POPULATION-CYCLES IN MICROTINES - THE SENESCENCE HYPOTHESIS

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.