, type B thorns on lateral 
branch from crown of very old A. inelanoceras (Panama, site 6). 
NUMBER I3 
49 
seed dispersed by the plant is viable, in contrast to 
the case with hard-seeded, swollen-thorn acacias 
where most seeds are killed by bruchids before the 
seeds are dispersed. In short, the average number of 
viable seeds produced per adult A. melanoceras is 
likely to be as great as for acacias of drier sites 
that initially produce many more seeds. 
A seedling of A. melanoceras (Figure 30~) can 
probably germinate and grow slowly to adulthood 
almost anywhere in the forest, in strong contrast to 
the seedlings of hard-seeded, swollen-thorn acacias, 
which must land in a fairly young stage of succes- 
sion to survive. This means that a certain large frac- 
tion of the viable seeds produced by dry-land acacias 
will not make it to the stage where they have any 
chance at all of surviving. Therefore, the apparently 
smaller seed crop of A. melanoceras may in fact not 
mean that any fewer saplings develop per adult 
than is the case with dry-land, swollen-thorn 
acacias. 
Throughout its range, I found A. melanoceras 
to be occupied only ,by Pseudomyrmex satanica 
(Figure 3413). (The ?Pseudomyrmex satan? cited by 
Standley in 1928 is a misspelling.) Wheeler (1942) 
likewise had no records of other obligate acacia-ants 
in A. melanoceTas. Since P. satanica is restricted to 
A. melanoceias, the ant will probably go extinct 
as the remaining forest in Panama is converted to 
agricultural land. P. satanica is probably a ?wet 
forest derivative? of the widespread P. ferruginea, 
which occupied A. collinsii in the drier parts of 
Panama, among other places. As will be discussed 
later, the part of the P. ferruginea population oc- 
cupying A. allenii on the Osa Peninsula and the 
Golfito region in Costa Rica probably represents 
an intermediate form of the same type of speciation 
event. P. satanica is easily distinguished from P. 
ferruginea in Panama by the former?s black color, 
habit of holding the gaster curled under the body 
(Figure 34~), and having multiple-queen colonies. 
Unoccupied adults of A. melanoceras are tenanted 
by the usual miscellaneous array of ants found in 
unoccupied swollen-thorn acacias: Crematogaster, 
Azteca, Camponotzis, Paracryptocerus, and Pseudo- 
myrmex. Since, as mentioned previously, unoccu- 
pied A. melanoceras seems to survive fairly well in 
well-developed forest, these colonies may attain 
unusually large size for ants tenanting swollen-thorn 
acacias. At times an Azteca or Crematogaster colony 
will take over all the thorns on the tree, but the 
ants? foraging on the acacia foliage is not thorough 
enough to prevent frequent damage by herbivores. 
P. satanica founding queens establish themselves in 
the thorns of A. melanoceras seedlings at the stage 
shown in Figure 30~. Until the ant colony has 
grown large enough to patrol the young shoot, 
growth is not rapid enough for the plant to have 
more than 3 to 6 intact thorns less than two years 
old) at any one time. Not only is the number of 
founding queens in the system small as compared 
with dry-land, swollen-thorn acacias, but one very 
rarely finds searching queens on the outside of the 
acacia. This indicates that few queens are produced 
by adult colonies or that the queens have a difficult 
time locating the widely spaced seedlings. Both 
possibilities are probably true, 
FIGURE 35.-Dormant shoot tip and ?crown? of A. melano- 
ceras seedling above point where latest swollen-thorns were 
produced (Panama, site 6). 
50 
SMITHSONIAN CON'I'RIBUTIONS TO BOTANY 
FIGURE 36.-Acacia melanoceras (Panama, site 6): A, petiolar nectaries characteristic of seedling 
leaves near swollen thorns; B, maximal development of petiolar nectaries in crown of old plant; 
c, minimal petiolar nectary development of large leaves subtended by type A thorns on lat- 
eral branches of 5-m-tall subadult. 
Leaf production on thc seedling acacia differs 
from that of dry-land acacias. The seedling pro- 
duces 1-4 leaves subtended by swollen thorns fol- 
lowed by a cluster of leaves without swollen thorns 
(Figure 35). By the time that swollen thorns have 
rotted off (about 1.5 years later), the leaves have 
been shed and the process is repeated if an ant 
colony has not become established. How long the 
seedling can continue this process is unknown, but 
I have found seedlings that have gone through 
this process at least five times. The seedling in 
Figure 30~ had done it at least four times, yet was 
only 53 cm tall. 
On the very young seedlings there may be no 
nectaries on the leaves (Figure 35) except those 
TABLE 9.-Distribution of leaflets among the 9 
leaves on a 40-cm-tall seedling of A. melanoceras at 
the time of production of its first swollen thorn 
(a total of 52 leaves had been produced by the 
seedling, and the thorn was on the 50th node) 
Rachis Petiole Number of Number of 
number length (mm) length (mm) pinnae pinnules 
Leaf 1 ~ 83 44 16 428 
108 
104 
117 
112 
113 
116 
108 
109 
22 22 
31 18 
30 24 
27 21 
24 28 
28 24 
30 22 
32 21 
604 
492 
660 
492 
174 
654 
598 
512 
NUMBER IS 
51 
280 
240 
200 
160 
120 
- 
z 
Y 
L4 
4 
UI 
cl 
0 
X 
F- 
u 
L 
m 
u *O 
el 40 
. 
. 
. 
. 
. . 
.* 
. 
. 
t 
m. 
.. .. 
. *. 
'. 
*.*. 
. 
*. .*. 
.. 
. ..*. . 
t 
.. 
0.- 
.* 
.* 
. 
. 
. 
. 
. 
. 
. .. 
. 
.* 
.. 
.' 
: **. . 
. . 
. 
t *. 
. 
. 
. 
. 
. 
I I I 1 I I 
0 5 10 15 20 25 
NUMBER OF NECTARIES PER LEAF 
FIGURE 37.-Distribution of 1256 petiolar nectaries among 24 leaves of A. melanoceras from 
sterile lateral branches in an adult crown (Panama, site 6). 
subtended by, or close to, swollen thorns (Figure 
36~). As the plant matures, the number of petiolar 
nectaries increases through the progression shown 
in Figures QA, B, c. The distribution of petiolar 
nectaries among the leaves (Figure 30~) in a mature 
acacia's crown (Figure 37) displays all the fre- 
quencies that can be observed by watching seedlings 
develop, but the majority of leaves have between 
5 and 20 nectaries. The leaves of A. rnelanoceras 
(Figure 30~) are at least three times the size of 
those found on dry-land acacias. The number of 
nectaries is higher on adults of A. melanoceras than 
on any other swollen-thorn acacia. On some A. 
melanoceras there are even nectaries on the pinnae. 
Owing to the shaded nature of its habitat, however, 
nectar production may be very low. 
Beltian body production by A. melanoceras is 
never fully developed, again probably from the 
relatively high cost to the plant of Beltian bodies 
in a habitat with low solar energy input and (per- 
haps) somewhat reduced herbivore loads. The first 
leaves on a seedling, as is usual with swollen-thorn 
acacias, have no Beltian bodies. By the time the 
leaves are developed to the level described in Table 
9, there are 1 to 3 Beltian bodies per new leaf; a 
dry-land acacia would have been producing 10 to 
20 per leaf by the time it had produced this many 
(52) leaves, even when growing in the shade. Even 
the very large and healthy acacia at site 6 was 
producing Beltian bodies on only about half its 
pinnules. 
On adult A. melanoceras, type B thorns appear 
on a small stubby branchlet (Figure 34~) which 
quickly elongates to 5 to 10 cm (Figure 34~). On 
large and healthy occupied acacias, such as the one 
at site 6, there may be many of these short shoots 
in relatively tight clusters (Figure 34c). On acacias 
in poorer condition, they are usually scattered along 
52 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
140 
120 
E 
E 
Y 
E 100 
W 
cl 1 .i. 
140 
,120 
s 
E 
- 
A 1.4 1.8 2.2 2.6 
i.101181111'' 
TOTAL THORN VOLUME (cc) 
r 
c. 
W 
cl 
g 100 
B 
0 
2 80 
1.0 1.4 1.8 2.2 2.6 3 .O 
TOTAL THORN VOLUME (cc) 
FIGURE 38.-Mature A. melanoceras (Canal Zone): A, distribution of total thorn volume among 
the 25 largest type B thorns (site 9); B, distribution of thorn volume among the 31 largest type 
B thorns (site 7). 
the branches (Figure 34~). The volumes of A. 
melanoceras type B thorns (Figure 38) are not spec- 
tacular and are rarely more than twice that of type 
A thorns. The similarity of the two different sets 
of thorns in Figure 38 is probably representative 
of variation in type B thorns throughout the range 
of A. melanoceras. 
Several aspects of the biology of A. melanoceras 
reflect its occupation by a multiple-queen species of 
ant: (1) As noted, there is not a great difference in 
size or toughness between type A and type B thorns. 
While P. satanica colonies do not have the great 
number of queens characteristic of P. venefica and 
the undescribed multiple-queen Pseudomyrmex in 
Acacia hindsii in western Mexico (Janzen, 1973), 
they do have 5 to 20 or more per colony and this 
means that the loss of a queen-thorn to a bird or 
other predator is not fatal to the colony. This 
means that the primary selective pressure for very 
large and strong type B thorns has been removed. 
(2) When there is the rare opportunity for an A. 
melanoceras to reproduce vigorously with root suck- 
ers, as at site 6, the multiple-queen colony can ex- 
pand indefinitely to provide patrolling for the 
increased surface area of foliage. Therefore, while 
A. melanoceras normally occurs as single plants 
that could easily be patrolled by a single-queen 
colony, when the situation arises, the plant can 
greatly increase in size (and presumably seed 
production). The acacia is effectively polymorphic 
for growth form. (3) The new searching queens 
may on occasion be absorbed by the colony 
that produced them rather than go off in 
search of seedlings. This may result in a 
lower survival of seedlings than would be the case 
with a single-queen ant species situation. On the 
other hand, the presence of several queens in a 
young seedling are all contributing workers to the 
development of the young plant. (4) When the 
opportunity occurs for the growth of a cluster of 
A. melanoceras shoots occupied by one multiple- 
queen colony, the shoots are free of competition 
from immediately adjacent vegetation. As is usu- 
ally the case with large multiple-queen colonies, 
P. satanica is especially effective at killing foliage 
that contacts its acacia. Incidentally, as the name 
suggests, P. satanica is very aggressive toward a wide 
variety of foreign objects. These ants are perhaps 
the most difficult of the obligate acacia-ant species 
to work with, primarily because they run inside 
NUMBER 13 53 
one?s clothing rather than just on top as is the case 
with the other species. As mentioned before, this 
means that a large cluster of A. melanoceras oc- 
cupied by a large colony of P. satanica tends to be 
in extremely good condition. 
4. Acacia allenii, new species 
Arbol usque ad 25 m alta; ramuli novelli minute 
velutini, glabrescentes; stipulae spinescentes inter- 
dum magnae, teretiusculae, porcatae, usque ad 4 
cm longae, 1 cm latae; folia 20-40 cm longa; nec- 
taria sessilia cupulata ad juncturas pinnarum 
omnium et 1-10 in petiolis; pinnae (5-) 10-20- 
jugae, 7-10 cm longae; pinnulae (1%) 20-30-jugae, 
oblongae, lineare, vel ellipticae, (3-) 5-10 mm 
longae, 1-4 mm latae, obtusae, glabrae; flores aurei 
in capitulo globoso; involucellum annulare in 
medio pedunculo vel inferiore, interdum basi; 
legumina (immatura) atrobrunnea, puberula, 
granulata, glabrescentes, lineares, 8 cm longa, 8 mm 
lata, complanata, acuta sessiles vel subsessiles. 
TYPE.-?. H. Janzen 1769, Costa Rica, Pun- 
tarenas Province, Osa Peninsula, bank of tributary 
of Rio Agua Buena, 3.5 mi SW of Rincon, 9 March 
1967 (holotype US 2639555; isotypes F, GH, 
Rica: Puntarenas: Forested hills near Golfito, Golfo 
Dulce, P. H. Allen 5B7 (US). 3.4 mi E of Golfito, 
Janzen 1658 (F, MICH, MO, NY, UC, US). 6 mi S 
of Rincon on the "Road to the  Pacific," Janzen
2000 (F, GH, MEXU, MICH, MO, NY, UC, US). 
Type tree 8 m tall with widely spaced branches 
and very dark green bipinnately compound leaves 
20 to 40 cm long. Pinnules 8 to 11 mm long, 2 to 
3.5 mm wide and up to 60 per pinna. Pinnae 7 to 
10 cm long and up to 40 per leaf. 2 to 10 widely 
spaced petiolar nectaries and 1 to 2 nectaries on 
rachis at bases of pinnae. Swollen stipular thorns 
dark brown and up to 4 cm long and 1 cm wide at 
base, external thorn walls with low and rounded 
longitudinal ridges. New leaves with orange-yellow 
food body on tip of each pinnule. Internode length 
5 to 10 cm on lateral branches. Sterile specimen. 
Fruits and flowers described below from other 
specimens. Janzen field collection number 1769. 
Types deposited in U. S. National Herbarium, 
Smithsonian Institution. 
This plant is named in honor of Paul H. Allen, 
who collected the first specimen (site 1) and did so 
MEXU, MICH, MO, NY, UC). PARATYPES.-cOSta 
much to aid plant biologists working in Central 
American wet forests. In his book (Allen, 1962) 
A. allenii is identified as A. melanoceras, a mistake 
easily made if one is working from herbarium speci- 
mens of either species. His specimen from site 1 
bears immature inflorescences but was not chosen as 
the type because its leaves were badly damaged and 
very small. This type of foliage is typical of flower- 
bearing branches but not sterile branches. Sexual 
material is of almost no use for identification or 
reference since plants bearing fruits or flowers are 
extremely rare in time and space. For these reasons, 
a sterile plant was chosen for the type (site 6). 
Specimens of mature pods are not available from 
any A. allenii, and the only good collection of 
flowers (site 5, no. 2000) is from a nearly leafless 
tree. The tree from which the type specimen was 
taken was growing approximately in the center of 
the geographic range of A. allenii (Figure 23), and 
along a creek bank in lightly disturbed ?rain 
forest,? which seems to be the normal habitat for 
A. allenii. Its foliage is very representative of sterile 
and mature plants of the species; the plant was 4 m 
tall and originated as a sucker shoot from a 22 cm 
diameter downed trunk that was at least 25 years 
old. I estimate that the shoot would have flowered 
for the first time within the next 3 to 4 years. It 
was occupied by a large colony of Pseudomyrinex 
ferruginea. 
Acacia allenii is very similar (and presumably 
very closely related) to A. melanoceras. I feel justi- 
fied in distinguishing it as a separate species on the 
following grounds: 
1. The population of A. allenii appears separated 
from that of A. melanoceras by at least 100 miles 
(and probably more) and by the Continental Di- 
vide. Despite careful searching, neither plant has 
been found in the wet and relatively undisturbed 
forests along the Pacific side of the mountains 
running through the western half of Panama. It 
seems that by the time one moves high enough in 
the mountains to find sufficient rainfall for A. 
allenii or A. melanoceras, it is too cold for the 
species. A. collinsii occupies the drier areas between 
the ranges of A. allensii and of A. melanoceras. In 
view of the long geographic distance between A. 
allenii and A. melanoceras, genetic exchange by 
either pollen or seed flow seems extremely improb- 
able. This is not meant to imply, however, that 
there would not be extensive exchange of genetic 
54 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
10 
u) 
-1 
z 
V 
t; 
25 
b 
a 
m 
a 
w 
I 
3 
X 
m 
40 
30 
u) 20 
-1 
x 
4 
l-l 
!!3 10 
a 
w 
3 
X 
B 
1 
13519 15 13 5 7 9 11 1315 
A NUMBER OF INFLORESCENCE-BEARING B NUMBER OF INFLORESCENCES PER AXIL 
AXILS PER BRANCHLET 
FIGURE 39.-Acacia allenii (Costa Rica, site 5): A, distribution of 242 inflorescence-bearing axils 
among 37 sexual branchlets from the crown of a 22-m-tall tree: B, distribution of 2255 in- 
florescences among the 242 axils in Figure A. 
FIGURE 40.-Acacia allenii (Costa Rica): A, left, type B thorns on the base of a lateral branch 
with terminal type A thorns: right, type B thorns on a short lateral branch from the main 
trunk (site 2): B, left, type A thorns on lateral branches and main trunk of 3-m sapling; right, 
type B thorns on short lateral branches of the same tree (site 5). 
NUMBER 13 
55 
material were this gap to be bridged in some man- 
ner, This seems unlikely, since both species become 
even more rare than they already are as their 
habitat comes under agricultural influence. 
2. The foliage of A. allenii is more similar to 
that of A. melanoceras than it is to any other 
acacia. However, leaves from mature A. allenii are 
easily distinguished from those of A. melanoceras, 
since a leaf of A. allenii usually has only 1 to 5 
petiolar nectaries and has pinnules with about twice 
the surface area of those of A. melanoceras. These 
two differences, along with the floral traits described 
below, are consistent throughout the population of 
A. allenii. 
3. Inflorescences of A. melanoceras are borne on 
long (up to 45 cm) sexual branchlets that tend to 
be bunched near the ends of sterile branches; in- 
floresences of A. allenii are borne on short (3 to 15 
cm) sexual branchlets scattered along major 
branches and therefore throughout the crown. The 
number of axils with inflorescences on a sexual 
branchlet (Figure 39, 42~) is about half that of 
A. melanoceras. While each sexual branch of A. 
melanoceras has 1 to 5 inflorescences per axil, each 
inflorescence-bearing axil on A. allenii has up to 
15 or 16 with a mean at about 9 for large samples. 
4. The type B swollen thorns of A. melanoceras 
are normally found on short branches, while those 
of A. allenii are much more likely to be strung 
along the lower parts of a branch that bears large 
leaves and type A thorns on its distal portion 
(Figure 40~). A. allenii thorns regularly have an 
inward projection that partly occludes the basal 
part of the thorn cavity. This is almost always 
absent from the thorns of A. melanoceras. 
5. The differences between A. allenii and A. 
melanoceras are of the same magnitude that can 
be encountered over the ranges of one widely spread 
species such as A. collinsii, A. cornigera, or A. 
hindsii. However, these widespread species appear 
to have continuous ranges and almost always the 
variation is clinal in nature. A. allenii and A. 
melanoceras each appear to be going its own way 
evolutionarily, and I have no reason to suspect 
that their similarity will persist through time. If 
for no other reason, they are likely to diverge since 
A. melanoceras is occupied exclusively by a multi- 
ple-queen ant colony while A. allenii is occupied by 
a single-queen species. There is always the possi- 
bility that P. ferruginea will evolve multiple-queen 
behavior and thereby evolutionarily ?catch up? 
with P. satanica in A. melanoceras. In the mean- 
time, however, A. melanoceras will likely have 
further diverged from A. allenii. 
6. When describing the ecology of these two 
populations, lumping them under one Latin name 
destroys resolution. To call them mutual ?sub- 
species? is to infer a common evolutionary origin, 
when in fact A. melanoceras may be a wet-forest 
derivative of the Panamanian portion of the A. 
collinsii population, while A. allenii may be a 
wet-forest derivative of the Costa Rican portion of 
the A. collinsii population. Even if one is in fact 
the result of a long-distance, overland-seed dispersal 
from the other?s population (the most likely origin), 
we still have no way of knowing which is the 
?parent? species and which the ?subspecies.? 
KEY COLLECTION LOCALITIES 
All specimens are from Puntarenas Province, 
Costa Rica. 
1. 
2. 
3. 
4. 
5. 
6. 
Forested hills near Golfito. 111-3-1951, P. H. Allen, no. 
5997 (USNH 2085794). Allen (1952) reports A. allenii as 
occasional in pastures around Golfito. 
2.5 mi E (inland) of Dominical. DHJ, VI-25-1967, no. 
1669, mp. 
3.4 mi E Golfito. DHJ, VI-28-1967, no. 1658, fl. 
11.0 mi E Golfito. DHJ, VI-28-1967, 11-28-1965, nc. 
Within 8 miles of Rincon along road to the Pacific and 
along highland road, Osa Peninsula. DHJ, 111-1-3-1965, 
no. 791, 803, 811, 812, 813; 111-10-1968, no. 2000, fl. 
TwE.-bank of tributary of Rio Agua Buena, 3.5 mi SW 
Rincon, Osa Peninsula. DHJ, 111-9-1967, no 1769. 
NATUML HISTORY 
Like A. melanoceras, A. allenii has a very nar- 
row geographic range (Figure 23) which probably 
reflects rather narrow tolerances of moisture and 
temperature. Its uppermost elevation record (160 
m at site 2) is probably representative for the entire 
range, and, incidentally, is probably about the same 
as that for A. melanoceras on the Atlantic side of 
Panama. A. allenii is occasionally found in some- 
what drier sites than A. melanoceras (such as land- 
slide succession, roadsides), but otherwise it appears 
to have the same ecological difficulty with invading 
disturbed habitats as detailed for A. melanoceras. 
Acacia allenii provides a somewhat better test 
than A. melanoceras of the prediction that both 
species are likely to become extinct with the intro- 
56 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 41 .-Sedling of A. allenii; note single swollen thorn 
in crown and old rotted thorn near bottom of photo (Costa 
Rica, site 5). 
duction of European-type agriculture to a particu- 
lar site. Allen (1952), based on experience around 
Golfito in the 1940s, when much primary forest was 
being cleared, reported that A. allenii was an oc- 
casional plant in brushy pastures. Today (1965- 
1970) it is totally absent from the pastures in the 
Golfito region but may still be encountered on 
the forest edge where fields are being pushed back 
into the forest, especially along the road between 
Golfito and the Pan-American highway. When the 
forest is cleared, the stumps of A. allenii produce 
small shrubby bushes. Like those of A. melanoceras 
under the same circumstances, these pasture shrubs 
may live for 1 to 4 years and attain a height of 2 
to 3 m-in the forest they live as long as 25 years 
and attain a height of 22 m. Since the ant colonies 
in these shrubs are quite ineffective at clearing the 
vegetation from around the acacia, pasture fires 
normally kill the acacia and its ant colony. In late 
stages of woody succession such as on landslides or 
undisturbed regeneration following logging, light 
ground fires kill neither the acacia nor ant colony 
(Janzen, 1967a). 
In the general area of Rincon on the Osa Penin- 
sula, healthy seedlings of A. allenii in the 0.5 to 2 
m size class occur at a density of at least 1 per 
hectare in old natural secondary succession (such as 
along creeks and rivers) ranging from 20 to 40 years 
of age. These young plants are often occupied by 
colonies of P. ferruginea with 25 to 300 workers. A 
high percentage of these seedlings appear to attain 
reproductive maturity since the same habitats have 
densities of adults as high as one per 5 hectares. 
When an area is cleared by logging and then al- 
lowed to return to forest, the density may be much 
higher, indicating that in uniform woody succes- 
sion, seedling survival is higher than in the frag- 
mented successional stages along rain-forest water- 
ways and landslides. For example, the lower part 
of the hillside immediately north of the airstrip 
near Rincon was almost clear-cut in the late 1940s 
and then allowed to regenerate undisturbed. The 
density of adult or subadult A. allenii in this site 
is about 3 per hectare. Along logging roads, A. 
allenii adults or subadults may attain a density as 
high as 4 per linear mile of road. 
The seedlings (e.g., Figure 41) of A. allenii are 
morphologically and physiologically very similar to 
those of A. melanoceras. For purposes of later com- 
parison with dry-land, swollen-thorn acacia seed- 
lings, I offer the following description of a 90 cm 
tall A. allenii seedling (site 5). The plant was 
growing in deep shade in undisturbed forest on 
level ground and had a tap root 22 cm long and 
8 mm in diameter at ground level. As do other 
swollen-thorn acacias, the rootlets had nodules on 
them. The taproot went straight down and had 8 
lateral roots of 1 to 2 mm diameter. The stem 
had narrowed to 5 mm diameter at 10 cm above 
the ground. Based on leaf scars, between 93 and 
110 leaves had been produced, 8 of which were on 
the plant. Between 10 and 14 swollen thorns had 
been produced, 2 of which were still intact and 
contained founding queens of P. ferruginea. The 
swollen thorns had the longitudinal shallow grooves 
characteristic of adult plants of both A. allenii and 
A. melanoceras. Their volumes were 0.8 and 0.9 cc. 
NUMBER 13 
FIGURE 42.-Acacia allenii (Costa Rica): A, sexual branchlet (site 2); B, mature seeds: seed in 
lower left had split its thin seed coat and was starting to germinate before it left the pod (site 
2); C, rachis nectaries on leaf subtended by a type A thorn (site 5). 
FIGURE 43.-Acacia allenii (Costa Rica, site 4): A, sapling that was growing in full shade; B, type 
A thorns in crown center of the sapling. 
57 
58 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
As with A. melanoceras, the seedling produced a 
whorl of leaves without swollen thorns which were 
either followed or preceded by several leaves with 
swollen thorns. The leaves were nearly as large as 
those of an adult, and radiated outward such that 
they did not overlap on a horizontal plane. Free- 
dom from overlap was aided by the leaf having a 
3 cm petiole about twice the length of the petiole 
on a mature leaf. The leaves ranged from 15 to 18 
cm in length, had pinnae 22 to 29 mm long, and 
had pinnules 6 to 7 mm long and 1.5 to 2.0 mm 
wide. Beltian bodies were on only the basal 3 to 6 
pinnules of each pinna, and were 2 to 4 mm long. 
These large Beltian bodies are about twice the 
length and 4 times the volume of those on a mature 
plant. There were no petiolar nectaries, but there 
was a single small nectary at the base of each pair 
of pinnae (as in Figure 42c). 
Once such a seedling is occupied by an active 
colony of ants, it will grow into a sapling in 2 to 3 
years such as the one in Figure 43~. As the sapling 
grows, it maintains a single-stem life form and 
produces virtually no root sucker shoots. In very 
young second growth, the plant becomes very bushy 
owing to insects eating off the growing shoot tips 
(again as with A. melanoceras). 
Seed-bearing adults have 12 to 20 growth rings 
and are probably 15 to 23 years old. They may 
attain a DBH of 35 cm and are generally 12 to 25 
m tall. The adults are almost never emergent to 
the general canopy (in strong contrast to dry-land 
species). Senescent canopies look like those figured 
for A. melanoceras. After the top has rotted out 
and the trunk fallen, the root crown sometimes 
produces a single sucker which may become oc- 
cupied and grow to adult status. The same happens 
if a healthy adult is cut, but if the sucker shoots 
are then removed, the stump usually fails to pro- 
duce more. It appears to have very limited food 
reserves (again, in contrast with dry-land acacias). 
As does A. melanoceras, A. allenii lives in a 
habitat (heavily shaded understory of wet forest) 
of less intense defoliation pressure as contrasted 
with the dry-land, swollen-thorn acacias. In an at- 
tempt to quantify this difference, an experiment of 
the type conducted extensively with A. cornigera 
in lowland eastern Mexico (Janzen, 1967b) was 
performed on the A. allenii population at site 5 
(cf. similar experiment for A. collinsii for direct 
comparative data for a dry-land species). During 
the two weeks preceding March 1, 1968, I searched 
intensively for adult or subadult A. allenii in site 
5. The 21 healthy plants located were in various 
kinds of vegetation and ranged from 5 to 20 m in 
height. None was fully insolated. On March 1, all 
were cut, leaving 60-cm-tall stumps in heavy to 
light shade. The control plot of 12 acacias was 
established by placing the acacia crown, with its ant 
colony, over or adjacent to the cut stump. The 
experimental plot of 9 acacias was established by 
dragging the crown at least 5 m from the stump 
and usually farther. Experimental and control 
plants were mixed in sequence, with the treatment 
determined by a flip of a coin. As anticipated, the 
ants-or the colony-abandoned 6 of the control 
acacias and reoccupied one of the experimental 
stumps by moving the colony 6.5 m from the old 
crown to the new sucker shoots from the stump. 
This left a final tally of 7 occupied stumps and 
14 unoccupied; these adjustments occurred within 
the first month after the stumps were cut. On 
September 17, 1968, six and a half months later, a 
variety of parameters were recorded (Table 10). 
The effect of an ant colony on sucker regeneration 
is obvious, but it is not nearly as dramatic as were 
this experiment to have been conducted with a 
dry-land acacia during six and a half months of the 
rainy season. These data, and some of the more 
obvious reasons for this difference are discussed in 
the following paragraphs. 
The diameter of the stump of a swollen-thorn 
acacia is weakly correlated with the amount of 
reserves the plant has for the production of sucker 
shoots. While the stumps of the occupied acacias 
appear to have smaller diameters, thus biasing the 
experiment in the favor of unoccupied shoots, in 
fact they are not statistically different. Further, 
differences in growth between occupied and un- 
occupied shoots for other reasons are usually far 
greater than those resulting from difference in 
stump size. Without much larger samples, it is im- 
possible to segregate out the effect of stump size. 
The tallest sucker shoot may originate at any 
point on the side of the 60 cm stump. For this 
reason, small samples tend to have a very high 
variance in the total height of the plant in the 
early stages of sucker regeneration. That the oc- 
cupied and unoccupied acacias have significantly 
different total heights only at the 0.025 (t-test) 
level is a reflection of this high variance. In this 
NUMBER 13 59 
TABLE 10.-General condition of occupied and un- 
occupied A. allenii after 6.5 months of sucker shoot 
growth from cut stumps (see text for details), Costa 
Rica, site 5 
Measurements 
Stump diameter (mm) 
average ........... 
s.d. ............... 
median ........... 
range ............. 
Maximum height of 
shoots (a) 
average ........... 
s.d. ............. 
median ........... 
range ............ 
Length of longest 
shoot (cm) 
average ........ 
s.d. ............... 
median .......... 
range ............ 
Number of suckers 
per stump 
average ........... 
s.d. ............... 
median ........... 
range ............. 
Number of intact 
shoot tips per stump 
average .......... 
s.d. ............... 
median ........... 
range ............. 
Number of damaged 
shoot tips per stump 
average .......... 
s.d. ............... 
median ........... 
range ............. 
Number of swollen 
thorns per shoot 
average ........... 
s.d. ............... 
median ........... 
range ............. 
Number of leaves 
per stump 
average ........... 
s.d. ............... 
mediah .......... 
range ............. 
Total live weight 
of leaves per stump 
(gm) ............... 
Occupied Unoccupied 
(n=7) (n i= 14) 
40 
17 
44 
18-67 
157 
73 
172 
41-245 
112 
105 
117 
18-202 
6.6 
1.8 
6 
4-9 
5.0 
2.0 
5 
3-7 
1.9 
1.6 
2 
0-5 
22 
22 
15 
0-57 
48 
20 
44 
16-74 
76 
56 
28 
52 
11-1 15 
81 
50 
85 
0-182 
34 
29 
37 
0-83 
10.6 
6.5 
13 
0-1 8 
1.8 
1.6 
1 
0-6 
12.0 
9.1 
11 
0-29 
20 
23 
5 
0-69 
32 
26 
27 
0-73 
10 
respect, the results are not outstandingly different 
from those expected in a drier habitat, except that 
in the dry land experiment this high variance 
would have disappeared within about 3 months of 
sucker regeneration. 
Much less so than with dry-land acacias, A. 
allenii suckers are not ?racing? to maintain a spot 
in the general canopy. They are already far below 
it and a difference of a few centimeters in height, 
between occupied and unoccupied, is not so im- 
portant as it is with an acacia whose survival to 
reproductive maturity depends on remaining inso- 
lated. With A. allenii, success in height increment 
is measured in the size of eventual seed crops, 
rather than in living or dying during a short 
period. 
Of course it is the length of the ?leading? sucker 
shoot that usually determines the height of the 
regenerating acacia, though sometimes the longest 
sucker originated low on the stump and this is in 
fact shorter than the tallest sucker. The significant 
difference at the 0.005 level (t-test) between the 
length of occupied and unoccupied suckers is the 
result of feeding damage by insects. If no shoot tips 
are damaged on the growing sucker, one or two 
quickly become the primary suckers and vertical 
elongation of the others nearly ceases. There is an 
occasional loss of a shoot tip on an occupied acacia 
(usually the moth larva, Coxina hadenoides, which 
appears to be host-specific on swollen-thorn acacias, 
cf. Janzen, 1967b). If the main sucker shoot is 
generally undamaged, however, a new vertically 
elongating branch is produced from a leaf axil high 
upon the main sucker shoot when the shoot tip is 
eaten and the ?single trunk? life form is not lost. 
If there is repeated loss of sucker shoot tips to in- 
sects, however, as is the case of unoccupied shoots, 
many short suckers are produced, none of which 
grow very tall. 
In striking contrast to sucker height, length of 
shoot, and condition of the shoot tips, the number 
of swollen thorns and leaves are not significantly 
different between the occupied and unoccupied 
acacias. However, the total live weight of leaves on 
the occupied acacias was 8 times as great as on the 
unoccupied acacias. 
All of the above data suggest that the following 
system is operating. On an unoccupied acacia, the 
insects that will feed on it encounter it at wide 
intervals. In between times, the plant has the 
60 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
chance to grow new sucker shoots to replace those 
eaten off. The long time between attacks by 
herbivores almost certainly reflects a low density of 
these insects. This in turn is probably brought 
about by the low productivity of the understory 
community and the absence of other mimosaceous 
legumes. This is in strong contrast to dry-land sites, 
where the insects arrive so frequently that most new 
shoots are eaten off before they have grown more 
than a centimeter. Phytophagous insects, however, 
find the A. allenii shoots often enough so that no 
one shoot ever becomes large enough to assume 
physiological dominance over the others. The result 
is a stump crowned with a scrawny clump of as 
many as 30 short shoots. Not only do suitable 
herbivores more rarely encounter the acacia, but 
when they do, they appear to feed primarily on 
the newest leaves and shoot tips. The old leaves are 
gradually nibbled at, but are rarely eaten in 
entirety. This results in the very great difference in 
leaf weight between the occupied and unoccupied 
acacias. A swollen thorn is edible only when very 
young, and thus if herbivores find the plant only 
occasionally, swollen thorns may accumulate almost 
as fast as damaged leaves. 
Judging from natural experiments of the types 
described above, unoccupied A. allenii eventually 
die. During the six and a half months of this 
experiment, however, only two (14 percent) of the 
unoccupied stumps died. In a dry-land experiment, 
the percentage would have been between 30 and 
60 percent during one rainy season. Lowered 
herbivore pressure is partly responsible, but the 
obvious shade tolerance of A. allenii is another 
factor. Suckers of dry-land, swollen-thorn acacias in 
shade, such as the A. allenii stumps received, would 
have ceased growth almost entirely within about 3 
months. In the dry areas, a major effect of the 
herbivores was to make the acacia lose the race for a 
spot in the sun. Such an interaction does not occur 
here. Incidentally, P. ferruginea has not lost its 
propensity to kill vegetation contacting the canopy 
FIGURE 44.4eneral postulated distribution and collection localities: 
A. chiapensis (stippled area): A. gentlei (cross-hatched area). 
NUMBER 13 
61 
of A. ullenii. Several of the occupied sucker shoots 
in the experiment grew through vine canopies and 
the ants literally chewed a hole through for the 
sucker shoot. 
5. Acacia chiapensis Safford 
Acacia chiapensis Safford, 1915:356. 
Acacia chiapensis is known only from Mexico 
(Figure 44) and the type specimen was collected at 
the same site as the type of A. collinsii (site 1). 
A. chiupensis is distinguishable from all other 
swollen-thorn acacias by the combination of globu- 
lar inflorescences, multiple and circular petiolar 
nectaries, semi-indehiscent seed pods, and straight 
or slightly curved thorns that are circular in cross 
section. It is easily distinguished from A. collinsii, 
since the latter species has elongate inflorescences 
and few petiolar nectaries. The type specimen of A. 
chiupensis is representative of its species throughout 
the range except that the single type A thorn is 
light brown, while A. chiupensis thorns are usually 
dark brown to black. The immature seed pods on 
the type specimen are full-sized and display the 
distinct ridge bordering the margins of the valves, 
which is characteristic of A. chiupensis, and absent 
from other swollen-thorn acacias. 
KEY COLLECTION LOCALITIES 
(ALL IN MEXICO) 
1. San Fernando, Chiapas. VII-12-1907, G. N. Collins, no. 
164. 
2. Shores of lake behind Presa Miguel Aleman, Temascal, 
Oaxaca (70 m) . XI-3-1963, DHJ, nc (occasional). 
3. Temascal, Oaxaca to La Granja, Veracruz (16 m), All 
year, DHJ, nos. 118, 130, 140, 154, 158, 164, 165, 170, 185, 
190, 193, 200, 204, 209, 214, 222, 230, 235, 242, 247, 266, 
277, 281 (common in permanently wet sites such as below 
the dam at Temascal, Rio Chichicazapa, and Arroyo En- 
medio, very rare otherwise). 
4. 3.4 to 8.5 mi E Papaloapan on Cd. Aleman-Loma Bonita 
Hgy., Oaxaca (25 m). XI41963, DHJ, nc (3 plants). 
5. 19.6 mi SW Tuxtepec on Tuxtepec-Oaxaca Hgy., Oaxaca. 
XI41963, DHJ, nc (1 plant). 
6. 26.4 mi SW Tuxtepec on Tuxtepec-Oaxaca Hgy., Oaxaca. 
XI-4-1963, DHJ, nc (common). 
7. 0.9 mi S Chontalpa on Cardenas-Presa Malpaso Rd., 
Tabasco (about 80 m). VI-17-1966, DHJ, nc (3 plants). 
8. 12.6 mi W Cardenas on Hgy. 180, Tabasco (30 m) . VI- 
15-1966, DHJ, nc (1 plant), fl. 
10. 
11. 
12. 
on Hgy. 185, Oaxaca (300-160 m). VIII-18-1967, DH], 
nos. 1500, 1510 (8 plants). 
Rio Cascabel, Oaxaca. 111-15-1934, C. D. Mell, no. 2239 
(US 1642830) fl. 
Upper Rio Coatzacoalcos, Oaxaca. 111-8-1930, C. D. Mell, 
no. 600 (US 1492290), fl. 
Ubero, Oaxaca (30-90 m). IV-1937, L. Williams, no. 
9124 (US 1741272, F 897560). 
NATURAL HISTORY 
Acacia chiupensis is the only dry-land lowland, 
swollen-thorn acacia that could be called rare, un- 
FIGURE 45.- Acacia chiafiensis (Mexico, site 3): A, emergent 
crown of an occupied 4-m-tall adult; B, two adults, 2-m tall, 
in an open field; c, looking up through crown of 5-m-tall 
~- I 
9. 15.1 mi S and 5.2 to 11.0 mi N turn-off to Matias Romero adult. 
62 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 46.--Acacia chiupensis (Mexico, site 3): A, stump (4 inches in diameter) with multiple 
sucker shoots; 8, branch pattern of 1.5-m-tall tree. 
less it is found that the rare, moist forest species 
A. mayana also occurs regularly in dry sites. It is 
often represented by only one or two plants where 
found. At both places where it was common (sites 
3 and 5), it was growing along or near watercourses 
in vegetation heavily disturbed at 2 to 4 year inter- 
vals by earth-moving machinery or logging; the sites 
were subjected to a severe dry season, but the 
acacias were growing in sites of locally high soil 
moisture. Based on the general condition of plants 
growing in a variety of sites, it appears that the 
pre-Colombian habitat of A. chiupensis was the 
evergreen or semideciduous forest along the edges 
of foothill streams running through deciduous 
forest. Road-building machinery appears to have 
been primarily responsible for movement out of 
this habitat into human disturbance sites. A. 
chiapensis has not invaded brushy pastures or up- 
land regeneration following clearing for cornfields 
or logging, in striking contrast to A. collinsii and 
A. cornigera, which probably occupied the same 
pre-Colombian habitat as A. chiupensis. 
Some of the ecology of A. chiupensis at site 3 was 
discussed briefly as a contrast with A. cornigera 
(Janzen, 196713) and A. globulifera is incorrectly 
listed there as a synonym of A. chiupensis. A more 
detailed discussion of A. chiupensis is offered here 
after having seen the plant growing at a variety of 
sites. 
Acacia chiapensis has a life form much like that 
of dry-land nonant-acacias such as A. farnesiana 
and A. macracantha (Figures 45~, B and 46~) but if 
given ample water will grow into a 8-m-tall tree. 
When repeatedly cut, a stump of A. chiupensis 
tends to form a large cluster of shoots from one 
root crown (Figure 46~), rather than also sprouting 
from lateral roots as the custom with A. cornigera. 
A. chiupensis is generally found in the general 
canopy or slightly emergent to it (e.g., Figure 45~); 
seedlings in the shade die even faster than those of 
A. cornigera and thus are rarely noticed. 
Acacia chiupensis resembles nonant-acacias in an- 
other respect. The seeds are surrounded by a dry 
pulp that is at best only slightly sweet and is prob- 
ably only rarely eaten by birds. Since the dehiscent 
pods (Figure 47c) do not open far enough for the 
seeds to fall out, the pod can either be wind dis- 
persed (perhaps up to 2 or 3 m), water dispersed, or 
rodent dispersed. None of these methods even ap- 
proaches being as effective as bird dispersal for 
NUMBER 13 
63 
FIGURE 47.-Acacia chiupensis 
(Mexico, site 3): A, flowering 
branch from a heavily shaded 
tree; B, flowering branch from 
a fully insolated tree; c, mature 
pods on a branch like that in 
figure A. 
rapidly invading newly created patches of young 
regeneration. All other swollen-thorn acacias, with 
the possible exceptions of A. cookii and A. ruddiue, 
are regularly bird dispersed. 
The flowers are pollinated by the wide variety of 
small bees that are available in deciduous forest 
habitats and contemporary second growth during 
the dry season. The inflorescences are borne in 
very dense clumps in the leaf axils of vegetative 
branches (as on A. cookii in the southern part of 
its range) produced in the early part of the previous 
rainy season. At any one site, the number of in- 
florescences per axil can vary strongly (Figures 
47~, B) and the largest numbers are found on 
branches that are fully insolated and from bushes 
that are fully insolated. Inflorescences generally 
set 1 to 5 pods, but may on occasion have as many 
as 10. 
The seedling ecology of A. chiupensis has not 
been examined but the morphology of young 
bushes indicate that it is more like that of nonant- 
acacias than like that of swollen-thorn acacias. It 
appears that the first swollen thorns often do not 
appear until the fiftieth or even later leaf, and 
even in fully insolated bushes, many leaves on 
lengthening lateral branches are not subtended by 
swollen thorns. This behavior persists even in very 
old A. chiupensis, resulting in branch segments 
with many nodes that lack swollen thorns. The 
canopy section in Figure 45c has about the maxi- 
mum number of thorns in it to be found in an 
A. chiapensis canopy. 
While the large leaves of mature A. chiupensis 
have large numbers of petiolar nectaries (Figure 
48~), sometimes making the petiole resemble that 
of A. melunocerus, the leaves on young plants have 
smaller numbers (e.g., Figure 48~), and I suspect 
that very young seedlings may only have one or zero 
petiolar nectaries per leaf. While nectar production 
of A. chiupensis is apparently very high, as with 
the other swollen-thorn acacias, Beltian body pro- 
duction is clearly reduced as compared with other 
64 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 46.-Petiolar nectaries of A. 
chiupensis leaves (Mexico, site 9): 
A, from fully insolated old crown; 
B, from a young sapling. 
FIGURE 49.-Acacia chiupensis (Mexico, site 3): A, type A thorns from lateral branch of adult; 
B, thorns intermediate between type A and type B on lateral branch from crown of tree in 
Figure 45c; c, type B thorns from crown of same tree. 
NUMBER 13 
130 - 
I 
a 
- 110 - 
I * 
W z 
w cl 
90 - 
0 
I 
65 
dry-land, swollen-thorn acacias such as A. cornigera, 
A. hindsii, and A. collinsii. A. chiupensis leaves on 
fully mature and well-insolated plants may have 
Beltian bodies on only half the pinnules of a large 
leaf subtended by a swollen thorn. 
There is strong variation in the degree of 
herbivore damage to occupied A. chiupensis-all 
occupied plants censused have been occupied by 
P. ferruginea or by a smaller undescribed species of 
obligate acacia-ant. Even plants with the largest ant 
colonies to be found on A. chiapensis usually have 
most shoot tips eaten, as with an unoccupied A. 
cornigera. The result is that plants of A. chiupensis 
are generally very shrubby and stunted in appear- 
ance. The cause appears to be that with the reduced 
number of Beltian bodies and reduced number of 
thorns, almost none of which have a large volume 
or could be regarded as type B thorns (Figures 49, 
50), the ant colony never grows very large. Since 
the colony may be old, its workers may be very 
aggressive, but there is too much acacia canopy to 
patrol effectively. A 2-m-tall A. chiapensis such as 
the one on the left in Figure 3a in Janzen (1967b) 
may have a maximum colony size of about 2,000 to 
4,000 worker P. ferruginea, while an adjacent A. 
cornigera of equal size and with an equal number 
of leaves will have between 5,000 and 10,000 worker 
ants. Younger plants of A. chiapensis character- 
istically have a colony size reduced to the same 
degree, with the result that they are slow to gain 
effective patrolling by ants, and thus have to be on 
thoroughly cleared ground in order to mature. 
When roads or dams are built, A. chiapensis be- 
comes established on the bare soil where there is 
no competition from fast-growing suckers from old 
stumps of woody plants (as would have been the 
case in areas cleared by fire or machete). At site 6, 
there was a nearly pure stand of 3- to 6-m-tall A. 
chiapensis covering a gravel island of about 2 acres 
in the middle of a river; a newly bulldozed site 
obviously simulates such a situation for primary 
succession. 
If an A. chiupensis is growing within 1 to 2 m of 
an A. cornigera, the latter species always contains 
the queen and the A. chiupensis serves as an 
auxiliary unit to the main colony. This means that 
during a dry season sufficiently severe to cause the 
A. chiupensis to drop its leaves, the ant colony is 
very likely to abandon it and retreat into the acacia 
with the queen. All of these observations suggest 
150 1 
t 
0 
b 
0 
a 
a 
i 
c 
I I I I I 
0.4 0.6 0.8 1.0 1.2 1.4 
TOTAL THORN VOLUME (cc) 
FIGURE 50.-Distribution of 89 cc of thorn volume among 100 
type A thorns from an adult occupied A. chiapensis (Mexico, 
site 3). 
that A. chiupensis is a marginal host for obligate 
acacia-ants, but that it is still dependent on the 
ants. However, it may not be as dependent as other 
dry-land, swollen-thorn acacias since its leaves con- 
tain cyanogenic glycosides that are also present in 
nonant-acacias (A. farnesiana) but absent from A. 
cornigera (Rehr et al., 1973). 
Not all A. chiupensis, however, demonstrate strong 
susceptibility to insect attack. At site 3, and only 
at this site, it appears that A. chiupensis may be 
hybridizing with nonant-acacias such as A. macra- 
cantha. Numerous plants occur there that are in- 
termediate in thorn, leaf, and branching morphol- 
ogy between these two species. A gradient may be 
recognized between the ?typical? A. chiupensis life 
form-dark green large leaves with large pinnules, 
swollen thorns subtending more than half of the 
large leaves on lengthening branches, new swollen 
thorns soft for several days allowing easy entry 
and with soft inner parenchyma allowing easy 
cleaning, fast-growing shoot tips, upright-cylindrical 
life form with a few major branches from a single 
trunk, and severe herbivore damage to uppermost 
shoot tips-and a life form like the shrubs in Fig- 
ures 45~ and 45~ which tend on all the above char- 
acters to be much more like A. macracantha. A. 
mucracantha is common in the area and shows the 
opposite traits of those listed above for A. chiapen- 
sis. It is particularly obvious that the forms inter- 
mediate between A. chiapensis and A. macracantha 
66 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
are more damaged by herbivores than ordinary 
nonant-acacias, but they are very weakly occupied, 
if at all. The lack of damage appears to be due to 
chemical and morphological defenses, since these 
only very mildly damaged bushes are often imme- 
diately adjacent to heavily damaged more typical 
A. chiapensis. 
The large stump of one of these intermediate 
acacias in Figure 47~ is particularly instructive in 
understanding the poor development of an ant 
colony in the apparent hybrids. It has produced at 
least 50 small sucker shoots since it was cut, but 
only five swollen thorns; sucker shoots of ?typical? 
A. chiapensis or A. cornigera from an equal-sized 
stump would have had a swollen thorn subtending 
almost every new leaf (e.g., Figure 53 in Janzen, 
1967b). Not only are there few swollen thorns, but 
they harden rapidly, and the queen of P. ferru- 
ginea has a very difficult time hollowing out enough 
of the inside of the thorn for its own body, to say 
nothing of enoligh space for brood and workers. 
Many of the new leaves on a stump like this have 
no Beltian bodies and the nectaries are very poorly 
developed, again in strong contrast to A. chiapensis 
or A. cornigera new sucker shoots. Not only are the 
suckers slow growing, but dominance is poorly de- 
veloped with the result that many shoots develop 
simultaneously. The dense shrub resulting eventu- 
ally matures in completely insolated sites-as might 
be expected of dry-land nonant-acacias-but ap- 
pears incapable of competing with dense vigorous 
vegetation on land cleared by fire or machete. 
In short, there appears to be a gradient in ability 
to deal with the surrounding vegetation from A. 
macracantha to the apparent hybrid to A. chiapen- 
sis to A. cornigera. Along this gradient, the ant 
becomes progressively more important to the plant 
in protecting its faster growing shoot tips that have 
progressively fewer chemical defenses. Both A. 
macracantha and A. cornigera represent adaptive 
peaks that are successful in vegetation types of 
great areal extent. The hybrid and A. chiapensis 
Contour lines at 1000, 5000, and 10,000 feet 
FIGURE 5l.-Collection localities of A. globulifera; its detailed collection records are too patchy 
to allow an estimation of the general distribution at this time. 
NUMBER I3 
67 
represent adaptive peaks that do well under much 
rarer combinations of vegetative competition and 
rainfall. It is not meaningful to say that one is any 
more or less successful than the other, except in 
terms of the number of acres of vegetation occupied. 
6. Acacia globulifera Safford 
Acacia globulifera Safford, 1915:360. 
Acacia donnelliana Safford, 1915:361. 
Myrmecodendrum globuliferum (Safford) Britton and Rose, 
Myrmecodendron donnellianum (Safford) Britton and Rose, 
1928:98. 
1928:93. 
Acacia globulifera is easily distinguished from 
other swollen-thorn acacias by the possession of 
spherical inflorescences, completely dehiscent pods 
with valves that curl back on themselves (as with 
A. hindsii), and 2 to 6 circular or oval petiolar 
nectaries (Figures 6, 7c). Its range (Figure 51) 
overlaps slightly with that of A. collinsii, A. 
cornigera, A. gentlei, A. cookii, and A. chiapensis, 
but it is very rarely encountered growing side by 
side with these plants. Herbarium specimens of 
A. globulifera are incorrectly listed as a synonym of 
A. chiupensis in Janzen (1967b), but a comparison 
of the petiolar nectaries and mature pods-barely 
indehiscent in A. chiapensis, fully indehiscent in 
A. globulifera-will easily separate them. The 
petiolar nectaries of A. globulifera may on occasion 
be confused with those of A. collinsii, but A. 
collinsii has elongate inflorescences. In field studies, 
failure to examine plants closely may result in mis- 
identifying A. globulifera as A. collinsii or as A. 
gentlei because it has a very similar life form to 
either of these when growing in their habitats. 
Safford's type specimen of A. globulifera (site 1) 
appears to have been taken from a fully insolated 
shrub growing in a locally exceptionally dry site, 
since its thorns are unusually light in color (beige) 
and short (the largest type A thorns are only 4 cm 
long), and its leaves only attain 8 cm in length. In 
the coastal areas of the Yucatan Peninsula such 
plants are usually found on old beach dunes; the 
more inland plants have thorns that are nearly 
black and twice as long, and have leaves up to 20 
cm long. The inflorescences on the type specimen 
are representative of A. globulifera throughout its 
range except that they have peduncles only 10 mm 
or less in length, and in wetter areas the peduncles 
of A. globulifera may attain a length of 20 mm. 
Safford's type specimen for A. donnelliana was 
collected from that portion of the A. globulifera 
population growing between San Pedro Sula and 
Santa Rosa, Honduras (site 24), and is representa- 
tive of the plants in that area growing in very wet 
habitats (swamps, creek edges); its thorns are nearly 
black, its leaves are up to 20 cm long, its inflores- 
cence peduncles are up to 18 mm long, and its 
sexual branchlets are very long (e.g., Figure 52~). 
KEY COLLECTION LOCALITIES 
Mexico: 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
Type of A. globulifera: Silam (Dzilam), north coast of 
Yucatan. April 1895, G. F. Gaumer, no. 1909. 
48 mi NE Puerto Real (Isla Aguada) on Mexico 180, 
Campeche. VI-22-1966, DHJ, nos. 611, 615, 617, 623 (OC- 
casional) , mp. 
11.3 mi N intersection of Mexico 185 and 190, on Hgy. 
185, Matias Romero, Oaxaca (290 m). VIII-18-1967, 
DHJ, no. 1502 (single plant) . 
11.8 mi N intersection of Mexico 185 and 190, on Hgy. 
185, Matias Romero, Oaxaca (290 m) . VIII-18-1967, 
DHJ, nc (occasional plants). 
25.8 mi N intersection of Mexico 185 and 190, on Hgy. 
185, Matias Romero, Oaxaca (250 m). VIII-18-1967, 
DHJ, nc (very abundant). 
26.3 mi N intersection of Mexico 185 and 190, on Hgy. 
185, Matias Romero, Oaxaca (240 m) . VIII-18-1967, 
DHJ, nc (3 plants). 
31.5 mi N intersection of Mexico 185 and 190, on Hgy. 
185, Matias Romero, Oaxaca (260 m). VIII-18-1967, DH], 
nc (1 plant). 
Chivelia, Oaxaca, Mexico. 111-6-1928, C. D. Me11 (US 
1406953), fl. 
Silam, Yucatan, Mexico (sea level). IV-1895, G. F. 
Gaumer, no. 655 (US 1267146), fl. 
Silam, Yucatan, Mexico (sea level). IV-1909, G. F. Gau- 
mer, (US 1267508) fl. 
Guatemala: 
11. 
12. 
13. 
14. 
15. 
16. 
Finca San Joaquin, slopes above Rio Negro, near San 
Cristobal de Verapaz, Alta Verapaz (1080 m). VII-14- 
1967, DHJ, nc (very abundant), mp. 
20.5 mi NE Guatemala City on CA9 (980 m). VIII-8- 
1967, DHJ, no. 1565 (occasional), fl., gp. 
26.7 mi to 26.9 mi NE Guatemala City on CA9 (700 m) . 
VIII-9-1967, DHJ, nc (abundant), gp. 
29.6 mi NE Guatemala City on CA9 (850 m). VIII-9- 
1967, DHJ, nc (rare). 
23.4 mi SW of turn off to Coban on Hgy. 9, Progreso, 
Dept. Progreso. VIII-5-1966, DHJ, no. 444. 
16.2 mi NE Guatemala City on CA9 (870). V-14-1965, 
DHJ, no. 1741 (1 plant). 
68 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
17. Sanarate, Depto. Progreso (900 m) . XI-9-1916, Popenoe, 
no. 720 (US 2089415, US 1319702), mp. 
18. La Libertad, Chicbul, Depto. Peten. IV-29-1933, C. L. 
Lundell, no. 3088 (US 1635206, F 738059), fl. 
19. Rio Teculutan, above Teculutan, Depto. Zacapa (250- 
275 m). 1-7-1942, J. A. Steyermark (F 1191231) fl., mp. 
20. Fallabon-Yaxha Rd., Depto. Peten. 111-22-1933, C. L. 
Lundell, no. 2214 (F 740476), mp. 
21. Lake Yaxha, Depto. Peten. 111-23-1933, C. L. Lundell, 
no. 2208 (F 738041). 
British Honduras: 
22. Vaca, El Cay0 Prov. 111-2-1938, P. H. Gentle, no. 2275 
(F 945039), fl. 
23. El Cayo, El Cayo. 111-5-13-1931, H. H. Bartlett, no. 11992 
(US 1493114), fl. 
Honduras: 
24. 
25. 
26. 
27. 
28. 
29. 
30. 
Type of A. donnelliana: San Pedro Sula, Cortes (200 m) . 
March 1888, C. Thieme, no. 5216. 
19.4 to 19.9 mi SW San Pedro Sula on Honduras 18 (road 
to Santa Rosa) Depto. Cortes (160 m). VIII-2-1967, 
DH], no. 1682, 1688 (8 plants), fl., gp. 
24.6 to 26.0 mi SW San Pedro Sula on Honduras 18 
(banks of the Rio Chamelecon; road to Santa Rosa), 
Depto. Santa Barbara (380 to 420 m). VIII-3-1967, DHJ, 
no. 1604, 1608 (occasional). 
28.4 mi SW Sula on Honduras 18 (road from San Pedro 
Sula to Santa Rosa), Depto. Copan (780 m) , VIII-3-1967, 
DH], no. 1630 (1 plant). 
10.4 to 16.3 mi (Cucuyagua) SW Santa Rosa on Hon- 
duras 18 (road to Nueva Ocotepeque), Depto. Copan 
(980-1180 m) . VIII-3-1967, DHJ, no. 1634 (occasional), 
gP. 
Yoro, Depto. Yoro (850 m). 1-26-1934, J. B. Edwards, no. 
P-746 (US 1828182, F 759556), fl. 
Santa Rosa de Copan, between El Portillo and San Juan 
Opoa, Depto. Copan (1000 m). IX-23-1963, A Molina, 
no. 12873 (F 1622365). 
NATURAL HISTORY 
As with the other widespread swollen-thorn 
acacias, it is difficult to characterize the contempo- 
rary or pre-Colombian habitats of A. globulifera 
in one statement. It is commonly encountered at 
elevations that represent the upper elevational 
range of swollen-thorn acacias (e.g., sites 11, 28, 30); 
however, all of these higher elevation sites are 
characterized by being dry and dusty even during 
the rainy season. It is only at these sites that A. 
globulifera is sympatric with no other swollen-thorn 
acacia, though at some lowland sites A. globulifera 
may be the only riparian species, with A. cornigera 
or A. collinsii on the hillsides (e.g., site 26). At lower 
elevations in the wetter part of the range (e.g., sites 
25, 26), it is most commonly encountered in 
riparian and swamp successional stages and oc- 
casionally on the edges of pastures and in roadside 
right-of-way. At sea level in the drier parts of the 
range (e.g., sites 2, 3-7) it is found on old sand 
dunes, rocky slopes, swamp edges, and badly eroded 
creek edges. No healthy plants have been found in 
habitats where they are heavily shaded, and almost 
all reproductive individuals have crowns that are 
emergent to the general canopy or are growing on 
nearly bare ground. One is left with the distinct 
impression that A. globulifera can withstand a very 
wide range of temperature and moisture regimens 
if it is moderately to heavily insolated. Since the 
largest individuals encountered have only been 3 m 
tall, and most are much shorter, this restricts the 
plant to young stages of succession, or to habitats 
with low or badly broken vegetative cover. At the 
higher elevation sites, A. globulifera is often asso- 
ciated with pines and oaks, and at sites 11 and 13 
became very common-up to 20 adults per acre- 
in old pastures and regenerating fields formed by 
clearing mixed pine-oak forest. 
Acacia globulifera is distinctly shrubby in life 
form, and like the other dry-land acacias may bear 
a large seed crop when only 1 to 2 m tall if well 
insolated. If growing in full sun continuously, A. 
globulifera generally does not grow over 2 m tall 
(Figure 53~) but if slightly shaded, it has a slightly 
more attenuated life form (Figure 53B), and its 
crown does not expand much laterally until it is 
emergent to the general canopy or otherwise inso- 
lated. 
The length of the sexual branchlets of A. globu- 
lifera appears to be more directly related to the 
health of the plant than is the case with other 
swollen-thorn acacias. For example, the very long 
branchlets in Figure 52~ are from a 2.5-m-tall acacia 
in its third year of sexual reproduction and fully 
insolated with a large crown and occupied by a very 
large ant colony. The same is true of the long 
sexual branchlets bearing immature pods in Figures 
54B and 54c. On the other hand, the very short 
sexual branchlets in Figure 53B are on the healthiest 
appearing branch in the senescent crown of a 4-m- 
tall acacia in a swamp. Though fully insolated, 
this 14-year-old acacia had lost its ant colony and 
was heavily damaged by herbivores. The extreme 
differences in number of inflorescences borne by the 
two branches in Figure 52 are documented in 
NUMBER 13 
69 
FIGURE 52.--Acacia globulifera (Honduras): A, elongate sexual branchlets from a very healthy 
adult (site 28): B, axillary inflorescences from the crown of a senescent tree (site 25). 
FIGURE 53.-Acacia globulifera (Hondu- 
ras): A, heavily insolated subadult (site 
26); B, main trunk of subadult growing 
under intense lateral competition (site 
28). 
70 
- 
- 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
60- 
a 
2 50- 
w 40 - 
30- 
U 
a 
I 
W 
u 
z 
U 
v) 
W 
ci 
U 
20- 
U 
0 
$10- 
z 
W 
cl 
FIGURE 34.-A, lateral branch of A. hindsii with large number of immature infructescences (Nica- 
ragua, site 85); B, long sexual branchlets of A. globulifera with heavy crop of immature in- 
fructescences (Guatemala, site 13) : c, the lateral branch bearing the infructescences in Figure B. 
0 
0 
0. 0: 
0 
I I 
5 15 25 
A NUMBER OF INFLORESCENCES PER BRANCHLET 
- 
32 
U 
z 
4 
IT 
L9 
m 
230 
a 
4 
W 
W 
U 
z 
0 
LL! 
a 
U 
4' 210 
5 190 
2 170 
E 
u, 150 
0 
32 
z 
W 
ci 
130 
0 
0 
0 
0 
0 
40 60 80 100 
NUMBER OF INFLORESCENCES PER BRANCHLET 
FIGURE 55.-Number of inflorescences per sexual branchlet in A. globulifera (Honduras): 
A, crown of a senescent tree that had an inflorescence crop of about 250 (site 25): B, crown of 
a healthy adult that had an inflorescence crop of about 12,000 (site 28). 
NUMBER 13 
71 
Figure 55; the size of the inflorescence crop is clearly 
proportional to the condition of the acacia. 
Acacia gEobulifera with many long sexual branch- 
lets usually bear large seed crops (Figures 54B, C, 
56). I suspect, however, this means that a large 
percent of the seeds are killed by bruchid beetles. 
As with the other dry-land acacias, the young green 
pods (Figure 112~) remain on the plant throughout 
the rainy season and do not mature until about 
the time the new flowers are produced during the 
following dry season. Mature undehisced pods of 
A. globulifera have not been collected and there- 
fore there is no information available on seeds. 
Since the pods curl open and yet stay on the tree 
for several months after all seeds are gone, I suspect 
that they are bird-dispersed as with A. hindsii. 
Seedling biology of A. globulifera has not been 
investigated except to note that they develop their 
first swollen thorns, Beltian bodies, and petiolar 
LO 
v) 
W 
u 
2 
w 
u 
v) 
W 
3 
s5 H 
U 
0 
CT 
W 
m 
9 
z: ~ 13 
1 
4 5 7 9 11 13 
NUMBER OF PODS PER INFLORESCENCE 
FIGURE 56.-Distribution of 207 immature seed pods among 
the infructescences from a healthy adult A globulifera (Hon- 
duras, site 28). 
FIGURE 57.-Acacia globulifera: A, main trunk covered with type B thorns bleached white by the 
sun (Guatemala, site 12); B, central axis of subadult with downward curved type A thorns 
(Honduras, site 28). 
72 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
120 
110 
- 
E 
E 100 
I 
X 
F 
z 
W 
4 
w 90 
2 80 
a 
0 
z 
k 
4 
70 
a 
t; 
b 
FIGURE 58.-Acacia globulifera: A, type B thorns (Honduras, site 26); B, left, type B thorns and 
on right, type A thorns (Guatemala, site 15). 
TOTAL THORN VOLUME (cc) 
FIGURE 59.-Distribution of 49 cc of thorn volume among 69 
type A thorns from lateral branches in the crown of a 
healthy adult A. globulifera (Honduras, site 28). 
nectaries very early in development, like other dry- 
land, swollen-thorn acacias. Also as with other 
dry-land, swollen-thorn acacias, seedlings of A. 
globulifera appear to die within 1 to 2 years if they 
are in heavy shade. 
Acacia globulifera produces large numbers of 
swollen thorns early in its development, and con- 
tinues to do so on all branches. The thorns are 
exceptional among swollen-thorn acacias in showing 
extreme differences in color from one habitat to 
another. Adding to the complexity is the fact that 
A. globulifera thorns bleach exceptionally easily in 
direct sunlight (e.g., Figure 57~). They are com- 
monly bicolored as well, with lighter tips than 
bases. Type B thorns are somewhat larger than type 
A thorns (Figures 57~, 58~, 59, 60), but not spec- 
tacularly so. They also do not differ in morphology 
from type A thorns in any outstanding manner; 
even the largest type B thorns are usually round 
in cross section. Type A thorns often curve down- 
ward (Figure 57~) or project outward almost at 
right angles from the branch bearing them (Figure 
58). 
Acacia globulifera shows greater variation in 
petiolar nectary morphology than any other species 
of swollen-thorn acacia. The major factor appears 
to be in the moisture of the ground where the 
individual plant is growing; the wetter the site, 
the longer the leaves. A great deal of the variation 
in nectary morphology can be viewed as simply the 
effect of lengthening or shortening the leaf at the 
NUMBER 13 
140 - 
120 - 
100 - 
73 
6 
6 
z 
U 
0 
X 
I+ 
160 -I 0 
ooo. 
0. 
n 
0 
0 
0 
1.0 2.0 3.0 
TOTAL THORN VOLUME (cc) 
FIGURE 60.-Distribution of 48 cc of thorn volume among 31 
type B thorns from the trunk of A. globulifera in Figure 
57A. 
time of formation. The effect of this variation in 
nectary size and number on the amount of nectar 
produced per leaf is unknown. There is no evidence 
that the ants react differently to any of these 
different forms of nectary. 
The impact of herbivore on unoccupied plants of 
A. globulifera has not been examined experi- 
mentally, but plants that lose their ant colony 
through natural causes are badly damaged by in- 
sects in the lowlands. At the upper end of the 
elevational distribution of ?4. globulifera, appar- 
ently healthy acacias in reproductive condition are 
encountered that obviously have been unoccupied 
for at least a year. They are often tenanted by other 
arboreal ants in the genus Camponotus. At these 
high elevations, where the weather is often dry, 
windy, and cool, herbivorous insect populations 
appear to be much reduced as compared with the 
lowlands and this is probably the reason why un- 
occupied A. globulifera is relatively free of insect 
damage at these sites. 
7. Acacia sphaerocephala Schlectendal 
and Chamisso 
Acacia sphaerocephala Schlectendal and Chamisso, 1830:594. 
Acacia ueracruzensis Schenck, 1913:362. 
Acacia dolichocephala Safford, 1915:355. 
Acacia sphaerocephala is easily distinguished 
from all other swollen-thorn acacias by having the 
elongate canoe-shaped petiolar nectaries found also 
on A. cornigera and A. mayana, but having a 
globular or thimble-shaped inflorescence. Sterile 
specimens of A. sphaerocephala are usually dis- 
tinguishable from A. cornigeya because the pinnules 
of A. sphaerocephala usually lack readily visible 
secondary venation (Rudd, 1964). As can be seen 
from its distribution (Figure Sl), A. sphaerocephala 
is probably a dry-land derivative of the widespread 
A. cornigera just as A. mayana is probably a wet- 
forest derivative of A. cornigera; the direction of 
this evolution cannot, however, be stated with 
certainty. 
Schlectendal and Chamisso?s type specimen of A. 
sphaerocephala has elongate internodes and mod- 
erate-sized thorns, indicating that it was collected 
a few meters inland from the beach dunes where 
most of the population of A. sphaerocephala now 
grows (site 1). While its spherical inflorescences are 
immature, they are representative of those found 
throughout the population of A. sphaerocephala 
except in the northern part of its range where 
some plants have more thimble-shaped inflores- 
cences. Rudd (1 964) has placed A. ueracruzensis and 
A. dolichocephala in synonymy with A. sphaero- 
cephala and I agree. The small maroon seed pods 
growing from a globular inflorescence, and the very 
thick thorns on the type of A. ueracruzensis identify 
it clearly as having been collected from the dense 
mat of A. sphaerocephala growing on the dunes 
immediately above the high-tide line south of the 
city of Veracruz (sites 2 and 6). At this site, the 
thorns of A. sphaerocephala may attain a diameter 
of 15 mm (e.g., Figure 65c) while equal-length 
thorns of A. cornigera are only 8 to 10 mm diameter 
at the base. Also, A. sphaerocephala thorns at this 
site (as at most sites) are bicolored with ivory to 
beige tips while those of A. cornigera are uniformly 
brown or gray. Safford?s type of A. dolichocephala, 
like that of A. sphaerocephala, was taken from the 
population just inland from the dunes and thus has 
as long internodes and thin thorns as are to be 
found on A. sphaerocephala. Its immature inflores- 
cences are slightly elongate but those of A. cornigera 
at the same site are 3 to 4 times as long. 
Acacia globulifera has been commonly misidenti- 
fied as A. sphaerocephala where the former species 
grows along the dunes sympatric with A. collinsii 
on the Yucatan Peninsula. Careful search has, how- 
74 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
1000 
R W 8. - I96 7 
1 I I # 
FIGURE 61.4eneral postulated distribution (stippled area) 
and collection localities of A. sphaerocephala. 
ever, located no A. sphaerocephala south of Alva- 
3. Type of A. dolichocephala: Along shore north of Vera- 
cruz, Veracruz (sea level). 1-24-1906, J. M. Greenman, 
rado, Veracruz, Mexico. _- 
KEY COLLECTION LOCALITIES 
(ALL IN MEXICO) 
no. 87. 
XI-10-1963, DHJ, nos. 1921, 1924 (common), mp. 
4. 5.1 to 38.1 mi SE Jalapa, Veracruz, along Mexico 140. 
5. 23.5 to 42.0 mi WNW Huatusco, Vcracruz, road from 
1. 
2. 
Type of A. sphaerocephala: Actopan, Veracruz (sea level). 
March, 1820, Schiede and Deppe, no. 684. 
Type of A. veracruzensis: sand dunes south of Veracruz, 
Veracruz (sea level). X-13-1908, Schenck, no. 916. 
Orizaba to Hgy. 140. XI-10-1963, DHJ, no. 1920 (com- 
mon) , mp. 
6. Within 1-2 km of the ocean beach, from the southern 
city limits of Veracruz, south to Anton Lizard0 (approxi- 
NUMBER 13 
75 
mately 15 miles north of Alvarado) , Veracruz. DHJ, nos. 
1922, 1925, .l926, 1927. 
7. 26.2 mi N Ciudad Valles to Antiguo Morelos on Mexico 
85 (100-300 m). VIII-25-1967, DHJ, nc. 
8. Antiguo Morelos to where the Tropic of Cancer crosses 
Mexico 85 (e.g., 1.8 mi S Llera on Hgy. 85; 22 mi s 
Ciudad Victoria, Tamps.). VI-10-1964, DHJ, no. 505. 
9. Ciudad Mante to Tampico (Mexico 80) (primarily in 
liver beds) and Tampico to the Tamuin area (e.g., 8 mi 
E Tampico on 110). 1-2-1964, DHJ, no. 504, Tamps. and 
SLP. 
10. Antiguo Morelos to Ciudad del Maiz, Tamps. and S.L.P. 
(occasional populations on dry southeast facing slopes) . 
11. 10.3 mi NE Ciudad Valles on Mexico 110, S.L.P. I-II- 
1964, DHJ, no. 1923. 
12. Tanquian, Partido de las Valles, Tamcanhuitz Dist., 
S.L.P. 1908, L.G. Cuevas, (US 692167), fl. 
13. Hda. Buena Vista, Tamaulipas, Mexico. VI-15-1919, E.O. 
Wooton; (US 989707). 
14. Between Tdla and Tampico. XII-1830, no. 728 (US 
40123), fl. 
15. Tampico, Tamaulipas (sea level). 1-1-31-1910, E. Palmer, 
no. 133 (US 764678), fl., mp. 
16. Between San Luis Potosi and Tampico (Huasteca Re- 
gion), 1878, E. Palmer (US 764678), fl., mp. 
17. Pueblo Viejo, 2 km S Tampico, Veracruz (sea level). 
V-23-31-1910, E. Palmer, no. 448a (US 463371), fl. 
18. Between Rio de Santa Maria and Rio de 10s Pescados, 
Veracruz. X-7-1908, Schenck, no. 837 (US 764677). 
19. 1 mi N Cd. Madero, Tamaulipas (sea level). 111-1-1961, 
R.M. King, no. 3990 (US 2365430), fl., mp. 
XII-28-1963. 
~~ATURAL HISTORY 
The distribution of Acacia sphaerocephala in- 
variably borders the dry edge of the A. cornigera 
distributions. At sites 2 and 6, it occupies the dunes 
and only extends inland to the wetter sites occupied 
by A. cornigera as scattered individuals. At sites 4 
and 5, A. sphaerocephala extends the distribution 
of swollen-thorn acacias into that part of the state 
of Veracruz with the most severe dry season. All 
along the northern part of the range of A. sphaero- 
cephala, the northernmost individuals are found in 
semidesert vegetation while the southernmost indi- 
viduals are scattered among the northernmost 
individuals of the A. cornigera population. A. 
sphaerocephala is the only species of swollen-thorn 
acacia that inhabits newly formed dunes, and it 
occupies some of the driest habitats of any swollen- 
thorn acacia population. It is the only Central 
American acacia involved in the early stages of 
beach succession. In the Isthmus of Tehuantepec 
on the Pacific side of Mexico, A. collinsii is also 
found on occasion in extremely dry habitats, but 
it is not clear if the low density population is 
reproducing itself or maintained by seed immigra- 
tion from wetter sites. 
For purposes of discussion, the A. sphaerocephala 
population can be divided into two ecotypes: the 
dune ecotype and the inland ecotype. The dune 
form occurs only on new dunes in the vicinity of 
the city of Veracruz (sites 3 to 6), while the inland 
form occurs throughout the remainder of the range 
and does not approach the ocean as closely as does 
the dune form, When seeds of the dune form are 
grown at well-watered inland sites, the resulting 
shrub closely resembles the inland ecotype except 
for having slightly shorter internodes and slightly 
thicker swollen thorns. On the dune, however, it is 
a very different plant and worthy of special dis- 
cussion. 
On the dunes closest to the ocean, the dune form 
of A. sphaerocephala forms a dense pure stand from 
0.2 to 1.8 m tall that generally crowns the leading 
edge of the dune (Figure 62~; Figure 10 in Sauer, 
1967). The origin of the mat of A. sphaerocephala 
is seed, but most of the plants in the larger mats 
are from sucker shoots from roots. The branches 
are so intertwined that the entire mat appears as 
though it were one crown. The largest mats may be 
30 m wide by 200 m long and contain at least 6,000 
major stems. 
The dune ecotype of A. sphaerocephala differs 
from other swollen-thorn acacias in not being oc- 
cupied by obligate acacia-ants when it grows on 
new dunes. Its success as an unoccupied swollen- 
thorn acacia in this habitat is apparently due to the 
continuous strong onshore winds, which probably 
make life very difficult for phytophagous insects 
and prevent their flying to the acacias in the first 
place. The absence of other adjacent shrubby vege- 
tation in this barren habitat also removes the source 
for most of the insects that defoliate unoccupied 
swollen-thorn acacias. Plants grown at inland sites 
from dune ecotype seeds are heavily defoliated if 
unoccupied, just as are adjacent unoccupied A. 
cornigera (experiments conducted 1963-1964, at 
Cotaxtla Exp. Sta. and Temascal, cf. Janzen, 1967b, 
for site details). Inland ecotypes of A. sphaero- 
cephala interact with ants in the same manner as do 
A. cornigera. 
Failure of Pseudomyrmex ferruginea (the only 
obligate acacia-ant at the site) to occupy the dune 
76 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
ecotype is due to the following reasons: (1) Search- 
ing queens appear to rarely arrive at the dune 
plants, probably because the prevailing winds blow 
from the dunes toward the inland swollen-thorn 
acacias (A. cornigera and A. sphaerocephala), and 
because they would have to cross many feet of very 
hot sand to walk there. Their absence is indicated 
by the almost complete lack of their characteristic 
entrance holes in thorns of seedlings and adult 
plants of the dune ecotype of A. sphaerocephala. 
(2) The dune ecotype produces almost no Beltian 
bodies and the few that are produced appear to be 
harvested by colonies of Crematogaster atrata Mayr 
or C. coruina Mayr that tenant the thorns in large 
colonies. These ants cut entrance holes at the base 
of the thorns, tend mealybugs (Pseudococcidae) 
and aphids (Aphididae) inside the thorns, and 
scavenge on the acacia surface. Their foraging is 
almost entirely nocturnal and very patchy in cov- 
erage. There is no evidence that they protect the 
acacia; inland A. sphaerocephala dies as fast from 
herbivore damage when tenanted by this ant as it 
does when unoccupied. (3) The Crematogaster ants 
thoroughly harvest the petiolar nectar produced by 
the dune ecotype of A. sphaerocephala and would 
probably be very severe competitors for the young 
FIGURE 62.-~, pasture dotted with 
young 1-2-m-tall A. sphaerocephala; 
the columnar structure is typical of 
pasture plants in this area (Mexico, 
site 9); B, beach dune created by dense 
mat of A. sphaerocephala (Mexico, 
site 6). 
queens attempting to start a colony. The total 
absence of Crematogaster, however, from those in- 
land acacias that are producing Beltian bodies and 
can be found by many searching queens of P. 
ferruginea, indicates that obligate acacia-ants can 
easily exclude Crematogaster in more ordinary 
habitats. (4) During the latter part of the dry sea- 
son most of the dune ecotype individuals of A. 
sphaerocephala drop their leaves (e.g., Figure 63c), 
and this behavior makes colony establishment and 
maintenance very difficult. 
As dune succession proceeds, the dense mats of 
A. sphaerocephala are broken up, and the popula- 
tion consists primarily of very flat-topped scattered 
individuals that rarely exceed 1.5 m in height (Fig 
ure 63~). Once the succession has reached a height 
of 3 to 5 m, A. sphaerocephala disappears com- 
pletely, though A. cornigera may still be common. 
Birds apparently disperse seeds continually into 
these inland sites, however, because if they are 
cleared, both A. cornigera and the inland form of 
A. sphaerocephala appear in the early successional 
stages. In the inland habitat, these two acacias are 
very difficult to tell apart until they bear flowers. 
Seedling development in situ of the dune ecotype 
has not been examined closely, but leaflet develop- 
NUMBER 15 
77 
FIGURE 63.-Acacia sphaerocephala (Mexico): 
A, adult in cattle pasture (site 4); B, adult 1 
m tall, leafless, in low vegetation several hun- 
dred meters behind the high-tide line (site 
6); c, leafless subadult on beach dune (site 
6). 
inent of a representative seedling from an inland 
plant is depicted in Figure 64. The gradual increase 
in leaf size and leaflet number with age is repre- 
sentative of dry-land, swollen-thorn acacias. It is 
characteristic of all the swollen-thorn acacias to 
have this short period of growth before developing 
the characteristic traits of a swollen-thorn acacia. 
In short, the seed probably lacks sufficient reserves 
to produce both a small amount of shoot and root, 
and swollen thorns, nectaries, and Beltian bodies. 
That these traits increase gradually in quantity 
indicates that the seedling continues to divide its 
new photosynthate between conventional vegetative 
growth and production of resources for the develop- 
ing ant colony. 
No matter in what habitat it is growing, a young 
78 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
525+ 
.O4?& 
28 5\ 
26 % 
16 i, 
/& 54 
# 30 
A34 
FIGURE 64.--Schematic representation of 46-day-old A. 
sphaerocephala seedling: the number of pinnules for each 
pinna are recorded on the figure and .the first petiolar nec- 
tary appeared on the 7th leaf with the first swollen thorn 
and Beltian bodies at the 11th node. A fully mature leaf 
on the 50th or later node may have as many as 20 pinnae 
with 50-70 pinnules on each (Mexico, site 6). 
FIGURE 65.-Acacia sphaerocephala (Mexico): A, sexual branchlet with type B thorns (site 9); 
B, type B thorns (two very large thorns) and type A thorns from the plants found on beach 
dunes (site 6): c, fruits and type A thorns (site 6). 
NUMBER 13 
79 
100 
90 
80 
h 
I 
= 
L 
T 
c 
K 
w 
- 
g 70 
n 
f 60 
t9 
L 
w 
-1 
cl 
4 
s 
c. 50 
. 
. 
5. . 
.... 
.. 
... . b 
. 
i 
*. .. 
0. 
. 
0 
. 
0 
. 
. 
. 
. 
. 
0. 
. 
. 
. 
e. 
I I I I I I I I 1 
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 
TOTAL VOLUME PER THORN (CC) 
FIGURE 66.-Distribution of 30 cc of thorn volume among the 59 thorns on a 50-cm-tall A. sphae. 
rocephala from the mat of shrubs on beach dunes (Mexico, site 6). 
A. sphaerocephala starts out with a single major 
trunk (e.g., Figure ~ZA), as with A. cornigera. These 
plants, however, are generally in dry and, therefore, 
relatively well-insolated sites and thus rapidly 
spread out laterally (Figure 63~). A. cornigera in 
the same sites tends to grow taller and more slender. 
Within 2 to 4 years after first becoming occupied 
by obligate acacia-ants, the young A. sphaero- 
cephala bears flowers. It is generally one year ahead 
of A. cornigera growing at the same site. After 
flowering, A. sphaerocephala rarely increases in 
height much in later years. The pods (Figure 65c) 
are produced in dense clusters, again as with A. 
cornigera, but generally have very thin walls- 
easily crushed between one's fingers-in contrast to 
the stiff and tough walls of A. cornigera at the same 
sites. A. cornigera has a morph with thin pod walls, 
but it is not sympatric with A. sphaerocephala. 
In an effort to document the thorn volume of 
A. sphaerocephala, and its relationship to external 
thorn dimensions, the swollen-thorns of three dif- 
ferent plants were measured. The young seedling 
(Figure 66) has the very small thorns (as compared 
with those of the adult in Figures 67 and 68) char- 
acteristic of young plants; in general, the larger the 
thorn, the higher it was up on the plant. For the 
inland ecotype (Figures 65~, 69~, B), the maximum 
thorn volume is 3.5 to 4.5 cc and probably repre- 
sents the maximum possible inside thorn diameter 
for optimal ant brood care. The great increase in 
thorn volume found in the dune ecotype (Figures 
65~, 69c) would be negatively adaptive for plants 
occupied by obligate acacia-ants, since it is pro- 
duced by inflating the thorn rather than increasing 
its length. It is very difficult for the ants to organize 
and care for brood in thorns where the thorn walls 
are more than $bout 4 to 6 mm apart. The ants of 
Crematogaster, however, that normally tenant these 
thorns are accustomed to living in large cavities in 
plants and build carton partitions in the thorns to 
keep the brood separate. One may envision this as 
the first possible step in the development of an 
80 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
.. 
I 
.. .? 
. * ..* 
.: 
.. . 
.. 
*. 
.. $;;:;:*? 
. I!: 
..:i!:: 
:l:il . 
. 
a:!!! 
FIGURE 67.-Relationship between thorn length, volume, and width for the 244 
.* *. : . thorns on a 2-m-tall adult A. sphaerocephala from the beach dune mat (Mexico, 
.. .I.. 
site 6): A, total thorn volume as a function of the total thorn length (sum of the 
:* 
two halves); B, total thorn volume as a function of the sum of the two halves of 
the thorn; c, relationship of total thorn length to the sum of the maximum width 
.?I:;. * 
... 
I 
5 10 15 20 25 ,b of the two halves of the thorn. 
c SUM 0? UAXIIJII LIDT? OF THE TWO THORN HALVES (mmj 
interaction in the New World as has evolved be- 
tween the African swollen-thorn acacias and Acacia turgida Safford in Wheeler, 142:plate 45. 
Acacia campecheana Schenck, 1913:361. 
Crematogaster. 
8. Acacia cornigera (L.) Willdenow 
Acacia cornigera (L.) Willdenow, 1806: 1080. 
Mimosa cornigera L., 1753:520. 
Acacia cornigera var. americana Dc., 1825:460. 
Acacia spadicigera Schlechtendal and Chamisso, 1830:594. 
Acacia cubensis Schenck, 1913:360. 
Acacia nicoyensis Schenck, 1913:360. 
Acacia hernandezii Safford, 1914:358. 
Acacia furcella Safford, 1914:359. 
Tauroceras spadicigerum (Schlechtendal and Chamisso) Brit- 
Tauroceras cornigerum (L.) Britton and Rose, 1928:86 [ex- 
Acacia interjecta Schenck, 1913:361. 
Acacia rossiana Schenck, 1913:361. 
ton and Rose, 1928:85. 
cluding synonymy]. 
Acacia cornigera is the best known of the swollen- 
thorn acacias. It is readily distinguished from all 
other swollen-thorn acacias by the unique combi- 
nation of elongated ?canoe-shaped? petiolar nec- 
taries (Figure 8~), very elongated infloresc, ences 
(Figure 12~), thorns usually round in cross section, 
and completely indehiscent seed pods. 
Linnaeus? type specimen of A. cornigera is a 
sterile branch representative of the eastern Mexican 
portion of the total population; it was collected 
from a cultivated plant in Holland, presumably 
grown from Mexican seed (Rudd, 1964). 
Rudd has compared pinnules of the type with 
those of A. sphaerocephala and found that the 
pinnules of the type have the readily visible 
secondary venation lacking on A. sphaerocephala 
NUMBER 13 
B 
r 20- 
E 
I i5. 
b 
: 
81 
.... -. .... 
................ 
.......... 
....... 
. ........ ..... 
. ...a ....... 
_..I 
..... 
.. a. 
.. 
15C 
12: 
p ii 
C 
.. 
.... 
...... 
....... 
............. 
... 
.. 
I 
1234567BD1011I2 L 2 3 4 5 6 7 0 9 10 11 
TOTAL THORN VOLUME Iccl B TOTAL TLiORN YOLJME lccl 
*. 
-. 
-2. : 
.* *. 
FIGURE 68.-Relationship between thorn length, volume, and width for the 200 
thorns from half of a 1.5-m-tall adult occupied A. sphaerocephala growing like a 
representative swollen-thorn acacia 200 m inland from the beach (contrast these 
data with those in Figure 67) (Mexico, site 6): A, total thorn volume as a function 
of the total thorn length (sum of the two halves); B, total thorn volume as a func- 
tion of the sum of the maximum width of the two halves of the thorn; c, relation- 
ship of total thorn length to the sum of the maximum width of the two halves of 
the thorn. 
*: .:.a. * 
*:. !i;.:* 
... 
'i,ili:i. 
i.1 ... 
::!i!.i . . 
. 11: 
.*. 
IS 20 25 10 35 
SUM OP MPXIMUM WIDTH OF THE MI0 THORN MLYES lmml 
FIGURE 69.-Acacia sphaerocephala (Mexico): A, black type A thorns on lateral branch (site 5); 
B, bicolor type A thorns from a lateral branch of an inland adult (site 6); c, bicolor type A 
slightly inflated thorns of plants growing in the dune mat (site 6). 
82 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
leaflets. Owing to the peculiar thorns of A. mayana, 
there is no doubt that the type does not belong to 
that species. 
The type specimen of A. spadicigera (site 1) is 
easily identified as A. cornigera because of its 
elongate inflorescences and thorns round in cross 
section. Its straight short thorns are representative 
of A. cornigera from lowland wet sites in Veracruz. 
Safford?s type specimen of A. hernandezii is un- 
mistakably from the northern wet lowlands portion 
of the A. cornigera population. A. cornigera is the 
only species of swollen-thorn acacia in the Rascon 
area (site 2). Safford?s leafless type specimen of A. 
furcella is representative of the A. cornigera popula- 
tion in the Lake Catemaco region (site 5). The type 
has the very dark brown thorns and widely spread 
type B thorns characteristic of highslevation, east- 
ern Mexican A. cornigera. The inflorescences of the 
types of both A. hernandezii and A. fnircella are 
representative of A. cornigera inflorescences. 
Schenck?s type of A. rossiana (site 4) is representa- 
tive of that portion of the A. cornigera population 
inland from Minatitlan, Veracruz, Mexico. I have 
been unable to locate his type specimen for A. 
campecheana; possibly it was destroyed at Berlin. 
He states that its leaves are like those of A. 
spadicigera and A. rossiana, and it was therefore 
most likely collected from that part of the A. 
cornigera population that extends along the coastal 
lowlands from Veracruz to Yucatan. Its name sug- 
gests that it was collected in Campeche, and there 
appear to be no swollen-thorn acacias in that area 
that could be confused with ?4. cornigera. Schenck 
described A. interjecta from material growing in 
the Singapore and Kew botanic gardens; the de- 
scription agrees with that of A. cornigera and the 
seed probably came from Mexico, since that appears 
to have been the source of most introduced A. 
cornigera seed. Schenck described A. nicoyensis 
from material representative of the southern end 
of the geographic range of A. cornigera (site 77). 
His type specimen has the white and slightly flat- 
tened type A thorn found on Costa Rican A. 
cornigera growing in very dry and insolated sites. 
The inflorescences and seed pods of the type of A. 
nicoyensis are unmistakably A. cornigera. 
Schenck?s type specimen of A. cubensis was taken 
from a cultivated plant (site 82); the white and 
partially recurved type B thorns indicate that the 
seed came from the A. cornigera population some- 
where north of Guatemala. A. cornigera apparently 
occurs only in gardens .in Cuba and its presence 
unlikely is due to natural introduction by birds or 
other means. The absence of ant entrance holes in 
the thorns of the type specimen of A. cubensis indi- 
cates that the ants have not yet been introduced 
to Cuba. 
Since A. turgida is supported only by a photo- 
graph (Wheeler, 1942), it probably represents a 
name that Safford intended to propose. I cannot 
find the locality (site 5) for the specimen, but it 
appears to have been collected from that rather 
distinctive portion of the A. cornigera population 
growing where the Pan-American highway crosses 
the Guatemala-Mexico border. 
KEY COLLECTION LOCALITIES * 
Mexico: 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
11. 
Type of A. spadicigera: La Laguna Verde, Veracruz, 
Mexico. March 1820, Schiede, no, 685. 
Type of A. hernandezii: Rascon, S.L.P., Mexico. VII-19- 
22-1905, E. Palmer, no. 669. 
Type of A. furcella: Catamaco, Veracruz, Mexico (300 
m). IV-26-1894, E.W. Nelson, no. 427. 
Type of A. rossiana: Santa Lucrezia, Isthmus of Tehu- 
antepec, Veracruz, Mexico, X-8-1906, H. Ross, no. 918. 
?Type? of A. turgida: Herba Santa, Chiapas, Mexico 
(see plate 45 in Wheeler, Bull. Mus. Comp. Zool. Har- 
vard, 90: 1-262). 
Guatemala-Mexico border (near Ciudad Cuahtemoc) to 
19.0 mi N on Hgy. 190, Chiapas (720-600-780 m) . VIII- 
7-1966, DHJ, no. 499; VIII-16-1967, DHJ, nc (common 
to rare). 
2.6 mi NE to 0.9 mi W Chiapa de Corzo on Hgy. 190, 
Chiapas (580-475 m) . VIII-17-1967, DHJ, nc (common 
to rare). 
9.7 mi W Tuxtla Gutierrez to 8 mi NE Cintalapa on 
Hgy. Chiapas 190 (860-1090-570 m) . VIII-18-1967, DHJ, 
nc (common to rare to occasional). 
13.5 to 9.0 mi E of intersection of Hgy. 190 and 185 (La 
Ventosa) , Oaxaca (80 m) . VIII-18-1967, DHJ, nc (com 
mon to rare). 
9.9 to 23.4 mi N of intersection of Hgy. 190 and 185 (La 
Ventosa) , Oaxaca (210-290-250 m). VIII-18-1967, DHJ, 
nc (common to occasional). 
33.3 mi N of intersection of Hgy. 190 and 185 (La Ven- 
tosa) to Acayucan, Oaxaca-Veracruz (23040 m) . VIII- 
18-1967, DHJ, nc (common). 
* Hundreds of collection records of A. cornigera in the 
U.S. National Herbarium and elsewhere are included within 
the inclusivc localities given here. For the sake of brevity, 
only type localities or collections made in sites not visited in 
this study are cited. 
NUMBER 13 83 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
22. 
23. 
24. 
25. 
26. 
27. 
28. 
29. 
30. 
31. 
32. 
33. 
34. 
Near Salina Cruz, Oaxaca (sea level). X-3-1965, DHJ, 
nc (occasional). 
Coatzacoalcos to Veracruz on Hgy. 180, Veracruz (0-350- 
0 m) . 111-21-1964, DHJ, no. 1935; VI-15-1966, DHJ, nc 
(occasional to common). 
8.3 mi SE bridge over Rio Coatzacoalcos, Veracruz, to 
Cardenas, Tabasco, on Hgy. 180 (0-40 m) . VI-15-1966, 
DHJ, nc (very rare, about 1-2 plants per 2 miles). 
Cardenas to 37.1 mi S, Tabasco (40-100 m) . VI-17-1966, 
DHJ, nc (rare to common). 
Cardenas to Paraiso, Tabasco (40-10 m) . VI-17-1966, 
DHJ, nc (rare). 
Cardenas to Ciudad del Carmen, Tabasco-Campeche 
(40-0 m) , VI-22-1966, DHJ, no. 494 (rare, and only on 
well drained sites), fl. 
Campeche to Ruinas Edzna (29 mi E on Hgy. 180), Cam- 
peche (0-50 m) . VI-22-1966, DHJ, nc (very rare to oc- 
casional). 
Hopelchen to Ticul on Hgy. 180, Campeche-Yucatan 
50-80). VI-28-1966, DHJ, nc (very rare to occasional). 
12.2 mi S Pet0 (Santa Rosa) on Hgy. to Cd. Chetumal, 
Yucatan-Quintana Roo (50-80 m). VI-29-1966, DHJ, nc 
(common), fl. 
Cd. Chetumal to 69.6 mi NW on Hgy. to Peto, Quintana 
Roo (0-20 m) . VI-30-1966, DHJ, nc (common), fl. 
30 mi S to Campeche on Hgy. 180, Campeche (0-80-0 
m). VI-25-1966, DHJ, no. 491; VII-8-1966, DHJ, nc 
(rare to common). 
3.2 mi E Cruz Grande to 19.1 mi NW Puerto Escondido 
on Hgy. 200, Guerrero-Oaxaca (50-250-70 m). VI-18- 
1967, DHJ, nc (rare to common to occasional) . 
9.3 mi N Playa Pie de la Cuesta to 16.5 mi SE Papanoa 
on Hgy. 200, Guerrero (0-20 m). VI-17-1967, DHJ, nc 
(very common in swamps, absent on dry hills). 
Alvarado to Tlacotalpan to Cd. Aleman to Las Tinajas 
to Paso del Toro, Veracruz (0-100-0 m). Throughout 
the year, DHJ, no. 1939 (common), flower February-May. 
Tierra Blanca to La Granja to Temascal, Veracruz- 
Oaxaca (20-70 m). Throughout the year, DHJ, nos. 
1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1974, 1975, 
1976, 1977, 1978, 1979, 1980, 1981 (very common). 
Shores of the lake behind Presa Miguel Aleman, Temas- 
cal, Oaxaca (70 m). X-20-1963, DHJ, no. 1937; XI-3- 
1963, DHJ, nc (occasional to very common). 
Cd. Aleman to Loma Bonita to Rio Playa Vicente, Vera- 
cruz-Oaxaca-Veracruz (50-100 m) . XI-4-1963, DHJ, no. 
1938 (common to very rare on red clay hills), 
Cd. Aleman, Veracruz to Valle Nacional, Oaxaca, on Hgy. 
to Oaxaca, XI-6-1963, DHJ, nc (common to occasional). 
22.8 to 27.5 mi SW Conejos (turn off of Hgy. 140 to 
Huatusco) on Hgy. to Huatusco to Cordoba, Veracruz. 
XI-10-1963, DHJ, nc (common to rare). 
4.3 mi E Cordoba, Veracruz on Hgy. 150, Veracruz (800-0 
m) . X-16-1963, DHJ, nc (rare to very common). 
Hgy. 150 SE to San Ignacio de la Llave, Veracruz (60-10 
m) . X-16-1963, DHJ, nc (common). 
Tampico to Tuxpam on Hgy. 105, Veracruz (0-100-0 m) . 
1-2-1964, DHJ, nos. 1942, 1943; VI-13-1966, DHJ, nc 
(common to rare). 
Tuxpan to 1.2 mi S Villa A. Camacho on Hgy. 130, Vera- 
35. 
36. 
37. 
38. 
39. 
40. 
41. 
42. 
43. 
44. 
45. 
46. 
cruz. 1-2-1964, DHJ, no. 1940; VI-13-1966, DHJ, nc (oc- 
casional to rare). 
Poza Rica to Nautla on Hgy. 180, Veracruz (50-0 m), 
1-20-1964, DHJ, nc (occasional to rare). 
6 mi SW Tamazunchale to Cd. Valles on Hgy. 85, San 
Luis Potosi (500-100 m) . 1-1-1964, DHJ, no. 1936; XII- 
31-1963, DHJ, nc (rare to common). 
36.6 mi E Rio Verde to Cd. Valles on Hgy. 86, San Luis 
Potosi (1140-100 m). VIII-24-1967, DHJ, nc (rare to 
common). 
Cd. Valles to 6.4 mi N Antiguo Morelos on Hgy. 85, San 
Luis Potosi-Tamaulipas (100-250 m) . VIII-24-1967, 
DHJ, nc (rare). 
Antiguo Morelos to 20 mi R?W Cd. del Maiz, Tamaulipas- 
San Luis Potosi (250-? m). XII-31-1963, DHJ, nc (com- 
mon on wetter hillsides) . 
Cd. Valles to Tampico on Hgy. 70, San Luis Potosi- 
Veracruz (100-10 m). VI-11-1966, DHJ, nos. 496, 497; 
VIII-24-1967, DHJ, nc (common to occasional). 
Pt. Talisman (Guatemala-Mexico border) to 3 mi N 
Arriaga on Hgy. 200, Chiapas (50-150 m). 1-11-1965, 
DHJ, no. 1047 (common). 
Presa Benito Juarez, near Jalapa de Marquez on Hgy. 
190, Oaxaca (200 m). IX-7-1968, DHJ, nc (1 plant). 
Pichucalco, Chiapas. 1-1907, GN. Collins and C.B. Doyle, 
no. 260 (US 692169) mp. 
3 mi S Pinola Las Rosas along road to Soyatitan, Venus- 
tiano Carranza (W of Comitan), Chiapas (1400 m). VII- 
27-1965, D.E. Breedlove, no. 11367 (US 2484541), fl. 
Zacualpan, Veracruz (1000 m) . VII-1917, C.A. Purpus, 
no. 7748 (US 884536), fl., and IX-1908, H. Schenck, no. 
836 (US 764665). 
Fortuno, Rio Coatzacoalcos. 111-1937, L. Williams (F 
897476), fl. 
British Honduras: 
47. Sta. Elena to Belize on main Hgy. (5-0 m) , VI-30-1966, 
DHJ, nc (rare to occasional), fl. 
48. El Cayo, El Cay0 Prov. V-5-1931, H.H. Bartlett, no. 
13007 (US 1493273), mp. 
Guatemala: 
49. 
50. 
51. 
52. 
53. 
54. 
55. 
Escuintla to Taxisco along Hgy. CA-7, Depto. Escuintla- 
Santa Rosa (330-100 m). 1-13-1965, DHJ, nc (occa- 
sional) , 
Escuintla, Drpto. Escuintla, to Puente Talisman, Depto. 
San Marcos (Guatamela-Mexico border) on Hgy. CA-7 
(330-50 m). VII-16-1966, DHJ, no. 466; 1-11-1965, DHJ, 
nos. 1045, 1046 (common to occasional), fl. 
Retalhuleu SW to Champerico, Depto. Retalhuleu (200- 
0 m) . VIII-1-1966, DHJ, nc (occasional). 
Retaljuleu to 14.0 mi NE on Hgy. S, Depto. Retalhuleu 
(200-900 m). VII-28-1966, DHJ, no. 464 (common to 
rare). 
Escuintla to 7.4 mi NE on Hgy. 3, Depto. Escuintla (330- 
880 m) . VIII-13-1967, DHJ, nc (common to occasional). 
0.3 mi SW to 4.1 mi NE Los Amates on Hgy. CA-9 (150- 
110 m). VIII-9-1967, DHJ, nc (common) . 
Finca San Joaquin, Slopes above Rio Negro, near San 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
84 
56. 
57. 
58. 
59. 
Cristobal de Verapaz, Depto. Alta Verapaz (1420-1080 
m) . VIII-14-1967, DHJ, nc (rare to common). 
14.9 mi SE to Guatemala-Mexico border (near Ciudad 
Cuauhtemoc) , Depto. Huehuetenango (900-1000-720 m) . 
VIII-16-1967, DHJ, nc (common). 
Patalul, Depto. Solola. 11-14-1906, WA. Kellerman, no. 
5915 (US 578841), fl. 
Finca Sepacuite, Depto. Alto Verapaz. IV-1905, O.F. Cook 
and R.F. Griggs, no. 8 (US 590344), mp. 
1-2 mi N Ocos, Depto. San Marcos (sea level). III-15- 
1940. ].A. Steyermark, no. 37867 (F 1035247). 
Honduras: 
60. 12.5 to 19.5 mi NW Siguatepec on Tegucigalpa-Puerto 
Cortes Hgy., Depto. Comayagua (910-540 m). VII-31- 
1967, DHJ, nc (comnion to very rare). 
61. 0.7 mi N Rio Lindo to 18.6 mi S Puerto Cortes on 
Tegucigalpa-Puerto Cortes Hgy., Depto. Cortes (90-10 
m) , VIII-2-1967, DHJ, nc (common to occasional). 
62. San Pedro Sula to 8.1 mi SW on Hgy. to Santa Rosa, 
Depto. Cortes (90-130 m) . VIII-2-1967, DHJ, nc (occa- 
sional along streams). 
63. 20.6 mi NE to 28.4 mi SW Sula on San Pedro Sula-Santa 
Rosa Hgy., Depto. Santa Barbara (260-780 m). VIII-3- 
1967, DHJ, nc (common to rare), fl. 
64. Vegas del Rio Agua, 3 km from Yoro, Depto. Yoro 
(1000 m), V-8-1956, A. Molina, no. 6807 (US 2412019), 
fl. 
El Salvador: 
65. From 11.2 to 26.3 mi N (Depto. Cuscatlan) San Salvador 
on San Salvador-Nueva Ocotopeque Hgy. (420-590 m) . 
VIII41967, DH], nc (common to rare). 
66. 6 mi E San Salvador on Interamerican Hgy., Depto. San 
Salvador (650 m) . 1-14-1965, DHJ, nc (occasional), 8. 
67. 29.7 to 31.6 mi E La Libertad on La Libertad-La Union 
Hgy., Depto. La Paz (50-80 m). VIII-5-1967, DHJ, nc 
(rare). 
68. Intersection of Hgy. from La Herradura with La Liber- 
tad-La Union Hgy., Depto. La Paz (20 m) . VIII-5-1967, 
DHJ, nc (1 plant). 
fig. 15.2 to 10.2 mi SE Santa Ana on Interamerican Hgy., 
Depto. Santa Ana (440-700 m). VIII-6-1967, DHJ, nc 
(common). 
70. San Vicente, Depto. San Vicente (350-500 m), III-2- 
1922, P.C. Standley, no. 21687 (US 137448, US 1137096), 
fl. 
71. Ateos, Depto. La Libertad. IV-17-1922, P.C. Standley, 
no. 23360 (US 1139047), fl. 
72. San Martin to Laguna de Ilopango, Depto. San Salvador. 
IV-1-1922, P.C. Standley, no. 22580 (US 1138317), fl. 
Nicaragua: 
73. Penas Blancas (Depto. Rivas) to 7.2 mi NW on Hgy. 2 
(50 m). VII-25-1967, DHJ, nc (common along rivers). 
74. 3.4 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VI-27- 
1967, DHJ, nc (30 acre patch), fl. 
75. 1.2 mi W Matagalpa to 3.6 mi N on Hgy. 3, Depto. Mata- 
galpa (650-880 m) . VII-28-1967, DHJ, nc (common), 8. 
76. 8 mi S Esteli on Hgy. 1 (940 m) . VII-28-1967, DHJ, nc 
(1 plant). 
Costa Rica: 
77. Type of A. nicoyensis: Shores of the Gulf of Nicoya, 
Guanacaste, Costa Rica (sea level). February 1900, Ton- 
duz, no. 13538. 
78. 20 mi NW turnoff of Interamerican Hgy. to Puntarenas, 
to Pefias Blancas (border of Costa Rica and Nicaragua) 
on Interamerican Hgy., Guanacaste Prov. (10-100 m) . 
January-September, periodically in 1963-1967, DH], nc 
(rare to common, patchy distribution and associated with 
wet spots), fl., February-April. 
79. Liberia to Playa COCO along Hgy. 21, Guanacaste Prov. 
(100-0 m). January-September, periodically in 1963-1967, 
DH], nc (rare to very common, primarily in wet spots), 
fl. and mp., in February. 
80. Hda. Palo Verde, COMELCO, Bagaces, Guanacaste Prov. 
(sea level). IV-2-1966, DHJ, nc (occasional), 
81. Filadelphia, Guanacaste Prov. 111-2-1946, Echeuerria, no. 
279 (F 1442853), mp. 
Cuba: 
82. Type of A. cubensis: North coast of Cuba. IV-21-1963, C. 
Wright, no. 2402. 
NATURAL HISTORY 
Acacia cornigeru (Figure 70) and A. collinsii 
have the greatest geographic and ecological range 
of all the swollen-thorn acacias. The riparian and 
swamp habitats that were occupied by A. cornigeru 
previous to human interference are in lowland areas 
that are ideal for slash-burn milpa agriculture. A. 
cornigera has probably been common in lowland 
agriculture fallow fields as long as Indians have 
lived in the Central American lowlands. Since oc- 
cupied A. cornigeru may reach reproductive ma- 
turity in 3 to 4 years, and since birds, people, and 
cattle distribute its seeds widely in second growth 
vegetation, A. cornigera populations have expanded 
widely into pastures, roadsides, and short-fallow 
fields generated by contemporary agricultural prac- 
tices. At many locations, A. cornigera is found in 
all types of disturbance sites, from creek bottoms to 
hilltops. Only A. sphaerocephala and A. collinsii 
are on occasion found in drier habitats than A. 
cornigera. The natural history of A. cornigeru in 
eastern 
Mexico has been described in detail (Jan- 
Zen, 1967b), and the comments in the remainder of 
this section are designed to highlight the way it 
differs in other parts of its range from that portion 
of the population. 
Local short-distance dispersals of A. cornigera by 
NUMBER IS 85 
humans make it difficult to understand its pre- 
Colombian range. For example, at site 42 in western 
Mexico, a single A. cornigera was found growing 
by a new hydroelectric dam. The closest native 
population of A. cornigera is several hundred miles 
away in the Isthmus of Tehuantepec and along the 
west coast in the Acapulco region (sites 23, 24) (also 
an introduction?). This plant probably arrived by 
either road-building machinery or through humans; 
farmers and Indians often eat the sweet pulp 
around the seeds and ingest many seeds simultane- 
ously (Janzen, 196713). The work crews on public 
construction in Mexico often come from several 
hundred miles away. If more than one plant was 
introduced in the area of the dam, it is likely that 
viable seeds will be produced. The bruchids that 
normally kill most A. cornigera seeds will be absent, 
at least at first, and this swollen-thorn acacia will 
likely spread down the watercourse away from the 
dam. It will probably not spread around the lake 
since the general area of the lake is extremely dry 
and water levels will fluctuate greatly. The single 
plant was occupied by Pseudomyrmex ferruginea. 
The queen probably came from the population of 
A. collinsii about 25 miles to the south or the 
population of A. hindsii 50 miles to the northwest. 
As a second example, in the central Yucatan Penin- 
sula, A. cornigera is only common around small 
settlements, cattle corrals, and old Indian ruins 
(sites 18 to 21). These small sites are also usually 
the wetter habitats in the region (the outlying areas 
are heavily occupied by A. collinsii), and it is im- 
possible to know if the presence of A. cornigeru 
is due to recent and frequent introduction by hu- 
mans and cattle, or by natural dispersal by birds. 
Deliberate long-distance introduction of A. 
cornigera may have some interesting repercussions 
in the future. Introduced to the southern tip of 
Florida as a garden plant, A. cornigera has oc- 
casionally escaped to form feral populations (e.g., 
Oneco and Chapman Field, US 1433083 and US 
764685 in the U.S. National Herbarium). These 
plants are severely damaged by the frosts that oc- 
casionally damage orange orchards, but they do 
flower and set fertile seeds if there are several 
together. Obligate acacia-ants have not been intro- 
duced, but it would be interesting to examine the 
interaction of occupied swollen-thorn acacias with 
the subtropical communities of southern Florida. 
Reproductively isolated from the Mexican parent 
genotype, A. cornigera might even develop a frost- 
resistant population in Florida. A. cornigeru has 
also escaped on Martinique, Guadeloupe, and Cuba 
in the Caribbean (US 2229384, US 1714928, US 
764657). Here, with a climate not unlike that of 
some parts of eastern lowland Mexico, the popula- 
86 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
tions (again without their ants) have a chance to 
expand into the native vegetation just as many 
other species of introduced trees have done on 
Caribbean islands. The central question here is 
whether the herbivore pressure in the island com- 
munities is strong enough to require occupation by 
obligate acacia-ants for the survival of a large breed- 
ing population of A. cornigera. All introductions 
to the Old World tropics appear to have been to 
botanical gardens, and I have no information on 
their escape into natural habitats. 
Acacia cornigera and A. collinsii occupy adjacent 
habitats over much of their range. Always A. corni- 
gera is in the wetter creek bottoms and swamp 
edges while A. collinsii is on the drier hillsides (e.g., 
Guanacaste Province, Costa Rica). In the wetter 
parts of the range, A. hindsii and A. cornigera often 
occur together (e.g., Pacific foothills of Guatemala) 
and here, A. cornigera is the drier hillside plant 
with A. hindsii along the watercourses. The distinc- 
tiveness of these separations by habitat was prob- 
ably once very clear. Current agricultural practices 
have rendered wetter habitats drier and more inso- 
lated by clearing the vegetation, with the conse- 
quence that the formerly separated swollen-thorn 
acacias now grow side by side. 
In the southern part of its range, the A. cornigera 
population is much more broken up into subpopu- 
lations than in eastern Mexico where it is found 
in all sorts of disturbance and climatic regimes 
from sea level to 1200 m, and from areas with a 
7-month dry season to areas with almost no obvious 
dry season. In habitats where one expects to find 
A. cornigera in the northern part of the range, one 
instead encounters A. collinsii and A. hindsii. To 
determine the role of competitive exclusion in this 
system, we need much more information on the 
parasites and predators (specifically, bruchid 
beetles) that these species have in common, and 
the ability of each to grow and maintain the largest 
possible obligate acacia-ant colony in the shortest 
period of time. 
Why A. cornigera has stopped its southward 
niovement in Guanacaste Province in Costa Rica 
is very enigmatic. A. collinsii continues south to 
Colombia with a nearly continuous population that 
occupies many habitats similar to those occupied by 
A. cornigera further north. It may well be that it 
has not stopped at all, but rather that it is in the 
process of slowly moving southward. The only 
possible ecological explanation that comes readily 
to mind is that the Guanacaste plants of A. 
cornigera are specialists at surviving the severe dry 
season there and therefore cannot advance slowly 
through the wet-forest area immediately down the 
coast from Puntarenas. What is needed is a long- 
distance dispersal of more than one seed simultane- 
ously from Guanacaste to the dry Pacific coast of 
Panama. 
The dependency of A. cornigera on a colony of 
obligate acacia-ants for survival to reproductive 
maturity has been well documented for lowland 
eastern Mexican habitats (Janzen, 1967a). Judging 
from the badly damaged condition of naturally un- 
occupied A. cornigera throughout its range, this 
dependency is the general case. It is clear, how- 
ever, that the intensity of damage to unoccupied 
A. cornigera varies from one habitat to another, 
just as it did from one season to another in the 
lowlands of eastern Mexico. High-elevation sites 
subjected to wet, cool, and windy weather (e.g., site 
75) produce the healthiest appearing unoccupied 
A. cornigera, but even here it is doubtful if an 
unoccupied tree can survive until reproductive 
maturity unless growing in a cluster of acacias in 
a large and well-grazed cattle pasture. In central 
highland Guatemala (e.g., site 53) and on the high- 
land Atlantic side of Honduras (site 60), A. 
cornigera is occasionally ?occupied? by a large 
colony of Camponotus planatus which appear to 
compete directly for thorns with the obligate acacia- 
ants colony. The C. planatus patrol the acacia quite 
effectively and the plants are healthy in appearance. 
How long an acacia can be occupied by C. planatus 
and survive is not clear. On some occasions, a 
colony of C. planatus and one of obligate acacia- 
ants occupy the same acacia, but usually in different 
branches. C. planatus occurs throughout much of 
the range of swollen-thorn acacias but in other areas 
does not appear to offer any protection to the 
acacia. It also lives in hollow twigs in many other 
species of tree. 
Acacia cornigera has gained the same ?freedom? 
from herbivorous insects gained by A. hindsii in 
areas of intense agriculture (e.g., site 74 in El 
Salvador); however, it is much rarer than A. hindsii 
in these sites and will probably become extinct in 
the near future. Seedlings are almost never en- 
countered. Without the ants to keep the vines off, 
the stiff and upright young shrub of A. cornigera 
NUMBER IS 87 
FIGURE 7l.-Acacia cornigera (Mexico, site 26): A, unoccupied seedling from heavily shaded 
site; B, unoccupied seedling from strongly insolated site. 
makes an excellent vine standard leading to its 
death by shading. The young plants of A. hindsii 
are more shade tolerant and less rigid than A. 
cornigera of the same age. 
As with A. hindsii and A. collinsii, the seedlings 
of A. cornigera are scattered throughout the habitat 
occupied by the parents. As stressed earlier (Janzen, 
1967b), they attain the height of 10 to 50 cm and 
die after 1 to 2 years if not occupied by a colony 
of obligate acacia-ants. When in heavily shaded 
sites, the seedlings tend to produce few swollen 
thorns (Figure 71~), while in fully insolated sites 
thorn production is much greater (Figure 71~). 
Once occupied, a variety of different shrub life 
forms may develop. In dry areas, about as much 
wood goes into the central trunk as into branches 
(e.g., Figure 72~), and the shrub ends up being very 
bushy in appearance (Figures 73~, B). Only very 
rarely does A. cornigera become a large tree to 
FIGURE 7P.-Acacia cornigera: A, crown of healthy adult after 
losing its leaves during the last part of the dry season (Nica- 
l5 in height; such are ragua, site 75); B, crown of healthy subadult growing in 
found in natural disturbance sites along rivers and marshy pasture (Honduras, site 61). 
88 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 73.-Acacia cornigera: A, pasture with adult (Mexico, site 23); B, type B and type A 
thorns in center of crown of adult (Nicaragua, site 75); c, type B thorns on main trunk of adult 
(Mexico, site 24). 
may at one time have made up a much larger 
proportion of the reproducing population than at 
present . 
Over most of the distribution of A. cornigera, 
sexual branchlets and their products are rather 
uniform; however, wetness of the habitat does ap- 
pear to influence this. In the Pacific foothills of 
Guatemala (site 50) the sexual branchlets may be 
20 to 50 an long (Figure 74~). In contrast to the 
situation at this wet site, most populations of A. 
cornigera produce sexual branchlets 1 to 10 cm long 
(Figure 74c). The inflorescence itself is not influ- 
enced by the amount of moisture at the site, except 
that at some of the driest sites, some of the shortest 
inflorescences were found (again suggesting that A. 
sphaerocephala is a dry-land offshoot of A. corni- 
gem). The number of florets per inflorescence is 
poorly correlated with the length of the inflores- 
cence (Figure 75) at any one site but over a variety 
of sites shows a weak correlation (Figure 76). 
NUMBER 13 89 
FIGURE 74.-Acacia cornigera: A, exceptionally long sexual branchlets (Guatemala, site 50); 
8, mature pods ripped open by birds (Guatemala, site 50); c, last year?s pods and this year?s 
inflorescences on short sexual branchlets in the crown of an adult (Nicaragua, site 75). 
The indehiscent pods of A. cornigera are split 
open by birds (Figure 75~) and the seeds dispersed 
by birds that frequent second-growth vegetation 
(Janzen, 1967b). Over most of the range of A. 
cornigera, the pod walls are thick and hard to 
break between ones fingers. They tend to have sharp 
and stiff long points on the end (Figures 74B, c); 
however, in riparian habitats at site 26, the pod 
walls are very thin and even split of their own 
accord when drying. The plants on the adjacent dry 
hills have the usual thick-walled pods. This differ- 
ence is genetic as shown by a lack of intermediates 
90 
0 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
0 
0 
LT Lu 
1 
3 
z 
m 
I 
26 28 30 35 311 3?6 38 4b 
LENGTH OF INFLORESCENCE (mm) 
FIGURE 75.-Distribution of 27,262 florets among 23 inflorescences from a single adult A. corni- 
gem; the inflorescences averaged about 35 mm in length (excluding peduncle) and 8.9 mm in 
width (Mexico, site 26). 
and occasional plants bearing the ?wrong? type of 
pods for the habitat where the plant is encountered. 
It is interesting that this site is only about 80 miles 
from that portion of the A. sphaerocephala popu- 
lation below the city of Veracruz that also has very 
thin pod walls. The number of seeds in A. cornigera 
pods is representative for dry-land, swollen-thorn 
acacias (Figure 77) but nearly all the seeds are 
killed by bruchid beetles. The very high predation 
on A. cornigera seeds by these beetles may be due 
to the high concentration of pod crops to be found 
in fields and roadsides; when the plants were widely 
spaced in natural disturbance sites, the bruchids 
might have a much more difficult time locating the 
pod crops. 
The thorns of A. cornigera display every color 
found in swollen-thorn acacias, but there is little 
intertree or intratree variation at any one site. In 
general the thorns in very hot and dry lowlands are 
white, ivory, or pale gray. At high elevations they 
are often very dark brown or nearly black. Though 
the correlation is not perfect, it is tempting to 
interpret these differences as being adaptive in 
respect to the ant colony. In lowland areas, tempera- 
tures in fully insolated thorns can be high enough 
to drive out the ants, and light-colored thorns may 
reflect radiation and slightly lower the temperature 
inside the thorn. It is perhaps significant in this 
connection that A. coZlinsii also has very light- 
colored thorns in dry and hot areas and dark thorns 
in high and wet sites. The dark thorns of A. 
cornigera could aid in warming up the ant colony 
on cool days and thereby increase the degree of 
patrolling by ants. It is significant in this respect 
that A. sphaerocephala has very dark thorns at the 
one high (cool) and dry site where it occurs, in 
strong contrast to the light-colored thorns it bears 
in dry lowland areas. All of the wet-forest swollen- 
NUMBER 13 
91 
In 
25 
20 
15 
VI 
8 
p 10 
E5 
I 
z 
a 
FIGURE 76-Distribution of 125,773 florets among 122 inflorescences from 16 adult A. cornigera 
from throughout the range of A. cornigera; there was no evidence of a geographical trend in 
size of inflorescence or number of florets per inflorescence. 
""'nnn n 
4 13 12 11 10 8 6 
NUMBER OF SEEDS PER PO0 
FIGURE 77.-Distribution of 1436 seeds among 100 pods from 
an adult A. cornigeru with a normally heavy seed crop of 
about 500 pods (Mexico, site 26). 
thorn acacias have very dark brown thorns, and 
this may be important in warming the thorns by 
occasional direct insolation. Bicolored thorns (Fig- 
ure 78c) are somewhat puzzling in this respect. 
They are most prominent in the small and insolated 
population of A. cornigera on the west coast of 
Mexico; here A. cornigera grows only in swamps, 
with A. collinsii on the extremely dry and hot ad- 
jacent hillsides. 
The spectacular "bull's horns" swollen-thorns of 
A. cornigera are extremely variable in shape, but 
in a consistent and often clinal manner. Many sites 
are characterized by a particular thorn type, and 
many of these thorn types resemble thorns found 
on other species of swollen-thorn acacia (though 
often not sympatrically). Type A thorns of A. 
cornigeru (Figures 78, 79) are usually round or 
nearly so in cross section and form a shallow to 
steep "V" with the branch wearing them. When 
flattened so that they superficially resemble A. 
hindsii thorns (Figure ~OD), they are still easily 
distinguished by having very thick and rough walls. 
Type B thorns (Figures 80, 81) are highly variable 
in form, but all are much larger in volume than 
type A thorns. There is no part of the range of 
A. cornigera where it is consistently occupied by 
multiple-queen species of obligate acacia-ants, so 
92 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 78.-Geographic variation in type A thorns from occupied adult A. cornigera: A, Mexico, 
site 33; B, Mexico, site 40; c, Mexico, site 24; D, Guatemala, site 56. 
FIGURE 79.-Geographic variation in type A thorns from occupied adult A. cornigera: A, Mexico, 
site 33; B, Mexico, site 36; c, Mexico, site 33. 
NUMBER 13 
FIGURE 80.-Geographic variation in type B thorns from occupied adult A. cornigera: A, Mexico, 
site 33; B, Guatemala, site 52; c, Mexico, site 84; D, Guatemala, site 56; E, Mexico, site 35. 
selection for large type B thorns is omnipresent. 
Type B thorns of A. cornigera are the hardest to 
break into of any of the swollen-thorn acacias. 
Several are often twisted together, making it diffi- 
cult to even pull one off the acacia. 
9. Acacia mayana Lundell 
Acacia mayana Lundell, 1937:210. 
Acacia mayana (Figure 17) is the most easily 
recognized of the swollen-thorn acacias, owing to 
the pair of bladelike longitudinal flanges extending 
from the thorn base to tip. The type specimen is 
representative of the other specimens examined, 
though it should be noted that there are only 6 
specimens at hand of this exceptionally rare swollen- 
thorn acacia. The leafless isotype of the U.S. Na- 
tional Herbarium was collected from the upper part 
of the canopy of a mature plant that has been re- 
producing at least 2 years, since it bears both 
flowers and fruit. The fruits and flowers of A. 
A 
1. 
2. 
3. 
4. 
3. 
mayana are similar to those of A. cornigera and A. 
sphaerocephala (and therefore very different from 
the other 9 species of swollen-thorn acacia), but the 
fruits differ from those of these two species by 
being longer, thinner, and having a long thin base 
or stalk before swelling out around the seeds. The 
inflorescences differ from those of A. cornigera and 
A. sphaerocephala in that they taper to a point, 
rather than having a blunt end. More specimens of 
mayana are needed for study. 
KEY COLLECTION LOCALITIES 
Type: Rio Pasion, San Diego, Peten, Guatemala. VI-10- 
1935, M. Aguilar, no. 495 (US 1688106), mp. 
Forest S of Finca Yalpemech, near Alta Verapaz-Peten 
boundary, Depto. Alta Verapaz, Guatemala (100-150 m) . 
111-25-1942, J. A. Steyermark, no. 45184 (F 1188721), mp. 
3 mi W Cardenas, Tabasco, Mexico (20 m). VI-18-1966, 
UHJ, no. 458. 
Fortuno, Coatzacoalcos River, Veracruz, Mexico (30-50 
m) . 111-1937, L. Williams, no 8425 (F 896540). 
Retiro, Tenosique, Tabasco, Mexico. VI-19-25-1935, E. 
Matuda, no. 3410 (F 1027159) (?shrub in virgin forest?). 
94 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 81.4eographic variation in type B thorns from occupied adult A. cornigera: A, Mexico, 
site 26; B, Mexico, site 28; c, Mexico, site 26; D, Mexico, site 41. 
6. Rio Icuolay, NNW Finca Cubilquitz to Quebrada Diablo, 
Depto. Alta Verapaz, Guatemala (300-350 m) ; 111-6-1942, 
1. A. Steyermark, no. 44726 (F 1148694). 
NAT~JRAL HISTORY 
Acacia mayana is by far the rarest of the swollen- 
thorn acacias. If the small number of collections 
represents the true density and spatial distribution 
of the species, it may well be self-pollinated in con- 
trast to the other swollen-thorn acacias. The only 
specimen that I have been able to find (site 3) was 
growing in the same type of old second-growth 
cornfield regeneration that commonly harbors A. 
cornigera, but with one exception. The forest was 
about 15 m tall and the single A. mayana was 
heavily shaded, yet growing very well; an A. 
cornigera in the same shade regime would have 
been spindly with undersized thorns. It is possible 
that A. mayana is a species of primary forest, such 
as A. melanoceras and A. allenii; Matuda recorded 
it at site 5 as a ?shrub in virgin forest.? If this is 
the case, then its rarity in areas under heavy agri- 
cultural pressure probably means that it will be- 
come extinct in the lowlands. 
If the above hypothesis is correct, then A. mayana 
is a mature and wet-forest edition of A. cornigera. 
It is interesting in this context that A. cornigera has 
perhaps produced both a beach-edge species (A. 
sphaerocephala) and a wet mature forest species. 
It is tempting to view the oddly shaped thorns of 
A. mayana as the result of a long past hybridization 
with a sword-thorn acacia such as A. macracantha. 
When A. macracantha hybridizes with A. chiapen- 
sis, the result is often a ?roundish? thorn in cross 
section, with longtitudinal shallow-sided ridges 
down each side of the thorn. On the other hand, 
A. mayana displays no other traits suggesting a 
history of introgression with other acacias. 
Whether the ridges are the result of introgres- 
sion, or a novel mutation, they have a possible 
adaptive value. A. cornigera thorns on heavily 
shaded shoots have weak walls and are easily opened 
by birds for the ant brood inside; apparently the 
starving shoot does not have the energy to make 
the thick-walled thorns that effectively protect the 
ant colony in more insolated microhabitats. The 
longitudinal ridges on A. mayana thorns may be a 
way to solve the strengthening problem without 
SUMBER I3 
95 
FIGURE 8P.-Acacia mayana: A, type B thorns (the two largest), the largest thorn contained the 
ant colony queen; B, type A thorns split by birds in search of ant brood (Mexico, site 5). 
putting a lot of bulk into the thorn wall (this bulk 
is also adaptive as insulation in heavy insolated 
sites) in a heavily shaded habitat. First, it is im- 
portant to note that the ant colony grows slowly 
at a comparatively large cost to the acacia in the 
shaded habitat, and the loss of thorns with brood 
will be proportionately larger to the acacia than in 
an insolated site. Second, the slow replacement rate 
of thorns in shaded sites, observed on all forest 
species of swollen-thorn acacias, means that the 
birds have a longer time per thorn to locate the 
tree and go after its ants. Figure 82 shows that 
birds can be a threat to the ant colony in A. mayana, 
and it is important to note that they opened all 
the thorns on the shoot except the few largest ones. 
All specimens of A. mayana examined have been 
occupied by mature colonies of P. ferruginea. In 
view of the small sample size, however, this should 
not be taken to indicate that it is a suitable host 
for only this species of obligate acacia-ant. The 
colony in the specimen at site 3 was as normal as 
can be expected of a colony that has just lost about 
three-fourths of its brood and workers to a bird. 
While the petiolar nectaries of A. mayana are 
nearly twice as long as those of A. cornigera and 
A. sphaerocephala, it should not be assumed that 
the ant colony gets twice as much nectar per leaf 
from the shaded plant. There are no data to show 
a correlation between nectary size and nectar pro- 
duction. 
A total of 12 pods from herbarium sheets was 
examined and there was no attack by bruchids. 
Owing to the small sample size, and the small sizes 
of the crops from which the pods were taken, this 
should not be taken as an indication that A. 
mayana is free from predispersal seed predation. 
On the other hand, if in fact it does occur as widely 
scattered individuals in mature forest, it is possible 
that the bruchids that commonly infest A. cornigera 
and A. sphaerocephala cannot cope with the very 
low prey density that such a rare population of 
swollen-thorn acacias presents (as with A. melano- 
ceras and A. allenii). 
10. Acacia gentlei Standley 
Acacia gentlei Standley, 1940:77. 
Acacia gentlei is the common acacia of wetter 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
sites in British Honduras and lowland northern 
Guatemala (Figure 44). It is easily distinguished 
from other swollen-thorn acacias by the combina- 
tion of elongate inflorescences, one or at the most 
two elongate petiolar nectaries, and a large number 
of very thin thorns. Where it is sympatric with A. 
collinsii and A. cornigera in northern British 
Honduras, it is easily distinguished from both by 
attaining heights of 3 to 8 m, while they are gen- 
erally 1.5 to 3 m in height when reproductive, and 
by having much darker green foliage that strikingly 
contrasts with the rust-red bark. Both are generally 
found in the drier sites, while A. gentlei is gen- 
erally in the wetter sites. Where the distribution of 
A. gentlei overlaps with that of A. ruddiae and A. 
cookii in northern Guatemalan wet forest, A. 
gentlei is again easily distinguished from the former 
by being occupied by obligate acacia-ants and by 
having Beltian bodies on the new foliage. It is 
distinguished from the latter by having oval petiolar 
nectaries and elongate inflorescences. Some speci- 
mens of A. globulifera have oval petiolar nectaries 
as well, but the spherical inflorescence of A. globu- 
lifera is very easily distinguished from the elongate 
one of A. gentlei. 
Standley?s type specimen of A. gentlei was obvi- 
ously collected from the upper part of the crown 
of a large acacia such as the one in Figure 83~, and 
it is representative of the population whose distri- 
bution is plotted in Figure 44. The thorns on the 
type are slightly redder than most thorns in nature 
from British Honduras; they look more like those 
common on the north side of Lake Izabal. The 
seed pods on the type specimen are not mature but 
are nearly full length (19 to 20 cm). 
KEY COLLECTION LOCALITIES 
British Honduras: 
1. 
2. 
3. 
4. 
5. 
Type of A. gentlei: British Honduras. P. Gentle, no. 185. 
13.5 mi S Sta. Elena, Corozal Prov., to 43.8 mi SE Orange 
Walk (on road to Belice), Belice Prov., British Honduras 
(10-30 m). VI-30-1966, DHJ, nos. 582, 575 (occasional to 
common) fl., mp. 
17.9 to 36.1 mi SW Belice (on road to Cayo), Cay0 Prov., 
British Honduras (50-100 m). VII-3-1966, DHJ, nc 
(common). 
2 mi S Sta. Elena, Corozal Prov., British Honduras (sea 
level), VII-4-1966, DHJ, nc (3 plants). 
Gallon Jug, British Honduras. 111-5-1938, D. A. Lan- 
caster, no. 31 (US 2219935) fl., mp. 
6. Maskall, British Honduras. 1-31-1934, P. H. Gentle, no. 
1121 (US 1635002), mp. 
7. Little Cocquericot, Belize River. 111-27-1933, C. L. 
Lundell, no. 4406 (F 738238). 
8. Valentin, El Cay0 Prov. VI-1936, C. L. Lundell, no. 6328 
(F 894493). 
9. Camp 6, El Cay0 Prov. 111-9-1938, P. H. Gentle, no. 2311 
(F 945128). 
10. Rio Grande (southern B. H.) (80 m). 111-20-1933, W. A. 
Schipp, no. 1142 (F 669139) ft. 
Guatemala: 
11. El Zapotillo, 1 km E El Estor, Depto. Izabal, Guatemala 
(10 m). V-16-1965, DHJ, nos. 657, 659, 665, 667, 671, 
690 (common), fl., mp. 
12. 16.1 mi SW (Puente El Lobo) Los Amates to 29.4 mi 
NE (Santa Rosa) Los Amates on CA-9, Depto. Izabal, 
Guatemala (200 to 50 m) . VIII-9-1967, DHJ, nc (com- 
mon but scattered). 
13. 1.0 to 9.4 mi SE Mariscos (S side Lake Izabal), Depto. 
Izabal, Guatemala (100 m). V-13-1965, DHJ, nc (com- 
mon). 
14. Uaxactun, Depto. Peten, Guatemala, IV-21-1931, H. H. 
Bartlett, no. 12676 (US 1493227, US 1493228), fl., mp. 
15. Aguacaliente, Depto. Santa Rosa. 1-25-1908, W. A. Kel- 
lerman, no. 7710 (F 224606) (locality doubtful). 
16. La Libertad, Depto. Peten. IV-1-1933, C. L. Lundell, no. 
2379 (F 738067) fl. 
17. N of Sebol, between Sebol and Carrizal, Depto. Alta 
Verapaz (200-300 m) . IV-18-1942, J. A. Steyermark, no. 
45762 (P 1180368, F 1180419). 
18. S of Sayaxche, Laguna Petexbatum. (Laguna Mexico), 
Depto. Peten (50 m). V-3-1942, J. A. Steyermark, no. 
46221 (F 1180409). 
19. 2-4 mi W Livingston, Depto. Izabal. IV-16-1940, J. A. 
Steyermark, no. 39546 (F 1038624). 
20. Fallabon-Yaxha Road, Depto. Peten. 111-1933, C. L. Lun- 
dell, no. 2195 (F 739642). 
21. 2.8 mi W Puerto Matias de Galvez (on road to Escobas) , 
Depto. Izabal (10 m) , VIII-10-1967, DHJ, nc. 
NATURAL HISTORY 
While A. gentlei is clearly a plant that was 
originally associated with wetter forest than extreme 
dry-land species such as A. cornigera, A. hindsii, and 
A. collinsii, it has survived very well in the face 
of human disturbance, in contrast to A. allenii, 
A. melanoceras, and A. ruddiae. Wherever the for- 
est is cut with at least a 15 to 20 year fallow rotation 
period, A. gentlei becomes very common, and has 
even become a roadside plant in the wetter parts of 
British Honduras. It is very common in fallow 
Indian milpas in the Izabal region of Guatemala. 
In Guatemala, it is commonly encountered in 
swamps and river-edge vegetation in forest undis- 
turbed by man for a long time. 
NUMBER 13 
97 
FIGURE BS.-Acacia gentlei (Guatemala, site 12): A, crown of 5.m-tall emergent adult; B, type B 
thorns on main trunk of adult. 
The obvious success of A. gentlei in moving from 
natural disturbance sites is probably due to two 
factors. First, the seeds are surrounded by a white 
and sweet pulp that is readily picked from the 
semidehiscent pods by birds such as kiskadees 
(Ptangus sulpheratus) and saltators (Saltator spp.). 
These, and other birds of second-growth vegetation, 
are probably responsible for frequent dispersal of 
seeds into newly cleared sites. Second, as will be 
discussed later, A. gentlei is quickly occupied and 
heavily patrolled by obligate acacia-ants. 
While A. gerttlei adults in reproductive condition 
are only occasionally encountered in primary forest 
(but slightly more often than are A. allenii or A. 
melanoceras), their seeds are probably moderately 
common. A. gentlei seeds are very hard, like those 
of other dry-land acacias, and remain viable at 
least 5 years and probably much longer. The pres- 
ence of A. gentlei seeds in the forest litter is sug- 
gested as the effect of the hurricane that leveled 
large expanses of wet forest to the southwest of 
Belice, British Honduras (site 3), in the late 1950s. 
In the woody regeneration that followed, by 1966 A. 
gentlei was scattered over the area at a density of 
about 10 reproducing adults per acre. There had 
clearly been enough seed and seedlings present to 
do this, despite the fact that the average density 
of A. gentlei in this type of forest, when undis- 
turbed, is about 0.1 tree per acre. 
Reproductive details of A. gentlei have not been 
recorded, but it sprouts very well from cut stumps. 
Seedlings grow well in very dense shade but appear 
to require a slight break in the general canopy be- 
fore they can grow large enough to reproduce. 
Seed crops are large and therefore comparable in 
size with the other dry-land acacias when fully 
insolated and mature. Adults are difficult to age 
because they show the least clear growth rings of 
any of the swollen-thorn acacias. Based on size, they 
probably live up to about 20 or 25 years and rarely 
have a trunk diameter in excess of 15 cm. 
Trees of A. gentlei are usually very heavily oc- 
98 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
h 
fi 
B 
- 150 
FIGURE 84.-Acacia gentlei: A, type A thorns from a lateral branch from the crown of a mature 
tree (Guatemala, site 11); B, type A thorns from the crown of a mature tree (British Honduras, 
site 2); c, type B thorns on the base of a lateral branch (British Honduras, site 2). 
X 
t-r 
t3 
2 
w 
cl 
2 
0: 
0 
X 
t-r 
130 
140 i 
a* 
a 
I I I I 
0.5 1.0 1.5 2.0 
TOTAL THORN VOLUME (cc) 
FIGURE 85.-Distribution of 15.4 cc of thorn volume among 
14 very large type A thorns from one A. gentlei (Guatemala, 
site 13). 
cupied by P. ferruginea or one of the two unde- 
scribed species of obligate acacia-ants found within 
its range. The young single-queen colonies appear 
to have little trouble becoming established- 
though in roadside areas of British Honduras, an 
unusually aggressive black morph of Pseudomyrmex 
gracilis often takes over young acacias. While the 
dark green foliage of A. gentlei closely resembles 
that of A. chiupensis in texture, color, and taste, A. 
gentlei lacks the herbivore damage so characteristic 
of A. chiupensis. This lack of herbivore damage is 
apparently because the ant colonies grow very large 
in A. gentlei; a single-queen colony of 15,000 work- 
ers is not uncommon in a 3-m-tall tree. 
Acacia gentlei shows that thin thorns (Figure 83~, 
84) alone do not lead to poor growth of an ant 
colony. As can be seen in Figure 85, even the very 
longest thorn has a very small thorn volume. Many 
thorns are only 1.5 to 3 mm inside diameter, barely 
allowing the worker ants to pass through. As a 
consequence there is a very small amount of brood 
per thorn, but the very large number of thorns per 
tree probably means that the total thorn volume 
available to the colony is about the same in equal- 
NUMBER 13 
FIGURE 8G.-Football-sized carton nests of Azteca ants in the 
crowns of 12-m-tall A. gentlei (Guatemala, site 12). 
sized trees of A. gentlei and other swollen-thorn 
acacias. The other thin-thorned species, A. cookii, 
does not appear to have an excessive number of 
swollen thorns, but the slightly larger diameter of 
A. cookii thorns may compensate for the lower 
number as contrasted with A. gentlei. 
Large senescent trees of A. gentlei growing in 
undisturbed (by humans) habitats are occasionally 
heavily tenanted by an unidentified species of ant 
in the genus Azteca. This ant genus, better known 
for its species occupying Cecropia and Cordia, is 
arboreal, predaceous on insects, and very aggressive 
to animals that shake the foliage near the nest. In 
A. gentlei the ants live both in the thorns previously 
opened up by the obligate acacia-ants and in hang- 
ing carton nests (Figure 86). While they do not 
systematically patrol the acacia leaves, they do 
forage on them, which probably results in the 
senescent tree living longer than normal. The obli- 
gate acacia-ant colony generally does very poorly in 
a senescent acacia since the ants have to have 
Beltian bodies and nectar in large quantities to 
maintain a colony large enough to effectively patrol 
the acacia. 
11. Acacia collinsii Safford 
Acacia collinsii Safford, 1910:677. 
Acacia yucatanensis Schenck, 1913:361. 
Acacia neisonii Safford, 1914:3G3. 
Acacia costaricensis Schenck, 1913:361. 
Acacia penonornenscs Safford, 1914:363. 
Acacia panamensis Schenck, 1913:362. 
Myrmecodendron collinsii (Safford) Britton and Rose, 1928: 
Myrmecodendron costaricense (Schenck) Britton and Rose, 
92. 
1928:95. 
Acacia collinsii is easily distinguished from all 
other swollen-thorn acacias by its unique broadly 
dome-shaped petiolar nectaries. It has elongated 
inflorescences and fully dehiscent pods (which it 
has in common only with A. hindsii). A. hindsii 
and A. collinsii may be distinguished at almost all 
localities by their thorns, since type A thorns from 
mature A. hindsii are always strongly flattened 
while type A thorns of A. collinsii are round in 
cross section. Of all the swollen-thorn acacias, A. 
collinsii and A. cornigera have the widest geo- 
graphical and ecological distributions (Figure 87) . 
Safford?s tjpe specimen of A. collinsii offers a 
classic example of the inadequacy of type specimens 
at representing an entire population. While Saf- 
ford?s type specimen is representative of the popu- 
lation of A. collinsii at site 1, the Chicoasen 
population of A. collinsii is quite different from A. 
collinsii over the remainder of its range. The type 
specimen appears to have been taken from a large 
adult tree in a partially shaded riparian site. Its 
leaves are as long as any found on A. collinsii (up 
to 16 cm long with the 10 pinnae spaced up to 3 
cm apart). The leaves have the unusually large 
number of 5 to 6 petiolar nectaries with the distal 
ones being as far as 2 mm apart (this makes one 
suspect a bit of introgression with A. globulifera, 
which also occurs at this site). The sexual branch- 
lets are up to 8 cm long (normal maximum) and 
the peduncles of the mature inflorescences range up 
to the exceptional length of 33 mm. The peduncles 
are also nearly 2 mm thick, while on most A. 
collinsii they are 1 mm or less thick. The thin type 
A thorns are round in cross section but twisted 
around the branch; this is characteristic of the A. 
collinsii population only in Chiapas, Mexico. 
Schenck?s type of A. yucatanensis (site 2) is repre- 
sentative of most of the population of A. collinsii 
in drier habitats, and especially the Yucatan portion 
of the population. The small leaves are specially 
characteristic of A. collinsii. His type of A. costari- 
censis is not available (apparently destroyed in 
Berlin) but a paratype from Costa Rica (site 66) is 
clearly drawn from that portion of the A. collinsii 
population at dry intermediate elevations in Costa 
Rica. If it were useful to designate a lectotype, this 
20. 
8? * 
0 
would be the appropriate specimen. Schenck?s de- 
scription of A. costaricensis applies unmistakably to 
A. collinsii and is very different from the other 
Costa Rican swollen-thorn acacias (A. cornigera, A. 
allenii, A. ruddiae). Britton and Rose (1928) refer 
to the type locality of A. costaricensis as ?Lagarto, 
Costa Rica? but do not give the source of these data; 
there is a ?Lagarto? in Puntarenas Province and A. 
collinsii is abundant there (sites 67 and 75). 
Acacia panamensis is clearly based on two species 
of plants. The description of the thorn and leaf- 
bearing branch clearly applies to A. collinsii (not 
to A. melanoceras, the only other central Pana- 
manian species of swollen-thorn acacia). Schenck 
(1913) notes that he is not sure that the inflores- 
cences belong to the sterile branch; the globular 
inflorescences could be from A. melanoceras (un- 
likely owing to their very thin peduncle) or, more 
likely, from one of the other nonant-acacias that 
grow commonly in the drier parts of Panama along 
with A. collinsii. A final complication is that the 
exact locality is given as ?Aljahuela? and I cannot 
SUMBER 13 
101 
locate such a site in Panama. This site most likely 
represents Alajuela in Alajuela Province, Costa 
Rica, which was a favorite collection site for Ger- 
man collectors around the turn of the century. A. 
collinsii is the only swollen-thorn acacia in the 
Alajuela region (sites 68, 69, 74). 
Safford?s type of A. nelsonii (site 3) is representa- 
tive of the A. collinsii population in the area north 
of the Isthmus of Tehuantepec, Mexico, but differs 
from more southerly populations in having large 
pinnules (up to 11 mm long and 3 mm wide) and 
somewhat flattened thorns (2 to 3 times as wide as 
deep in cross section). Flattened thorns are, how- 
ever, even better developed in the A. collinsii popu- 
lation from central Costa Rica to Panama. The 
inflorescences of the type of A. nelsonii are normal, 
but the thorns on the same branch have been 
weathered gray rather than being the usual beige 
color of younger thorns from western Mexico. 
Safforcl?s type of A. penonomensis (site 76) differs 
from the remainder of the A. collinsii population 
on the dry Pacific side of Panama in that its type A 
thorns are not quite as flattened as are those on 
many A. collinsii at sites 82 to 84. The seed pods 
on the type of A. penonomensis are representative 
of those found in the drier parts of the range of A. 
collinsii; these pods have stiff woody valves that 
only curl a slight amount during dehiscion. 
KEY COLLECTION LOCALITIES 
Mexico: 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
Type of A. collinsii: (between San Fernandino and) 
Chicoasen, Chiapas, Mexico (320 m). 1-13-1907, G. N. 
Collins and C. B. Doyle, no. 180 (US 692159). 
Type of A. yucatanensis: Yucatan, Mexico. 1895, G. F. 
Gaumer, no. 353 (US 571618). 
Type of A. nelsonii: Acapulco, Guererro, Mexico (sea 
level). IV-30-1903, E. W. Nelson, no. 7024 (US 399366). 
3.6 mi NW, border station between Mexico and Guate- 
mala (La Mesilla) to 8.4 mi S La Trinitaria on Hgy. 
190, Chiapas (680-720-600-1060 m) . VIII-7-1966, DHJ, 
nos. 441, 443; VIII-17-1967, DHJ, no. 1713 (very com- 
mon to occasional) . 
11.8 mi E (near turnoff to Villa Hermosa) Chiapa de 
Corzo to 13.8 mi SW Tuxtla Gutierrez on Hgy. 190, 
Chiapas (10904.50-1090 m). VIII-17-1967, DHJ, nos. 
424, 43.5 (common to occasional). 
13.8 mi SW, Tuxtla Gutierrez to 17.9 mi SW Las Cruces 
(turnoff to Arriaga) on Hgy. 190 (1090-530-820 m), 
Chiapas. VIII-18-1967, DHJ, nc (common to exceed- 
ingly rare). 
1-2 mi N Gabriel Zamora on Hgy. 37, Michoacan (820 
m) . VI-10-20-1968, DHJ, nc (very common). 
8. 
9. 
10. 
11. 
12. 
13. 
14. 
15. 
16. 
17. 
18. 
19. 
20. 
21. 
13.4 to 23.5 mi E Zanatepec on Hgy. 190, Oaxaca (50 
m) . VIII-18-1968, DHJ, nc (very rare to common), fl. 
42.1 mi E Zanatepec on Hgy. 190, Oaxaca (60 m) . VIII- 
18-1967, DHJ, nc (1 plant). 
3 mi N Soyalo on Tuxtla Guticrrez.Villa Hermosa Hgy., 
Chiapas (- 1100 m) . VII-10-1966, DHJ, nc (common), 
fl. 
10 to 14 W Tchuantepec (Hgy. 190), Oaxaca (100 m). 
1-10-1965, DHJ, nos. 1845, 1950. IX-3-1965, DHJ, nos. 
1779, 1780, 1783 (common). 
Pucrto Escondido to 3.5 mi N on road to Oaxaca, 
Oaxaca (10-120 m) . VI-19-1967, DHJ, nc (common to 
very rare) . 
Puerto Escondido to 18.4 mi RTV (Hgy. ZOO), Oaxaca 
(10-80 m). VI-19-1967, DHJ, nc (common to very 
rare). 
Hills in and around Acapulco, Guerrero (0-100 m). 
1?1-18-1967, DHJ, nc (occasional) . 
Acapulco to 1.3 mi SE Papanoa, Guerrero (0-30 m). 
VI-16-1967, DHJ, nc (common on dry hillsides, ab- 
sent from swamps). 
48 mi NE Isla Aguada (across from Puerto Real, Cam- 
peche) to Merida (Yucatan), on Hgy. 180 (0-30 m), 
VI-22-1966, DHJ, nos. 428, 431, 433, 438, 606 (occasional 
to very abundant.) 
Ruinas Edzna north to Hgy. 180 (east of Campeche), 
Campechc (15-40 m). VI-25-1966, DHJ, nc (rare to 
occasional). 
Becal south to Hgy. 180, Campcche (1540 m) . VI-26- 
1967, DHJ, nc (rare to occasional). 
Merida to 14.2 mi E (Hgv. 180), Yucatan (10-20 m). 
VI-28-1966, DHJ, no. 455 (rare). 
Mum (Yucatan) to Felipe Carillo Puerto (road to Cd. 
Chetumal) , Quintana Roo (10-20 m). VI-29-1966, DHJ, 
nc (very rare). 
Felipe Carillo Puerto to Cd. Chetumal, Quintana Roo 
(20-0 m). VI-30-1966, DHJ, no. 453 (very rare to oc- 
casional) . 
22. 9.6 mi E San Marcos on Hgy. 200, Oaxaca (50 m). VI- 
18-1967, DHJ, nc (1 plant). 
23. La Ventosa, 7 km E Salina Cruz, Oaxaca (sea level). 
VII-3-1959, K.M. King, no. 1302 (US 2362102), fl. 
24. Petatlan, Montes de Oca, Guerrero. VI-17-1937, G. B. 
Hinton, no. 10333 (US 1805541, F 959109). 
25. Tuxpena, Campcche. 11-2-1932, C. L. Lundell, no. 1279 
(US 1494445), mp. 
26. Sudzal, Yucatan. IV-21-1956, 0. G. Enriquez, no. 564 
(US 2448079). 
27. Chichen Itza, Yucatan. V-30-1929, J. Bequaert, no. 2 
(US 1477467), ff., mp. 
28. Isla Cozumel, Quintana Roo. XII-5-1966, V. E. Rudd, 
no. 2036 (US 2518023). 
29. San Vicente, Chiapas. IV-16-1904, E. A. Goldman, no. 
862 (US 470663), mp. 
30. Berriozabal, on road to San Fernando, Chiapas (950 m). 
XII-18-1949, F. Aliranda, no. 5847 (US 2367591), mp. 
31. Chorreadcro de Tuxtla, 5.6 mi E Chiapa de Corzo (800 
ni). 11-16-1965, D. E. Breedlove, no. 9018 (F 1627187), 
fl. 
102 SMITHSOKIAN COSTRIBUTIONS TO BOTANY 
32. Chichen Itza, Yucatan. V-30-1929, J. Bequaert, no. 2 
(F 710846). 
33. Izamal, Yucatan. 11-21-1906, 
J. M. Gaurner, no. 379 (F 
189844), fl. 
34. Salina Cruz, Oaxaca (sea level). IV-28-1910, Orcutt, no. 
3451 (F 280154), mp. 
British Honduras: 
35. 23.6 mi S Orange Walk to Belize, Belize Prov. (10-0 m). 
VII-2-1966, DHJ, nc (rare to common). 
36. 10.1 to 34.4 mi SIV Belize on road to El Cayo, Belize 
Prov. (30-100 m). VII-3-1966, DHJ, nc (rare to occa- 
sional). 
37. Duck Rum, Pine Ridge, El Cay0 Prov. 11-17-1931, 
H. H. Rartlett, no. 11546 (US 1493376, US 1493375), 
mP* 
Guatemala: 
38. 2.6 mi S\V Puente Rio Hondo on Hgy. CA-9 (near 
turnoff to Zacapa), to 1.8 mi NE Pucnte El Lob0 on 
Hgy. CA-9, Depto. Zacapa (200-100 m) . VIII-9-1967, 
UHJ, nos. 1794, 1797 (very common to occasional). 
39. ?District of Peten.? IV-8-1933, C. L. Lundelf, no. 2614 
(F 738050). 
Honduras: 
40. El Espino (Sicaragua) to 0.4 mi W on Honduras- 
Siraragua border on Interamerican Hgy., Depto. Cho- 
luteca (880-930 m). VII-29-1967, DHJ, nc (4 plants). 
41. 21.6 mi I$? San Marcos de Colon on Interamerican Hgy., 
to Choluteca, Depto. Choluteca (450-10 m). VIII-31- 
1965, DHJ, 110. 1788; V-11-1965, DHJ, no. 1807; VII- 
29-1967, DHJ, nc (very common). 
42. Choluteca (Depto. Choluteca) to Jicaro Galan (Depto. 
L?alle) on Interamerican Hgy. (10-40 m) . VII-29-1967, 
DHJ, no. 1803 (common to rare). 
43. Jicaro Galan to 3.8 mi NE (Hyg. 1) , Depto. Valle (40- 
80 m). VII-29-1967, DHJ, nc (rare). 
44. Comayagua to 3 mi S on Hgy. 1, Depto. Comayagua 
(550 m). VII-31-1967, DHJ, nc (rare to common), fl. 
43, Jicaro Galan to El Amatillo (Honduras-El Salvador 
border) on Interamerican Hgy. (40-200 m) . 1-15-1965, 
DHJ, no. 1841; V-12-1965, DHJ, nc (occasional to rare), 
fl. 
46. Juticalpa, Depto. Olancho. 111-5-16-1949, P. C. Stand- 
ley, no. 17580 (F 1378910), mp. 
47. Rio Guayainbre, Valle Jamastran, Depto. El Paraiso 
(350 in). 111-13-1956, A. Molina, no, 7364 (F 1599625). 
48. Danli, Dcpto. El Paraiso (700-800 m) . 11-11-23-1949, 
P. C. Standley, no. 16506 (F 1378924). 
El Salvador: 
49. El Amatillo (Honduras-El Salvador border) to 7.1 mi 
IV (on road to La Union), Depto. La Union (200-190 
in). V-12-1965, DHJ, no. 1801 (rare), fl. 
Xicaragua: 
50. Penas Blancas (Depto. Rivas) to Jinotepe (Depto. 
Carazo) (Hgy. 2) (50-500 m) . V-9-1965, DHJ, no. 1822; 
51. 
52. 
53. 
54. 
56. 
57. 
58. 
59. 
60. 
61. 
62. 
63. 
64. 
65. 
1-16-1965, DHJ, nos. 1830, 1932; VII-26-1967, DHJ, nc 
(occasional to very common). 
La Virgin to San Juan del Sur (Hgy. 16), Rivas Prov. 
(50-0 m). V-10-1965, DHJ, nos. 1814, 1815, 1819, 1921 
(common), fl, mp. 
Masachapa to 14.7 mi NE (Hgy. 8), Depto. Managua 
(0-480 m) . VII-26-1967, DHJ, nc (common to rare) . 
15.2 mi SW Interamerican Hgy. (Depto. Managua) on 
road to Chinandega (Hgy. 12) to 5 mi SE Leon (Depto. 
Leon), (30-450 in). VII-27-1967, DHJ, nc (occasional 
to rare). 
Puerto Somoza to 6.4 mi SE (junction with Hgy. 12), 
Depto. Leon (0-30 mi S Tipitapa on Hgy. 1, Depto. 
Managua (50 m). V-10-1965, DHJ, no. 1812; VII-28- 
1967, DHJ, nc (1 unoccupied plant). 
4.2 mi N Tipitapa on Hgy. 1, Managua (75 m). VII- 
28-1967, DHJ, nc (1 unoccupied plant), fl. 
San Benito 
(N of Tipitapa on Hgy. 1) to 16.7 NE 
(Hgy. y) (100 m). VII-28-1967, DHJ, nc (extremely 
common). 
San Bcnito (Depto. Managua) to Sebaco (Depto. Mata- 
galpa) on Hgy. 1 (100-420-350-500 m). VII-28-1967, 
DHJ, nc (very common to very rare at 420 m to com- 
mon) , fl. 
Sebaco to 1.6 mi N Matagalpa (Hgy. 3), Depto. Mata- 
galpa (500-730 m) . VII-28-1967, DHJ, nc (common to 
occasional), fl. 
Sebaco (Depto. Matagalpa) to Esteli (Depto. Esteli) on 
Hgy. 1 (500-1040-850 m). VII-28-1967, DHJ, nc (corn. 
mon to very rare at 1040 m to common). 
Esteli (Depto. Esteli) to El Espino on the Honduras- 
Nicaragua border (Depto. Madriz) 011 Hgy. 1 (850-530- 
880 m) . 1-15-1965, DHJ, no. 1825; V-11-1965, DHJ, no. 
1808, 1810; VII-29-1967, DHJ, nc (common). 
Laguna de Masaya, Depto. Masaya (300 m) . VII-6-1923, 
W. R. Maxon, no. 7739 (US 1181452), 8. 
Granada, Depto. Granada (250 m) . VII-2-1923, W. R. 
Alnxon et al., no. 7397 (US 1181308) ; IV-21-1916, J. D. 
Mandragou (US 764653 and CS 764655), fl; and II-12- 
1903, C. F. Baker, no. 2546 (US 862954), fl. 
La Libertad, Depto. Chontales, V-29-VI-1-1947, P. C. 
Standley, no. 8826 (F 1288080). 
Braggman?s Bluff (Puerto Cabezas), Yulu Village, Depto. 
Zelaya (100 m) , (F 585006). 
Costa Rica: 
66. Potential lectotjpe of A. costaricensis: Alajuela, Ala- 
juela, Costa Rica (920 m). 1896, J. D. Smith, no. 6488 
(cf. Schenck, 1913). 
67. 18.4 mi to 21.4 mi E Palmar Sorte, Puntarenas Prov. 
(90 m) . V-23-1963, DHJ, no. 1852; VI-26-1967, VIII- 
23-1965, V-15-1965, 111-21-1966, VII-21-1963, DHJ, nc 
(occasional), in fl. 
68. Escazu to 3.6 in 11? Santaigo de Puriscal (Hgy. 7), Ala- 
juela Prov. (1000-600 m). VII-17-1967, 11-15-1965, 
DHJ, no. 1862; 111-2-1965 (fl), DHJ, nc (occasional to 
rare). 
69. Along the electric railroad from Antonio to Orotina, 
Alajuela Prov. (900 to 50 in). VIII-1963, DHJ, nc (oc- 
casional). 
SUMBER 19 
103 
70. 
71. 
72. 
i3. 
74. 
75. 
15.0 mi NTV turnoff of Interamerican Hgy. to Punta- 
renas, to Peiias Blancas (Border of Costa Rica and 
Nicaragua), Guanacaste Prov. (10-100 m) . January- 
September, periodically in 1963-1967, DH], nos. 1757, 
1759, 1762, 1763, 1826 (rare to exceedingly common), 
fl. January to April. 
Liberia to Playa COCO (Hgy. 21), Guanacaste Prov. 
(100-0 m) , January-September, periodically in 1963- 
1967, DHJ, no. 1858 (occasional to very common), fl. 
January to April. 
Hda. Palo Verde (about 18 mi S. Bagaces), Guanacaste 
Prov. (sea level). IV-2-1966, DHJ, nc (common), fl. 
Canas to 6 mi NE Guanacaste Prov. (80-300 m) . IV-15- 
1965, DHJ, nc (occasional), fl. 
Alajuela Prov. (920 m) . 111-1896, J. D. Smith, no. 6488 
(US 943274), fl. 
Lagarto, Puntarenas Prov. (sea level). XII-1891, Ton- 
duz, no. 4810 (US 943276). 
Panama: 
76. 
77. 
78. 
79. 
80. 
81. 
82. 
83. 
84. 
85. 
86. 
87. 
88. 
81). 
90. 
Type of A. penonomensis: Penome, Cocle, Panama (15- 
300 m). 11-23-1908 to 111-22-1908, R. S. Williams, no. 
113. 
4.8 mi NE Tucuman Airport on Panama-Chepo Hgy., 
Panama Prov. (20 m). VII-1-1967, DHJ, no. 1704 (very 
rare), gp. 
Vicinity of Madden Dam, Madden Lake (50 m). VII-3- 
1967, DHJ, no. 1653 (common), gp, fl. 
Miraflores Locks to Paraiso and Corozal area, Canal 
Zone, VII-8-1967, DH] and Wheeler (1942), nc (rare). 
Arraijuan to 4.1 mi E on Interamerican Hgy., Canal 
Zone. VII-9-1967, DHJ, nc (occasional). 
3.2 mi SIV La Chorrera, Panama Prov. (100 m) , VII-I- 
1967, DHJ, nc (rare). 
5 mi N. Bejuco (Panama Prov.) to Santiago (Veraguas 
Prov.) along the Interamerican Hgy. (10-50 m) . VII-1- 
1967, DHJ, nos. 1698, 1700, 1702 (rare to occasional). 
Interamerican Hgy. (Herrara Prov.) to 3.1 mi S Pedasi 
(Los Santos Prov.) on Azuero Peninsula (5-25 m) . VI- 
30-1967, DHJ, no. 1695 (occasional to common). 
Santiago (Veraguas Prov.) to David (Chiriqui Prov.) 
along Interamerican Hgy. (50-200 m). VII-11-1967, 
DHJ, nc (very rare to occasional). 
2.1 mi SE Sona to Santiago, Veraguas Prov. (50 m). 
1'1-29-1967, DHJ, nc (rare). 
Remedios (Chiriqui Prov.) to 19.9 mi StV Jorones 
(Veraguas Prov.) on Sona-Remedios Road. VI-29-1967, 
DHJ, no. 1666 (occasional). 
David to Pedregal, Chiriqui Prov. (0-30 m). VI-29- 
1967, DHJ, nc (rare). 
David to 8.5 mi S Boquete, Chiriqui Prov. (30-620 m) . 
VI-28-1967, DHJ, nc (rare to occasional), gp. 
David to 2.7 mi NIV on Interamerican Hgv., Chiriqui 
Prov. (90 m) , VI-28-1967, DHJ, nc (rare to occasional). 
Along Quebrada Ocho Paso, Rio Tonosi, Tonosi, Los 
Santos Prov. (20 m). 11-26-1963, W. L. Stern et nl., no. 
1831 (US 2490276), fl., mp. 
Colombia:* 
* Colombian localities are not included in Figure 87. 
91. 
92. 
93. 
94. 
95. 
96. 
97. 
98. 
99. 
100. 
101. 
Str-ams of Usiacuri and Pital, in mountains SIV of 
Baranquilla, Depto. Atlantico. V-26-1928, E. Nienzeyer 
(US 1496168). 
Malambo to Sabanagrande, Depto. Atlantico (60 m) . 
1-1936, A. Dugand, nos. 117, 404 (US 1619541). 
Megua, Depto. .4tlantico (50 m) . 1-13-1941, A. Dugand 
and Jaramillo, no. 2796 (US 1799137). 
Usiacuri, Baranquilla, Depto. Atlantico (250 m). VII- 
1927, B. Elias, no. 245 (US 1342935), fl. 
Arroyo Jubilado, Depto. Atlantico (50-60 m) . XII-24- 
1933, A. Dugand, no. Ilia, 40u (US 1573571). 
Cartagena, Depto. Bolivar. 1919, B. Heriberto, no. 39 
(US 1041093). 
Isla de Providencia (Caribbean Sea). IV-29 to V-9-1948, 
G. R. Proctor, no. 3430 (US 1979268), fl., nip.; I-24- 
1967, S. Anderson; VIII-23 to VIII-29-1970, DHJ, nc 
(very common). 
Providence Island (Caribbean Sea). VII-19-26-1966, 
.I. H. Torres R., no. 238 (COL 112189). 
,Tuanmina, Baranquilla Dist., Depto. Atlantico (20 m). 
VII-5-1963, H. S. McKee, no. 10433 (COL 98405). 
Valencia de Jesus, Valledupar, Magdalena (30 m). II- 
14-1948, R. Roinero-Castaneda, no. 793 (COL 44486). 
Arroyo de Megua, Depto. Atlantico (50 m) . 1-13-1941, 
A. Dugand and R. Jni-amillo, no. 2769 (COL 05500). 
NATURAL HISTORY 
Over its wide geographical and ecological range 
(Figure 87), A. collinsii is found in habitats ranging 
from moderately wet to very dry, but the majority 
of the population is found in habitats that appear 
generally too dry for other swollen-thorn acacias 
(except A. sphaeroccpJiala with its peculiarly local 
distribution). These habitats ale often too dry for 
agriculture on hillsides (8 month dry season), and 
as a result, the majoiity of the contemporary popu- 
lation of A. colliiisii lives in shrubby vegetation 
that is overgrazed by cattle and goats (Figure 88). 
In the moister paits of its habitat, the natural 
vegetation has been almost entirely destroyed to 
plant cornfields, but where this is done with a 5 to 
25 year fallow period (e.g., sites 67 and 88), A. 
collinsii is a common part of the regeneration. In 
the very dry parts of its range, such as the east side 
of the Yucatan Peninsula and the Tehuantepec 
area, A. collinsii probably grows in vegetation simi- 
lar to that occupied before the advent of European 
agriculture except for graiing by goats. 
TVherever A. collinsii and othei species of swollen- 
thorn acacias occur in the same general area, A. 
collinsii is always in the driest subhabitat (and thus 
mixed in with several nonant-acacias). This is par- 
ticularly striking at a place such as site 15 where it 
104 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 88.-Typical pasture habitat of A. collinsii (Nicaragua, site 60). 
is common on the dry ridges that extend into the 
swampy coastal flatland occupied by A. cornigera. 
Also, in the very dry lowlands in northeastern 
Guatemala (site 38), A. collinsii occupies the dry 
rocky pastures, while A. hindsii, A. gentlci, and 
(rarely) A. cornigera occupy the swamps and stream 
bottoms and banks. Since much of the Yucatan 
Peninsula lacks surface watercourses, it would lack 
swollen-thorn acacias in most of the eastern drier 
area if it were not for A. collinsii. While A. collinsii 
is apparently sympatric with either A. cornigern 
(e.g., site 17) or A. globziliferu (e.g., site 16), the 
latter two species are always in a local wet site, 
such as a lagoon running along the beach or the 
site of a cenote or well. 
Acacia collinsii appears rarely to be moved about 
by contemporary human activities, and the geo- 
graphic distribution recorded in Figure 87 is prob- 
ably little changed since Europeans began disturb- 
ing Central American vegetation. Like A. cornigera 
and A. hindsii, however, A. collinsii is probably 
much more common now than in pre-Colombian 
times, since it grows very well in the dry and 
shrubby pastures that have replaced much of low- 
land Central America's deciduous forests. The fail- 
ure of humans to move the seeds of A. collinsii is 
probably due in part to the fact that the pods 
dehisce so that the edible pulp is either taken by a 
bird or drops to the ground (in contrast to the 
indehiscent pods of A. cotnigera that can be easily 
harvested, and carried about). Also, A. collinsiz lives 
in habitats that are so dry that earth-moving equip- 
ment is less likely to pick up mud with seeds in it 
than is the case for either A. cornigein or A. hindsii. 
Finally, and perhaps most important, A. collinsii 
appears to occupy L irtually all the suitable habitats 
in the dry lowlands on both coasts of Central 
America. The conspicuous exceptions to this state- 
ment are the dry northern portions of the east and 
west coasts of Mexico, occupied by A. cornigera and 
A. sphaerocephala, and by A. hindsii, respectively; 
human-movement patterns in these pal ts of hlexico 
are not at ,all favorable to contemporary transport 
of A. collinsii further north than at present. 
The failure of A. collinsii to spread throughout 
the di y lowlands of Colombia is the most perplexing 
aspect of its distribution. There are two possibili- 
ties, neither of which can be discussed fully with 
the data at hand. It may be a relatively recent 
introduction from Panama or further north along 
the coast; old coastal ports such as Acapulco, La 
Libertad, Puntarenas, and Panama City are well 
within the range of A. collinsii. On the other hand, 
the twin dry seasons in Colombia may be inimical 
to the development of A. collinsii. Jesus Idrobo 
(personal communication) reports that it is most 
commonly found in swamps in Colombia, a habitat 
rarely occupied by A. collinsii in Central America. 
SUMBER 13 105 
FIGLIRE 89.--Acncia collinsii: A, left, unoccupied sucker regeneration and on the right, occupied 
sucker-shoot regeneration of the same age (Costa Rica, site 70); B, occupied lateral branch end; 
contrast with Figure c (Costa Rica, site 67); c, unoccupied lateial branch end growing near 
Figure B. 
IVhile swollen-thorn acacias may reach immedi- 
ately offshore islands (eg., occupied A. collinsii on 
Isla Co/umel, site 28), there is only one case where 
the ant >< acacia interaction appears to have be- 
come n:iturall~~ established on an island far from 
the Central ;\merican coast. A. colliiisii is a com- 
iiion member 01 the flora of Isla Providencia (sites 
9i antl 98) where it is occupied by Pseudoiugmzex 
ferruginen. This island is about 100 miles off the 
Caribbean coast of Nicaragua, and even though the 
acacia is completely wild, there is no guarantee that 
it was not introduced bl, postColombian human 
activities. Isla Providencia has been in contact with 
Europeans for at least 200 years. Even if the acacia 
was recently introduced, it is most likely that the 
ants got there of their own accord; even the most 
avid gardener is unlikely to bring an acacia with an 
ant colony on a boat from the mainland to Isla 
Providencia. The Isla Providencia population of 
A. collinsii is morphologically representative of the 
mainland population, with the exception that the 
aeed pods are the smallest 1 have seen. Sot only are 
they very hea\.ily ~voody (like those on the dry 
Pacific side of Panama) but range from 2 to 3 cm 
long and 4 to 8 inm thick. Almost all of the vegeta- 
tion on Isla Pi-ovidencia has been cut many times 
and is repeatedly disturbed by livestock antl cultiva- 
tion; however, even in equally disturbed sites on the 
mainland, the population density of adult A. col- 
liiisii is higher on this island than I have ever 
encountered in mainland sites of comparable land 
use. This high abundance is probably attributable 
106 
SMITHSOSIAS COSTRIBLTIOSS TO BOTAXY 
FILVRE SO.--Accccic~ collinsii (Panama, site 83) : A, crown of adult with clumps of type A thoins 
caused by an unknown disease; B, close-up of the thorn cluster from the crown. 
FIGURE 91.--Acacia co11imii: A, two seedlings (compound leaces) with their first pair of leaves 
(Costa Rica, site 70); B, sucker spiout from burned stump (Costa Rica, site 70); c, unoccupied 
2-4-year-old seedlings (Panama, site 88). 
NUMBER 13 
107 
to several factors: (1) Based on two small seed 
samples taken in August 1969, the bruchids that 
kill a high percentage of the mainland seeds have 
not arrived on the island; each of the seeds pro- 
duced is therefore still alive when dispersed. (2) 
Based on sweep samples taken during the rainy sea- 
son (late August 1969) the herbivorous insect density 
on the island is far lower than found in any main- 
land lowland habitat during the rainy season; this 
means that the primary environmental challenge 
that unoccupied seedlings face is competition from 
other plants, (3) Since the number of plant species 
is very low on the island, most of the usual com- 
petitors with A. collinsii are absent from the 
habitat; in particular, vines are missing. The ants 
appeared to be as aggressive as their mainland 
counterparts and attacked vegetation contacting the 
A. collinsii crown as usual. The only major herbi- 
vore on A. collinsii that appears to have arrived on 
Isla Providencia is the noctuid moth (Cocina had- 
enoides) whose larva can protect itself against the 
ants and eats the shoot tips of occupied swollen- 
thorn acacias (Janzen, 196713). On the mainland, 
this larva invades the acacia often enough to cause 
occasional multiple branching in the shrub's de- 
velopment. On Isla Providencia, its damage was 
intense, causing nearly all the acacias to take on a 
shrubby life form. 
'M'hile A. collinsii lives in a drier habitat than A. 
cornigern, it appears to have a dependence on 
obligate acacia-ants as great as that of A. cornigern. 
A brief test of this was conducted with the hilltop 
portion of the A. collinsii population two miles 
southeast of Canas, Guanacaste Province, Costa 
Kica (site 70). While there are three species of 
obligate acacia-ants in the area (P. ferruginca, P. 
belti, P. nigrocincta), only those plants occupied 
by P. belti were used in tlie experiment. The hill- 
top site was covered with regenerating pasture 
shrubs (Baziliinin, Ci,oton, Cassia, Lithen, Acacia, 
etc.) and about 40 adults of A. collinsii to the acre. 
On February 16, 1967, 52 plants of A. collinsii in 
the 2 to 3 m class (3 to 4 years of occupied growth) 
were cut off 60 cm above the ground. The acacia 
crown was left next to 25 of the stumps, and for 
27 stumps tlie crown was dragged at least 10 m 
away. After the usual movements of P. belti colonies 
following this disturbance, 9 of the stumps in the 
control plot lost their colonies and none of those 
in the experimental plot gained one (if this ex- 
FIGURE 92.--Acncia collinsii: A, ten-year-old adult (Panama, 
site 83); B, crown of adult (Sicaragua, site 54); c, left, 7-year- 
old clump occupied by Pseudoniyrrnex belti; right, 7-year-old 
tt'w occupied by Pseudomyrmex ferruginea (Sicaxagua, 57). 
periment had been conducted during the rainy 
season, when colonies are larger and the ground is 
not so hot, 5 to 10 of the stumps in the experimental 
plot would have regained their colonies despite the 
108 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 93.-Acacia collinsii (Panama): A, central crown of adult (site 78); B, crown of 10-m-tall 
tree (site 88) : c, senescent crown of 10-m-tall tree (site 88). 
distance to which they were removed). By July 19, 
1967, about five months later, the mean length of 
the longest sucker shoot from each of the 25 stumps 
in the control plot was 25.1 cm (s.d. 10 cm) while 
the same measurement was 5.1 cm (s.d. 4 cm) in 
the experimental plot. These differences are highly 
significant (Mann-Whitney U test) and differ in no 
major way from the results obtained with occupied 
and unoccupied sucker regeneration of A. cornigera 
and P. ferrziginea in eastern Mexico (Janzen, 
1967b). If we consider only the 16 members of the 
control plot that were occupied, the results are more 
dramatic; the mean length of the longest sucker 
shoot is 36.4 cm (s.d. 14 cm). The 9 unoccupied 
suckers of the control plot also had a mean length 
of 5.1 cm (s.d. 12 cm). Figure 89~ illustrates a 
representative shoot from each of the occupied and 
unoccupied stumps of this date. Figures 898 and c 
compare the branch ends of large lateral branches 
from naturally occupied and unoccupied adult 
acacias in a much wetter habitat in southern Costa 
Rica (site 67). 
Since nearly all the regeneration was during the 
dry season, this low total amount of sucker-shoot 
development was expected. That no stump died 
during the course of the experiment suggests 
strongly that substantial energy reserves were stored 
in the lower stump and root system. One year 
later, on July 12, 1968, only 4 of the stumps un- 
occupied on July 19, 1967, were still alive; all the 
plants that had been occupied in 1967 were still 
alive. By July 14, 1969, all the occupied shoots had 
approximately regained their original height (1967) 
or grown taller, and there was no trace of the 36 
stumps unoccupied on July 19, 1967. 
Acacia collinsii has a small number of leaf and 
stem galls just as do other swollen-thorn acacias. 
At site 86 in Panama, however, the population of 
NUMBER 13 
109 
FIGURE 94.-Acacia collinsii (Costa Rica, site 68): A, adult 
growing in open pasture, August 1, 1963; B, same tree crown, 
July 1, 1967; the lack of change is due to lack of interplant 
competition. 
A. collinsii is afflicted with an outstanding disease. 
Almost all the acacias have thick clusters of swollen 
thorns scattered thiougli the crown (Figure 90). 
These are the consequence of some agent that 
causes extreme branch proliferation, yet each inter- 
node is extremely shoit. The number of thorns 
produced by the acacia is far greater than the ants 
can use, with the consequence that many of the 
thorns in the cluster have no entrance holes. The 
potentially negative effect of this excess of thorns 
on seed production has not been examined. Young 
seedling and sucker shoot development of A. collinsii 
is very similar to that of A. cornigera and A. hinclsii. 
The bird-dispersed seed germinates at the beginning 
of each rainy season (Figure 91~) and, if fully inso- 
lated, forms into a small bush that produces its 
first swollen thorn on the 12th to 17th leaf (about 
two months old). Suckers from stumps in recently 
FIGURE 95.-Acacia collinsii: A, crown of adult growing in 
area with very severe dry season (Nicaragua, site 54); B, 
crown of tree growing in area with very mild dry season 
(Costa Rica, site 67). 
burned areas do the same (Figure ~IB), but the 
larger the stump, the more the first foliage resembles 
that of the mature plant. As the months pass with- 
out a new colony developing in the young plants, 
they gain in height at the rate of about 10 cm per 
year and suffer very heavy damage from defoliating 
insects (Figure 91c). In most open habitats, A. 
collirisii grows into a bushy shrub once occupied 
(Figures 92~, B). If it is in dense regeneration, it 
may become a small tree that gradually loses crown 
size and senesces when 15 to 20 years old (Figure 
93). It may retain this shape for many years (Fig- 
ure 94). 
There is considerable variation in branching pat- 
110 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 96.-Acacia collinsii (Nicaragua): A, dense crown of adult caused by shoot damage by 
Pseudomyrmex ferruginea (site 57) : B, diffuse crown of adult occupied by Pseudomyrmex nigro- 
cincta (site 51). 
terns of A. collinsii in fully insolated habitats. In 
the very driest sites, where A. collinsii may almost 
be regarded as a member of the ?climax? forest, 
the plant has only a few major branches, and the 
new leaves each year are produced in axillary tufts 
or on very short new branches (Figure 95~). In 
the wettest sites, the canopy is uniformly dense 
owing to the production of a large number of small 
lateral branches (Figure 95~). A second type of 
crown variation is produced by the interaction be- 
tween the eastern Nicaraguan portion of the 
Pseudomyrmex ferrziginea population and A. col- 
linsii. This species of ant in this area prunes the 
shoot tips of the swollen-thorn acacia it occupies. 
The consequence is that two adjacent A. coZlinsii 
of the same age-one occupied by P. fer7ziginea 
and the other by P. belti or P. nigrocincta-will be 
extremely different in height (Figure 92c). If the 
shoot is not occupied by P. ferruginea until it is 
1 to 2 m tall, it produces a ??lollypop?? life form 
(Figure 96). As discussed in Janzen (1969c), one 
possible explanation for the ants? behavior is that 
birds preying on ant brood cannot get into the 
dense crown to get the queen-thorn; in this area, A. 
collinsii does not have the well-developed type B 
thorns which are normally adequate to protect the 
queen froni birds. 
Acacia collinsii, like A. cornigera, bears its first 
flowers at the end of three years of occupied growth. 
Seeds therefore appear in the fourth dry season 
after the plant is occupied, meaning that A. collinsii 
can exist as a reproducing population in cultivated 
sites with only four years fallow. Its long inflores- 
cences are normally born in short axillary clusters 
(Figure 98~) but in very wet sites may be produced 
on long sexual branchlets (Figure 97A). There 
appears to be a general cline in increasing inflores- 
cence length from the northern to the southern end 
of the range of A. collinsii. The woody pods are 
usually born 1 to 6 per inflorescence; upon 
SUMBER 13 
111 
FICL?RE 97.--Acacia collinsii: A, exceptionally well-developed sexual branchlet (Mexico, site 4); 
B, type A thorn at base of leaf with three droplets of nectar from petiolar nectaries (Costa Rica, 
site 65); c, newly grown type B thorns and fully developed branch on tree in Figure B. 
dehiscing, the seeds drop partly out and are 
wrapped in sweet yellow pulp which is picked off 
by birds in flight. This dispersal mechanism is 
common to the entire population, but there is 
considerable variation as to how woody the pods 
are and how much they curl upon opening. In no 
areas do the pods curl as extremely as those of A. 
hindsii and A. globulijern. 
Type A thorns on A. collinsii are easily dis- 
tinguished from those of all other swollen-thorn 
acacias, but the differences are difficult to charac- 
terize. A. collinsii bears the shortest type A thorns 
to be found on a mature swollen-thorn acacia. They 
are usually round in cross section and give the 
impression of being ?fat? (Figures 98, 99). When 
flattened (Figures 98~, 99~) like A. hindsii, they 
still have much thicker walls than would an A. 
hindsii thorn of the same size. .4. collinsii probably 
shows the most extreme variation on type A thorn 
morphology of any of the swollen thorn acacias. 
Most of the population of A. collinsii bears thorns 
that are ivory to beige, but in wet or high elevation 
sites they are often dark brown with a maroon or 
reddish cast. There is usually a prominent bump 
on the inner wall of each thorn near the base; this 
may aid the ants in making use of the large volume 
of space at the base of the ?fatter? thorn types 
(e.g., Figure 98~). 
Type B thorns of A. collinsii (Figures 100-103) 
are easily as variable in shape as those of A. 
cornigera and A. hiiadsii. As is the case with these 
latter two species, there is little intrapopulation 
variation in this trait, but of course there is rather 
great variation associated with the age of the acacia. 
Type B thorns are generally produced at the ex- 
pense of the leaf they subtend (Figure 97c) and are 
produced very rapidly once branch elongation 
starts. It is common to find all the type B thorns 
on a short branch to be at approximately the same 
stage in hardening. While type B thorns may appear 
112 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 98.--Geographic variation in type A thorns from occupied adult A. collinsii: A, Costa 
Rica, site 68; B, Nicaragua, site 61; c, D, Mexico, site 16. 
FIGURE 99.-Geographic variation in type A thorns from occupied adult A. collinsii: A, Mexico, 
site 11; i%, c, Mexico, site 16; D, Nicaragua, site 61; E, Panama, site 86. 
NUMBER 13 
113 
FIGURE 100.-Type B thorns around main trunk of A. collinsii: A, Panama, site 88; B, Costa Rica, 
site 67. 
to converge on those of A. coriiigera or A. hindsii, 
A. colliiisii is not sympatric with those species 
where this occurs. 'Cl'hile the very largest type B 
thorns to be found on A. collinsii (Figure 104) 
have ii large volume and are therefore similar to 
the largest on A. hiiidsii, in general both type A 
and type 13 thorns of A. colliiisii are small in 
volume when compared with swollen-thorn acacias 
in general. 
12. Acacia hindsii Bentham 
Acacia hindsii Bentham, 1842:504. 
Acacia bwsaria Schenck, 1913:363. 
Amcin tepicnna Safford, 1914:366. 
Acacia sinaloemis Safford, 1914:365. 
Myrrttecodendron lzindsii (Bcntham) Britton and Rose, 1928: 
91. 
A mature Acacia hindsii is distinguished from all 
other swollen-thorn acacias by its very flat and dis- 
tended thorns. On occasion the thorns of A. collinsii 
display superficially similar distension, but A. 
hindsiz has small diameter petiolar nectaries rather 
than broad dome-shaped ones as on A. collinsii. 
A. Iiiizdsti, A. collinsii, and A. cornigera are the 
three most widely distributed swollen-thorn acacias. 
A. hiiidsii, however, is essentially a plant on the 
Pacific lowlands and foothills (Figure 105). 
1 was unable to examine the type specimen of A. 
hindsii, but A. hindsii is the only species of swollen- 
thorn acacia in the Nanzanillo Bay area (site l), 
and the t) pe description agrees closely with the 
swollen-thorn acacia population in that area. The 
northernmost record of A. collinsii (A. collinsii site 
7) is from slightly to the south of site 1, but at this 
site it has cylindrical thorns and is not sympatric 
with A. hindsii. A. hindsii is not to be confused 
with its hybrids with nonant-acacias such as A. 
pennatula, e.g., A. oaxacanum, A. hirtipes, and A. 
114 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 101.-Geographic variation 
in type B thorns of occupied adult 
A. collinsii: A, B, Mexico, site 4; c, 
Nicaragua, site 51; D, Costa Rica, 
site 67; E, Mexico, site 11. 
FIGURE 102.-Geographic variation in type B thorns of occupied adult A. collinsii: A, Costa Rica, 
site 67; B, Nicaragua, site 51; c, Mexico, site 4. 
NUMBER 13 
140 - 
130 - 
2120 - 
g110 - 
2 100 - 
2 90- 
6 
X 
z 
W 
4 
I 
0 
X 
I- 
I- 
0 
I- 
115 
FIGURE 103.4eographic variation in type B thorns of occupied adult A. collinsii: A, Panama, 
site 82: B, Mexico, site 11: c, Guatemala, site 38. 
c 
1- I I 
TOTAL THORN VOLUME (cc) 
2 4 6 8 10 
FIGURE 104.-Distribution of thorn volume among the largest 
type B thorns of A. collinsii (Costa Rica, site 67). 
standleyi. None of the hybrids has a full comple- 
ment of Beltian bodies, large nectaries or fully de- 
veloped swollen thorns. 
Shenck's description of the type of A. bursaria 
applies without doubt to the A. hindsii population 
at site 49, rather than to A. cornigera, which is the 
only other swollen-thorn acacia at the type locality. 
The thorns on the type specimen of A. bursaria are 
exceptionally large type A thorns, of the kind often 
found on large and very healthy A. hindsii in the 
wetter parts of Guatemala. 
The vegetative branch chosen for the type speci- 
men of A. tepicana is a representative young sucker 
shoot or central axis from a young seedling before 
it became occupied by obligate acacia-ants. The 
thorns are not flattened, but this is the case with 
most young A. hindsii. Their very light color is 
also representative of young A. hindsii in areas 
where the adults have very dark colored thorns. 
The peduncle with mature fruits accompanying the 
type specimen was taken from a representative 
mature A. hindsii. A. hindsii is the only swollen- 
thorn acacia found as far north as Acaponeta in 
western Mexico (site 2). 
Safford's type specimen for A. sinaloensis is a 
representative branch from the upper crown of a 
normal mature A. hindsii of the form common at 
the type locality (site 3). Type A thorns of all de- 
grees of flattening occur in such a crown and the 
116 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FICIIRF. 105.-Postulated general distribution (stippled area) and collection localities of A. hindsii. 
type specimen has thorns representing the smaller 
end of the spectrum. The seed pod accompanying 
the type is a normal A. hiizdsii pod. A. hindsii is 
the only swollen-thorn acacia found as far north 
as Villa Union, Sinaloa, Mexico. 
KEY COLLECTION LOCALITIES 
Mexico 
1. Type of A. hindsii: Shore of Manzanillo Bay, Colima, 
Mexico (sea level). 1836-39 (or 1841; Sir George Taylor, 
in litt.), R. C. Ilinds, no. 248. 
2. Type of A. tepicana: Acaponeta, Nayarit, Mexico (30 m). 
IV-10-1910. J. N. Rose, P. C. Standley, and P. G. Russell, 
no. 14357. 
3. Type of A. sinaloensb: Villa Union, Sinaloa, Mexico (10 
m). IV-2-1910, J. N. Rose, P. C. Standley, and P. G. Rus- 
$ell, no. 13972. 
4. Arriaga to 13.2 mi N on Hgy. 200, Chiapas (10-550 m) . 
1-9-1965, DHJ, nc (common). 
5. Tehuantepec to 21.2 mi E on Hgy. 190, Oaxaca (10-30 
m). 1-10-1965, DHJ, no. 1041; IX-2-1965, DHJ, nc; 
VIII-7-1966, DHJ, no. 415 (occasional to rare). 
6. Near Salina Cruz, Tehuantepec, Oaxaca (sea level). IX- 
l-1965, no. 1934 (occasional to common). 
7. Tapilula to 3 mi E, Chiapas (700 in). VII-10-1966, DHJ, 
nc (occasional). 
8. 12.2 mi SW El Camaron to 10.2 mi W turn into Tex- 
quisistlan on Hgy. 190, Oaxaca (925-735 m). 1-10-1965, 
DHJ, nos. 1033, 1034. 
9. Puerto Escondido to 36.9 mi N on Hgy. to Oaxaca, Oax- 
aca (sea level to 940 m). VI-19-1967, DHJ, nc (occa- 
sional to ra:e) . 
10. Las Cruces (just inland from Acapulco) to Puerto Es- 
condido on Hgy. 200, Gucrerro-Oaxaca (20-300-10 m) . 
VI-18-1967, DHJ, nc (common to occasional). 
11. 14.0 mi S Chilpancingo to Las Cruccs, Guererro (1175-30 
m) . 1-7-1965. DHJ, nos. 1031, 1035; DHJ, nc (common). 
12. Playa de Pie de la Cuesta (just W of Acapulco) to 13.6 
mi SE Papanoa on Hgy. 200, Guererro (sea level). VI- 
17-1967, DHJ, nc (common to very rare, only along up- 
per edges of swamp sites). 
13. 21.G mi SW Autlan to Barra de Navidad, Jalisco (450-0 
m). 1-5-1965, DHJ, nos. 1026, 1027, 1028, 1029, 1030 
(common to rare, only in wetter sites) . 
14. Barra de Navidad to Tecoman, Jalisco-Colima (50-0 m.) 
1-5-1965, DHJ, nc (rare-common) . 
15. 12 mi NW Tepic to 28.4 mi SE Mazatlan on Hgy. 15, 
NUMBER 13 
117 
Nayarit-Sinaloa (700-5-70 m). IX-14-1963, DHJ, no. 
1745; 1-2-1965, DHJ, no, 1011; VI-8-1967, DHJ, nc (rare 
to common to rare) , fl. 
16. Hgy. 12 to San Blas on Hgy. 46, Nayarit (200-0 m). I- 
13-1965, DHJ, nos. 1019, 1020, 1021, 1022, 1023, 1024, 
1025. 
17. 23.5 mi SW El Palinito to 9.0 mi SW, Concordia on Hgy. 
40 (1089-10 m). 1-2-1965, DHJ, nos. 1005, 1006, 1012, 
1013, 1014, 1015, 1016, 1017; VI-6-1967, DHJ, nc (com- 
mon to rare). 
18. Palo Blanco, San Ignacio, Sinaloa (340 m). IV-18-1918, 
M. N. Montes and H. E. Salazar, no. 286 (US 1035197). 
19. Puerto Angel, Oaxaca (sea level), IV-12-1933, C. V. 
Morton and E. Makrinius, no. 2614 (US 1585606), fl. 
20. Cafetal Concordia, Oaxaca (400-650 m) . IV-15-1933, 
C. V. Morton and E. Makinius, no. 2427 (US 1585440), 
fl. 
21. Pochutla, Oaxaca (50 m) . IV-19-1917, no. 3146, C. Con- 
zatti (US 763852), fl. 
22. Huatulco, Oaxaca (150 m). 11-1941, B. P. Reko, no. 6010 
(F 1510963), fl. 
23. 13.7 mi NE La Mira to Playa Azul and to Melchor 
Ocampo on Hgy. 37, Michoacan (200-0 m). VI-12-1968, 
DHJ, nc (common). 
24. Guatemala-Mexico border (near Ciudad Cuauhtemoc) 
to 3.6 mi N on Hgy. 190, Chiapas (720-680 m). VIII-6- 
1966, DHJ no, 416; VIII-16-1967, DHJ, nc (common to 
rare). 
25. 5.1 mi SW Cintalapa (Chiapas) to 20.9 mi W Tapana- 
tepec (Oaxaca) on Hgy. 190 (610-820-110 m) . VIII-18- 
1967, DHJ, nc (common to rare). 
25a. 23.3 mi S Pichucalco, on Villa Hermosa-Chiapa de Corzo 
road, Chiapas (1000 m) . VII-10-1966, DHJ, no. 420, 564. 
26. 29.3-35.0 mi W Tapanatepec, Oaxaca (100-20 m) . VIII- 
18-1967, DHJ, nc (occasional). 
27. 6.8-4.1 m S of turn into Matias Romero, on Hgy. 185, 
Oaxaca (230 m) . VIII-18-1967, DHJ, nc (3 plants). 
28. Guatemala-Mexico border (Pte. Talisman) to Arriaga 
on Hgy. 200, Chiapas. 1-10-1965, DHJ, nc (common). 
Guatemala: 
29. 6.8 mi SW (1370 m) Guatemala City to Escuintla, Depto. 
Esquintla on Hgy. 2. 1-12-1965, DHJ, no. 1050; VIII-12- 
1967, DHJ, nc (common), 
30. Shores of Lake Amatitlan and surrounding hills (1300- 
1350 m). Depto. Guatemala. VIII-11-1967, DHJ, nc 
(common). 
31. Escuintla, Depto. Escuintla (330 m) to Guatemala- 
Mexico border (Pte. Talisman), Depto. San Marcos (50 
m) , 1-12-1965, DHJ, nos. 1048, 1049 (rare to common). 
32. Retalhuleu SW to Champerico, Depto. Retalhuleu (200- 
0 m) . VIII-1-1966, DHJ, nc (occasional to rare). 
33. Retalhuleu to 15.2 mi NE on Hgy. 9S, Depto. Retalhuleu 
(200-1000 m). VII-18-1966, DHJ, no. 400; VII-16-1966, 
DHJ, no. 404; VII-28-1966, DHJ, nc (common to rare). 
34. 16.5 mi SE to Guatemala-Mexico border (Ciudad 
Cuauhtemoc), Depto. Huehuetenango (900-1000-720 m) 
on Interamerican Hgy. VIII-16-1967, DHJ, nc (com- 
mon). 
35. 9.5 mi SW to 6.2 mi NE Puente El Lob0 on Hgy. 4, 
Depto. Zacapa (150 m). V-14-1965, DHJ, nos. 1929, 
1930; V-20-1965, DHJ, no. 1931; VIII-9-1967, DHJ, nc 
(very rare except as clumps on river banks). 
36. Guatemala-El Salvador border (25 m; Pijije) , Depto. 
Jutinpa to Escuintla (330 m), Depto Escuintla on Hgy. 
6E. 1-13-1965, DHJ, nc (rare to common), fl. 
37. Guatemala.El Salvador border (San Cristobal), Depto. 
Jutinpa to 15.8 mi SE Guatemala City on Hgy. 2, Depto. 
Guatemala (520-1250 m) , VII-7-1967, DHJ, nc (common 
to rare to common). 
38. Tikal, Depto. Peten (100-500 m). IV-2-1922, 0. F. Cook 
and R. D. Martin, no. 170 (US 1084640), mp. 
39. San Carlos, Depto. Retalhuleu (650 m) . 111-22-1921, 
Rojas, no. 241 (US 1014948), fl. 
40. Santa Barbara, Depto. Solola (400 m) . VIII-1891, W. C. 
Shannon, no. 578 (US 943298). 
41. Pancajche, Depto. Alta Verapaz (360 m). IV-5-1939, 
P. C. Standley, no. 70648 (F 978766), fl. 
42. narranca Hondo, lower slopes Volcan de Fuego, Depto. 
Sacatepequez (1800 m) , XII-16-1938, P. C. Standley, no. 
60262 (F 994870), gp. 
43. Ocos, Depto. San Marcos (sea level). 111-15-1940, J. A. 
Steyermark, no. 37779 (F 105266). 
44. Between Finca Pirineos and Finca Soledad, lower south- 
facing slopes of Volcan Santa Maria, between Santa 
Maria and Calahuache, Depto. Quezaltenango (1300- 
1400 m). 1-5-1940, J. A. Steyermark, no. 33599 (F 
1051210). 
45. Near Calahuache, Depto. Quezaltenango (1020 m) . III- 
1-1939, P. C. Standley, no. 67107 (F 987673), mp. 
46. Rio Chiquimula, 1.5 mi NE Chiquimula, Depto. Chiqui- 
mula (400 in). X-21- 1939, J. A. Steyermark, no. 30123 
(F 1038583). 
47. Finca El Porvenir, Rio Cabus, SW slope Volcan Taju- 
mulco, Depto. San Marcos (1000-1300 m) . 111-12-1940, 
J. A. Steyermark, no. 37550 (F 1049161). 
48. Along Rio Gaucalate, Depto. Escuintla (500 m). XI-28- 
1938, P. C. Standley, no. 58188 (F 993751), gp. 
49. Type of A. bursaria: San Felipe, Retalhuleu, Guatemala 
(300 m). February, 1878, Bernouilli and Cario, no. 1129 
or Laguna Amatitlan, Depto. Guatemala, Guatemala 
(1100 m). 1890, J. D. Smith, no. 2304 (Sir George Taylor, 
in litt.) . 
Honduras: 
50. 13.1 mi W San Marcos de Colon (850 m) to 11.6 mi NE 
Choluteca (330 m) on Interamerican Hgy., Depto. Cho- 
luteca. 1-14-1965, DHJ, no. 1053; V-11-1965, DHJ, nos. 
1928, 1933; VII-29-1967, DHJ, nc (rare to common to 
rare). 
51. 9.4 mi NE Choluteca on Interamerican Hgy., Depto. 
Choluteca (300 m) . VII-29-1967, DHJ, nc (1 plant). 
52. 4.3 mi NE Jicaro Galan, Depto. Valle (70 m) to 21.6 mi 
S Tegucigalpa, Depto. Francisco Morazan (1000 m) on 
Hgy. 1. VII-30-1967, DHJ, nc (rare to common to.rare). 
53. Honduras-El Salvador border (El Amatillo) to 15.5 mi 
E on Interamerican Hgy., Depto. Valle (50-100 m). I-15- 
1965, DHJ, nc (very rare). 
54. 13.1 mi W Zambrano, Depto. Francisco Morazan (830 m) 
to 7.8 mi SE Villa de San Antonio, Depto. Comayagua 
118 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
55. 
56. 
57. 
58. 
59. 
60. 
61, 
62. 
63. 
64. 
65. 
66. 
67, 
68 
(750 m) on Tegucigalpa-Puerto Cortes Hgy. VII-31-1967, 
DHJ, nc (common). 
7.8 mi SE to 4.7 mi N (550 m) Villa de San Antonio on 
Tegucigalpa-Puerto Cortes Hgy., Depto. Francisco Mora- 
zan. VII-31-1967, DHJ, nc (occasional to very rare). 
5.3 to 6.3 mi NW Comayagua on Tegucigalpa-Puerto 
Cortes Hgy., Depto. Comayagua (510 m). VII-31-1967, 
DHJ, nc (rare). 
13.2 mi NW Comayagua on Tegucigalpa-Puerto Cortes 
Hgy. (850 m). VII-31-1967, DHJ, nc (1 plant). 
10.7 mi NW (820 m) Siguatepec, Depto. Comayagua to 
16.4 mi S (50 m) San Pedro Sula, Depto. Cortes on 
Tegucigalpa-Puerto Cortes Hgy. VIII-2-1967, DHJ, nc 
(common to occasional). 
4.6 mi SW (90 m) to 12.9 mi SW (150 m) San Pedro 
Sula, Depto. Cortes on road to Santa Rosa. VIII-2-1967, 
DH], nc (occasional). 
24.4 mi SW (240 m, bank of Rio Chamelecon) San Pedro 
Sula, Depto. Cortes to 10 mi N Santa Rosa. VIII-2-1967, 
DHJ, nc (occasional to very rare). 
4.4 mi SW Santa Rosa, Depto. Copan (940 m) , VIII-3- 
1967, DHJ, nc (1 plant). 
4.4 mi NE (1310 m) Nueva Ocotepeque to Honduras-El 
Salvador border (850 m) on Santa Rosa-San Salvador 
Hgy., Depto. Ocotepeque. VIII-4-1967, DHJ, nc (rare to 
common). 
Amapala, Isla Tigre, Depto. Valle (less than 250 m) . II- 
14-1922, P. C. Standley, no. 20738 (US 1136566, US 
1136557), fl. 
Siguatepec, Depto. Comayagua (1080-1400 m). II-14-27- 
1928, P. C. Standley, no. 56333 (US 1409186), fl. 
Las Limas, Depto. Comayagua (1000 m) . IV-16-1932, 
J. B. Edwards, no. P-89 (F 713842), fl. 
Vicinity of Siguatepeque, Depto. Comayagua (1080-1400 
m) . 11-14-27-1928, P. C. Standley, no. 56333 (F 581757), 
fl. 
La Lima, Depto. Cortes (30 m). IV-11-20-1947, P. C. 
Standley and J. Chacon, no. 7548 (F 1268590), fl. 
Near Lancetilla, Depto. Atlantida. VIII-10-1934, T. G. 
Yuncker, no. 5099 (F 749502). 
El Salvador: 
69. 
70. 
71. 
72. 
73. 
74. 
Honduras-El Salvador border (850 m) to 10 mi S (1170 
m), Depto. Chalatenango on Santa Rosa-San Salvador 
Hgy. VIII-4-1967, DHJ, nc (occasional to very rare). 
11.7 mi S (1130 m) Honduras-El Salvador border (Depto. 
Chalatenango) to 9.3 mi N San Salvador (580 m) on 
Santa Rosa-San Salvador Hgy. VIII-4-1967, DHJ, nc 
(rare to very common to rare). 
Intersection just E of La Libertad (sea level) to 11.8 mi 
N on Hgy. to Interamerican Hgy. (910 m), Depto. La 
Libertad. VIII-5-1967, DHJ, nc (rare to very common). 
La Libertad, Depto. La Libertad to La Union, Depto. La 
Union (sea level to 30 m) on La Libertad-Zacatecoluca- 
Usulutan-La Union Hgy. VIII-6-1967, DHJ, nc (common 
to occasional to rare), 
La Herradura to La Libertad-Zacatecoluca Hgy., Depto. 
LaPaz. VIII-6-1967, DHJ, nc (very rare) . 
41.5 mi SE San Miguel to La Union on San Miguel-La 
n? 
13. 
76. 
77. 
78. 
79. 
80. 
81. 
82. 
83. 
84. 
Union Hgy., Depto. La Union. V-12-1965, DHJ, no. 1932; 
1-15-1965, DHJ, nc (rare), 
La Libertad, Depto. La Libertad to El Salvador-Guate- 
mala border (La Hachadura) , Depto. Ahuachapan. 1-16 
1965, DHJ, no. 1051 (common to rare). 
Sonsonate (280 m) SE to Interamerican Hgy#, Depto 
Sonsonate. VIII-6-1967, DHJ, nc (common to accasional). 
Sonsonate through Armenia to San Salvador-Guatemala 
Hgy. (280-920-450 m). VIII-6-1967, DHJ, nc occasional 
to rare). 
Intersection of San Salvador-Guatemala City Hey., with 
Sonsonate-Armenia Hgy. to Guatemala-San Salvador bor- 
der (San Cristobal) , Depto. Santa Ana (450-810-520 m) . 
VIII-6-1967, DHJ, nc (very rare to common to occa- 
sional) , 
San Martin de Laguna Ilopango, Depto. San Salvador. 
IV-1-1922, P. C. Standley, no. 22622 (US 1138360), fl. 
2 km N7V Monte Cristo, 15 km XE San Miguel, Depto. 
Morazan (150 m). XII-7-1941, J. iM. Tucker, no. 486 
(US), mp. 
Laguna de Olomega, Depto. San Miguel (75 m). II-20- 
1922, P. C. Standley, no. 21027 (US 1136842). 
San Vicente, Depto. San Vicento (350-500 m) . III-2-11- 
1022, P. C. Standley, no. 21291 (US 1137447), fl. 
Ahuachapan, Depto Ahuachapan (700-1100 m) . I-16- 
1925, P. C. Standley and E. Padilla, no. 2497 (F 1256801). 
2 km NW Monte Cristo, 15 km NE San Miguel (150 m) . 
XII-7-1941, J. M. Tucker, no. 486 (F 1492748). 
Nicaragua: 
85. 3.4 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VII- 
27-1967, DHJ, nc (30 acre patch), gp. 
86. 4.9 mi SE Leon on Hgy. 12, Depto. Leon (20 m). VII-27- 
1965, DHJ, nc (1 plant). 
87. Ameya (near Pto. Morazan) Depto. Chianandega (sea 
level). VI-19-21-1923, W. R. Maxon et a!., no. 7176 
(US 1180890). 
NATURAL HISTORY 
Being widespread geographically (Figure 105) on 
soils and in climates very suitable to seasonal agri- 
culture, A. hindsii is difficult to characterize 
ecologically since nearly all the semideciduous and 
deciduous forests that it occupied before the advent 
of European agriculture have been destroyed. As 
McVaugh (1966) has stressed, when both of these 
forest types are cleared, A. hindsii is a common 
member of the shrubby regenerating vegetation. In 
contrast to those swollen-thorn acacias that live in 
natural disturbance sites in wet or rain forests (A. 
allenii, A. melanoceras, perhaps A. mayana), the 
A. hindsii population density has very likely in- 
creased greatly since the advent of European agri- 
culture, except in those areas where Indian slash- 
burn agriculture was common on a system with at 
NUMBER I3 
119 
least ten years fallow per field. Based on examina- 
tion of A. hindsii throughout its contemporary 
range, I suspect that its pre-Colombian ecological 
distribution was the following. The plant was ex- 
tremely common along rivers and mangrove swamps 
in deciduous and seinideciduous forests and formed 
large thickets where occupied by multiple-queen 
species of obligate acacia-ants (Janzen, 1967a, 1973). 
Scattered large trees surrounded by sucker shoots 
from lateral roots occurred on dry hillsides, be- 
coming progressively rarer the more intense and 
the longer the tlry season. Along the wetter margins 
of its range (usually higher as well) it was found 
only along watercourses and other natural dis- 
turbance sites. In the very driest parts of its range 
(e.g., where the Tropic of Cancer crosses western 
Mexico), it occurred only along watercourses and 
in the foothills where the slightly cooler weather 
compensated for a lack of rain equal to that in the 
well-drained parts of the coastal plain. In no places 
did it occur as a shrub in the semidesert conditions 
as A. collinsii may do on rare occasions. In short, 
A. hiizdsii was a plant of locally wet sites in the 
drier habitats of the Pacific side of Central America 
between Nicaragua and the Tropic of Cancer. 
Throughout this range it invaded wet sites, as wet 
as those occupied by A. gentlei, A. cookii, A. may- 
ana, A. globulifera on the Atlantic coast. Like these 
four swollen-thorn acacias, it was restricted to 
natural disturbance sites such as river banks in the 
wetter habitats. The wetter sites on the Pacific side, 
however, tend to experience a much more severe 
tlry season than do sites on the Atlantic side with 
roughly equal annual rainfall. That the plant had 
to be able to survive this dry season, and thus could 
move into drier sites fairly readily, had been in- 
strumental in preventing the very widespread popu- 
lation of A. hindsii from differentiating into many 
species. In other words, drying of the general 
weather would not be so likely to split that popula- 
tion of A. hindsii into as many small and isolated 
populations as it would a species widely distributed 
along the wetter Atlantic side of Central America. 
The restriction of A. hindsii to the Pacific side 
of Central America is very puzzling. It has been 
collected east of the Continental Divide at only 7 
sites, and 5 of these (sites 7, 27, 35, 55, 56) could 
well be the result of seed dispersal via road building 
machinery; at these sites the population is restricted 
in the area within a mile of the highway. At Tikal 
and Lancetilla (sites 38 and 68) there has been 
much traffic by biologists from other parts of Cen- 
tral America, and the collections could well be 
recent introductions. A. hiizdsii is certainly not the 
common swollen-thorn acacia at these sites; how- 
ever, there were reproducing trees at each of the 5 
sites that I examined, and if they are not very recent 
introductions, it is most enigmatic that they have 
not spread from these sites over much of Caribbean 
Central America. Even if these 7 sites represent 
recent introductions, it is puzzling that the many 
natural introductions that must have occurred in 
the p.ast have not resulted in established popula- 
tions of A. hindsii on the Atlantic side of Central 
America. Being bird dispersed, the seeds surely 
must have been carried across the Continental Di- 
vide. Since isolated greenhouse plants of A. hindsii 
never set seed despite the fact that they flower 
profusely, it is likely that they are self-sterile. Suc- 
cessful establishment across the Continental Divide 
would then be exceedingly difficult except with 
simultaneous multiple-seed dispersal as could easily 
occur via the mud caked on road-building equip- 
ment. Low-income workers on the roads also eat 
the sweet pulp around the seeds and may have dis- 
persed some seeds in this manner. 
The dependency of A. hindsii on obligate acacia- 
ants for survival to reproductive maturity appears 
no different from that of A. cornigera, though ex- 
periments have not been conducted. Naturally un- 
occupied plants of A. hindsii are heavily damaged 
by herbivorous insects except under two circum- 
stances: one natural and the other artificial. At 
the upper elevation limits of their distribution (e.g., 
sites 8, 37) in windy cool habitats, it appears that 
herbivorous insect pressure is low enough so that 
only during the warmest months of the year are 
unoccupied A. hindsii subject to severe defoliation. 
A. hindsii is one of the small number of species of 
woody second-growth plants that has managed to 
survive along the well-traveled dirt roads on low- 
land coastal El Salvador (site 72). These roads lie 
between cotton fields that are frequently sprayed 
with parathion and other very toxic insecticides. 
Not only have all the insects been eliminated from 
the roadside strips of vegetation by the combination 
of dust and insecticides, but the colonies of obligate 
acacia-ants have been gone so long that a large 
A. hindsii can show no sign of ever being located 
by searching queens. The thorns only stay on the 
120 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 106.-Acacia kindsii: A, unoccupied 1-year-old sucker shoot (Mexico, site 6); B, unoccupied 
2-year-old seedling (Mexico, site 9); c, occupied 2-year-old sucker shoot (Honduras, site 59); D, 
lateral branch from crown of occupied adult (Mexico, site 6). 
NUMBER 13 121 
acacia for a maximum of about 4 years, so this 
means that the ants have been missing at least that 
long. Since these fields are 10 to 30 years older than 
that, I suspect that the ants and most other insects 
have been missing equally long. The dust along 
other Central American roads is generally not as 
bad, since those with as much traffic as El Salva- 
doran roads are usually paved. Despite the absence 
of herbivorous insects, the A. hindsii do very poorly 
because they are frequently used as vine standards 
by wild plants. At site 85 in Nicaragua, the south- 
ernmost local population of A. hindssii displays an 
extreme case of insecticide protection. This 30-acre 
patch of nearly pure A. hindsii (with scattered A. 
FIGURE 107.-~, B, Acacia hindsii (Mexico, site 6) : A, pair of 2-m-tall subadult occupied sucker 
shoots; B, adult 4-m-tall occupied tree, with the life form characteristic of open pastures; c, 
brushy pasture with a 3-m-tall occupied A. hindsii on the left and A. pennatvla at right of 
center (Guatemala, site 37). 
122 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE lOS.-Acacia hindsii: A, looking up through the crown of an occupied adult (Mexico, 
site 17) ; B, fully leafed crowm of an occupied subadult (Guatemala, site 35). 
cornigera) was almost certainly established from 
seed brought by man and is an island in the center 
of many hundreds of acres of cotton. The acacias 
are of all ages antl many are reproducing. Three 
species of obligate acacia-ants are in the foothills 25 
miles east, but after searching for 2 hours at site 85 
I could find no evidence that any searching queen 
had located the patch during the past 4 years. If 
any did, they were killed by insecticide before they 
could attempt to cut an entrance hole in a thorn. 
There were almost no other insects in this patch of 
vegetation, antl almost every plant had no herbivore 
damage; cotton in this area is almost uneconomical 
to grow because of the severity of insect damage. 
Seedlings of A. hindsii in natural disturbance 
sites, and in brushy pasture, tend to attain a height 
of 10 to 40 cm over 1 to 3 years of growth unoc- 
cupied by obligate acacia-ants. Such unoccupied 
seedlings are very stunted in appearance (Figures 
106~, B) with damaged leaves and many short 
branches that grew a few cm before having the 
shoot tip eaten off by an insect. In contrast to the 
heavily shaded unoccupied seedlings of A. allenii 
and A. melanoceras, whose slow growth is as much 
from a lack of sunlight as defoliation, the slow 
growth of unoccupied A. hiudsii seedlings can be 
attributed almost entirely to defoliation by insects. 
It appears that somewhat less than 1 percent of the 
seedlings of A. hindsii ever become occupied; most 
die within the first 4 years. Cursory examination of 
habitats where they are common indicated that the 
fully insolated unoccupied seedlings live longest, 
despite the dryness of the site. This indicated that 
once established, energy relationships during the 
wet season are more important than desiccation 
during the dry season. 
Once occupied by a single-queen colony, the 
young seedling often produces a small intact bush 
(Figure 1OGc) with multiple branches but quickly 
one of the vertical branches becomes the central 
trunk (Figures 107~, c). The central trunk grows 
rapidly upward; there may be 1 to 2 m of vertical 
elongation during a 5-month iainy season. During 
the second and third years of occupied growth, the 
crown begins to fill in with small branches (Figures 
107c, 108~) if not under strong lateral competition 
from the crowns of other plants. If there is much 
adjacent woody vegetation, the vertical elongation 
KUMBER 13 
123 
of the central trunk may continue until the tree is 
6 to 12 m tall. Lateral branches are long and bear 
large leaves and swollen thorns (Figure 106~). Old 
A. hindsii almost invariably have dense crowns 
(Figures 107~, 108~). The development described 
for A. hindsii is very similar to that for A. cornigera. 
In areas where niultiple-queen species of obligate 
acacia-ants are primary occupants of A. hindsii, the 
development is somewhat different. Here, most new 
FIGURE 109.-A clump (clone?) of adult 
A. hindsii occupied by a multiple- 
queen species of obligate acacia-ant 
(Mexico, site 16). 
plants are sucker shoots from lateral roots of older 
acacias and are occupied by part of the colony in 
the parent shoot. This produces a dense thicket 
(Figure 109) of A. hindsii that can cover as much 
as several acres. In a new habitat, where the A. 
hirtdsii is becoming established through seeding, 
the early stages of shoot development are not much 
different from the case with single-queen obligate 
acacia-ants, but within a few years, the thicket be- 
124 
40 - 
35 - 
- 
v 2 30- 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
W 
u 
z 
W 
V 
V) 
W 
E 
0 
25' 
I+ 
8 
0 
0 
n 
0 n 
.... 0 
0 
0. 0 
0 
0 
0 
PI, , , , 
W 
cl 
150 200 250 300 
NUMBER OF FLORETS PER INFLORESCENCE 
FIGURE 110.-Distribution of 4314 florets among 20 inflores- 
cences from one adult A. hindsii (Mexico, site 5). 
gins to form. The multiple-queen situation prevails 
in areas with particularly severe dry seasons coupled 
with wet seasons very favorable to acacia growth, as 
in western Mexico. 
Acacia hindsii does not bear flowers until the 
fifth to seventh year after becoming occupied, and 
v) 
W 
40 
u 
v) 
W 
0 
I4 
a 30 
k 20 
a 10 
W 
B 
2 
1234 6 
many trees occupied by multiple-queen species do 
not produce their largest seed crops until they are 
10 to 30 years old. In contrast, A. cornigera and 
A. collinsii bear flowers in the fourth to sixth year 
of occupied growth and have their heaviest seed 
crops when 6 to 15 years old. The inflorescences of 
A. hindsii, like those of other dry-land acacias, are 
visited by many species of small wild bees. The 
inflorescences vary greatly in length, but not so 
greatly in the number of florets per inflorescence, 
(Figure 110) , but a maximum of 8 pods per inflo- 
rescence (Figures 11 l~, 112~) is observed in most pod 
crops. When more than 2 pods develop on a single 
inflorescence, they become so tangled when they 
dehisce (Figure 14~) that not all the seeds are ex- 
ternally visible to the birds that disperse them. The 
number of seeds per pod (Figure 111~) is repre- 
sentative of acacias in general and swollen-thorn 
acacias in particular. As with other dry-land acacias, 
these seeds are heavily attacked by bruchid beetles. 
The swollen thorns of A. hindsii show very high 
variation in volume, as do those of A. collinsii. The 
volume of type A thorns (Figure 113) tends to be 
about the same throughout the range. In areas 
where there are only multiple-queen ant colonies, 
all thorns are type A and rather monotonous in 
form. Where single-queen colonies are present, how- 
ever, and A. hindsii is represented primarily by 
well-defined individual trees, there is great varia- 
tion in thorn volume. The volume for some of the 
largest type B thorns encountered are graphed in 
6 
I4 
0 
62 
B 
E 
A NUMBER OF PODS PER INFLORESCENCE B NUMBER OF SEEDS PER POD 
FIGURE 111.--Acacia hindsii (Guatemala, site 29): A, distribution of 187 mature seed pods among 
100 inflorescences on on,e tree; B, distribution of 268 seeds among 22 of the mature pods. 
NUMBER 13 
125 
FIGURE 112.-Infructesccnccs with immature pods: A, A. globulifera (Guatemala, sitc 13) ; R, A. 
hindsii (Honduras, site 59). 
Figure 114, and in contrast to the large volumes 
for the dune ecotype of A. sphncrocephala, the 
large space is all useful to the ants because the thorn 
is flattened such that the thorn walls are only 2 to 
4 mm apart. 
The thorns of A. hiiidsii are highly variable in 
shape, as are those of A. cornigera and A. collinsii. 
Almost all seedlings have thorns that are cylindrical 
or only slightly flattened in cross section (e.g., 
Figure 106~). Thorns on young sucker shoots are 
the same (Figure 115~, middle). This difference 
between young and old thorns is probably asso- 
ciated with maximizing thorn volume and wall 
strength with a fixed small amount of energy to be 
expended on the thorn. Type A thorns in the 
crowns of mature A. hindsii show strong geographic 
variation (Figures 82~, 83c, 115, 116, 117, 118~, C) 
but have low variation at any particular locality. 
Their geographic variation in morphology is as 
great as that displayed between different species of 
more geographically restricted swollen-thorn acacias. 
- 130 
E 
B 
3: 
I-. 
0 
z 
1 
120 
5110 
g 100 
2 
LL: 
I I I I 1 1 I 
0.8 1.0 1.2 1.4 1.6 1.8 2.0 
TOTAL THORN VOLUME (cc) 
FIGURE llS.-Distribution of thorn volume among type A 
thorns from rcprescntative latcral branchcs in the crown of 
an adult occupied A. hintlsii (Mexico, site 17). 
126 
170 - 
h 
8 
E 
X 
b 
W 
z 
w 
cl 
- 150- 
E 130- 
0 
X 
+ 
cl 
0 
b 
;s 110- 
SMITHSONIAN CONTRIBUTIONS TO BOTANY 
I I I I 
150 
h 
Y 
n 
< 110- 
k 
0 
** 
8. 
B 
FIGURE 114.-A. hindsii (Mexico): A, distribution of 84 cc of thorn volume among 17 of the 
largest type B thorns on an occupied adult (site 9); B, distribution of 62 cc of thorn volume 
among 11 of largest type B thorns on an occupied adult (site 11). 
FICIRE ll5.-Geographic variation in typc A thorns from occupied adult A. hindsii: A, Mexico, 
site 17; R, Guatemala, site 33; C, Honduras, site 50; D, Guatemala, site 25A. 
NUMBER 13 
I27 
FIGURE 116.-Geographic varia- 
tion in type A thorns from oc- 
cupied adult A. hindsii: A, 
Mexico, site 11, B, Mexico, site 
13; c, Guatemala, site 33. 
FIGURE 117.-Geographic var- 
iation in type A thorns from 
occupied adult A. hindsii: A, 
Mexico, site 8; B, Guatemala, 
site 35; c, Mexico, site 13; D, 
Guatemala, site 35. 
128 SMITHSONIAN CONTRIBUTIONS TO BOTANY 
FIGURE 118.-Acacia hindsii (Mexico): A, type B thorns on short lateral branches from main 
trunk of an occupied adult (site 9) : B, white type A thorns on main trunk and lateral branches 
of occupied subadult (site 17); c, black type A thorns on main trunk and lateral branches of 
occupied subadult (site 17). 
Much of the variation is probably adaptive in the 
context of the developmental physiology of the 
lateral branch, rather than the ant colony. It is 
possible that the details of the thorn shape are 
nonadaptive, but unlikely, since so much of it is 
clinal in nature. The production of type B thorns 
appears highly adaptive in the context of multiple- 
queen colonies; the single-colony queen lives in the 
type B thorns. There is, however, a great deal of 
geographic variation in type B thorn shape (Figures 
118~, 119) that probably had the same basis as that 
displayed by type A thorns. 
NUMBER 13 
FIGURE 119.-Geographic variation in type B thorns from occupied adult A. hindsii: A, B, F, Mex- 
ico, site 11; c, Guatemala, site 31; D, Mexico, site 13. E, Type B thorns of A. collinsii that are 
convergent in shape on those of A. hindsii. 
129 
130 
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