Relationship Between Body Condition and Vulnerability to Predation in Red Squirrels and Snowshoe Hares

We examined the relationship between physical condition and vulnerability to predation in 2 species of mammals having different life history traits. We predicted that predators would be more likely to kill substandard individuals disproportionately in red squirrels (Tamiasciurus hudsonicus) than in snowshoe hares (Lepus americanus), given that sciurids apparently are less susceptible to predation. We also predicted that differences would exist between patterns of substandard-prey selection among groups of predators of red squirrels but not among predators of snowshoe hares because of differential predator efficiencies in capturing elusive prey. Through radio tracking, we found that substandard squirrels (n = 113) were disproportionately vulnerable to predation, whereas hares (n = 125) were killed irrespective of condition. Although the relationship between condition and vulnerability to predation, relative to predator groups, did not differ significantly for either species of prey, the difference was qualitatively greater in squirrels than in hares. These results support the notion that predators are more likely to kill substandard individuals disproportionately when targeting prey species that are difficult to capture and, moreover, that the tendency for particular species of predators to take substandard prey may be a reflection of the predator's hunting strategy and efficiency.