Does mass change of primiparous bighorn ewes reflect reproductive effort?

Abstract

Reproductive effort during a female's first breeding attempt could affect subsequent fitness, particularly in species that reproduce before completing body growth. We analyzed 26 years of data on marked bighorn (Ovis canadensis) ewes to assess how variation in first reproductive effort affected other life-history traits. We measured reproductive effort as the residual of the regression of mass of primiparous ewes in late lactation on their mass 1 year earlier. Survival of the first-born lamb to weaning reduced maternal mass gain, suggesting a trade-off between reproduction and growth. Mass gain during the year of primiparity therefore appears to reflect reproductive effort. Lower mass gain was associated with lower adult mass and longevity, two important determinants of lifetime reproductive success. Reproductive effort at first parity therefore appears to lower residual reproductive value. Over their lifetime, females with low mass gain as primiparae produced proportionately more daughters than did females with high mass gain. Reproductive effort at first reproduction was not heritable, and may affect the evolutionary potential of adult mass and longevity, two fitness-related traits that are highly heritable in the study population.