Ecological and social determinants of birth intervals in baboons

Birth rates in primates have long been proposed to result from an interacti on between ecological and social factors. We analyzed a variety of social a nd environmental variables to determine which ones best ex-plain the observ ed variation in interbirth intervals across 14 baboon populations. Both the number of females in the group and mean annual temperature were found to b e important, and a multivariate equation containing the quadratic component s of both these variables accounts for almost all the observed variance in interbirth intervals. The quadratic relationship with temperature is explai ned in terms of the energetic costs of maintaining a stable body temperatur e at both low and high temperatures. The quadratic relationship with number of females results from relationships with bath food availability and the costs of increasing intragroup competition as group size increases. Althoug h females may be able to exert a certain degree of choice in their reproduc tive scheduling decisions, they are ultimately constrained by limits impose d upon them by the complex interactions between their ecological and demogr aphic environment.