Ecological and social determinants of birth intervals in baboons

Birth rates in primates have long been proposed to result from an interaction between ecological and social factors. We analyzed a variety of social and environmental variables to determine which ones best explain the observed variation in interbirth intervals across 14 baboon populations. Both the number of females in the group and mean annual temperature were found to be important, and a multivariate equation containing the quadratic components of both these variables accounts for almost all the observed variance in interbirth intervals. The quadratic relationship with temperature is explained in terms of the energetic costs of maintaining a stable body temperature at both low and high temperatures. The quadratic relationship with number of females results from relationships with both food availability and the costs of increasing intragroup competition as group size increases. Although females may be able to exert a certain degree of choice in their reproductive scheduling decisions, they are ultimately constrained by limits imposed upon them by the complex interactions between their ecological and demographic environment.