Ontogenetic changes and sexual dimorphism in the cranium and mandible of the Atlantic walrus (Odobenus rosmarus rosmarus L.).

Walruses have been an important subsistence and cultural resource for humans and have been exploited for millennia across their distribution. This exploitation has contributed to severe declines in several populations and local extirpations. The study of walrus remains from archaeological sites can provide insights into past exploitation strategies and contribute to improving current conservation practices. The ability to differentiate disarticulated skeletal remains by age and sex can provide insights into past herd compositions. While sexing is a well-established approach for mandibles, a clear morphological key to determine such differentiation in crania is currently lacking. Here, linear measurements and geometric morphometrics of mandibles (n = 121) and crania (n = 116) of the Atlantic walrus (Odobenus rosmarus rosmarus) are used to describe the age and sex differences. Suture fusion states are strongly associated with age. There are clear size and shape differences between adults and neonates, with juveniles showing a gradual transition between states. Sexual dimorphism is clearly present in adults, but not detected in juveniles and neonates. Linear discriminant analysis successfully assigned sexes to 19 out of 21 (90.5%) DNA-sexed adult test specimens. Comparatively, only two out of five juvenile test specimens were correctly sexed using morphometrics. We suggest that aging specimens using suture fusion states, followed by classification between adult males and females, provides the most accurate results. This classification technique will facilitate future osteological studies on walruses, for example, to understand their ecomorphology and to provide insight into historical age- and sex-based exploitation and population impacts through time.