A body mass/rainfall relationship in baboons, Papio, is often treated as a well-established socioecological principle. This paper tests its reality in 29 populations representing five of six recognized phylogenetic baboon species. Contrary to previous findings from fewer cases, mean adult body mass was not significantly related to mean annual rainfall (MAR) across the whole genus in either gender. A positive mass/rainfall relationship is seen in chacma baboons, Papio ursinus, and anubis or olive baboons, Papio anubis, but only if the two species are considered separately. An explanatory hypothesis in terms of year-to-year predictability of food resources (rather than absolute productivity and its surrogate, MAR) is advanced for further testing. Unlike variables such as group size and time budgets, interpopulational body mass differences are likely to be largely ‘evolutionary’ rather than ‘phenotypic’. As such, they may reflect population history and adaptation to ancient ecosystems as much as to extant environments. This being so, plausible explanations of the interpopulational distribution of mean body mass and other similar variables are more likely to be found if analysis incorporates biogeographic and phylogenetic histories, and taxonomic subdivisions.