MASS-DEPENDENT AND DENSITY-DEPENDENT REPRODUCTIVE SUCCESS AND REPRODUCTIVE COSTS IN A CAPITAL BREEDER

For capital breeders, mass may affect reproductive potential. Reproduc tive expenditure may reduce future reproductive potential, particularl y when resources are scarce. To test the hypothesis that reproductive success and the costs of reproduction vary according to mass and popul ation density, we analyzed 25 yr of data on bighorn ewes (Ovis canaden sis). The number of adult females was first limited by yearly removals , then allowed to triple. We found no survival costs of reproduction f or ewes aged 4-7 yr. For ewes aged 8-14 yr, survival was density depen dent for barren ewes but not for ewes that weaned lambs. Failure to la mb was rare and negatively correlated with fertility the following yea r. At low population density, lactation had a negative effect on mass gain but had a limited reproductive cost. At high density, heavy ewes had higher reproductive success than light ewes, and the reproductive cost and somatic costs of reproduction increased. The cost of reproduc tion was greater for light than for heavy ewes. Survival of weaned lam bs to 1 yr was affected by population density but not by maternal mass or previous reproductive success. In large mammals, manipulations of reproductive effort are problematic, but long-term monitoring of indiv idual mass and reproductive success under varying conditions of resour ce availability can provide insights into the evolution of life histor ies.