Early Survival of Juvenile Snowshoe Hares

Juvenile snowshoe hares (Lepus americanus) were radio—tagged at birth on one food addition grid and one control grid, to determine early juvenile survival, the effects of the food addition on survival, and proximate causes of mortality. Indices of survival were also estimated by live—trapping on these grids and on a replicate pair of grids. The overall 30—d survival rates of radio—tagged leverets were 0.46, 0.15, and 0.43 for the first, second, and third litters of the year, respectively. There were no differences between early juvenile survival on the food addition and control grids in any of the litter groups. The main proximate cause of juvenile mortality was predation by small mammalian predators, the most important being red squirrels and arctic ground squirrels. Seventy percent of early juvenile mortality occurred during the first 5 d after birth. Survival of littermates was not independent; litters tended to all live or die as a unit more often than expected by chance. Fifty—one percent of litters had no known survivors after 14 d of age. Individual survival rates were negatively related to litter size and positively related to body size at birth, and litter size was negatively correlated with body size. These correlations were most closely related to differences in life history traits among litters born at different times of the summer, rather than to trade—offs among traits within litter groups.