Determinants of the Synchrony of Reproduction in Townsend's Voles, Microtus townsendii

Abstract

To determine whether synchronization of birth by female Townsend's voles occurs in natural populations as a result of environmental or social factors, the temporal pattern of reproduction was monitored in two populations of known relatedness. reproduction was most intense from April to July and less intense during the infestation by botflies in August and September. Synchronization of births was not related to the distance between females's nests or activity centers. This suggests that habitat characteristics of home-ranges do not cause the synchronization of parturitions. There was no correlation between the degree of relatedness of females and the synchronization of the birth of their litters. Among nearest neighbours, pairs of sisters and mothers and their daughters reproducing in the year of their birth had their litters significantly more synchronized than other females. Thus neighbouring relatives reproduce more synchronously than unrelated neighbours. Because related females often breed in communal home-ranges they could benefit from synchrony through a reduction of the risk of their pups being killed by close-relatives. Voles are thought to be unable to recognize their relative's offspring and it is possible that, like female wild mice, female Townsend's voles are infanticidal during pregnancy and non-infanticidal during lactation. The degree of synchrony of the spring onset of reproduction changed between years and was not higher among females of high average relatedness. Thus the synchrony of the onset of reproduction in spring is more a consequence of differences in the condition of individual females than determined by the extent of female territoriality.