in certain groups are equally beautiful, and are equally provided with
ornaments which are usually confined to the male sex. In other groups both
sexes are equally plain-coloured and unornamented. Lastly, in some few
anomalous cases, the females are more beautiful than the males. We shall
often find, in the same group of birds, every gradation from no difference
between the sexes, to an extreme difference. We shall see that female
birds, like female insects, often possess more or less plain traces or
rudiments of characters which properly belong to the males and are of use
only to them. The analogy, indeed, in all these respects between birds and
insects is curiously close. Whatever explanation applies to the one class
probably applies to the other; and this explanation, as we shall hereafter
attempt to shew in further detail, is sexual selection.
CHAPTER XII.
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 Societe 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'etalage de leurs
vives couleurs chercher a attirer l'attention des femelles, lesquelles ne
paraissaient indifferentes a ce manege, 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