The cause of population cycles in microtines (voles and lemmings) rema
ins an enigma. I propose a new solution to this problem based on a cru
cial feature of microtine biology, shifts in age structure, that has b
een 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 decl
ine and density-dependent social inhibition of maturation of young. De
clines become inevitable as populations composed of older animals surv
ive and reproduce poorly because of the effects of senescence, possibl
y interacting with the experiences of peak density and I present both
theoretical and empirical evidence for this hypothesis. Although a var
iety of physiological systems deteriorate with aging, I focus on a cru
cial one - the inability of older animals to effectively maintain home
ostasis in the face of environmental challenges because of a progressi
ve 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 owin
g to extrinsic factors such as intense predation. Six testable predict
ions are made that can falsify this hypothesis.