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Secondary Seed Dispersal by Ants of Ricinus communis(Euphorbiaceae) in the Atlantic Forest in Southeastern Brazil:
Influence on Seed Germination
by
Valéria Forni Martins1, Paulo Roberto Guimarães Jr.1,2, Rogério Rosa da Silva3 &João Semir4

ABSTRACT

our dictionary does not recognize these bolded
words, are they spelled correctly?

Some seeds rely on fleshy lipid-rich appendages as attractants to ants. By
carrying those seeds to their nest, ants play a major role as seed dispersers
and can provide suitable conditions for seed germination. Ricinus communis(the commonly known castor) seeds are primarily dispersed by autocory
but probably present secondary dispersal by ants, once they bear a lipid-richelaiosome. The following questions were addressed: (1) are ants legitimate
dispersers of castor seeds?; (2) which ant species interact with the seeds?;
and (3) is germination success higher in a predicted scenario met by seeds
discarded from ant nest? We compared the removal of seeds with and without
elaiosome and we determined which ant species interact with castor seeds
and whether there was seed predation by ants. We performed experiments
in controlled conditions to evaluate seed germination response to light, temperature and elaiosome. Seeds with elaiosome were removed preferentiallyand no predation was found, indicating that ants are legitimate seed dispersers. A large coterie of ants (20 species) interacts with castor seeds, especiallyMyrmicinae species. Elaiosome removal enhanced germination success, aswell as alternated temperatures and absence of light. Therefore, by discarding
seeds without elaiosome on the pile mound, ants may submit castor seeds toconditions that enhance germination.
Keywords: ant nest, castor, elaiosome, forest edge, myrmecochory, light,
temperature.
Programa de Pós-Graduação em Ecologia, Instituto de Biologia, Universidade Estadual de Campinas,
Campinas, SP, BRAZIL. E-mails: vfm@unicamp.br, paulomiudo@uol.com.br.
2
Integrative Ecology Group, Estación Biológica de Doñana, CSIC, Sevilla, SPAIN.
3
Museu de Zoologia da Universidade de São Paulo, São Paulo, BRAZIL. E-mail: rogeriorosas@gmail.com.
4
Departamento de Botânica, Instituto de Biologia, Universidade Estadual de Campinas, C.P. 6109,
13083-970, Campinas, SP, BRAZIL. E-mail: semir@unicamp.br.
1

Sociobiology Vol. 47, No. 1, 2006

INTRODUCTION
Seed dispersal by ants, or myrmecochory, is widespread and occurs in over
60 plant families (van der Pijl 1972, Beattie 1985). Seeds of myrmecochorous
species bear an elaiosome, a fleshy lipid-rich appendage that attracts ants (vander Pijl 1972, Beattie 1985, Holldöbler & Wilson 1990). The ants usually
carry these seeds to their nest, eat the elaiosome, and discard the seed often in
viable condition (Berg 1975, Davidson & Morton 1981, Beattie 1985, Passos& Oliveira 2002). Ants may collect seeds from parental plants (Lu & Mesler
1981, Stiles 1992), soil (Oliveira et al. 1995, Guimarães & Cogni 2002), or
frugivore feaces (Stiles 1992, Pizo et al. 2005). The interaction between ants
and seeds may result in: (1) modification of seed deposition spectrum and of
seed bank dynamics (Levey & Byrne 1993, Pizo & Oliveira 1998, 1999); (2)germination facilities to seeds originally attached to pulp (Culver & Beattie
1978, Oliveira et al. 1995, Leal & Oliveira 1998); (3) sapling establishment(Culver & Beattie 1978, Levey & Byrne 1993), and (4) influences on spatial
patterns of species primarily dispersed by vertebrates (Böhning-Gaese et al.
1999), autocory or abiotic vectors (van der Pijl 1972) and, thereafter, affects
plant distribution patterns (Willson 1992, Levey & Byrne 1993, Wang &Smith 2002).
Seeds discarded from ant nest may benefit from nutrients (e.g. Passos &Oliveira 2002) and high humidity (Farji-Brener & Ghermandi 2004) available in that microsite, but other abiotic factors may also influence germination success (Fenner 1985). Light and temperature are often regarded inphysiological studies, without any further attempt to relate such conditions
in which germination is enhanced with seed dispersal. Besides, ants may also
affect germination success by changing diaspore features: although studies
have shown that pulp removal by ants enhances germination, especially due
to reduced fungal attack (e.g. Pizo & Oliveira 2001, Guimarães & Cogni
2002), little is known about the influence of elaiosome removal by ants on
the germination success (e.g. Passos & Ferreira 1996).
In this study we investigated the interaction between seeds of the castor
plant Ricinus communis L. (Euphorbiaceae) and ants in a forest edge of theAtlantic Forest in southeastern Brazil. Castor is a shrub or treelet that occurs
worldwide as a colonizing species. The seeds are primarily dispersed by auto-

Martins, V.F. et al. — Seed Dispersal by Ants of Ricinus communiscory (e.g. Stebbins 1974, Fenner 1985, Willson 1992) but probably present
secondary dispersal by ants, once they bear a lipid-rich elaiosome (van der Pijl
1972). Although castor seed is often regarded in literature as typically myrmecochorous (e.g. van der Pijl 1972), no study to the extent of our knowledge
has documented the interaction between ants and those seeds. We addressed
the following questions: (1) are ants legitimate dispersers of castor seeds?;(2) which ant species interact with the seeds?; and (3) is germination successhigher in a predicted scenario met by seeds discarded from ant nest?

METHODS
Study area

Fieldwork was undertaken between October 2002 and February 2003 andconsisted of castor seed removal experiments in three areas located on theedge of a fragmented forest, the Reserva Municipal Mata de Santa Genebra(22°48’34’’-50’13’’S and 47°06’15’’-07’33’’W), Campinas, Southeast Brazil.
This fragment has approximately 250 ha of residual Atlantic Forest vegetation,in which mesophilous semideciduous forest is predominant (Leitão-Filho1995). The criteria to define the study areas was presence of great abundanceof castor plants in clusters which were not connected between the areas.

Experiment
Seed removal by ants on the forest edge

In order to determine whether the elaiosome of castor seeds is attractive
to ants, we compared the removal of seeds with and without elaiosome. One
intact seed and one without the elaiosome (manually removed) were placedside by side (Byrne & Levey 1993) on white filter paper (4 cm x 4 cm). The
white filter paper allows ants interacting with seeds to be more easily visualized (Pizo & Oliveira 2001). There is no interference of filter paper on ants’behavior, once they do not seem to distinguish between filter paper and thelitter around it (Byrne & Levey 1993). We placed 30 samples of filter paperwith seeds around each of the three study areas. There was a 5 m distance
between samples, ensuring that ants of different colonies explore seeds in
different samples (Pizo & Oliveira 2000, 2001). We observed the experimenton an hourly basis between 0830 h and 1830 h and we counted removed
seeds, which were not replaced. We repeated the experiment once for eachSociobiology Vol. 47, No. 1, 2006

area. We used chi-square test to test for differences among seed removal inboth treatments (Zar 1996).
In order to determine whether ants remove castor seeds exclusively to consume the elaiosome and to identify ant species interacting with the seeds, wecreated two experimental groups. The first one was composed by 15 samples
consisting of two intact seeds on white filter paper (4 cm x 4 cm). The second
experimental group was identical but the fact that the elaiosome was removed.
Samples of both groups were placed in alternate fashion around each of thethree study areas. We observed the experiment on an hourly basis between
0830 h and 1830 h and we collected ants interacting with seeds. We usedsoft brush to collect ants and we preserved the specimens in alcohol 70%.
We replaced seeds removed during the experiment and we repeated the experiment once for each area. The specimens were deposited in the Museu deZoologia da Universidade de São Paulo (MZUSP).
Each time both experiments were checked and there were ants interactingwith seeds, we registered what was happening at the moment. So, no further
effort was made to determine ants’ behavior or dispersal distances.

Seed germination in controlled conditions

In order to determine germinative responses of castor seeds to temperature,light and presence or absence of elaiosome, we performed eight germination
treatments. We used two germination cameras with white light, one with aconstant temperature of 30o C and other with alternate temperatures of 20o
C and 30o C. We regulated both cameras for uninterrupted 12 hour lightand 12 hour darkness. In the alternated temperature camera, dark period
was coincident with 20o C period. In order to reproduce natural conditions
with more fidelity, we did not perform any kind of pre treatment with seeds,
such as application of fungicide.
The eight germination treatments consisted in combinations of light (present or absence), temperature (alternated or constant) and elaiosome (present
or absence). Seeds kept in constant darkness were placed in Petri dishes inside
black plastic bags, and seeds exposed to light were placed in Petri dishes insidetransparent plastic bags, only for humidity retention. There were five Petridishes for each treatment, each dish with 25 castor seeds newly harvested,totalizing 125 seeds per treatment. We humidified the Petri dishes with

Martins, V.F. et al. — Seed Dispersal by Ants of Ricinus communis

distilled water whenever necessary. We considered a seed germinated when
there was protrusion of the hipocotile-radicular axis. Seeds kept in constantdarkness were observed inside dark room with green light. We carried out theexperiment for 30 days, recording the number of germinated seeds.
Although we did not measure microhabitats conditions on soil, inside ant
nest and on the pile mound, we assumed three different scenarios to castor
seed germination based on literature: (1) seeds with elaiosome exposed tolight and alternated temperatures, as a result of no interaction between castor
seeds and ants; (2) seeds without elaiosome kept in both constant darkness
and temperature, for those removed to the ant nest but not discarded outside it on a pile mound; (3) seeds without elaiosome exposed to light andalternated temperatures, as a result of being discarded outside ant nest after
elaiosome removal. The effects of light, temperature and presence of elaiosome on germination were investigated using a nominal logistic regression(Dobson 2002).

RESULTS
Ants removed 75.6% of the intact castor seeds and only 40.6% of seeds
without elaiosome in the three study areas (χ2 = 18.990, df = 1, α = 0.05).
Seeds without elaiosome were always removed from the same sample fromwhich intact seeds had been already removed. Ants used the elaiosome as ahandle to carry the seeds to their nest. We did not observe ants detaching theelaiosome from seeds in order to take only the appendage to their nest. Smaller
ants were recorded recruiting many nest mates to samples, while bigger ants
used to remove seeds alone or in small groups of two or three individuals.
No removal was observed when only seeds without elaiosome were placed on
filter paper in the second experiment. Therefore, ants might regard only theelaiosome as food source, without consuming the seed itself. We performed
44 collections of ants interacting with intact castor seeds. We found 20 ant
species of four subfamilies and the more abundant species was the Myrmecinae
Pheidole sp.1, which occurred in 40.91% of collects (Table 1).
We strongly believe that ants were the only agents removing castor seeds
in the study areas. This is because these seeds are highly toxic (Windor 2004)
and we are not aware of any specialized predator or any other possible vector
removing seeds in Santa Genbra. After all, this fragment is extremely affectedSociobiology Vol. 47, No. 1, 2006

Table 1. Occurence of subfamilies and species of ants interacting with seeds of Ricinus communis in three study
areas in Southeast Brazil.
Subfamily

Occurence (%)

			
			
			
			
			
			
			
Myrmicinae
93.18
			
			
			
			
			
			
			
Formicinae
2.27
Ectatomminae
2.27
			
Ponerinae
6.82
			SpeciesOccurence (%)

Pheidole sp.1
Pheidole sp.2
Pheidole sp.3
Pheidole sp.4
Pheidole sp.5
Pheidole sp.6
Pheidole sp.7
Pheidole sp.8
Pheidole sp.9
Solenopsis sp.1
Solenopsis sp.2
Solenopsis sp.3
Solenopsis sp.4
Oligomyrmex eidmanni
Atta sexdens piriventris
Camponotus silvestrii
Ectatomma edentatum
Odontomachus chelifer
Pachycondyla striata
Pachycondyla marginata

40.91
11.36
9.09
6.82
4.55
4.55
2.27
2.27
2.27
6.82
4.55
9.09
2.27
2.27
4.55
2.27
2.27
2.27
2.27
2.27

by anthropogenic disturbance and, especially at the edge, no mammals suchas rodents may be seen during the hours at which the experiments were carried out.
The nominal logistic regression was significant (χ2 = 365. 44, P < 0.0001)
and all factors tested markedly affected the probability of germination. The
probability of germination of castor seeds was enhanced by absence of light(χ2 = 64.57, P < 0.0001, odds ratio = 3.88), by alternated temperatures (χ2= 228.07, P < 0.0001, odds ratio = 15.24) and by removal of the elaiosome(χ2 = 64.57, P = 0.003, odds ratio = 1.62).

DISCUSSION
Castor seeds with elaiosome are more attractive to ants than seeds without
elaiosome, a pattern demonstrated before by other authors for myrmecochorous seeds (O’Dowd & Hay 1980, Davidson & Morton 1981, Lu & Mesler
1981, Stiles 1992, Byrne & Levey 1993). Almost all ants regard elaiosomes
as food sources, as demonstrated by the fact that ants from four subfamilies(Formicinae, Myrmecinae, Ectatomminae and Ponerinae) removed seeds withelaiosome. For some species, lipidic compounds found in the elaiosome act asMartins, V.F. et al. — Seed Dispersal by Ants of Ricinus communis

physiological constituents and behavioral releasers (Beattie 1985, Marshall etal. 1979), Although some ants such as harvest ants are primarily granivores(Holldöbler & Wilson 1990), in our study no ant removed seeds when only
seeds without elaiosome were placed on filter paper on the second experiment, suggesting that ants are acting as legitimate dispersers of castor seeds.
We believe ants may have removed seeds without elaiosome when seeds withand without this appendage were placed on filter paper on the first experiment
due to the chemical stimuli emitted by the elaiosome of the intact seed, inducing ants to remove both seeds in the sample. Besides, Berg (1975) describedthat inexperienced workers may carry seeds without elaiosome to their nest,
whereas more experienced workers exclusively remove intact seeds.
Castor seeds without elaiosome are found on the pile mound. Seeds
discarded outside ant nest after elaiosome removal is a pattern recorded
for ant-dispersed plants (Berg 1975, Handel 1978, Culver & Beattie 1978,
Horvitz & Beattie 1980, Horvitz 1981, Davidson & Morton 1981). Outside the ant nest, discarded seeds are exposed to other agents transportation(Stebbins 1974, Horvitz & Beattie 1980, O’Dowd & Hay 1980), but may as
well germinate on the pile mound, where soil usually shows higher concentration of organic matter and nutrients than its vicinities (Culver & Beattie
1978, Davidson & Morton 1981, Horvitz 1981, Horvitz & Schemske 1986,
Higashi et al. 1989).
It is often found in literature that seed cleaning by ants increases germination success by allowing water and nutrients to enter the seed or by reducing
pathogen attack that begins in the fleshy portion of the seed (Augspurger
1990, Oliveira et al. 1995, Pizo & Oliveira 1998, Guimarães & Cogni 2002,
Passos & Oliveira 2002). Studies show that the elaiosome of castor seed plays
an important role in absorbing water from the soil and transferring it to therest of the seed during germination, thus, enhancing germination success (Lisciet al. 1995, Bianchini & Pacini 1996). Although, our results do not confirm
this and are more accordingly to the results of Lagôa & Pereira (1987). Theseauthors found that elaiosome removal of castor seed speeded germination
rate, since inhibitory substances are found in the elaiosome. Thus, the removalof this appendage by ants may be determinant to the germination of viable
seeds in natural systems (Lagôa & Pereira 1987).

Sociobiology Vol. 47, No. 1, 2006

The higher germination success of seeds in the absence of light and underalternate temperatures suggests that the best scenarios for castor seed germination is a mixture of the predicted ones and is met by seeds discarded
from ant nest, but covered by a thin layer of litter, or eventually abandoned
inside the nest, but in shallow depths. In these situations castor seeds may
benefit from temperature and light conditions. Many species that respondto alternate temperatures form seed banks, an important mechanism that
limits seedling emergence to suitable sites for sapling establishment (Fenner
1985, Pons 1992). Therefore, castor seed germinative response and its highlongevity (I.F.M. Válio, pers. comm.) indicate seed bank formation.
In conclusion, the elaiosome of castor seed is attractive to different species of ants. By discarding seeds without elaiosome on the pile mound, ants
constraint the intrinsic inhibition for seed germination and may submit theseeds to conditions that enhance germination success. Ungerminated seeds on
the pile mound may be incorporated into soil to form seed banks, a temporaldispersal mechanism still to be studied for castor.

ACKNOWLEDGMENTS
The authors wish to thank Dr. Ivany F.M. Válio for suggestions and technical assistance at the Laboratório de sementes do Departamento de Fisiologia
Vegetal da Universidade Estadual de Campinas, and the Muzeu de Zoologia
da Universidade de São Paulo. Thanks are also to Fundação José Pedro de
Oliveira for logistic support in the field work. V.F. Martins and P.R. Guimarães
were supported by FAPESP grants. please spell out acronym

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