Reproduction, juvenile survival and movements of snowshoe hares at a cyclic population peak

Snowshoe hare (Lepus americanus) populations were provided with supplemental food on two study grids in the southwest Yukon to examine the effects of food on reproduction and juvenile growth. Timing of parturition, pregnancy rates, litter sizes, male breeding condition, and juvenile growth rates were measured on the food grids and on two control grids during two summers at a cyclic peak in hare numbers. The main effects of food addition were to increase hare densities 2.1- to 2.7-fold, to advance the timing of breeding by about a week in one year, and to increase the mean size of third litter 30% in one year relative to the controls. There were no significant differences in pregnancy rates, litter sizes in five of six litter groups, length of male breeding season, or juvenile growth rates between hare populations on the food and control grids. Third litter stillborn rates were higher, and third litter juveniles grew slightly more slowly on food grids relative to those on controls, possibly because of higher densities. This study suggests that food is not a proximate factor limiting hare reproduction and early juvenile growth at the observed peak hare densities. Juvenile snowshoe hares were radio-tagged at birth on one food addition grid and one control grid, to determine early juvenile survival rates, the effects of the food addition on these rates, and the proximate causes of mortality. Indices of survival were estimated by live-trapping on these grids, and on one additional set of grids. Thirty-day survival rates 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 days after birth. Survival of littermates was not independent; Utters 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 days of age. Individual survival rates were negatively related to litter size, positively related to body size at birth, and litter size was negatively correlated with body size, suggesting trade-offs as predicted by life history theory. The number of recruits per litter, and the probability of total litter failure, did not differ significantly over the observed range of litter sizes. The radio-tagged juveniles were also followed to examine pre-dispersal movements, maternal-juvenile interactions, and timing of natal dispersal. Hare Utters stayed at their nest sites for an average of 2.7 days, after which each individual hare usually found a separate hiding place from its littermates. Juvenile hares ranged progressively farther from their nest sites as they grew, up to the age of 20 days. From 20 to 35 days of age, leverets stayed approximately 75 m from their nest sites, after which time their movements again increased. Observations at nest sites suggested that adult female hares nursed their litters only once per day, shortly after twilight. Some females aggressively defended their newborn litters before the juveniles left the nests. Natal dispersal of juvenile hares began shortly after weaning at 24-28 days of age. Many third litter juveniles were nursed for a longer period of at least 29-40 days. Juvenile males may disperse sooner and travel farther than females from their natal ranges.

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