Primate Social Organization 85to face from a branch and rubbing their erect penises to-
gether as if crossing swords. Among females, genital rub-
bing is particularly common.
As described in this chapter’s Original Study, the pri-
mary function of most of this sex, both hetero- and homo-
sexual, is to reduce tensions and resolve social conflicts.
Bonobo sexual activity is very frequent but also very brief,
lasting only 8 to 10 seconds. Since the documentation of
sexual activities among bonobos, field studies by prima-
tologists are now recording a variety of sexual behaviors
among other species as well. This chapter’s Biocultural
Connection describes a variation on sexuality in orang-
utans that has been described as disturbing.
In gorilla families, the dominant silverback tends to
have exclusive breeding rights with the females, although he
will sometimes tolerate the presence of a young adult male
and allow him occasional access to a low-ranking female.
Generally by the time a young male becomes a silverback
he must leave “home,” luring partners away from other es-
tablished groups, in order to have reproductive success.
Field studies have revealed variation in the typical go-
rilla pattern of a single dominant male. There are gorilla
groups in Uganda and Rwanda in which there are multiple
silverback males. Still, in one gorilla group with more than
one adult male studied in Rwanda, a single dominant male
fathered all but one of ten juveniles.^10
Although the vast majority of primate species are not
monogamous—bonded exclusively to a single sexual
partner—in their mating habits, many smaller species of
New World monkeys, a few island-dwelling populations
of leaf-eating Old World monkeys, and all of the smaller
apes (gibbons and siamangs) appear to mate for life with a
single individual of the opposite sex. None of these species
is closely related to human beings, nor do monogamous
species ever display the degree of sexual dimorphism—
anatomical differences between males and females—that
is characteristic of our closest primate relatives or that was
characteristic of our own ancient ancestors.
Evolutionary biologists, dating back to Charles Dar-
win himself,^11 have proposed that sexual dimorphism (for
example, larger male size in apes, beautiful feathers in
peacocks) relates to competition among males for access
to females. Females only evolved by what Canadian pri-
matologist Linda Fedigan has called the “coat-tails theory”Ovulation is also concealed in humans, by the absence of
genital swelling at all times.
Concealed ovulation in humans and bonobos may
play a role in separating sexual activity for social and
pleasurable reasons from the purely biological task of re-
production. In fact, among bonobos (as among humans)
sexuality goes far beyond male–female mating for pur-
poses of biological reproduction. Primatologists have ob-
served virtually every possible combination of ages and
sexes engaging in a remarkable array of sexual activities,
including oral sex, tongue-kissing, and massaging each
other’s genitals.^9 Male bonobos may mount each other, or
one may rub his scrotum against that of the other. They
have also been observed “penis fencing”—hanging faceBecause geladas spend far more time sitting than upright, signaling
ovulation through genital swelling is nowhere near as practical as
signaling it through the reddening of a patch of furless skin on their
chests. This way it is easy for other members of the group to see that
they are fertile even while they are foraging.© Biosphere/Gunther Michel/Peter Arnold, Inc.©^
Bios
phere
/Gun
ther
Mi
che
l/P
eter
Arno
ld,
Inc
.
(^10) Gibbons, A. (2001). Studying humans—and their cousins and parasites.
Science 292 , 627.
(^11) Darwin, C. (1936). The descent of man and selection in relation to sex.
New York: Random House (Modern Library). (orig. 1871)
(^9) de Waal, F. B. M. (2001). The ape and the sushi master (pp. 131–132). New
York: Basic.
monogamous Mating for life with a single individual of the
opposite sex.