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Supplemental Note
ON SEXUAL SELECTION IN RELATION TO MONKEYS.
Reprinted from NATURE, November 2, 1876, p. 18.
In the discussion on Sexual Selection in my 'Descent of Man,' no case
interested and perplexed me so much as the brightly-coloured hinder ends
and adjoining parts of certain monkeys. As these parts are more brightly
coloured in one sex than the other, and as they become more brilliant
during the season of love, I concluded that the colours had been gained as
a sexual attraction. I was well aware that I thus laid myself open to
ridicule; though in fact it is not more surprising that a monkey should
display his bright-red hinder end than that a peacock should display his
magnificent tail. I had, however, at that time no evidence of monkeys
exhibiting this part of their bodies during their courtship; and such
display in the case of birds affords the best evidence that the ornaments
of the males are of service to them by attracting or exciting the females.
I have lately read an article by Joh. von Fischer, of Gotha, published in
'Der Zoologische Garten,' April 1876, on the expression of monkeys under
various emotions, which is well worthy of study by any one interested in
the subject, and which shews that the author is a careful and acute
observer. In this article there is an account of the behaviour of a young
male mandrill when he first beheld himself in a looking-glass, and it is
added, that after a time he turned round and presented his red hinder end
to the glass. Accordingly I wrote to Herr J. von Fischer to ask what he
supposed was the meaning of this strange action, and he has sent me two
long letters full of new and curious details, which will, I hope, be
hereafter published. He says that he was himself at first perplexed by the
above action, and was thus led carefully to observe several individuals of
various other species of monkeys, which he has long kept in his house. He
finds that not only the mandrill (Cynocephalus mormon) but the drill (C.
leucophaeus) and three other kinds of baboons (C. hamadryas, sphinx, and
babouin), also Cynopithecus niger, and Macacus rhesus and nemestrinus, turn
this part of their bodies, which in all these species is more or less
brightly coloured, to him when they are pleased, and to other persons as a
sort of greeting. He took pains to cure a Macacus rhesus, which he had
kept for five years, of this indecorous habit, and at last succeeded.
These monkeys are particularly apt to act in this manner, grinning at the
same time, when first introduced to a new monkey, but often also to their
old monkey friends; and after this mutual display they begin to play
together. The young mandrill ceased spontaneously after a time to act in
this manner towards his master, von Fischer, but continued to do so towards
persons who were strangers and to new monkeys. A young Cynopithecus niger
never acted, excepting on one occasion, in this way towards his master, but
frequently towards strangers, and continues to do so up to the present
time. From these facts Von Fischer concludes that the monkeys which
behaved in this manner before a looking-glass (viz., the mandrill, drill,
Cynopithecus niger, Macacus rhesus and nemestrinus) acted as if their
reflection were a new acquaintance. The mandrill and drill, which have
their hinder ends especially ornamented, display it even whilst quite
young, more frequently and more ostentatiously than do the other kinds.
Next in order comes Cynocephalus hamadryas, whilst the other species act in
this manner seldomer. The individuals, however, of the same species vary
in this respect, and some which were very shy never displayed their hinder
ends. It deserves especial attention that Von Fischer has never seen any
species purposely exhibit the hinder part of its body, if not at all
coloured. This remark applies to many individuals of Macacus cynomolgus
and Cercocebus radiatus (which is closely allied to M. rhesus), to three
species of Cercopithecus and several American monkeys. The habit of
turning the hinder ends as a greeting to an old friend or new acquaintance,
which seems to us so odd, is not really more so than the habits of many
savages, for instance that of rubbing their bellies with their hands, or
rubbing noses together. The habit with the mandrill and drill seems to be
instinctive or inherited, as it was followed by very young animals; but it
is modified or guided, like so many other instincts, by observation, for
Von Fischer says that they take pains to make their display fully; and if
made before two observers, they turn to him who seems to pay the most
attention.
With respect to the origin of the habit, Von Fischer remarks that his
monkeys like to have their naked hinder ends patted or stroked, and that
they then grunt with pleasure. They often also turn this part of their
bodies to other monkeys to have bits of dirt picked off, and so no doubt it
would be with respect to thorns. But the habit with adult animals is
connected to a certain extent with sexual feelings, for Von Fischer watched
through a glass door a female Cynopithecus niger, and she during several
days, "umdrehte und dem Mannchen mit gurgelnden Tonen die stark gerothete
Sitzflache zeigte, was ich fruher nie an diesem Thier bemerkt hatte. Beim
Anblick dieses Gegenstandes erregte sich das Mannchen sichtlich, denn es
polterte heftig an den Staben, ebenfalls gurgelnde Laute ausstossend." As
all the monkeys which have the hinder parts of their bodies more or less
brightly coloured live, according to Von Fischer, in open rocky places, he
thinks that these colours serve to render one sex conspicuous at a distance
to the other; but, as monkeys are such gregarious animals, I should have
thought that there was no need for the sexes to recognise each other at a
distance. It seems to me more probable that the bright colours, whether on
the face or hinder end, or, as in the mandrill, on both, serve as a sexual
ornament and attraction. Anyhow, as we now know that monkeys have the
habit of turning their hinder ends towards other monkeys, it ceases to be
at all surprising that it should have been this part of their bodies which
has been more or less decorated. The fact that it is only the monkeys thus
characterised which, as far as at present known, act in this manner as a
greeting towards other monkeys renders it doubtful whether the habit was
first acquired from some independent cause, and that afterwards the parts
in question were coloured as a sexual ornament; or whether the colouring
and the habit of turning round were first acquired through variation and
sexual selection, and that afterwards the habit was retained as a sign of
pleasure or as a greeting, through the principle of inherited association.
This principle apparently comes into play on many occasions: thus it is
generally admitted that the songs of birds serve mainly as an attraction
during the season of love, and that the leks, or great congregations of the
black-grouse, are connected with their courtship; but the habit of singing
has been retained by some birds when they feel happy, for instance by the
common robin, and the habit of congregating has been retained by the black-
grouse during other seasons of the year.
I beg leave to refer to one other point in relation to sexual selection.
It has been objected that this form of selection, as far as the ornaments
of the males are concerned, implies that all females within the same
district must possess and exercise exactly the same taste. It should,
however, be observed, in the first place, that although the range of
variation of a species may be very large, it is by no means indefinite. I
have elsewhere given a good instance of this fact in the pigeon, of which
there are at least a hundred varieties differing widely in their colours,
and at least a score of varieties of the fowl differing in the same kind of
way; but the range of colour in these two species is extremely distinct.
Therefore the females of natural species cannot have an unlimited scope for
their taste. In the second place, I presume that no supporter of the
principle of sexual selection believes that the females select particular
points of beauty in the males; they are merely excited or attracted in a
greater degree by one male than by another, and this seems often to depend,
especially with birds, on brilliant colouring. Even man, excepting perhaps
an artist, does not analyse the slight differences in the features of the
woman whom he may admire, on which her beauty depends. The male mandrill
has not only the hinder end of his body, but his face gorgeously coloured
and marked with oblique ridges, a yellow beard, and other ornaments. We
may infer from what we see of the variation of animals under domestication,
that the above several ornaments of the mandrill were gradually acquired by
one individual varying a little in one way, and another individual in
another way. The males which were the handsomest or the most attractive in
any manner to the females would pair oftenest, and would leave rather more
offspring than other males. The offspring of the former, although
variously intercrossed, would either inherit the peculiarities of their
fathers or transmit an increased tendency to vary in the same manner.
Consequently the whole body of males inhabiting the same country would tend
from the effects of constant intercrossing to become modified almost
uniformly, but sometimes a little more in one character and sometimes in
another, though at an extremely slow rate; all ultimately being thus
rendered more attractive to the females. The process is like that which I
have called unconscious selection by man, and of which I have given several
instances. In one country the inhabitants value a fleet or light dog or
horse, and in another country a heavier and more powerful one; in neither
country is there any selection of individual animals with lighter or
stronger bodies and limbs; nevertheless after a considerable lapse of time
the individuals are found to have been modified in the desired manner
almost uniformly, though differently in each country. In two absolutely
distinct countries inhabited by the same species, the individuals of which
can never during long ages have intermigrated and intercrossed, and where,
moreover, the variations will probably not have been identically the same,
sexual selection might cause the males to differ. Nor does the belief
appear to me altogether fanciful that two sets of females, surrounded by a
very different environment, would be apt to acquire somewhat different
tastes with respect to form, sound, or colour. However this may be, I have
given in my 'Descent of Man' instances of closely-allied birds inhabiting
distinct countries, of which the young and the females cannot be
distinguished, whilst the adult males differ considerably, and this may be
attributed with much probability to the action of sexual selection.
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