Exploring factors shaping the craniomandibular shape of extant walrus

As both iconic Arctic megafauna and long-exploited marine resources, walruses (Odobenus rosmarus) have left an abundant osteological record across the Holarctic. However, distinguishing sex, ontogenetic stage, and subspecies assignation from disarticulated skeletal elements remains challenging. Quantifying cranial and mandibular shape variation while disentangling the effects of individual intrinsic factors (sexual dimorphism, ontogeny, geographic distribution) but also their combined influence offers new insights into these key axes of morphological differentiation. Here, two-dimensional geometric morphometric analyses are applied to 166 skulls and 143 mandibles representing the Atlantic (O. r. rosmarus) and Pacific (O. r. divergens) subspecies. Analyses revealed that sexual dimorphism is the dominant morphological signal: males develop more compact and reinforced skulls and mandibles, reflecting strong constraints to grow tusks and handle combat stresses, whereas females retain more gracile morphologies. Juveniles closely resemble adult females, with males diverging later through delayed hypermorphic growth. Subspecific comparisons confirmed significant differences, with Pacific walruses exhibiting more robust morphologies and greater developmental canalization, while Atlantic walruses show higher variability, likely linked to heterogeneous ecological conditions. Population-level analyses highlighted additional structuring, with Pacific groups tightly constrained and Atlantic groups occupying a broader morphospace. Unlike previous studies that examined sexual dimorphism, ontogeny, or subspecific differences separately, our study evaluates all three factors within a unified framework, revealing a hierarchical organization of craniomandibular variation shaped by sexual selection, developmental timing, and ecological divergence. These results provide new insights into walrus evolutionary history and their differential vulnerability to past human exploitation and ongoing Arctic environmental change.