Early development, adult mass, and reproductive success in bighorn sheep

Despite considerable empirical and theoretical work on the individual and population consequences of early development, little is known about the correlations between early mass and adult size or lifetime reproductive success of free-ranging mammals. Using a 26-year study of bighorn sheep (Ovis canadensis), we examined how mass as a lamb and mass gain as a yearling affected adult mass for both sexes, horn length of males and lifetime reproductive success of females at different population densities. Mass as a 3-week-old lamb was either weakly or not correlated with adult mass, horn length of adult males, or the number of lambs weaned over a ewe's lifetime. Weaning mass was correlated with most of these variables when the number of ewes in the population was taken into account. When weaning mass was controlled through partial correlation, mass as a yearling was correlated with adult mass of ewes but not with ewe reproductive success or with adult mass or horn length of rams. Lamb mass and number of ewes explained more of the variance in adult characteristics for males than for females. Our results suggest that mass gain during lactation, possibly but not necessarily related to the amount of maternal care received, affects adult mass and reproductive success. Females appear better able than males to compensate for poor early development, likely by postponing their first reproduction. Mass gain over several years and the number of ewes in the population strongly affect adult mass of both sexes and therefore can have profound effects on reproductive success of this long-lived species with a multi-year growth period.