Population dynamics and litter size of the montane vole, Microtus montanus
Population dynamics of the montane vole, Microtus montanus, in northwestern Wyoming have been studied since 1969. Four peaks and four troughs in population density and four peaks and four troughs in litter size have been documented to date. There is a significant correlation between the peaks and the troughs of the two parameters: the peaks and the troughs in litter size precede the peaks and troughs in population density by 1 year. There were only two exceptions to this relationship in 15 years. Changes in litter size are, therefore, a useful predictor of the direction, but not, however, of the magnitude of the change in population density in the ensuing year. Litter size was also correlated with population cycle phase, suggesting the existence of qualitative differences between populations from different cycle phases. However, the factors that underlie the predictive value of litter size and the correlation of litter size with cycle phase remain unknown.