Reproductive Ecology of Female Chimpanzees

Abstract

An important adaptive problem for mammals in general, and primates in particular, is how females can manage the high costs of reproduction in the face of fluctuating energetic supplies. For many species, the best solution is to breed seasonally such that high costs are temporally coincident with predictable periods of resource abundance. This is an unreliable strategy for some primates, such as chimpanzees (Pan troglodytes), for which large body size forces an increase in dietary complexity and prolonged reproductive efforts. Here, I review data on reproductive function in chimpanzees, a species that demonstrates a risk-averse reproductive strategy wherein reproductive investment is allocated in accordance with maternal condition. Life history parameters for chimpanzees indicate that most females produce very few surviving offspring. However, comparisons between captive and wild populations and within wild populations illustrate that variation in resource access leads to highly variable reproductive success. Focused hormonal studies have demonstrated these effects at a proximate level, with energetic influences on female dispersal, receptivity, cycle quality, conception success, and lactational amenorrhea. Downstream of these effects, female reproductive function affects sexual attractiveness, and by virtue of males' own optimal reproductive strategies, can lead to coercive aggression and decreased foraging efficiency. Because of their extreme reproductive costs, female chimpanzees utilize a highly conservative reproductive strategy, one that minimizes the costs of ecological variation but makes them vulnerable to sexual conflict and costs of sociality.

First molar eruption, weaning, and life history in living wild chimpanzees

“…There has been modest population growth since the initiation of behavioral observations in 1988, suggesting that this community is not being negatively affected by anthropogenic effects of human encroachment, disease, or severe food shortages. Infant mortality also appears to be comparable to other sites (save for Mahale) (22). Limited data suggest that Kanyawara females may cease nursing infants earlier than other eastern African chimpanzee populations (34,38).…”

“…However, effectively nothing is known about variation in M1 emergence ages within populations or among species or subspecies of wild great apes (8,9). Moreover, most assessments of the predictive value of M1 emergence fail to consider normal variation in life history characters such as weaning age or interbirth intervals (20), which are influenced by ecology and maternal energetics (15,16,19,(21)(22)(23).…”

Faster reproductive rates trade off against offspring growth in wild chimpanzees

“…This is counterintuitive, given that, as a general rule, chimpanzees exhibit a conservative pattern of reproductive investment, with high sensitivity to energetic stress and among the longest birth intervals of any animal (37,111). In female chimpanzees, as in humans, maternal energy reserves are important determinants of reproductive function (39,111), perhaps because maintaining maternal health is vital for offspring survival through long periods of dependency.…”

“…We first assessed whether a juvenile chimpanzee's body size was predicted by the interbirth interval preceding its birth and/or that following its birth. Reproductive rates in this population are strongly dependent on interindividual, as well as temporal, variation in resource access (37)(38)(39)(40)(41). Thus, although a trade-off between interbirth interval and juvenile size was predicted, it remained plausible that some mothers could afford to produce higher-quality infants at a faster rate.…”

“…Chimpanzees are not weaned until ∼4-5 y of age and do not reach sexual maturity until their early teens (36,37). Unlike humans, chimpanzee infants are rarely provisioned by their mothers or by other group members after weaning.…”