Body size plasticity in North American black and brown bears

Body size reflects realized physiological niche width at the population level and provides insights into the potential resiliency of a species to natural and anthropogenic perturbations to ecosystems. We analyzed patterns of American black and brown bear body size through a meta-analysis of data from 18 studies conducted across North America, to evaluate the effects of species, sex, latitude, sympatry, and harvest. We used a bootstrap model selection procedure, which accounted for differences in sample size and population variation between studies, to investigate patterns in body mass. As expected, we found that brown bears were generally larger than black bears and both species were sexually dimorphic (i.e., males were generally larger than females). Black bear body size was not related to latitude, whereas brown bear body size was greatest at intermediate latitudes, possibly due to the presence of salmon. Neither sympatry nor harvest was associated with body size for either species at the geographic scale of our meta-analyses, but both may warrant consideration at the local scale. Body size, as an index of population health, reflects phenotypic plasticity within species and populations and may serve as a useful indicator of niche utilization. Indeed, understanding large-scale physiological patterns such as these can assist in understanding past, present, and future changes to realized niches and subsequent resiliency of species and populations.