Selfish mothers: Reproductive expenditure and resource availability in bighorn ewes

Abstract

When resources are scarce, iteroparous females may value their subsequent survival and reproduction over their current off-spring's development and survival. Field data to test this hypothesis are scant because it is difficult to determine whether reduced development of juveniles when resources are scarce is due to maternal restraint or constraint. During a 24-year study o bighorn sheep (Ovis canadensis), lamb mass near the time of weaning was very weakly correlated with maternal mass. A weak maternal mass effect persisted for body mass of yearlings of both sexes. As the number of adult ewes tripled, summer mass gain by lambs decreased about 22%, while summer mass gain by mothers decreased only 9%. Maternal expenditure (the residual of the regression of lamb mass and maternal mass in mid-September) was much lower at high than at low population density. For individual females, maternal expenditure was correlated with winter mass loss, but had no other overt short-term costs. Our results suggest that most bighorn ewes adopt a conservative maternal care strategy and reduce maternal care when resources are scarce to favor their own mass gain over the development of their lambs.