Ims (1987a) hypothesized that the spatial distribution pattern of male
voles may be influenced by the temporal distribution of females. When
females enter estrus synchronously (i.e. are clumped in time) they re
present a defendable resource and males should defend a territory cont
aining a cluster of females. When females breed asynchronously, males
should not be territorial, In this study we examined the role of femal
e breeding synchrony on the spacing strategies of male meadow voles Mi
crotus pennsylvanicus (Ord, 1815). Maternity was determined by the tra
nsfer of a unique combination of radionuclides from mother to offsprin
g. The date of conception (to the nearest week) was determined by the
weight at first capture of newly recruited voles. The number of litter
s conceived per week ranged from zero to four. The null hypothesis tha
t the number of litters conceived per week was distributed randomly co
uld not be rejected in 8 of 9 grid-years (4 years on 3 grids). This fi
nding of breeding asynchrony in a vole species with nonterritorial mal
es is consistent with Ims' hypothesis.