Differential body condition and vulnerability to predation in snowshoe hares

Abstract

Summary Selection of prey in poor body condition allegedly is a widespread phenomenon in predator–prey systems. Yet, it remains unclear to what extent such selection reflects impeded prey escape ability due to severe malnutrition and/or state‐dependent risk taking following lesser changes in prey body condition. In systems where prey food availability is adequate, few individuals should be severely malnourished and thus condition‐sensitive predation should be related principally to changes in nutritional status that occur irrespective of an individual's absolute body condition. I studied predation in snowshoe hares ( Lepus americanus , Erxleben) when food was abundant, to test the prediction that change in body condition would contribute, more than would absolute body condition, to vulnerability to predation. Hares ( n = 618) were radio‐marked and monitored for survival while body condition was indexed during bimonthly livecaptures. A subsample of animals ( n = 36) was equipped with motion‐sensitive transmitters and activity patterns were examined in relation to the body condition index and vulnerability to predation. Manipulation of hare nutritional status via food supplementation improved hare body condition but did not affect vulnerability to predation; reduction of nematode parasite numbers did not improve hare condition but reduced predation among female hares. Reproductive status affected predation in both sexes, with recently pregnant females and non‐scrotal males being less vulnerable to predation. Heavier males also tended to experience lower predation rates. Predation rates for both female and male hares were higher among individuals having recently experienced condition deterioration but not among animals considered to be in poor absolute condition. Yet, the parameter representing change in condition was the last to enter survival models, indicating a relatively modest effect of this factor on hare vulnerability to predation. Mammalian but not avian predators tended to kill hares in declining condition, and hare activity patterns were not correlated either to change in condition or to survival. These findings indicate that nutritional status may influence vulnerability to predation even when food is abundant, but that such effects occur indirectly through changing body condition. This implies that hares are likely to forage in a risk‐prone manner when subject to worsening condition.