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

Several hypotheses have been proposed to explain cyclic fluctuations in abu ndance of some small mammal populations. These hypotheses have been controv ersial, however, and there is no consensus among biologists as to why popul ation cycles occur. In a demographically based model, we tested the potenti al influence of phase-specific changes in life history traits (age at matur ity, fertility, juvenile survival and adult survival) on population cycles. Our demographic model considers, and is logically consistent with, the emp irical pattern of population characteristics during a cycle. The essence of the model is that phase-specific changes in age at maturity, abetted secon darily by changes in juvenile survival, result in cyclic fluctuations in po pulation size. Changes in adult survival and fertility may play a minor rol e, but they are neither necessary nor sufficient by themselves to generate population cycles. Phase-specific changes in age at maturity might be relat ed to primary changes in the quality of the ecological and social environme nt that permit particularly high densities.