DETERMINANTS OF THE SYNCHRONY OF REPRODUCTION IN TOWNSEND VOLES, MICROTUS-TOWNSENDII

To determine whether synchronization of birth by female Townsend's vol es occurs in natural populations as a result of environmental or socia l factors, the temporal pattern of reproduction was monitored in two p opulations of known relatedness. reproduction was most intense from Ap ril to July and less intense during the infestation by botflies in Aug ust and September. Synchronization of births was not related to the di stance between females's nests or activity centers. This suggests that habitat characteristics of home-ranges do not cause the synchronizati on of parturitions. There was no correlation between the degree of rel atedness of females and the synchronization of the birth of their litt ers. Among nearest neighbours, pairs of sisters and mothers and their daughters reproducing in the year of their birth had their litters sig nificantly more synchronized than other females. Thus neighbouring rel atives reproduce more synchronously than unrelated neighbours. Because related females often breed in communal home-ranges they could benefi t from synchrony through a reduction of the risk of their pups being k illed by close-relatives. Voles are thought to be unable to recognize their relative's offspring and it is possible that, like female wild m ice, 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 f emales of high average relatedness. Thus the synchrony of the onset of reproduction in spring is more a consequence of differences in the co ndition of individual females than determined by the extent of female territoriality.