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Age at first reproduction has been recognized as the most significant life-history parameter in determining population growth rates. Females in fluctuating rodent populations show great phenotypic variation in this parameter with precocious breeding, defined as pre-weaning fertile mating, being among the most extreme cases of early reproduction in mammals. Here we present the results of experiments in which 14-d-old female common voles (Microtus arvalis) were paired with adult males. Precocious breeding was common and coupled with unexpectedly large litters. We consider this reproductive feature to be adaptive in strictly seasonal environments with greater variation in length of the breeding season, with high resource availability, and a low level of intraspecific competition. Under these conditions, linkage between precocious breeding and high fertility has evolved as part of the most effective life-history strategy which produces the high intrinsic rates of population increase typical of small rodent populations. Seasonality of the environment was suggested to be the main factor behind the geographic pattern in small rodent cyclicity.
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