The cause of population cycles in microtines (voles and lemmings) remains an enigma. I propose a new solution to this problem based on a crucial feature of microtine biology, shifts in age structure, that has been ignored until now. Empirical evidence indicates that age structure must shift markedly towards older animals during declines because of three characteristics of the previous peak year: a shortened breeding season, total replacement of the breeding population from peak to decline and density-dependent social inhibition of maturation of young. Declines become inevitable as populations composed of older animals survive and reproduce poorly because of the effects of senescence, possibly interacting with the experiences of peak density and I present both theoretical and empirical evidence for this hypothesis. Although a variety of physiological systems deteriorate with aging, I focus on a crucial one - the inability of older animals to effectively maintain homeostasis in the face of environmental challenges because of a progressive deterioration in the endocrine feedback mechanisms involved in the hippocampal-hypothalamic-pituitary-adrenal axis. Microtine populations will not exhibit cycles where age structure shifts are prevented owing to extrinsic factors such as intense predation. Six testable predictions are made that can falsify this hypothesis.
