Factors influencing sexual behavior and reproductive patterns in
chimpanzees
Janette Wallis
Recent analysis of data from Gombe National Park, Tanzania has
provided new insights into the complex nature of chimpanzee sexual behavior and
reproductive patterns. For example, although genital swelling in pregnancy and lactation
is relatively rare, and therefore most copulation involves cyclic females, there is no
significant difference in the copulation rate of cycling, pregnant, or lactating females
exhibiting genital swellings (1). Moreover, adult males copulate most often with
multiparous females during the latter half of their estrous swelling (near ovulation) yet
show no such time preference with infertile females. Assessment of consort data indicates
lower probability of conception than previously thought (2) and it now appears that
consort mating is a highly selective activity; some chimpanzees are more likely to
participate in consort ships than are others. Several critical periods of a female's
reproductive life are sensitive to contact with female companions. Both the first full
estrous cycle and the first postpartum cycle often occur in synchrony with the cycles of
companions. These events are also under seasonal influence; at Gombe, they occur most
often during the late dry season. Recent publications have suggested that the seasonal
changes in food availability may produce the seasonal patterns seen in reproductive
parameters, either indirectly via olfactory cueing from social contacts, or directly via
phytoestrogen content of plant foods (2,3). The presentation will include a brief
comparison of the data from Gombe chimpanzees to those from chimpanzees living in Budongo
Forest, Uganda (4). Although a slight seasonal pattern of sexual behavior is observed at
Gombe, the pattern at Budongo is much more dramatic, though less clearly associated with
rainfall patterns. For both populations, the number of males is positively correlated with
number of estrous females present in feeding parties, and these are predictably seasonal.
Combined, these findings suggest that chimpanzee sexual behavior and reproductive patterns
are more complex than previously thought, indicating a need for further study.
Wallis, J.; Bettinger, T.; and Mkono, H. Sexual behavior of chimpanzees: Males'
response to cyclic, pregnant, and lactating females with anogenital swellings, in prep.
Wallis, J. A survey of reproductive parameters in the free-ranging chimpanzees of Gombe
National Park. Journal of Reproduction and Fertility 109:297-307, 1997.
Wallis, J. Seasonal influence on reproduction in chimpanzees of Gombe National Park.
International Journal of Primatology 16(3): 435-451, 1995.
Wallis, J. and Reynolds, V. Seasonal aspects of sociosexual behavior in two chimpanzee
populations: A comparison of Gombe (Tanzania) and Budongo (Uganda), in prep.
