Jonathan Van Elslander, Garth Mowat, Andrew Derocher

Somatic growth is affected by individual factors and resources that have long-term consequences for survival and reproductive success. We evaluated how diet, habitat-related environmental conditions, and density influence somatic growth using a 48-year dataset of 18,928 human-killed grizzly bear (Ursus arctos) skull measurements across British Columbia, Canada. Growth was modeled as a function of sex, age, precipitation, season length, diet, and cohort density using non-linear Bayesian von Bertalanffy growth models. We assigned dietary compositions to bears using stable isotope analysis (n = 1,592) and predictive diet maps for the remaining individuals (n = 17,336). We found dietary meat had the strongest positive effect on growth, followed by season length and cohort, consistent with variation in habitat productivity. Males growing under maximal habitat conditions (~95% meat-based diets, long growing seasons, recent cohorts), compared to limiting conditions (0% meat-based diets, short growing seasons, earlier cohorts), reached 95% of their asymptotic (t95) skull length at similar ages but were 6.9 cm (20.4%) longer and 3.6 cm (15.7%) wider. Females reached t95 of skull length at similar ages and were 7.0 cm (23.3%) longer and 3.9 cm (21.5%) wider. Terrestrial meat increased skull growth, length, and width more than fish sources. Our results demonstrate that spatial variation in habitat-driven diet composition and growing-season length can drive substantial differences in somatic development in a long-lived omnivore, with implications for managing nutritionally important habitat resources to promote fitness potential under ongoing climate and land-use change.