SECONDARY SEXUAL CHARACTERS OF FISHES, AMPHIBIANS, AND REPTILES.
FISHES: Courtship and battles of the males—Larger size of the
females—Males, bright colours and ornamental appendages; other strange
characters—Colours and appendages acquired by the males during the
breeding-season alone—Fishes with both sexes brilliantly
coloured—Protective colours—The less conspicuous colours of the female
cannot be accounted for on the principle of protection—Male fishes
building nests, and taking charge of the ova and young.
AMPHIBIANS: Differences in structure and colour between the sexes—Vocal
organs.
REPTILES: Chelonians—Crocodiles—Snakes, colours in some cases
protective—Lizards, battles of—Ornamental appendages—Strange
differences in structure between the sexes—Colours—Sexual differences
almost as great as with birds.
We have now arrived at the great sub-kingdom of the Vertebrata, and
will commence with the lowest class, that of fishes. The males of
Plagiostomous fishes (sharks, rays) and of Chimaeroid fishes are
provided with claspers which serve to retain the female, like the
various structures possessed by many of the lower animals. Besides the
claspers, the males of many rays have clusters of strong sharp spines
on their heads, and several rows along “the upper outer surface of
their pectoral fins.” These are present in the males of some species,
which have other parts of their bodies smooth. They are only
temporarily developed during the breeding-season; and Dr. Gunther
suspects that they are brought into action as prehensile organs by the
doubling inwards and downwards of the two sides of the body. It is a
remarkable fact that the females and not the males of some species, as
of Raia clavata, have their backs studded with large hook-formed
spines. (1. Yarrell’s ‘Hist. of British Fishes,’ vol. ii. 1836, pp 417,
425, 436. Dr. Gunther informs me that the spines in R. clavata are
peculiar to the female.)
The males alone of the capelin (Mallotus villosus, one of Salmonidae),
are provided with a ridge of closely-set, brush-like scales, by the aid
of which two males, one on each side, hold the female, whilst she runs
with great swiftness on the sandy beach, and there deposits her spawn.
(2. The ‘American Naturalist,’ April 1871, p. 119.) The widely distinct
Monacanthus scopas presents a somewhat analogous structure. The male,
as Dr. Gunther informs me, has a cluster of stiff, straight spines,
like those of a comb, on the sides of the tail; and these in a specimen
six inches long were nearly one and a half inches in length; the female
has in the same place a cluster of bristles, which may be compared with
those of a tooth-brush. In another species, M. peronii, the male has a
brush like that possessed by the female of the last species, whilst the
sides of the tail in the female are smooth. In some other species of
the same genus the tail can be perceived to be a little roughened in
the male and perfectly smooth in the female; and lastly in others, both
sexes have smooth sides.
The males of many fish fight for the possession of the females. Thus
the male stickleback (Gasterosteus leiurus) has been described as “mad
with delight,” when the female comes out of her hiding-place and
surveys the nest which he has made for her. “He darts round her in
every direction, then to his accumulated materials for the nest, then
back again in an instant; and as she does not advance he endeavours to
push her with his snout, and then tries to pull her by the tail and
side-spine to the nest.” (3. See Mr. R. Warington’s interesting
articles in ‘Annals and Magazine of Natural History,’ October 1852, and
November 1855.) The males are said to be polygamists (4. Noel
Humphreys, ‘River Gardens,’ 1857.); they are extraordinarily bold and
pugnacious, whilst “the females are quite pacific.” Their battles are
at times desperate; “for these puny combatants fasten tight on each
other for several seconds, tumbling over and over again until their
strength appears completely exhausted.” With the rough-tailed
stickleback (G. trachurus) the males whilst fighting swim round and
round each other, biting and endeavouring to pierce each other with
their raised lateral spines. The same writer adds (5. Loudon’s
‘Magazine of Natural History,’ vol. iii. 1830, p. 331.), “the bite of
these little furies is very severe. They also use their lateral spines
with such fatal effect, that I have seen one during a battle absolutely
rip his opponent quite open, so that he sank to the bottom and died.”
When a fish is conquered, “his gallant bearing forsakes him; his gay
colours fade away; and he hides his disgrace among his peaceable
companions, but is for some time the constant object of his conqueror’s
persecution.”
The male salmon is as pugnacious as the little stickleback; and so is
the male trout, as I hear from Dr. Gunther. Mr. Shaw saw a violent
contest between two male salmon which lasted the whole day; and Mr. R.
Buist, Superintendent of Fisheries, informs me that he has often
watched from the bridge at Perth the males driving away their rivals,
whilst the females were spawning. The males “are constantly fighting
and tearing each other on the spawning-beds, and many so injure each
other as to cause the death of numbers, many being seen swimming near
the banks of the river in a state of exhaustion, and apparently in a
dying state.” (6. The ‘Field,’ June 29, 1867. For Mr. Shaw’s Statement,
see ‘Edinburgh Review,’ 1843. Another experienced observer (Scrope’s
‘Days of Salmon Fishing,’ p. 60) remarks that like the stag, the male
would, if he could, keep all other males away.) Mr. Buist informs me,
that in June 1868, the keeper of the Stormontfield breeding-ponds
visited the northern Tyne and found about 300 dead salmon, all of which
with one exception were males; and he was convinced that they had lost
their lives by fighting.
[Fig. 27. Head of male common salmon (Salmo salar) during the
breeding-season. [This drawing, as well as all the others in the
present chapter, have been executed by the well-known artist, Mr. G.
Ford, from specimens in the British Museum, under the kind
superintendence of Dr. Gunther.]
Fig. 28. Head of female salmon.]
The most curious point about the male salmon is that during the
breeding-season, besides a slight change in colour, “the lower jaw
elongates, and a cartilaginous projection turns upwards from the point,
which, when the jaws are closed, occupies a deep cavity between the
intermaxillary bones of the upper jaw.” (7. Yarrell, ‘History of
British Fishes,’ vol. ii. 1836, p. 10.) (Figs. 27 and 28.) In our
salmon this change of structure lasts only during the breeding-season;
but in the Salmo lycaodon of N.W. America the change, as Mr. J.K. Lord
(8. ‘The Naturalist in Vancouver’s Island,’ vol. i. 1866, p. 54.)
believes, is permanent, and best marked in the older males which have
previously ascended the rivers. In these old males the jaw becomes
developed into an immense hook-like projection, and the teeth grow into
regular fangs, often more than half an inch in length. With the
European salmon, according to Mr. Lloyd (9. ‘Scandinavian Adventures,’
vol. i. 1854, pp. 100, 104.), the temporary hook-like structure serves
to strengthen and protect the jaws, when one male charges another with
wonderful violence; but the greatly developed teeth of the male
American salmon may be compared with the tusks of many male mammals,
and they indicate an offensive rather than a protective purpose.
The salmon is not the only fish in which the teeth differ in the two
sexes; as this is the case with many rays. In the thornback (Raia
clavata) the adult male has sharp, pointed teeth, directed backwards,
whilst those of the female are broad and flat, and form a pavement; so
that these teeth differ in the two sexes of the same species more than
is usual in distinct genera of the same family. The teeth of the male
become sharp only when he is adult: whilst young they are broad and
flat like those of the female. As so frequently occurs with secondary
sexual characters, both sexes of some species of rays (for instance R.
batis), when adult, possess sharp pointed teeth; and here a character,
proper to and primarily gained by the male, appears to have been
transmitted to the offspring of both sexes. The teeth are likewise
pointed in both sexes of R. maculata, but only when quite adult; the
males acquiring them at an earlier age than the females. We shall
hereafter meet with analogous cases in certain birds, in which the male
acquires the plumage common to both sexes when adult, at a somewhat
earlier age than does the female. With other species of rays the males
even when old never possess sharp teeth, and consequently the adults of
both sexes are provided with broad, flat teeth like those of the young,
and like those of the mature females of the above-mentioned species.
(10. See Yarrell’s account of the rays in his ‘History of British
Fishes,’ vol. ii. 1836, p. 416, with an excellent figure, and pp. 422,
432.) As the rays are bold, strong and voracious fish, we may suspect
that the males require their sharp teeth for fighting with their
rivals; but as they possess many parts modified and adapted for the
prehension of the female, it is possible that their teeth may be used
for this purpose.
In regard to size, M. Carbonnier (11. As quoted in ‘The Farmer,’ 1868,
p. 369.) maintains that the female of almost all fishes is larger than
the male; and Dr. Gunther does not know of a single instance in which
the male is actually larger than the female. With some Cyprinodonts the
male is not even half as large. As in many kinds of fishes the males
habitually fight together, it is surprising that they have not
generally become larger and stronger than the females through the
effects of sexual selection. The males suffer from their small size,
for according to M. Carbonnier, they are liable to be devoured by the
females of their own species when carnivorous, and no doubt by other
species. Increased size must be in some manner of more importance to
the females, than strength and size are to the males for fighting with
other males; and this perhaps is to allow of the production of a vast
number of ova.
[Fig. 29. Callionymus lyra. Upper figure, male; lower figure, female.
N.B. The lower figure is more reduced than the upper.]
In many species the male alone is ornamented with bright colours; or
these are much brighter in the male than the female. The male, also, is
sometimes provided with appendages which appear to be of no more use to
him for the ordinary purposes of life, than are the tail feathers to
the peacock. I am indebted for most of the following facts to the
kindness of Dr. Gunther. There is reason to suspect that many tropical
fishes differ sexually in colour and structure; and there are some
striking cases with our British fishes. The male Callionymus lyra has
been called the gemmeous dragonet “from its brilliant gem-like
colours.” When fresh caught from the sea the body is yellow of various
shades, striped and spotted with vivid blue on the head; the dorsal
fins are pale brown with dark longitudinal bands; the ventral, caudal,
and anal fins being bluish-black. The female, or sordid dragonet, was
considered by Linnaeus, and by many subsequent naturalists, as a
distinct species; it is of a dingy reddish-brown, with the dorsal fin
brown and the other fins white. The sexes differ also in the
proportional size of the head and mouth, and in the position of the
eyes (12. I have drawn up this description from Yarrell’s ‘British
Fishes,’ vol. i. 1836, pp. 261 and 266.); but the most striking
difference is the extraordinary elongation in the male (Fig. 29) of the
dorsal fin. Mr. W. Saville Kent remarks that this “singular appendage
appears from my observations of the species in confinement, to be
subservient to the same end as the wattles, crests, and other abnormal
adjuncts of the male in gallinaceous birds, for the purpose of
fascinating their mates.” (13. ‘Nature,’ July 1873, p. 264.) The young
males resemble the adult females in structure and colour. Throughout
the genus Callionymus (14. ‘Catalogue of Acanth. Fishes in the British
Museum,’ by Dr. Gunther, 1861, pp. 138-151.), the male is generally
much more brightly spotted than the female, and in several species, not
only the dorsal, but the anal fin is much elongated in the males.
The male of the Cottus scorpius, or sea-scorpion, is slenderer and
smaller than the female. There is also a great difference in colour
between them. It is difficult, as Mr. Lloyd (15. ‘Game Birds of
Sweden,’ etc., 1867, p. 466.) remarks, “for any one, who has not seen
this fish during the spawning-season, when its hues are brightest, to
conceive the admixture of brilliant colours with which it, in other
respects so ill-favoured, is at that time adorned.” Both sexes of the
Labrus mixtus, although very different in colour, are beautiful; the
male being orange with bright blue stripes, and the female bright red
with some black spots on the back.
[Fig. 30. Xiphophorus Hellerii. Upper figure, male; lower figure,
female.]
In the very distinct family of the Cyprinodontidae—inhabitants of the
fresh waters of foreign lands—the sexes sometimes differ much in
various characters. In the male of the Mollienesia petenensis (16. With
respect to this and the following species I am indebted to Dr. Gunther
for information: see also his paper on the ‘Fishes of Central America,’
in ‘Transact. Zoological Soc.’ vol. vi. 1868, p. 485.), the dorsal fin
is greatly developed and is marked with a row of large, round,
ocellated, bright-coloured spots; whilst the same fin in the female is
smaller, of a different shape, and marked only with irregularly curved
brown spots. In the male the basal margin of the anal fin is also a
little produced and dark coloured. In the male of an allied form, the
Xiphophorus Hellerii (Fig. 30), the inferior margin of the caudal fin
is developed into a long filament, which, as I hear from Dr. Gunther,
is striped with bright colours. This filament does not contain any
muscles, and apparently cannot be of any direct use to the fish. As in
the case of the Callionymus, the males whilst young resemble the adult
females in colour and structure. Sexual differences such as these may
be strictly compared with those which are so frequent with gallinaceous
birds. (17. Dr. Gunther makes this remark; ‘Catalogue of Fishes in the
British Museum,’ vol. iii. 1861, p. 141.)
[Fig.31. Plecostomus barbatus. Upper figure, head of male; lower
figure, female.]
In a siluroid fish, inhabiting the fresh waters of South America, the
Plecostomus barbatus (18. See Dr. Gunther on this genus, in
‘Proceedings of the Zoological Society,’ 1868, p. 232.) (Fig. 31), the
male has its mouth and inter-operculum fringed with a beard of stiff
hairs, of which the female shows hardly a trace. These hairs are of the
nature of scales. In another species of the same genus, soft flexible
tentacles project from the front part of the head of the male, which
are absent in the female. These tentacles are prolongations of the true
skin, and therefore are not homologous with the stiff hairs of the
former species; but it can hardly be doubted that both serve the same
purpose. What this purpose may be, it is difficult to conjecture;
ornament does not here seem probable, but we can hardly suppose that
stiff hairs and flexible filaments can be useful in any ordinary way to
the males alone. In that strange monster, the Chimaera monstrosa, the
male has a hook-shaped bone on the top of the head, directed forwards,
with its end rounded and covered with sharp spines; in the female “this
crown is altogether absent,” but what its use may be to the male is
utterly unknown. (19. F. Buckland, in ‘Land and Water,’ July 1868, p.
377, with a figure. Many other cases could be added of structures
peculiar to the male, of which the uses are not known.)
The structures as yet referred to are permanent in the male after he
has arrived at maturity; but with some Blennies, and in another allied
genus (20. Dr. Gunther, ‘Catalogue of Fishes,’ vol. iii. pp. 221 and
240.), a crest is developed on the head of the male only during the
breeding-season, and the body at the same time becomes more
brightly-coloured. There can be little doubt that this crest serves as
a temporary sexual ornament, for the female does not exhibit a trace of
it. In other species of the same genus both sexes possess a crest, and
in at least one species neither sex is thus provided. In many of the
Chromidae, for instance in Geophagus and especially in Cichla, the
males, as I hear from Professor Agassiz (21. See also ‘A Journey in
Brazil,’ by Prof. and Mrs. Agassiz, 1868, p. 220.), have a conspicuous
protuberance on the forehead, which is wholly wanting in the females
and in the young males. Professor Agassiz adds, “I have often observed
these fishes at the time of spawning when the protuberance is largest,
and at other seasons when it is totally wanting, and the two sexes shew
no difference whatever in the outline of the profile of the head. I
never could ascertain that it subserves any special function, and the
Indians on the Amazon know nothing about its use.” These protuberances
resemble, in their periodical appearance, the fleshy carbuncles on the
heads of certain birds; but whether they serve as ornaments must remain
at present doubtful.
I hear from Professor Agassiz and Dr. Gunther, that the males of those
fishes, which differ permanently in colour from the females, often
become more brilliant during the breeding-season. This is likewise the
case with a multitude of fishes, the sexes of which are identical in
colour at all other seasons of the year. The tench, roach, and perch
may be given as instances. The male salmon at this season is “marked on
the cheeks with orange-coloured stripes, which give it the appearance
of a Labrus, and the body partakes of a golden orange tinge. The
females are dark in colour, and are commonly called black-fish.” (22.
Yarrell, ‘History of British Fishes,’ vol. ii. 1836, pp. 10, 12, 35.)
An analogous and even greater change takes place with the Salmo eriox
or bull trout; the males of the char (S. umbla) are likewise at this
season rather brighter in colour than the females. (23. W. Thompson, in
‘Annals and Magazine of Natural History,’ vol. vi. 1841, p. 440.) The
colours of the pike (Esox reticulatus) of the United States, especially
of the male, become, during the breeding-season, exceedingly intense,
brilliant, and iridescent. (24. ‘The American Agriculturalist,’ 1868,
p. 100.) Another striking instance out of many is afforded by the male
stickleback (Gasterosteus leiurus), which is described by Mr. Warington
(25. ‘Annals and Mag. of Nat. Hist.’ Oct. 1852.), as being then
“beautiful beyond description.” The back and eyes of the female are
simply brown, and the belly white. The eyes of the male, on the other
hand, are “of the most splendid green, having a metallic lustre like
the green feathers of some humming-birds. The throat and belly are of a
bright crimson, the back of an ashy-green, and the whole fish appears
as though it were somewhat translucent and glowed with an internal
incandescence.” After the breeding season these colours all change, the
throat and belly become of a paler red, the back more green, and the
glowing tints subside.
With respect to the courtship of fishes, other cases have been observed
since the first edition of this book appeared, besides that already
given of the stickleback. Mr. W.S. Kent says that the male of the
Labrus mixtus, which, as we have seen, differs in colour from the
female, makes “a deep hollow in the sand of the tank, and then
endeavours in the most persuasive manner to induce a female of the same
species to share it with him, swimming backwards and forwards between
her and the completed nest, and plainly exhibiting the greatest anxiety
for her to follow.” The males of Cantharus lineatus become, during the
breeding-season, of deep leaden-black; they then retire from the shoal,
and excavate a hollow as a nest. “Each male now mounts vigilant guard
over his respective hollow, and vigorously attacks and drives away any
other fish of the same sex. Towards his companions of the opposite sex
his conduct is far different; many of the latter are now distended with
spawn, and these he endeavours by all the means in his power to lure
singly to his prepared hollow, and there to deposit the myriad ova with
which they are laden, which he then protects and guards with the
greatest care.” (26. ‘Nature,’ May 1873, p. 25.)
A more striking case of courtship, as well as of display, by the males
of a Chinese Macropus has been given by M. Carbonnier, who carefully
observed these fishes under confinement. (27. ‘Bulletin de la Societé
d’Acclimat.’ Paris, July 1869, and Jan. 1870.) The males are most
beautifully coloured, more so than the females. During the
breeding-season they contend for the possession of the females; and, in
the act of courtship, expand their fins, which are spotted and
ornamented with brightly coloured rays, in the same manner, according
to M. Carbonnier, as the peacock. They then also bound about the
females with much vivacity, and appear by “l’étalage de leurs vives
couleurs chercher a attirer l’attention des femelles, lesquelles ne
paraissaient indifférentes a ce manège, elles nageaient avec une molle
lenteur vers les males et semblaient se complaire dans leur voisinage.”
After the male has won his bride, he makes a little disc of froth by
blowing air and mucus out of his mouth. He then collects the fertilised
ova, dropped by the female, in his mouth; and this caused M. Carbonnier
much alarm, as he thought that they were going to be devoured. But the
male soon deposits them in the disc of froth, afterwards guarding them,
repairing the froth, and taking care of the young when hatched. I
mention these particulars because, as we shall presently see, there are
fishes, the males of which hatch their eggs in their mouths; and those
who do not believe in the principle of gradual evolution might ask how
could such a habit have originated; but the difficulty is much
diminished when we know that there are fishes which thus collect and
carry the eggs; for if delayed by any cause in depositing them, the
habit of hatching them in their mouths might have been acquired.
To return to our more immediate subject. The case stands thus: female
fishes, as far as I can learn, never willingly spawn except in the
presence of the males; and the males never fertilise the ova except in
the presence of the females. The males fight for the possession of the
females. In many species, the males whilst young resemble the females
in colour; but when adult become much more brilliant, and retain their
colours throughout life. In other species the males become brighter
than the females and otherwise more highly ornamented, only during the
season of love. The males sedulously court the females, and in one
case, as we have seen, take pains in displaying their beauty before
them. Can it be believed that they would thus act to no purpose during
their courtship? And this would be the case, unless the females exert
some choice and select those males which please or excite them most. If
the female exerts such choice, all the above facts on the ornamentation
of the males become at once intelligible by the aid of sexual
selection.
We have next to inquire whether this view of the bright colours of
certain male fishes having been acquired through sexual selection can,
through the law of the equal transmission of characters to both sexes,
be extended to those groups in which the males and females are
brilliant in the same, or nearly the same degree and manner. In such a
genus as Labrus, which includes some of the most splendid fishes in the
world—for instance, the Peacock Labrus (L. pavo), described (28. Bory
Saint Vincent, in ‘Dict. Class. d’Hist. Nat.’ tom. ix. 1826, p. 151.),
with pardonable exaggeration, as formed of polished scales of gold,
encrusting lapis-lazuli, rubies, sapphires, emeralds, and amethysts—we
may, with much probability, accept this belief; for we have seen that
the sexes in at least one species of the genus differ greatly in
colour. With some fishes, as with many of the lowest animals, splendid
colours may be the direct result of the nature of their tissues and of
the surrounding conditions, without the aid of selection of any kind.
The gold-fish (Cyprinus auratus), judging from the analogy of the
golden variety of the common carp, is perhaps a case in point, as it
may owe its splendid colours to a single abrupt variation, due to the
conditions to which this fish has been subjected under confinement. It
is, however, more probable that these colours have been intensified
through artificial selection, as this species has been carefully bred
in China from a remote period. (29. Owing to some remarks on this
subject, made in my work ‘On the Variation of Animals under
Domestication,’ Mr. W.F. Mayers (‘Chinese Notes and Queries,’ Aug.
1868, p. 123) has searched the ancient Chinese encyclopedias. He finds
that gold-fish were first reared in confinement during the Sung
Dynasty, which commenced A.D. 960. In the year 1129 these fishes
abounded. In another place it is said that since the year 1548 there
has been “produced at Hangchow a variety called the fire-fish, from its
intensely red colour. It is universally admired, and there is not a
household where it is not cultivated, IN RIVALRY AS TO ITS COLOUR, and
as a source of profit.”) Under natural conditions it does not seem
probable that beings so highly organised as fishes, and which live
under such complex relations, should become brilliantly coloured
without suffering some evil or receiving some benefit from so great a
change, and consequently without the intervention of natural selection.
What, then, are we to conclude in regard to the many fishes, both sexes
of which are splendidly coloured? Mr. Wallace (30. ‘Westminster
Review,’ July 1867, p. 7.) believes that the species which frequent
reefs, where corals and other brightly-coloured organisms abound, are
brightly coloured in order to escape detection by their enemies; but
according to my recollection they were thus rendered highly
conspicuous. In the fresh-waters of the tropics there are no
brilliantly-coloured corals or other organisms for the fishes to
resemble; yet many species in the Amazons are beautifully coloured, and
many of the carnivorous Cyprinidae in India are ornamented with “bright
longitudinal lines of various tints.” (31. ‘Indian Cyprinidae,’ by Mr.
M’Clelland, ‘Asiatic Researches,’ vol. xix. part ii. 1839, p. 230.) Mr.
M’Clelland, in describing these fishes, goes so far as to suppose that
“the peculiar brilliancy of their colours” serves as “a better mark for
king-fishers, terns, and other birds which are destined to keep the
number of these fishes in check”; but at the present day few
naturalists will admit that any animal has been made conspicuous as an
aid to its own destruction. It is possible that certain fishes may have
been rendered conspicuous in order to warn birds and beasts of prey
that they were unpalatable, as explained when treating of caterpillars;
but it is not, I believe, known that any fish, at least any fresh-water
fish, is rejected from being distasteful to fish-devouring animals. On
the whole, the most probable view in regard to the fishes, of which
both sexes are brilliantly coloured, is that their colours were
acquired by the males as a sexual ornament, and were transferred
equally, or nearly so, to the other sex.
We have now to consider whether, when the male differs in a marked
manner from the female in colour or in other ornaments, he alone has
been modified, the variations being inherited by his male offspring
alone; or whether the female has been specially modified and rendered
inconspicuous for the sake of protection, such modifications being
inherited only by the females. It is impossible to doubt that colour
has been gained by many fishes as a protection: no one can examine the
speckled upper surface of a flounder, and overlook its resemblance to
the sandy bed of the sea on which it lives. Certain fishes, moreover,
can through the action of the nervous system change their colours in
adaptation to surrounding objects, and that within a short time. (32.
G. Pouchet, ‘L’Institut.’ Nov. 1, 1871, p. 134.) One of the most
striking instances ever recorded of an animal being protected by its
colour (as far as it can be judged of in preserved specimens), as well
as by its form, is that given by Dr. Gunther (33. ‘Proc. Zoolog. Soc.’
1865, p. 327, pl. xiv. and xv.) of a pipe-fish, which, with its reddish
streaming filaments, is hardly distinguishable from the sea-weed to
which it clings with its prehensile tail. But the question now under
consideration is whether the females alone have been modified for this
object. We can see that one sex will not be modified through natural
selection for the sake of protection more than the other, supposing
both to vary, unless one sex is exposed for a longer period to danger,
or has less power of escaping from such danger than the other; and it
does not appear that with fishes the sexes differ in these respects. As
far as there is any difference, the males, from being generally smaller
and from wandering more about, are exposed to greater danger than the
females; and yet, when the sexes differ, the males are almost always
the more conspicuously coloured. The ova are fertilised immediately
after being deposited; and when this process lasts for several days, as
in the case of the salmon (34. Yarrell, ‘British Fishes,’ vol. ii. p.
11.), the female, during the whole time, is attended by the male. After
the ova are fertilised they are, in most cases, left unprotected by
both parents, so that the males and females, as far as oviposition is
concerned, are equally exposed to danger, and both are equally
important for the production of fertile ova; consequently the more or
less brightly-coloured individuals of either sex would be equally
liable to be destroyed or preserved, and both would have an equal
influence on the colours of their offspring.
Certain fishes, belonging to several families, make nests, and some of
them take care of their young when hatched. Both sexes of the bright
coloured Crenilabrus massa and melops work together in building their
nests with sea-weed, shells, etc. (35. According to the observations of
M. Gerbe; see Gunther’s ‘Record of Zoolog. Literature,’ 1865, p. 194.)
But the males of certain fishes do all the work, and afterwards take
exclusive charge of the young. This is the case with the dull-coloured
gobies (36. Cuvier, ‘Regne Animal,’ vol. ii. 1829, p. 242.), in which
the sexes are not known to differ in colour, and likewise with the
sticklebacks (Gasterosteus), in which the males become brilliantly
coloured during the spawning season. The male of the smooth-tailed
stickleback (G. leiurus) performs the duties of a nurse with exemplary
care and vigilance during a long time, and is continually employed in
gently leading back the young to the nest, when they stray too far. He
courageously drives away all enemies including the females of his own
species. It would indeed be no small relief to the male, if the female,
after depositing her eggs, were immediately devoured by some enemy, for
he is forced incessantly to drive her from the nest. (37. See Mr.
Warington’s most interesting description of the habits of the
Gasterosteus leiurus in ‘Annals and Magazine of Nat. History,’ November
1855.)
The males of certain other fishes inhabiting South America and Ceylon,
belonging to two distinct Orders, have the extraordinary habit of
hatching within their mouths, or branchial cavities, the eggs laid by
the females. (38. Prof. Wyman, in ‘Proc. Boston Soc. of Nat. Hist.’
Sept. 15, 1857. Also Prof. Turner, in ‘Journal of Anatomy and
Physiology,’ Nov. 1, 1866, p. 78. Dr. Gunther has likewise described
other cases.) I am informed by Professor Agassiz that the males of the
Amazonian species which follow this habit, “not only are generally
brighter than the females, but the difference is greater at the
spawning-season than at any other time.” The species of Geophagus act
in the same manner; and in this genus, a conspicuous protuberance
becomes developed on the forehead of the males during the
breeding-season. With the various species of Chromids, as Professor
Agassiz likewise informs me, sexual differences in colour may be
observed, “whether they lay their eggs in the water among aquatic
plants, or deposit them in holes, leaving them to come out without
further care, or build shallow nests in the river mud, over which they
sit, as our Pomotis does. It ought also to be observed that these
sitters are among the brightest species in their respective families;
for instance, Hygrogonus is bright green, with large black ocelli,
encircled with the most brilliant red.” Whether with all the species of
Chromids it is the male alone which sits on the eggs is not known. It
is, however, manifest that the fact of the eggs being protected or
unprotected by the parents, has had little or no influence on the
differences in colour between the sexes. It is further manifest, in all
the cases in which the males take exclusive charge of the nests and
young, that the destruction of the brighter-coloured males would be far
more influential on the character of the race, than the destruction of
the brighter-coloured females; for the death of the male during the
period of incubation or nursing would entail the death of the young, so
that they could not inherit his peculiarities; yet, in many of these
very cases the males are more conspicuously coloured than the females.
In most of the Lophobranchii (Pipe-fish, Hippocampi, etc.) the males
have either marsupial sacks or hemispherical depressions on the
abdomen, in which the ova laid by the female are hatched. The males
also shew great attachment to their young. (39. Yarrell, ‘History of
British Fishes,’ vol. ii. 1836, pp. 329, 338.) The sexes do not
commonly differ much in colour; but Dr. Gunther believes that the male
Hippocampi are rather brighter than the females. The genus Solenostoma,
however, offers a curious exceptional case (40. Dr. Gunther, since
publishing an account of this species in ‘The Fishes of Zanzibar,’ by
Col. Playfair, 1866, p. 137, has re-examined the specimens, and has
given me the above information.), for the female is much more
vividly-coloured and spotted than the male, and she alone has a
marsupial sack and hatches the eggs; so that the female of Solenostoma
differs from all the other Lophobranchii in this latter respect, and
from almost all other fishes, in being more brightly-coloured than the
male. It is improbable that this remarkable double inversion of
character in the female should be an accidental coincidence. As the
males of several fishes, which take exclusive charge of the eggs and
young, are more brightly coloured than the females, and as here the
female Solenostoma takes the same charge and is brighter than the male,
it might be argued that the conspicuous colours of that sex which is
the more important of the two for the welfare of the offspring, must be
in some manner protective. But from the large number of fishes, of
which the males are either permanently or periodically brighter than
the females, but whose life is not at all more important for the
welfare of the species than that of the female, this view can hardly be
maintained. When we treat of birds we shall meet with analogous cases,
where there has been a complete inversion of the usual attributes of
the two sexes, and we shall then give what appears to be the probable
explanation, namely, that the males have selected the more attractive
females, instead of the latter having selected, in accordance with the
usual rule throughout the animal kingdom, the more attractive males.
On the whole we may conclude, that with most fishes, in which the sexes
differ in colour or in other ornamental characters, the males
originally varied, with their variations transmitted to the same sex,
and accumulated through sexual selection by attracting or exciting the
females. In many cases, however, such characters have been transferred,
either partially or completely, to the females. In other cases, again,
both sexes have been coloured alike for the sake of protection; but in
no instance does it appear that the female alone has had her colours or
other characters specially modified for this latter purpose.
The last point which need be noticed is that fishes are known to make
various noises, some of which are described as being musical. Dr.
Dufosse, who has especially attended to this subject, says that the
sounds are voluntarily produced in several ways by different fishes: by
the friction of the pharyngeal bones—by the vibration of certain
muscles attached to the swim bladder, which serves as a resounding
board—and by the vibration of the intrinsic muscles of the swim
bladder. By this latter means the Trigla produces pure and long-drawn
sounds which range over nearly an octave. But the most interesting case
for us is that of two species of Ophidium, in which the males alone are
provided with a sound-producing apparatus, consisting of small movable
bones, with proper muscles, in connection with the swim bladder. (41.
‘Comptes-Rendus,’ tom. xlvi. 1858, p. 353; tom. xlvii. 1858, p. 916;
tom. liv. 1862, p. 393. The noise made by the Umbrinas (Sciaena
aquila), is said by some authors to be more like that of a flute or
organ, than drumming: Dr. Zouteveen, in the Dutch translation of this
work (vol. ii. p. 36), gives some further particulars on the sounds
made by fishes.) The drumming of the Umbrinas in the European seas is
said to be audible from a depth of twenty fathoms; and the fishermen of
Rochelle assert “that the males alone make the noise during the
spawning-time; and that it is possible by imitating it, to take them
without bait.” (42. The Rev. C. Kingsley, in ‘Nature,’ May 1870, p.
40.) From this statement, and more especially from the case of
Ophidium, it is almost certain that in this, the lowest class of the
Vertebrata, as with so many insects and spiders, sound-producing
instruments have, at least in some cases, been developed through sexual
selection, as a means for bringing the sexes together.
AMPHIBIANS.
URODELA.
[Fig. 32. Triton cristatus (half natural size, from Bell’s ‘British
Reptiles’). Upper figure, male during the breeding season; lower
figure, female.]
I will begin with the tailed amphibians. The sexes of salamanders or
newts often differ much both in colour and structure. In some species
prehensile claws are developed on the fore-legs of the males during the
breeding-season: and at this season in the male Triton palmipes the
hind-feet are provided with a swimming-web, which is almost completely
absorbed during the winter; so that their feet then resemble those of
the female. (43. Bell, ‘History of British Reptiles,’ 2nd ed., 1849,
pp. 156-159.) This structure no doubt aids the male in his eager search
and pursuit of the female. Whilst courting her he rapidly vibrates the
end of his tail. With our common newts (Triton punctatus and cristatus)
a deep, much indented crest is developed along the back and tail of the
male during the breeding-season, which disappears during the winter.
Mr. St. George Mivart informs me that it is not furnished with muscles,
and therefore cannot be used for locomotion. As during the season of
courtship it becomes edged with bright colours, there can hardly be a
doubt that it is a masculine ornament. In many species the body
presents strongly contrasted, though lurid tints, and these become more
vivid during the breeding-season. The male, for instance, of our common
little newt (Triton punctatus) is “brownish-grey above, passing into
yellow beneath, which in the spring becomes a rich bright orange,
marked everywhere with round dark spots.” The edge of the crest also is
then tipped with bright red or violet. The female is usually of a
yellowish-brown colour with scattered brown dots, and the lower surface
is often quite plain. (44. Bell, ‘History of British Reptiles,’ 2nd
ed., 1849, pp. 146, 151.) The young are obscurely tinted. The ova are
fertilised during the act of deposition, and are not subsequently
tended by either parent. We may therefore conclude that the males have
acquired their strongly-marked colours and ornamental appendages
through sexual selection; these being transmitted either to the male
offspring alone, or to both sexes.
ANURA OR BATRACHIA.
With many frogs and toads the colours evidently serve as a protection,
such as the bright green tints of tree frogs and the obscure mottled
shades of many terrestrial species. The most conspicuously-coloured
toad which I ever saw, the Phryniscus nigricans (45. ‘Zoology of the
Voyage of the “Beagle,”’ 1843. Bell, ibid. p. 49.), had the whole upper
surface of the body as black as ink, with the soles of the feet and
parts of the abdomen spotted with the brightest vermilion. It crawled
about the bare sandy or open grassy plains of La Plata under a
scorching sun, and could not fail to catch the eye of every passing
creature. These colours are probably beneficial by making this animal
known to all birds of prey as a nauseous mouthful.
In Nicaragua there is a little frog “dressed in a bright livery of red
and blue” which does not conceal itself like most other species, but
hops about during the daytime, and Mr. Belt says (46. ‘The Naturalist
in Nicaragua,’ 1874, p. 321.) that as soon as he saw its happy sense of
security, he felt sure that it was uneatable. After several trials he
succeeded in tempting a young duck to snatch up a young one, but it was
instantly rejected; and the duck “went about jerking its head, as if
trying to throw off some unpleasant taste.”
With respect to sexual differences of colour, Dr. Gunther does not know
of any striking instance either with frogs or toads; yet he can often
distinguish the male from the female by the tints of the former being a
little more intense. Nor does he know of any striking difference in
external structure between the sexes, excepting the prominences which
become developed during the breeding-season on the front legs of the
male, by which he is enabled to hold the female. (47. The male alone of
the Bufo sikimmensis (Dr. Anderson, ‘Proc. Zoolog. Soc.’ 1871, p. 204)
has two plate-like callosities on the thorax and certain rugosities on
the fingers, which perhaps subserve the same end as the above-mentioned
prominences.) It is surprising that these animals have not acquired
more strongly-marked sexual characters; for though cold-blooded their
passions are strong. Dr. Gunther informs me that he has several times
found an unfortunate female toad dead and smothered from having been so
closely embraced by three or four males. Frogs have been observed by
Professor Hoffman in Giessen fighting all day long during the
breeding-season, and with so much violence that one had its body ripped
open.
Frogs and toads offer one interesting sexual difference, namely, in the
musical powers possessed by the males; but to speak of music, when
applied to the discordant and overwhelming sounds emitted by male
bull-frogs and some other species, seems, according to our taste, a
singularly inappropriate expression. Nevertheless, certain frogs sing
in a decidedly pleasing manner. Near Rio Janeiro I used often to sit in
the evening to listen to a number of little Hylae, perched on blades of
grass close to the water, which sent forth sweet chirping notes in
harmony. The various sounds are emitted chiefly by the males during the
breeding-season, as in the case of the croaking of our common frog.
(48. Bell, ‘History British Reptiles,’ 1849, p. 93.) In accordance with
this fact the vocal organs of the males are more highly-developed than
those of the females. In some genera the males alone are provided with
sacs which open into the larynx. (49. J. Bishop, in ‘Todd’s Cyclopaedia
of Anatomy and Physiology,’ vol. iv. p. 1503.) For instance, in the
edible frog (Rana esculenta) “the sacs are peculiar to the males, and
become, when filled with air in the act of croaking, large globular
bladders, standing out one on each side of the head, near the corners
of the mouth.” The croak of the male is thus rendered exceedingly
powerful; whilst that of the female is only a slight groaning noise.
(50. Bell, ibid. pp. 112-114.) In the several genera of the family the
vocal organs differ considerably in structure, and their development in
all cases may be attributed to sexual selection.
REPTILES.
CHELONIA.
Tortoises and turtles do not offer well-marked sexual differences. In
some species, the tail of the male is longer than that of the female.
In some, the plastron or lower surface of the shell of the male is
slightly concave in relation to the back of the female. The male of the
mud-turtle of the United States (Chrysemys picta) has claws on its
front feet twice as long as those of the female; and these are used
when the sexes unite. (51. Mr. C.J. Maynard, ‘The American Naturalist,’
Dec. 1869, p. 555.) With the huge tortoise of the Galapagos Islands
(Testudo nigra) the males are said to grow to a larger size than the
females: during the pairing-season, and at no other time, the male
utters a hoarse bellowing noise, which can be heard at the distance of
more than a hundred yards; the female, on the other hand, never uses
her voice. (52. See my ‘Journal of Researches during the Voyage of the
“Beagle,”’ 1845, p. 384.)
With the Testudo elegans of India, it is said “that the combats of the
males may be heard at some distance, from the noise they produce in
butting against each other.” (53. Dr. Gunther, ‘Reptiles of British
India,’ 1864, p. 7.)
CROCODILIA.
The sexes apparently do not differ in colour; nor do I know that the
males fight together, though this is probable, for some kinds make a
prodigious display before the females. Bartram (54. ‘Travels through
Carolina,’ etc., 1791, p. 128.) describes the male alligator as
striving to win the female by splashing and roaring in the midst of a
lagoon, “swollen to an extent ready to burst, with its head and tail
lifted up, he springs or twirls round on the surface of the water, like
an Indian chief rehearsing his feats of war.” During the season of
love, a musky odour is emitted by the submaxillary glands of the
crocodile, and pervades their haunts. (55. Owen, ‘Anatomy of
Vertebrates,’ vol. i. 1866, p. 615.)
OPHIDIA.
Dr. Gunther informs me that the males are always smaller than the
females, and generally have longer and slenderer tails; but he knows of
no other difference in external structure. In regard to colour, be can
almost always distinguish the male from the female, by his more
strongly-pronounced tints; thus the black zigzag band on the back of
the male English viper is more distinctly defined than in the female.
The difference is much plainer in the rattle-snakes of N. America, the
male of which, as the keeper in the Zoological Gardens shewed me, can
at once be distinguished from the female by having more lurid yellow
about its whole body. In S. Africa the Bucephalus capensis presents an
analogous difference, for the female “is never so fully variegated with
yellow on the sides as the male.” (56. Sir Andrew Smith, ‘Zoology of S.
Africa: Reptilia,’ 1849, pl. x.) The male of the Indian Dipsas cynodon,
on the other hand, is blackish-brown, with the belly partly black,
whilst the female is reddish or yellowish-olive, with the belly either
uniform yellowish or marbled with black. In the Tragops dispar of the
same country the male is bright green, and the female bronze-coloured.
(57. Dr. A. Gunther, ‘Reptiles of British India,’ Ray Soc., 1864, pp.
304, 308.) No doubt the colours of some snakes are protective, as shewn
by the green tints of tree-snakes, and the various mottled shades of
the species which live in sandy places; but it is doubtful whether the
colours of many kinds, for instance of the common English snake and
viper, serve to conceal them; and this is still more doubtful with the
many foreign species which are coloured with extreme elegance. The
colours of certain species are very different in the adult and young
states. (58. Dr. Stoliczka, ‘Journal of Asiatic Society of Bengal,’
vol. xxxix, 1870, pp. 205, 211.)
During the breeding-season the anal scent-glands of snakes are in
active function (59. Owen, ‘Anatomy of Vertebrates,’ vol. i. 1866, p.
615.); and so it is with the same glands in lizards, and as we have
seen with the submaxillary glands of crocodiles. As the males of most
animals search for the females, these odoriferous glands probably serve
to excite or charm the female, rather than to guide her to the spot
where the male may be found. Male snakes, though appearing so sluggish,
are amorous; for many have been observed crowding round the same
female, and even round her dead body. They are not known to fight
together from rivalry. Their intellectual powers are higher than might
have been anticipated. In the Zoological Gardens they soon learn not to
strike at the iron bar with which their cages are cleaned; and Dr. Keen
of Philadelphia informs me that some snakes which he kept learned after
four or five times to avoid a noose, with which they were at first
easily caught. An excellent observer in Ceylon, Mr. E. Layard, saw (60.
‘Rambles in Ceylon,’ in ‘Annals and Magazine of Natural History,’ 2nd
series, vol. ix. 1852, p. 333.) a cobra thrust its head through a
narrow hole and swallow a toad. “With this encumbrance he could not
withdraw himself; finding this, he reluctantly disgorged the precious
morsel, which began to move off; this was too much for snake philosophy
to bear, and the toad was again seized, and again was the snake, after
violent efforts to escape, compelled to part with its prey. This time,
however, a lesson had been learnt, and the toad was seized by one leg,
withdrawn, and then swallowed in triumph.”
The keeper in the Zoological Gardens is positive that certain snakes,
for instance Crotalus and Python, distinguish him from all other
persons. Cobras kept together in the same cage apparently feel some
attachment towards each other. (61. Dr. Gunther, ‘Reptiles of British
India,’ 1864, p. 340.)
It does not, however, follow because snakes have some reasoning power,
strong passions and mutual affection, that they should likewise be
endowed with sufficient taste to admire brilliant colours in their
partners, so as to lead to the adornment of the species through sexual
selection. Nevertheless, it is difficult to account in any other manner
for the extreme beauty of certain species; for instance, of the
coral-snakes of S. America, which are of a rich red with black and
yellow transverse bands. I well remember how much surprise I felt at
the beauty of the first coral-snake which I saw gliding across a path
in Brazil. Snakes coloured in this peculiar manner, as Mr. Wallace
states on the authority of Dr. Gunther (62. ‘Westminster Review,’ July
1st, 1867, p. 32.), are found nowhere else in the world except in S.
America, and here no less than four genera occur. One of these, Elaps,
is venomous; a second and widely-distinct genus is doubtfully venomous,
and the two others are quite harmless. The species belonging to these
distinct genera inhabit the same districts, and are so like each other
that no one “but a naturalist would distinguish the harmless from the
poisonous kinds.” Hence, as Mr. Wallace believes, the innocuous kinds
have probably acquired their colours as a protection, on the principle
of imitation; for they would naturally be thought dangerous by their
enemies. The cause, however, of the bright colours of the venomous
Elaps remains to be explained, and this may perhaps be sexual
selection.
Snakes produce other sounds besides hissing. The deadly Echis carinata
has on its sides some oblique rows of scales of a peculiar structure
with serrated edges; and when this snake is excited these scales are
rubbed against each other, which produces “a curious prolonged, almost
hissing sound.” (63. Dr. Anderson, ‘Proc. Zoolog. Soc.’ 1871, p. 196.)
With respect to the rattling of the rattle-snake, we have at last some
definite information: for Professor Aughey states (64. The ‘American
Naturalist,’ 1873, p. 85.), that on two occasions, being himself
unseen, he watched from a little distance a rattle-snake coiled up with
head erect, which continued to rattle at short intervals for half an
hour: and at last he saw another snake approach, and when they met they
paired. Hence he is satisfied that one of the uses of the rattle is to
bring the sexes together. Unfortunately he did not ascertain whether it
was the male or the female which remained stationary and called for the
other. But it by no means follows from the above fact that the rattle
may not be of use to these snakes in other ways, as a warning to
animals which would otherwise attack them. Nor can I quite disbelieve
the several accounts which have appeared of their thus paralysing their
prey with fear. Some other snakes also make a distinct noise by rapidly
vibrating their tails against the surrounding stalks of plants; and I
have myself heard this in the case of a Trigonocephalus in S. America.
LACERTILIA.
The males of some, probably of many kinds of lizards, fight together
from rivalry. Thus the arboreal Anolis cristatellus of S. America is
extremely pugnacious: “During the spring and early part of the summer,
two adult males rarely meet without a contest. On first seeing one
another, they nod their heads up and down three or four times, and at
the same time expanding the frill or pouch beneath the throat; their
eyes glisten with rage, and after waving their tails from side to side
for a few seconds, as if to gather energy, they dart at each other
furiously, rolling over and over, and holding firmly with their teeth.
The conflict generally ends in one of the combatants losing his tail,
which is often devoured by the victor.” The male of this species is
considerably larger than the female (65. Mr. N.L. Austen kept these
animals alive for a considerable time; see ‘Land and Water,’ July 1867,
p. 9.); and this, as far as Dr. Gunther has been able to ascertain, is
the general rule with lizards of all kinds. The male alone of the
Cyrtodactylus rubidus of the Andaman Islands possesses pre-anal pores;
and these pores, judging from analogy, probably serve to emit an odour.
(66. Stoliczka, ‘Journal of the Asiatic Society of Bengal,’ vol. xxxiv.
1870, p. 166.)
[Fig.33. Sitana minor. Male with the gular pouch expanded (from
Gunther’s ‘Reptiles of India’)’]
The sexes often differ greatly in various external characters. The male
of the above-mentioned Anolis is furnished with a crest which runs
along the back and tail, and can be erected at pleasure; but of this
crest the female does not exhibit a trace. In the Indian Cophotis
ceylanica, the female has a dorsal crest, though much less developed
than in the male; and so it is, as Dr. Gunther informs me, with the
females of many Iguanas, Chameleons, and other lizards. In some
species, however, the crest is equally developed in both sexes, as in
the Iguana tuberculata. In the genus Sitana, the males alone are
furnished with a large throat pouch (Fig. 33), which can be folded up
like a fan, and is coloured blue, black, and red; but these splendid
colours are exhibited only during the pairing-season. The female does
not possess even a rudiment of this appendage. In the Anolis
cristatellus, according to Mr. Austen, the throat pouch, which is
bright red marbled with yellow, is present in the female, though in a
rudimental condition. Again, in certain other lizards, both sexes are
equally well provided with throat pouches. Here we see with species
belonging to the same group, as in so many previous cases, the same
character either confined to the males, or more largely developed in
them than in the females, or again equally developed in both sexes. The
little lizards of the genus Draco, which glide through the air on their
rib-supported parachutes, and which in the beauty of their colours
baffle description, are furnished with skinny appendages to the throat
“like the wattles of gallinaceous birds.” These become erected when the
animal is excited. They occur in both sexes, but are best developed
when the male arrives at maturity, at which age the middle appendage is
sometimes twice as long as the head. Most of the species likewise have
a low crest running along the neck; and this is much more developed in
the full-grown males than in the females or young males. (67. All the
foregoing statements and quotations, in regard to Cophotis, Sitana and
Draco, as well as the following facts in regard to Ceratophora and
Chamaeleon, are from Dr. Gunther himself, or from his magnificent work
on the ‘Reptiles of British India,’ Ray Soc., 1864, pp. 122, 130, 135.)
A Chinese species is said to live in pairs during the spring; “and if
one is caught, the other falls from the tree to the ground, and allows
itself to be captured with impunity”—I presume from despair. (68. Mr.
Swinhoe, ‘Proc. Zoolog. Soc.’ 1870, p. 240.)
[Fig. 34. Ceratophora Stoddartii. Upper figure; lower figure, female.]
There are other and much more remarkable differences between the sexes
of certain lizards. The male of Ceratophora aspera bears on the
extremity of his snout an appendage half as long as the head. It is
cylindrical, covered with scales, flexible, and apparently capable of
erection: in the female it is quite rudimental. In a second species of
the same genus a terminal scale forms a minute horn on the summit of
the flexible appendage; and in a third species (C. Stoddartii, fig. 34)
the whole appendage is converted into a horn, which is usually of a
white colour, but assumes a purplish tint when the animal is excited.
In the adult male of this latter species the horn is half an inch in
length, but it is of quite minute size in the female and in the young.
These appendages, as Dr. Gunther has remarked to me, may be compared
with the combs of gallinaceous birds, and apparently serve as
ornaments.
[Fig. 35. Chamaeleo bifurcus. Upper figure, male; lower figure, female.
Fig. 36. Chamaeleo Owenii. Upper figure, male; lower figure, female.]
In the genus Chamaeleon we come to the acme of difference between the
sexes. The upper part of the skull of the male C. bifurcus (Fig. 35),
an inhabitant of Madagascar, is produced into two great, solid, bony
projections, covered with scales like the rest of the head; and of this
wonderful modification of structure the female exhibits only a
rudiment. Again, in Chamaeleo Owenii (Fig. 36), from the West Coast of
Africa, the male bears on his snout and forehead three curious horns,
of which the female has not a trace. These horns consist of an
excrescence of bone covered with a smooth sheath, forming part of the
general integuments of the body, so that they are identical in
structure with those of a bull, goat, or other sheath-horned ruminant.
Although the three horns differ so much in appearance from the two
great prolongations of the skull in C. bifurcus, we can hardly doubt
that they serve the same general purpose in the economy of these two
animals. The first conjecture, which will occur to every one, is that
they are used by the males for fighting together; and as these animals
are very quarrelsome (69. Dr. Buchholz, ‘Monatsbericht K. Preuss.
Akad.’ Jan. 1874, p. 78.), this is probably a correct view. Mr. T.W.
Wood also informs me that he once watched two individuals of C. pumilus
fighting violently on the branch of a tree; they flung their heads
about and tried to bite each other; they then rested for a time and
afterwards continued their battle.
With many lizards the sexes differ slightly in colour, the tints and
stripes of the males being brighter and more distinctly defined than in
the females. This, for instance, is the case with the above Cophotis
and with the Acanthodactylus capensis of S. Africa. In a Cordylus of
the latter country, the male is either much redder or greener than the
female. In the Indian Calotes nigrilabris there is a still greater
difference; the lips also of the male are black, whilst those of the
female are green. In our common little viviparous lizard (Zootoca
vivipara) “the under side of the body and base of the tail in the male
are bright orange, spotted with black; in the female these parts are
pale-greyish-green without spots.” (70. Bell, ‘History of British
Reptiles,’ 2nd ed., 1849, p. 40.) We have seen that the males alone of
Sitana possess a throat-pouch; and this is splendidly tinted with blue,
black, and red. In the Proctotretus tenuis of Chile the male alone is
marked with spots of blue, green, and coppery-red. (71. For
Proctotretus, see ‘Zoology of the Voyage of the “Beagle”; Reptiles,’ by
Mr. Bell, p. 8. For the Lizards of S. Africa, see ‘Zoology of S.
Africa: Reptiles,’ by Sir Andrew Smith, pl. 25 and 39. For the Indian
Calotes, see ‘Reptiles of British India,’ by Dr. Gunther, p. 143.) In
many cases the males retain the same colours throughout the year, but
in others they become much brighter during the breeding-season; I may
give as an additional instance the Calotes maria, which at this season
has a bright red head, the rest of the body being green. (72. Gunther
in ‘Proceedings, Zoological Society,’ 1870, p. 778, with a coloured
figure.)
Both sexes of many species are beautifully coloured exactly alike; and
there is no reason to suppose that such colours are protective. No
doubt with the bright green kinds which live in the midst of
vegetation, this colour serves to conceal them; and in N. Patagonia I
saw a lizard (Proctotretus multimaculatus) which, when frightened,
flattened its body, closed its eyes, and then from its mottled tints
was hardly distinguishable from the surrounding sand. But the bright
colours with which so many lizards are ornamented, as well as their
various curious appendages, were probably acquired by the males as an
attraction, and then transmitted either to their male offspring alone,
or to both sexes. Sexual selection, indeed, seems to have played almost
as important a part with reptiles as with birds; and the less
conspicuous colours of the females in comparison with the males cannot
be accounted for, as Mr. Wallace believes to be the case with birds, by
the greater exposure of the females to danger during incubation.