Fat deposits of463 Red fox vixens shot during autumn-winter were monitored throughout a four-year cycle of vole abundance (1975-79) in south-central Sweden. The growth ofjuveniles during the same period was indexed by lower jaw length during winter. The reproduction of foxes in the study area was correlated with vole abundance. Fat deposits built up during autumn and were depleted during winter. Subadults had consistently smaller fat deposits than adults. There was no positive correlation between vole abundance and mean fat deposits in autumn or degree of depletion during winter. The mean amount of fat deposited during autumn was correlated with the frequency ofoccurrence offruit and berries in fox stomachs. The degree of depletion of fat deposits during winter was correlated with mean snow depth. It is hypothesized that the availability of fruit and berries (a major source of carbohydrates in the diet of foxes) determines the amount of fat reserves built up during autumn, whereas the energy requirement of moving in deep snow determines the degree of depletion during winter. Juvenile growth was correlated with vole abundance but growth later in life seemed to compensate for this variation.
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