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

Despite considerable empirical and theoretical work on the individual and p opulation consequences of early development, little is known about the corr elations between early mass and adult size or lifetime reproductive success of free-ranging mammals. Using a 26-year study of bighorn sheep (Ovis cana densis), we examined how mass as a lamb and mass gain as a yearling affecte d 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 a dult males, or the number of lambs weaned over a ewe's lifetime. Weaning ma ss was correlated with most of these variables when the number of ewes in t he population was taken into account. When weaning mass was controlled thro ugh 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 le ngth of rams. Lamb mass and number of ewes explained more of the variance i n adult characteristics for males than for females. Our results suggest tha t mass gain during lactation, possibly but not necessarily related to the a mount of maternal care received, affects adult mass and reproductive succes s. Females appear better able than males to compensate for poor early devel opment, likely by postponing their first reproduction. Mass gain over sever al years and the number of ewes in the population strongly affect adult mas s of both sexes and therefore can have profound effects on reproductive suc cess of this long-lived species with a multi-year growth period.