Endocrinology and ecology of wild female chimpanzee reproduction

Human females show remarkable sensitivity of ovarian steroid production to current ecological conditions. While animals apply a variety of strategies for altering reproduction in the face of energy shortage, humans may have evolved their atypical system in response to increased reproductive costs and unpredictable resource availability. My research asks whether chimpanzees, by virtue of similar ecological constraints, also respond to energy availability by modulating ovarian function. I used non-invasive techniques to monitor ovarian hormones in 81 wild chimpanzee females from three populations at Kibale National Park and Budongo Forest Reserve in Uganda and at Gombe National Park in Tanzania. Urine and fecal sampling were sufficiently sensitive to distinguish significant variation in hormone production on a number of levels. Conception cycles were characterized by elevated ovarian function in the swelling and post-swelling periods. Sexual behavior was distributed nonrandomly, with increased copulations during the fertile days of the cycle and during cycles which led to conceptions. I found significant variation in ovarian function among the three populations; this variation followed predictions based on broad differences in diet and long-term reproductive parameters. Within populations, I found systematic variation among females who utilized different core areas, suggesting that uneven resource distribution across habitats could produce variation in female reproductive success. Both cycling and non-cycling females also responded to temporal variation in dietary quality. This variation was related not to total fruit consumption, but to utilization of site-specific preferred food items. The number of sexually receptive females was correlated with dietary composition. Seasons of low fruit utilization produced abrupt declines in ovarian steroid levels. Utilization of highly seasonal, preferred foods was associated with transient increases in ovarian function. Most significantly, females experienced fewer cycles to conception when dietary quality remained high throughout their cycling period. These results demonstrate the influence of local ecology on ovarian hormones and relate this to consequences on female attractivity, fecundity, and long-term reproductive success. These data are discussed in the context of a set of ecological circumstances that are expected to increase reproductive sensitivity to maternal energetic conditions.