84 CHAPTER 4 | Primate Behavior
frequent sexual activity—initiated by either the male or
the female—occurs during estrus, the period when the
female is receptive to impregnation. In chimpanzees, es-
trus is signaled by vivid swelling of the skin around the
genitals. Bonobo females, by contrast, appear as if they
are fertile at all times due to their constantly swollen
genitals and interest in sex. Gorillas appear to show less
interest in sex compared to either chimps or bonobos.
By most human standards, the sexual behavior of
chimps is promiscuous. A dozen or so males have been
observed to have as many as fifty copulations in one day
with a single female in estrus. For the most part, females
mate with males of their own group. Dominant males try
to monopolize females in full estrus, although cooperation
from the female is usually required for this to succeed. In
addition, an individual female and a lower-ranking male
sometimes form a temporary bond, leaving the group to-
gether for a few “private” days during the female’s fertile
period. Interestingly, the relationship between reproduc-
tive success and social rank differs for males and females.
In the chimpanzee community studied by Goodall, about
half the infants were sired by low- or mid-level males.
Although for females high rank is linked with successful
reproduction, social success for males—achieving alpha
status—does not translate neatly into the evolutionary
currency of reproductive success.
In contrast to chimpanzees, bonobos (like humans)
do not limit their sexual behavior to times of female fer-
tility. The constant genital swelling of bonobos, in effect,
conceals the females’ ovulation, or moment when an
egg released into the womb is receptive for fertilization.adults tends to be quite restrained. Friendship or close-
ness between adults and infants is more evident. Among
bonobos, chimpanzees, gorillas, and orangutans, as
among most other primates, the mother–infant bond is
the strongest and longest lasting. It may endure for many
years—commonly for the lifetime of the mother. Gorilla
infants share their mothers’ nests but have also been seen
sharing nests with mature, childless females. Bonobo,
chimpanzee, and gorilla males are attentive to juveniles
and play a role in their socialization. Bonobo males even
carry infants on occasion. Their interest in a youngster
does not elicit the nervous reaction from the mother that
it does among chimps; chimp mothers may be reacting
to the occasional infanticide on the part of chimpanzee
males, a behavior never observed among bonobos.
Sexual Behavior
Most mammals mate only during specified breeding sea-
sons occurring once or twice a year. While some primates
have a fixed breeding season tied to a simultaneous in-
crease in body fat, or to the consumption of specific plant
foods, many primate species are able to breed at any time
during the course of the year. Among the African apes, as
with humans, no fixed breeding season exists. In chimps,
Grooming is an important
activity among all catarrhine
primates, as shown here in
a group of chimps grooming
each other in a pattern
known as the Domino Effect.
Such activity is important
for strengthening bonds
among individual members of
the group.
© Gunter Ziesler/Peter Arnold, Inc.estrus In some primate females, the time of sexual receptivity
during which ovulation is visibly displayed.
ovulation Moment when an egg released from the ovaries
into the womb is receptive for fertilization.