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Reproduction and pre-weaning juvenile survival in a cyclic population of snowshoe hares Stefan, Carol Irene


Reproductive output and pre-weaning survival were estimated in a cyclic population of snowshoe hares (Lepus americanus) in the Kluane Lake region of southwest Yukon Territory. Data collected by five researchers were collated over eight years (1989 - 1996). Pregnant hares were captured and held in cages until they gave birth, so that reproductive characteristics could be measured. Newborn hares were radio-tagged to estimate survival rates from birth. Pregnancy rate, litter size and neonate size fluctuated significantly throughout the cycle, changing about two years before corresponding changes in density. The lowest and highest values for all measured parameters were recorded during the decline and increase phases, respectively. Pregnancy rates were nearly 100% in litters born early in the breeding season, but declined up to 20% in the last litter of the year. The number of litters produced in a breeding season varied between two (decline phase) and four (low, early increase). Litter sizes varied among litters within a year, with larger litters being born later in the breeding season. Litter sizes also differed among years, ranging from a mean of 3.8 during the decline to 5.5 during the increase. The weight and size of neonates varied by 5-33% among years. Predation was the primary cause of death of leverets in all phases of the cycle except the decline, when exposure and starvation claimed most young hares. Juvenile survival was highly variable among litter groups at peak hare densities and extremely poor during the decline, particularly Litter 2. Survival was still variable but higher during the low, and was consistently high during the increase. Red squirrels were the primary predators of leverets

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