$Unique_ID{how01095} $Pretitle{} $Title{Descent Of Man, The Chapter 15.2} $Subtitle{} $Author{Darwin, Charles} $Affiliation{} $Subject{sexes female species male birds females color vol cases males} $Date{1874} $Log{} Title: Descent Of Man, The Book: Part II: Sexual Selection Author: Darwin, Charles Date: 1874 Chapter 15.2 Notwithstanding the foregoing objections, I cannot doubt, after reading Mr. Wallace's excellent essay, that looking to the birds of the world a large majority of the species in which the females are conspicuously colored (and in this case the males with rare exceptions are equally conspicuous) build concealed nests for the sake of protection. Mr. Wallace enumerates ^911 a long series of groups in which this rule holds good; but it will suffice here to give as instances the more familiar groups of kingfishers, toucans, trogons, puff-birds (Capitonidae), plantain - eaters plantain-eaters (Musophagae), woodpeckers and parrots. Mr. Wallace believes that in these groups, as the males gradually acquired through sexual selection their brilliant colors, these were transferred to the females and were not eliminated by natural selection owing to the protection which they already enjoyed from their manner of nidification. According to this view, their present manner of nesting was acquired before their present colors. But it seems to me much more probable that in most cases, as the females were gradually rendered more and more brilliant from partaking of the colors of the male, they were gradually led to change their instincts (supposing that they originally built open nests) and to seek protection by building domed or concealed nests. No one who studies, for instance, Audubon's account of the differences in the nests of the same species in the Northern and Southern United States, ^912 will feel any great difficulty in admitting that birds, either by a change (in the strict sense of the word) of their habits, or through the natural selection of so-called spontaneous variations of instinct, might readily be led to modify their manner of nesting. [Footnote 911: "Journal of Travel," edited by A. Murray, vol. i, p. 78.] [Footnote 912: See many statements in the "Ornithological Biography." See also some curious observations on the nests of Italian birds, by Eugenio Bettoni, in the "Atti della Societa Italiana," vol. xi, 1869, p. 487.] This way of viewing the relation, as far as it holds good, between the bright colors of female birds and their manner of nesting receives some support from certain cases occurring in the Sahara Desert. Here, as in most other deserts, various birds and many other animals have had their colors adapted in a wonderful manner to the tints of the surrounding surface. Nevertheless there are, as I am informed by the Rev. Mr. Tristram, some curious exceptions to the rule; thus the male of the Monticola cyanea is conspicuous from his bright-blue color, and the female almost equally conspicuous from her mottled brown and white plumage; both sexes of two species of Dromolaea are of a lustrous black; so that these three species are far from receiving protection from their colors, yet they are able to survive, for they have acquired the habit of taking refuge from danger in holes or crevices in the rocks. With respect to the above groups in which the females are conspicuously colored and build concealed nests, it is not necessary to suppose that each separate species had its nidifying instinct specially modified; but only that the early progenitors of each group were gradually led to build domed or concealed nests, and afterward transmitted this instinct, together with their bright colors, to their modified descendants. As far as it can be trusted the conclusion is interesting, that sexual selection, together with equal or nearly equal inheritance by both sexes, have indirectly determined the manner of nidification of whole groups of birds. According to Mr. Wallace, even in the groups in which the females, from being protected in domed nests during incubation, have not had their bright colors eliminated through natural selection, the males often differ in a slight and occasionally in a considerable degree from the females. This is a significant fact, for such differences in color must be accounted for by some of the variations in the males having been from the first limited in transmission to the same sex; as it can hardly be maintained that these differences, especially when very slight, serve as a protection to the female. Thus all the species in the splendid group of the Trogons build in holes; and Mr. Gould gives figures ^913 of both sexes of twenty-five species, in all of which, with one partial exception, the sexes differ sometimes slightly, sometimes conspicuously in color - the males being always finer than the females, though the latter are likewise beautiful. All the species of king-fishers build in holes, and with most of the species the sexes are equally brilliant, and thus far Mr. Wallace's rule holds good; but in some of the Australian species the colors of the females are rather less vivid than those of the male; and in one splendidly colored species the sexes differ so much that they were at first thought to be specifically distinct. ^914 Mr. R. B. Sharpe, who has especially studied this group, has shown me some American species (Ceryle) in which the breast of the male is belted with black. Again, in Carcineutes, the difference between the sexes is conspicuous; in the male the upper surface is dull-blue banded with black, the lower surface being partly fawn-colored, and there is much red about the head; in the female the upper surface is reddish-brown banded with black, and the lower surface white with black markings. It is an interesting fact, as showing how the same peculiar style of sexual coloring often characterizes allied forms, that in three species of Dacelo the male differs from the female only in the tail being dull-blue banded with black, while that of the female is brown with blackish bars; so that here the tail differs in color in the two sexes in exactly the same manner as the whole upper surface in the two sexes of Carcineutes. [Footnote 913: See his "Monograph of the Trogonidae," first edition.] [Footnote 914: Namely Cyanalcyon. Gould's "Hand-book to the Birds of Australia," vol. i, p. 133; see also pp. 130, 136.] With parrots, which likewise build in holes, we find analogous cases; in most of the species both sexes are brilliantly colored and indistinguishable, but in not a few species the males are colored rather more vividly than the females, or even very differently from them. Thus, besides other strongly marked differences, the whole under surface of the male king lory (Aprosmictus scapulatus) is scarlet, while the throat and chest of the female is green tinged with red; in the Euphema splendida there is a similar difference, the face and wing coverts moreover of the female being of a paler blue than in the male. ^915 In the family of the tits (Parinoe), which build concealed nests, the female of our common blue tomtit (Parus coeruleus) is "much less brightly colored" than the male; and in the magnificent Sultan yellow tit of India the difference is greater. ^916 [Footnote 915: Every gradation of difference between the sexes may be followed in the parrots of Australia. See Gould's "Hand-book," etc., vol. ii, pp. 14-102.] [Footnote 916: Macgillivray's "British Birds," vol. ii, p. 433. Jerdon, "Birds of India," vol. ii, p. 282.] Again, in the great group of the woodpeckers, ^917 the sexes are generally nearly alike, but in the Megapicus validus all those parts of the head, neck and breast which are crimson in the male are pale-brown in the female. As in several woodpeckers the head of the male is bright crimson, while that of the female is plain, it occurred to me that this color might possibly make the female dangerously conspicuous whenever she put her head out of the hole containing her nest, and consequently that this color, in accordance with Mr. Wallace's belief, had been eliminated. This view is strengthened by what Malherbe states with respect to Indopicus carlotta; namely, that the young females, like the young males, have some crimson about their heads, but that this color disappears in the adult female, while it is intensified in the adult male. Nevertheless, the following considerations render this view extremely doubtful; the male takes a fair share in incubation, ^918 and would be thus almost equally exposed to danger; both sexes of many species have their heads of an equally bright crimson; in other species the difference between the sexes in the amount of scarlet is so slight that it can hardly make any appreciable difference in the danger incurred; and lastly, the coloring of the head in the two sexes often differs slightly in other ways. [Footnote 917: All the following facts are taken from M. Malherbe's magnificent "Monographie des Picidees," 1861.] [Footnote 918: Audubon's "Ornithological Biography," vol. ii, p. 75; see also the "Ibis," vol. i, p. 268.] The cases, as yet given, of slight and graduated differences in color between the males and females in the groups, in which, as a general rule, the sexes resemble each other, all relate to species which build domed or concealed nests. But similar gradations may likewise be observed in groups in which the sexes as a general rule resemble each other, but which build open nests. As I have before instanced the Australian parrots, so I may here instance, without giving any details, the Australian pigeons. ^919 It deserves especial notice that in all these cases the slight differences in plumage between the sexes are of the same general nature as the occasionally greater differences. A good illustration of this fact has already been afforded by those kingfishers in which either the tail alone or the whole upper surface of the plumage differs in the same manner in the two sexes. Similar cases may be observed with parrots and pigeons. The differences in color between the sexes of the same species are, also, of the same general nature as the differences in color between the distinct species of the same group. For when in a group in which the sexes are usually alike the male differs considerably from the female he is not colored in a quite new style. Hence we may infer that within the same group the special colors of both sexes when they are alike, and the colors of the male when he differs slightly or even considerably from the female, have been in most cases determined by the same general cause; this being sexual selection. [Footnote 919: Gould's "Hand-book to the Birds of Australia," vol. ii, pp. 109-149] It is not probable, as has already been remarked, that differences in color between the sexes, when very slight, can be of service to the female as a protection. Assuming, however, that they are of service, they might be thought to be cases of transition; but we have no reason to believe that many species at any one time are undergoing change. Therefore we can hardly admit that the numerous females which differ very slightly in color from their males are now all commencing to become obscure for the sake of protection. Even if we consider somewhat more marked sexual differences, is it probable, for instance, that the head of the female chaffinch, the crimson on the breast of the female bullfinch, the green of the female greenfinch, the crest of the female golden-crested wren have all been rendered less bright by the slow process of selection for the sake of protection? I cannot think so; and still less with the slight differences between the sexes of those birds which build concealed nests. On the other hand, the differences in color between the sexes, whether great or small, may to a large extent be explained on the principle of the successive variations acquired by the males through sexual selection, having been from the first more or less limited in their transmission to the females. That the degree of limitation should differ in different species of the same group will not surprise any one who has studied the laws of inheritance, for they are so complex that they appear to us in our ignorance to be capricious in their actions. ^920 [Footnote 920: See remarks to this effect in my work on "Variation under Domestication," vol. ii, chap. xii.] As far as I can discover there are few large groups of birds in which all the species have both sexes alike and brilliantly colored, but I hear from Mr. Sclater that this appears to be the case with the Musophagae or plantaineaters. Nor do I believe that any large group exists in which the sexes of all the species are widely dissimilar in color. Mr. Wallace informs me that the chatterers of South America (Cotingidoe) offer one of the best instances; but with some of the species in which the male has a splendid red breast the female exhibits some red on her breast; and the females of other species show traces of the green and other colors of the males. Nevertheless we have a near approach to close sexual similarity or dissimilarity throughout several groups; and this, from what has just been said of the fluctuating nature of inheritance, is a somewhat surprising circumstance. But that the same laws should largely prevail with allied animals is not surprising. The domestic fowl has produced a great number of breeds and sub-breeds, and in these the sexes generally differ in plumage; so that it has been noticed as an unusual circumstance when in certain sub-breeds they resemble each other. On the other hand, the domestic pigeon has likewise produced a vast number of distinct breeds and sub-breeds, and in these, with rare exceptions, the two sexes are identically alike. Therefore if other species of Gallus and Columba were domesticated and varied it would not be rash to predict that similar rules of sexual similarily and dissimilarity depending on the form of transmission would hold good in both cases. In like manner the same form of transmission has generally prevailed under nature throughout the same groups, although marked exceptions to this rule occur. Thus within the same family, or even genus, the sexes may be identically alike or very different in color. Instances have already been given in the same genus, as with sparrows, fly-catchers, thrushes and grouse. In the family of pheasants the sexes of almost all the species are wonderfully dissimilar, but are quite alike in the eared pheasant or Crossoptilon auritum. In two species of Chloephaga, a genus of geese, the male cannot be distinguished from the females except by size; while in two others the sexes are so unlike that they might easily be mistaken for distinct species. ^921 [Footnote 921: The "Ibis," vol. vi, 1864, p. 122.] The laws of inheritance can alone account for the following cases in which the female acquires late in life certain characters proper to the male, and ultimately comes to resemble him more or less completely. Here protection can hardly have come into play. Mr. Blyth informs me that the females of Oriolus melanocephalus and of some allied species when sufficiently mature to breed differ considerably in plumage from the adult males; but after the second or third moults they differ only in their beaks having a slight greenish tinge. In the dwarf bitterns (Ardetta), according to the same authority, "the male acquires his final livery at the first moult; the female not before the third or fourth moult; in the meanwhile she presents an intermediate garb, which is ultimately exchanged for the same livery as that of the male." So, again, the female Falco peregrinus acquires her blue plumage more slowly than the male. Mr. Swinhoe states that with one of the drongo shrikes (Dicrurus macrocercus) the male, while almost a nestling, molts his soft brown plumage and becomes of a uniform glossy greenish-black; but the female retains for a long time the white striae and spots on the axiliary feathers; and does not completely assume the uniform black color of the male for three years. The same excellent observer remarks that in the spring of the second year the female spoon-bill (Platalea) of China resembles the male of the first year, and that apparently it is not until the third spring that she acquires the same adult plumage as that possessed by the male at a much earlier age. The female Bombycilla carolinensis differs very little from the male, but the appendages, which like beads of red sealing-wax ornament the wing-feathers, ^922 are not developed in her so early in life as in the male. In the male of an Indian paroquet (Paloeornis javanicus) the upper mandible is coral-red from his earliest youth, but in the female, as Mr. Blyth has observed with caged and wild birds, it is at first black and does not become red until the bird is at least a year old, at which age the sexes resemble each other in all respects. Both sexes of the wild turkey are ultimately furnished with a tuft of bristles on the breast, but in two-year-old birds the tuft is about four inches long in the male and hardly apparent in the female; when, however, the latter has reached her fourth year, it is from four to five inches in length. ^923 [Footnote 922: When the male courts the female, these ornaments are vibrated, and "are shown off to great advantage," on the outstretched wings: A. Leith Adams, "Field and Forest Rambles," 1873, p. 153.] [Footnote 923: On Ardetta, Translation of Cuvier's "Regne Animal," by Mr. Blyth, foot-note p. 159. On the Peregrine Falcon, Mr. Blyth, in Charlesworth's "Mag. of Nat. Hist.," vol. i, 1837, p. 304. On Dicrurus, "Ibis," 1863, p. 44. On the Platalea, "Ibis," vol. vi, 1864, p. 366. On the Bombycilla, Audubon's "Ornitholog. Biography," vol. i, p. 229. On the Palaeornis, see, also, Jerdon, "Birds of India," vol. i, p. 263. On the wild turkey, Audubon, ibid, vol. i, p. 15; but I hear from Judge Caton that in Illinois the female very rarely acquires a tuft. Analogous cases with the females of Petrocossyphus are given by Mr. R. Sharpe, "Proc. Zoolog. Soc.," 1872, p. 496.] These cases must not be confounded with those where diseased or old females abnormally assume masculine characters, nor with those where fertile females, while young, acquire the characters of the male, through variation or some unknown cause. ^924 But all these cases have so much in common that they depend, according to the hypothesis of pangenesis, on gemmules derived from each part of the male being present, though latent, in the female; their development following on some slight change in the elective affinities of her constituent tissues. [Footnote 924: Of these latter cases Mr. Blyth has recorded (Translation of Cuvier's "Regne Animal," p. 158) various instances with Lanius, Ruticilla, Linaria and Anas. Audubon has also recorded a similar case ("Ornith. Biog.," vol. v, p. 519) with Pyranga oestiva.] A few words must be added on changes of plumage in relation to the season of the year. From reasons formerly assigned there can be little doubt that the elegant plumes, long pendant feathers, crests, etc., of egrets, herons and many other birds, which are developed and retained only during the summer, serve for ornamental and nuptial purposes, though common to both sexes. The female is thus rendered more conspicuous during the period of incubation than during the winter; but such birds as herons and egrets would be able to defend themselves. As, however, plumes would probably be inconvenient and certainly of no use during the winter, it is possible that the habit of moulting twice in the year may have been gradually acquired through natural selection for the sake of casting off inconvenient ornaments during the winter. But this view cannot be extended to the many waders, whose summer and winter plumages differ very little in color. With defenseless species, in which both sexes, or the males alone, become extremely conspicuous during the breeding-season - or when the males acquire at this season such long wing or tail feathers as to impede their flight, as with Cosmetornis and Vidua - it certainly at first appears highly probable that the second moult has been gained for the special purpose of throwing off these ornaments. We must, however, remember that many birds, such as some of the birds of paradise, the Argus pheasant and peacock, do not cast their plumes during the winter; and it can hardly be maintained that the constitution of these birds, at least of the Gallinaceae, renders a double moult impossible, for the ptarmigan moults thrice in the year. ^925 Hence it must be considered as doubtful whether the many species which moult their ornamental plumes or lose their bright colors during the winter, have acquired this habit on account of the inconvenience or danger which they would otherwise have suffered. [Footnote 925: See Gould's "Birds of Great Britain."] I conclude, therefore, that the habit of moulting twice in the year was in most or all cases first acquired for some distinct purpose, perhaps for gaining a warmer winter covering; and that variations in the plumage occurred during the summer were accumulated through sexual selection, and transmitted to the offspring at the same season of the year; that such variations were inherited either by both sexes or by the males alone, according to the form of inheritance which prevailed. This appears more probable than that the species in all cases originally tended to retain their ornamental plumage during the winter, but were saved from this through natural selection, resulting from the inconvenience or danger thus caused. I have endeavored in this chapter to show that the arguments are not trustworthy in favor of the view that weapons, bright colors, and various ornaments are now confined to the males owing to the conversion, by natural selection, of the equal transmission of characters to both sexes, into transmission to the male sex alone. It is also doubtful whether the colors of many female birds are due to the preservation, for the sake of protection, of variations which were from the first limited in their transmission to the female sex. But it will be convenient to defer any further discussion on this subject until I treat, in the following chapter, of the differences in plumage between the young and old.