Studies of plasticity in vole reproductive traits in response to changing predation risk have produced conflicting results. Predator-induced breeding suppression observed in laboratory experiments has not been fully supported by other studies. Here, data from a 9-y field study of Microtus oeconomus in a strongly fluctuating population and under variable predation risk imposed by a specialist predator, Mustela nivalis, were used to assess plasticity of reproduction. The age (body mass) at maturity and 2 indices of the reproductive effort of females were measured during the summer. In years of high vole density, increased predation risk resulted in early maturation and increased reproductive effort of females. In years of low vole density the effect of increased predation risk was not evident, probably because the reproductive performance of females was significantly enhanced by low density alone. These results do not support the predator-induced breeding suppression hypothesis, but are in agreement with models that predict increased investment in reproduction under decreased probability of survival in fluctuating environment.
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Vol. 14 • No. 3