Females are the ecological sex: sex-specific body mass ecogeography in wild sifaka populations (Propithecus spp.).

Previous work in primates has shown that body size often covaries with ecological parameters related to resource or energy availability in the environment. This relationship may differ for males and females as access to resources has greater importance for reproductive success in females. We test the hypotheses that (1) female body mass may be more tightly constrained than male body mass by ecological variables, and (2) female body mass may respond more strongly than male body mass to changes in ecological variables (i.e., population-specific female mass may vary more across an ecological gradient than male mass). Specifically, we investigate the relationship between climatic variables and sex-specific body mass in Propithecus, a genus in which species-specific body mass has already been demonstrated to covary significantly with climatic variables. Data from 733 wild sifakas are used to identify sex-specific body mass for 27 populations representing all nine described sifaka species, and climatic data for each population are derived from the WorldClim database. We use phylogenetic generalized least squares models to demonstrate that body mass in both sexes is significantly correlated with annual rainfall and number of dry months. Furthermore, coefficients of determination are always higher for female models, and coefficients for each climatic variable are higher for females in all significant models. These results support the two hypotheses tested, indicating that ecological forces can have a greater impact on female mass than on male mass in primates.