44THE S.A. ORNITHOLOGIST

October, 1957

NEOTENY AND THE EVOLUTION OF THERATITES
The origin of the large flightless birds
and whether they are descended from flyingancestors have long been debatable questions. (1) (2)Nothing has been found in the fossil
record which might indicate the nature of
ratite phylogeny; although Wetmore (3) has
painted a vivid picture of an "ostrichlikeform" Eleutheromis heloeticus Schaub (1941)
which was named on the basis of a "fairly
well preserved pelvis" from the Eocene ofSwitzerland. This bird, which lived perhaps50 million years ago, is said to have had"affinities with the carinate groups, while
being at the same time of unquestioned
ratite stock."
Wetmore regards it as "alink that decides the long debate on theancestry of the struthious birds, since it
indicates clearly that these flightless groups
of modern times have come from flying
ancestors . . ."(l) TheEvolution of Ratites, by Sir Gavinde Beer, F.R.S. 1956. Bull. Brit. Mus.
(N.H.) zs«, 4, ii, p. 57·70, pl. 5·9.

(2)

The word Ratite is used in a vernacularsense only, nowadays, for what must be
regarded as a polyphyletic group composed of the following orders: Struthiones, Apteryges, Casuarii, Rheae, andCrypturi; some would combine the last
two which are South American.
TheRatites are also referred to by various
authors as "Struthionids," or "Palaeognaths",(3)

Wetmore, A. 1950. Presidential Address.
The Xth Internat. Ornith. Congress.
Uppsala.

The opposite view was strongly presented
by the late P. R. Lowe in a series of papers
between the years 1928 and 1944, and heclaimed that his morphological studies proved
the 'primitiveness' of the characters ofpresent-day Ratites which he described as'dinosaurian'.
The features which Loweregarded as primitive included, among
others, the sternum, the sternal callosities,
wing, cranial sutures which persist at least
until senescence, fluffy downlike feathers
without barbules in the adult, and the formof the palate.
This last-named structure
is often quoted in connection with the higher
It has been usedclassification of birds.
to define the Ratites as a separate group,
the super-order Palaeognathae, but as pointed
out in the recent work of McDowell (1948)
and Hofer (1949) the "palaeognathous"
palate is undefinable, and it is possible torecognise at least four well- defined palatal
types among the Ratites.
Lowe described the Struthious birds as"surviving relics" and was of the opinion
that their evolution towards flight "was permanently retarded or compromised by somefactor or other millions of years ago before
flight had been evolved" in birds. At thepresent time the concensus of zoologists isthat Lowe's suggestions are not acceptable.
The question is taken up by Sir Gavin deBeer (1956), who considers that, now thatthe anatomy of the fossil toothed-bird
Archaeopteryx is adequately known, a rigorous analysis of the characters of the Ratites
in the light of conditions shown by primitive
birds indicates that they are birds whichhave "abrogated" the power of flight. Indeed, the' unavoidable conclusion is thatthey have degenerated from flying birds, while

October, 1957

THE S·:A. ORNITHOLOGIST

still retaining a nu~ber ?£ st~ucturalfeatures in common with flying birds orCarinates and which are intimately connectedwith the requirements of active flight. Amongthese are the fused condition of the wmg
bones in the carpal region to form a very
characteristic avian structure (the carpometacarpus), the short tai~ a~d 'pygostyle, the ala
spuria or 'bastard wing (in Rhea), and ~nenlarged cerebellum.
It now seems certainthat the large cerebellum in Carinates is connected with the need for a proper meansof controlling equilibrati~n and flight. .A
similar development of this part of the brain
occurred in the Mesozoic flying reptiles orPterodactyls, where are found many analogies in structure with birds, but indepenIn Archaeopteryx. on thedently derived.
other hand, the cerebellum was small andthere is good reason. to believe that it was
not an active- flier, although it possessedfeathers exactly identical to those of
Carinates. For one thing, the forelimb orwing skeleton differed markedly from thatof all flying birds, and could not have exer~ed
or withstood the pressures connected WIthactive flight.
The tail was very long, aslong as the rest of the hody, with 20
elongated. vertebrae, and 15 pairs of rectrices,
one pair to each of the hinder vertebrae, andarranged to form a palm-like tail. Doubtless, Archaeopteryx could do little morethan glide, and if we regard this primitive
Jurassic bird as the type of structure fromwhich Carinates evolved, then many of thecharacters in which the last-named differ
from it must be attributed to adaption toactive flight.
In Rhea, where the wing iswell-developed, the young show' differentiationof primary and secondary remiges as wellas wing coverts and a 'bastard wing' (seede Beer, plate 7). This last-named structure
is formed by the attachment of a small
number of feathers to the "first" digit ofthe "hand" and seeing that in Garinates it
has the sole purpose of maintaining the slipstream during flight, its presence in Rhea
would appear to be highly significant.
The occurrence of vestiges of diastataxy
or aquintocubitalism in young Apteryx andother Ratites may be further evidence that
their ancestors were not cursorial. Diastataxy is a condition, found in a large majorityof flying birds, in which the fifth cubital(I,e. fifth secondary attached to the ulna) isabsent, although the corresponding majorcovert' 'is present.
It is supposed to be45

connected with the peculiar method of folding
the wing in flying birds, and is a primit.ivecondition. The presence of the fifth cubitalin some birds, such as cuckoos, is a secondary
readjustment. In n;any ways .th~ course of
Ratite morphogeneSIs IS so similar to theCarinate that it would be inexplicable unlessthey were descended from flying birds.
At one time the belief was firmly heldthat the presence of large flightless b!rds inthe continents of the southern hemisphere
was evidence of a much greater extent ofthese land masses in the geologic past, or atleast of former land bridges or connections.
It is now recognised that the large flightlessbirds are not very closely related, and thatthe forms confined to the various regions
It is
may have arisen independently.
unnecessary, therefore, to postulate former
land connections to explain their distribution, and the theory has been finally disposedof, for the benefit of geologists, by Prof. E.
Mayr (1944).
However, the presence ofmoas and kiwis in New Zealand still requires.
proper explanation. Geologists maintain that
New Zealand has had no land connections
with any continent since early Jurassic times(perhaps 150 million years ago), which
would mean that the ancestors of the apterygious forms must have flown there.
This
is a question which lends itself tospeculation.
Recently there have beenhints, from various quarters, that the land
surfaces of the unstable Pacific regions wereonce more extensive than they are at present,
in which case it is unnecessary and even alittle misleading to mention the present
isolation of New Zealand in connection withRatite evolution as was done by Broom(1947) .
According to Dr. de Beer the "primitivefeatures" of the Ratites are, in fact, persistent.juvenile characters whose presence can be
explained by a process known as neoteny.
Neoteny is said to occur when the adultanimal hears features so that it resembles
the young form of its ancestors. It is now
considered that this process may have played
an important part in the evolution of many
species, including Man. The classic exampleof neoteny is the North American Tiger
Salamander Amblystoma tigrinum; which in
one part of its range (the United States) isa lung-breather and spends much of its timeon land. In Mexico, this same species (butknown as the Axolotl) usually becomessexually mature' and breeds in the large-

THE S.A. ORNITHOLOGIST
headed, . gill.breathi~g larval stage; meta.
morphosis can be .mduced by feeding on
the ~ormone thyroxm.- The axolotl is only
~artIally neotenous, but there are other newt.
like forms (e.g, Proteus) in which the conditio~ is permanent.
Neoteny, which isbeheved to be under genetic control can bedefined a,s precocious development' of thereproductive organs accompanied by arelative reduction in the speed of bodily
growth.
Usually it is not limited to one
specific feature, neotenic structures inexternal form perhaps accompanying others
not so easily observed.
Neoteny has been
described as a phenomenon which offers ameans of escape from extinction for animals
highly specialised in the adult condition,
providing, of course, the young' stages areless or differently adapted and possess agen.e complex which will sooner or later
affect the rates of different developmentalprocesses.
In the Ratites the possession of downlikefeathers, persistent cranial sutures, and atype of palate formation associated with areduced mobility of the upper jaw, are structures which in Carinates are temporary andembryonic or juvenile, and therefore thestruthious birds can be regarded as "gigantic
neotenous chickens."
Such a view is in
direct contradiction of the now discarded"Theory of Recapitulation" which has been
attributed to Haeckel (1866) but was in fact
propounded more than half a century before.
This so-called "law" stated in effect that thedevelopmental history of the individual
repeats the evolutionary history of thespecies.
As pointed out by Dobzhansky(1955) the. "law" does contain an important"kernel of truth," hut if followed literally

October, 1957'

then many so-called evolutionary sequences
deduced from comparative morphology aref~lse .and should be read in the opposite
direction,
Dr. de Beer's welcome contribution to theproblem of bird origins is a re-affirmationof his belief in the importance of neoteny
and paedomorphosis as major evolutionary
processes and one which he has consistently
held for many years. Much of the evidence
is .purely circumstantial and it might beobjected that the" validity of the conclusions
r~ached is open to question pending thediscovery of more fossils.
Wetmore as aresult of the finding of the Eocene' fossil
Eleutherornis helveticus) is satisfied that thestruthious birds have come from flying
ancestors and Craigie (1936, 1940), working
on !he .cerebral cortex of birds, found nothing
to indicate that they had an origin distinct
from Carinates.
The principle of neotenysuggests that the evolutionary changes whichgave rise to the Ratites were retrogressive incharacter but associated with adaption to aparticular mode of life.
Although the superficial resemblances hetween the largest forms are strong there seems
to be little doubt that here is an excellent
example of convergent evolution and thevarious orders are not closely related, except
through ancestral groups which may havepossessed genetic constitutions with similar
Edinger (1942),
potentialities for change.
after a general survey of living and fossil
giant animals, has made the interesting suggestion that large size, plumage charactersand other peculiarities in the Ratites may
be the result of pituitary disturbances.H. T. CONDON.