Is Skull Shape Related to Aggressive Displays in Pinnipeds?

Abstract

Some pinniped species display aggression as male‐male vocal displays only. In highly‐aggressive species, outright combat is waged between males, and in other cases aggression can occur between males and females of the same species. Are these differences in behavior manifested in skeletal dimorphism? To answer this, we compared species' aggressive behaviors with skull proportions measured from photographs of museum specimens, including 26 species of seals (n = 143) and five species of fur seals and sea lions (n = 39).We predicted that there would be a difference in skull shape between males of highly‐aggressive and less‐aggressive species, and also between males and females of species with male‐male combat. On average, the highly‐aggressive species did have relatively wider zygomatic arches (p < 0.001), likely to allow the emplacement of large jaw‐closing muscles, but no difference in relative facial length between these two groups (p = 0.14). Most highly‐aggressive males also had substantial nuchal crests to allow fierce rear‐and‐slam behaviors. Males that were more aggressive also had wider zygomatic arches than less‐aggressive females (p < 0.001). We found a characteristic shape for “less‐aggressive” species, but the skull proportions of highly‐aggressive species were not uniform. We found no difference in skull shape for species in which both sexes displayed similar aggressive levels. From these trends, we were able to reconstruct the relative aggression of several extinct pinnipeds.