Mass-dependent reproductive strategies in wild bighorn ewes: a quantitative genetic approach

In the Ram Mountain bighorn sheep (Ovis canadensis) population, ewes differ ing by more than 30% in body mass weaned lambs with an average mass differe nce of only 3%. Variability in adult body mass was partly due to additive g enetic effects, but inheritance of weaning mass was weak. Maternal effects could obscure genetic effects in the phenotypic expression of weaning mass, particularly if they reflected strategies of maternal expenditure that var ied according to ewe mass. We performed a quantitative genetic analysis to assess genetic and environmental influences on ewe mass and on maternal exp enditure. We used the mean daughters/mother regression method and Derivativ e Free Restricted Maximum Likelihood models to estimate heritability (h(2)) of ewe mass and indices of maternal expenditure. We found additive genetic effects on phenotypic variation in maternal mass, in lamb mass at weaning (absolute maternal expenditure) and in weaning mass relative to maternal ma ss at weaning (relative maternal expenditure). Heritability suggests that m aternal expenditure has the potential to evolve. The genetic correlation of ewe mass and absolute maternal expenditure was weak, while ewe mass and re lative maternal expenditure were strongly negatively correlated. These resu lts suggest additive genetic effects on mass-dependent reproductive strateg ies in bighorn ewes. Mass-dependent reproductive strategies could affect la mb survival and phenotypic variation in adult mass. As population density i ncreased and reproduction became costlier, small females reduced maternal e xpenditure more than large females. Constraints on reproductive strategy im posed by variations in resource availability are therefore likely to differ according to ewe mass. A general trend for a decrease in maternal expendit ure relative to maternal size in mammals suggests that size-dependent negat ive maternal effects may be common.