Linking Sexual Dimorphism and Sociality in Rhinoceroses: Insights from Teleoceras proterum and Aphelops malachorhinus from the Late Miocene of Florida

Abstract

A strong relationship between sexual dimorphism and the degree of polygyny (i.e., the degree to which males compete for mates) is not apparent in living perissodactyls. For instance, in both monomorphic and dimorphic species of rhinos, about half of male mortality is attributable to tusk and horn mediated combat. Males of the North American Miocene rhinoceros Teleoceras had delayed tusk (i2) eruption, prolonged tusk root growth, and highly sharpened tusk facets maintained by honing on the upper incisor, thus predicting high levels of intermale aggression similar to living rhinos. The sex biases and elevated male mortality rates found in Teleoceras assemblages from Nebraska and Florida seem to confirm this prediction. However, the degree of body size dimorphism in these assemblages varies. Therefore, the intensity of intermale competition seems unrelated to the magnitude of body size dimorphism in Teleoceras. Male individuals of Aphelops, a sympatric rhino, experienced more finite tusk growth and tusks were blunted with age due to the ancestral loss of the upper honing incisor, thus predicting lower levels of intermale competition. The Aphelops fossil assemblage from the Love Bone Bed of Florida is not sex-biased, shows more balanced sex-specific mortality rates, and seems to confirm the prediction of reduced intermale competition, thus suggesting a type of sociality that is different from both Teleoceras and modern rhinos. However, the same assemblage exhibits a degree of sexual dimorphism in tusk and body size that are is not demonstrably different from Teleoceras. Thus, we are left with a perplexing relationship between dimorphism and sociality for rhinos, where levels of intermale competition seem uncorrelated to the degree of sexual dimorphism in both living and extinct species.