Population cycles in small mammals: the role of age at sexual maturity

Several hypotheses have been proposed to explain cyclic fluctuations in abundance of some small mammal populations. These hypotheses have been controversial, however, and there is no consensus among biologists as to why population cycles occur. In a demographically based model, we tested the potential influence of phase-specific changes in life history traits (age at maturity, fertility, juvenile survival and adult survival) on population cycles. Our demographic model considers, and is logically consistent with, the empirical pattern of population characteristics during a cycle. The essence of the model is that phase-specific changes in age at maturity, abetted secondarily by changes in juvenile survival, result in cyclic fluctuations in population size. Changes in adult survival and fertility may play a minor role, but they are neither necessary nor sufficient by themselves to generate population cycles. Phase-specific changes in age at maturity might be related to primary changes in the quality of the ecological and social environment that permit particularly high densities.