INDIVIDUAL DIFFERENCES, LONGEVITY, AND REPRODUCTIVE SENESCENCE IN BIGHORN EWES

Within individual iteroparous mammals, a high rate of reproduction in early life may occur at the cost of decreased reproduction near the end of life, leading to reproductive senescence(definition). Using long-term data on marked individuals from two populations of bighorn sheep (Ovis canadensis), we tested for the existence of reproductive senescence and of trade-offs between longevity and early reproductive success in ewes, which have an observed maximum life-span of 19 yr. Lamb production decreased in older ewes, while weaning success for parous ewes was independent of age in one population and decreased with age in the other. The age-related decrease in lamb production followed a slight decrease in body mass, which began at ∼11 yr of age. Reproductive senescence in this species appears to begin 6–7 yr after the onset of survival senescence. Longevity was positively related to late-summer body mass at 6 yr. Contrary to reproductive cost theories, there was no negative relationship between early and late reproductive success or between early reproductive success and longevity; instead, those relationships tended to be weakly positive. Longevity had a strong positive influence on lifetime reproductive success in both populations. Since the longest lived ewes were among the heaviest as young adults and had high reproductive success throughout their lifetime, longevity appears to be state-dependent; only ewes of higher phenotypic quality survived long enough to reach reproductive senescence. Models that assume that survival to old age is random with respect to phenotype are therefore not applicable to reproductive senescence in bighorn sheep.