Presence of fathers and concurrent pregnancy have been shown to influe
nce maternal behavior in rodents, but often are confounded in studies
of species that mate during postpartum estrus. I examined parent-young
interactions in red-backed voles (Clethrionomys gapperi) to document
patterns of parental care in this species and to determine whether the
separate factors of presence of father and concurrent pregnancy influ
enced maternal behavior. Experiments were conducted in a seminatural e
nvironment with observations occurring every other day from day 1 to d
ay 20 of Life of the young. Although male and female red-backed voles
always built and maintained separate nests, there was substantial vari
ation among females in their response to fathers in the vicinity of th
e natal nest. Whereas some females permitted fathers in the nest soon
after birth, others reacted aggressively and prevented fathers from in
teracting with young throughout most of the preweaning period. Such va
riation in behavior of females appeared responsible, at least in part,
for variation among individual males in level of paternal care. Overa
ll, females caring for young in the presence of fathers spent less tim
e nursing and groomed their young less frequently than did females rea
ring young in the absence of fathers. Lower maternal care in the fathe
r present condition may reflect reduced maternal workload in the pairs
in which males spent considerable time in the nest with young or disr
uption caused by presence of males in the pairs in which females aggre
ssively excluded males from the natal nest. The maternal behavior of p
regnant females did not differ from that of nonpregnant females.