Rensch’s rule states that in species with male-biased sexual dimorphism the differences in body size will be most pronounced in larger species, while the opposite is true in species with female-biased sexual dimorphism. There is conflicting evidence in the current literature concerning whether ungulates (hoofed mammals) follow Rensch’s rule. There are two primary forms of sexual dimorphism in ungulates, sexual size dimorphism (SSD) in body mass, and secondary sexual characteristics (SSC) in their weaponry. In this study, data were
extracted from the literature for 334 species of ungulates to determine whether they follow Rensch’s rule, either in terms of their SSD or SSC. The magnitude of sexual dimorphism was measured and represented by a sexual dimorphism index (SDI). While consistency with Rensch’s rule was determined by whether the slope of the allometric relationship between male and female body size exceeded one. The results from this study demonstrated that ungulates follow Rensch’s rule for body mass but not weaponry length. Different ecological variables; habitat type, foraging strategy and social grouping, were shown to have a significant effect on the SSD exhibited by ungulates, but only habitat type and social grouping had a significant influence on SSC. When groups of ungulate species defined by their ecological variables were tested for allometry to
Rensch’s rule independently, the body mass of grazing and omnivorous species along with gregarious and sexually segregated species were found to be inconsistent with Rensch’s rule. Although they are sexually dimorphic groups of ungulates, the cost of increased SSD with species mass outweighed any benefits to their reproductive success. When the weaponry length of ungulate groups was tested independently, they were still found to be inconsistent with Rensch’s rule. These dimorphic weapons are multipurpose and so there are different selection pressures beyond reproductive success influencing the relationship between species mass and sexual dimorphism. It is the finding of this study that the SSD of ungulate species exhibiting male-biased sexual dimorphism are greater in larger species and those with female-biased SSD express greater dimorphism in smaller species. Furthermore, that this trend is driven by the social structure, habitat type and foraging strategy of the species.