1Evolutionary Anthropology, Duke University, 2Jane Goodall Institute
April 16, 2016 2:15, A 703/704
Published reports of maturation milestones in female chimpanzees are typically derived from captive populations, in which development is known to be accelerated. Data from wild populations reflect development subject to evolutionary pressures and should be prioritized for use in modeling hominin evolution. But because most females transfer before breeding, precise age at first birth is rarely known in the wild and data from known-aged females come from a small number of non-dispersing individuals. Here we report maturation milestones and explore sources of variance for 36 wild female chimpanzees of known age, including seven dispersing females, born in Gombe National Park, Tanzania. Using Kaplan-Meier survival analysis including censored intervals, we find an average age of 11.6 years for first maximal sexual swelling (which is generally closely associated with menarche) and 14.9 years for first birth. These values exceed previously published averages for wild chimpanzees by one or more years. Even in this larger sample, age at first birth is likely underestimated due to the disproportionate number of non-dispersing females, which, on average, give birth two years earlier than dispersing females. Log-rank comparison tests show that age at first maximal sexual swelling is delayed in firstborn females and those either orphaned before 8 years or born to low-ranking mothers. Age at first birth is also delayed in females orphaned before 8 years. These data provide improved estimates of maturation milestones in a population of wild female chimpanzees for use in anthropological study and indicate the importance of maternal factors in development.
Funding provided by the Jane Goodall Institute, Duke University, University of Minnesota, the National Science Foundation (9021946, 9319909, 0431141,0452315, 1052693) and the National Institutes of Health (R01-AI058715).