History of the World/History of the World (Bureau Development, Inc.)(1992).BIN/dp/0109/01095.txt

Text File | 1992-10-11 | 23KB | 367 lines

$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.