COS 177-1 - New estimates indicate that males are not larger than females in most mammals

Abstract: Sexual size dimorphism (SSD) is a salient trait in many mammals and has motivated a large body of research into mating strategies and sexual selection. Ralls published reviews in the 1970’s on rates of SSD in mammals, concluding that most species were not extremely sexually dimorphic in body size, that species with little sexual size dimorphism were extremely numerous in the most species-rich mammalian orders, and that larger females were more common than generally assumed. However, the narrative that larger males are the norm in mammals – upheld since Darwin’s Descent of Man – still dominates to this day, with some even misciting Ralls as having supported it. The prevalence of the ‘larger males’ narrative is likely driven by a long-standing research focus on taxa with larger males and on male mating strategies in the study of sexual selection. In addition, the use of crude and arbitrary measures of dimorphism and taxonomically-biased data has hindered an accurate estimation of rates of SSD in mammals in meta-analyses, largely because mean body masses are the only data that are broadly available and because SSD research has focused on a few, relatively species-poor taxa. Newly-available datasets on sex-segregated means and variances in adult body mass across a large number of mammalian species allowed us to revisit the question with greater statistical rigor. We also collected additional data from primary sources, focusing on sampling species from each mammalian order and family according to its species richness. We obtained these data on wild, non-provisioned populations of >400 mammalian species. Our results indicate that although males tend to be larger than females when dimorphism occurs, males are not statistically larger than females in most mammal species, supporting Ralls’s original conclusions. Our results suggest that it is time to retire the ‘larger males’ narrative and to put other common and related assumptions in sexual selection research under greater scrutiny.