Sexual dimorphism of head, teeth, flipper and body size in northern elephant seals ( Mirounga angustirostris ) throughout ontogeny

Abstract

Sexual dimorphism, the difference in appearance between males and females of the same species, can shape physiology, behaviour and fitness. Yet, few species have age- and sex-specific data to characterize its development. Northern elephant seals (Mirounga angustirostris) are an extremely sexually dimorphic marine vertebrate for which detailed measurements are taken throughout life. To investigate the ontogeny of sexual dimorphism in juvenile elephant seals, we assessed differences in body size, tooth size, craniofacial and flipper morphology directly and photogrammetrically from 60 male and female seals ≤4 years old. Males and females maintained similar age-specific masses, but males grew in length more rapidly than females, with males growing 5.31 cm more per year, according to model predictions. We found evidence of early onset sexual dimorphism in the snouts and flippers of juvenile seals, with males growing larger and more quickly than females. Overall, we found that sexual dimorphism developed even earlier than sexual maturity in northern elephant seals. These results help explain differences in behaviour and fitness between male and female juvenile elephant seals early in life and highlight the importance of understanding the ontogeny of sexual dimorphism in large marine vertebrates.