DIFFERENTIAL MORTALITY OF SYMPATRIC SNOWSHOE HARES AND COTTONTAIL RABBITS IN CENTRAL WISCONSIN

During 1990 and 1991 we livetrapped and radio-collared cottontail rabb its (Sylvilagus floridanus) and snowshoe hares (Lepus americanus) on f ive sites (three in lowland deciduous cover, two in conifer bogs) in c entral Wisconsin. The survival rates of these sympatric cottontails an d snowshoes were equally high during snow-free periods from mid-March through October. With the onset of persistent snow cover (mean depth 2 0 cm) and low temperatures (mean daily minimum -14-degrees-C) in Novem ber 1991, the 60-day survival rate of cottontails plummeted from 0.89 to 0.18 (P < 0.01) whereas snowshoe survival declined only moderately from 0.84 to 0.63 (P = 0.19). This interspecific difference reflected continued high survival of snowshoes (0.80), but not of cottontails (0 .15), in the conifer bogs. Predation, primarily by coyotes, was the pr oximate cause of 37 of 39 rabbit and hare deaths, and was thus the ove rwhelming determinant of survival. We speculate that where persistent snow and low temperatures characterize winter, the cottontail's greate r foot loading, brown coloration, and escape behavior markedly increas e its vulnerability to predators. This probably explains the absence o f cottontails from the forests of northern Wisconsin.