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.

                         THE END