The Descent of Man – Day 90 of 151


Triton cristatus (half natural size, from Bell's 'British Reptiles'). Upper figure, male during the breeding season; lower figure, female.

Figure 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.



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.)

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