Does mass change of primiparous bighorn ewes reflect reproductive effort?

Reproductive effort during a female's first breeding attempt could affect s ubsequent fitness, particularly in species that reproduce before completing body growth. We analyzed 26 years of data on marked bighorn (Ovis canadens is) ewes to assess how variation in first reproductive effort affected othe r life-history traits. We measured reproductive effort as the residual of t he 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 gai n 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 val ue. Over their lifetime, females with low mass gain as primiparae produced proportionately more daughters than did females with high mass gain. Reprod uctive effort at first reproduction was not heritable, and may affect the e volutionary potential of adult mass and longevity, two fitness-related trai ts that are highly heritable in the study population.