Observations on the Nature of the Decline in Numbers of Two Microtus Populations

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Abstract
Microtus populations are known to be subject to marked fluctuations in numbers, with periods of peak abundance alternating with periods of scarcity, and recurring at approximately four-year intervals. In 1952 Chitty advanced an hypothesis to account for the ‘crash’ of the voles at the end of each period of abundance, explaining the severe mortality at this stage of the cycle as resulting from conditions to which the parents had been exposed during the previous (i.e. peak) breeding season, and not from abnormal environmental conditions affecting the progeny at that time or during the decrease. This view is developed more fully by Chitty (1954). The hypothesis is based on evidence obtained during a study of vole populations in Wales where severe mortality occurred among the first-born young during the peak breeding season. Young born later in the season had a higher survival rate at first,...