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. Using long-term data on marked individ uals from two populations of bighorn sheep (Ovis canadensis), we tested for the existence of reproductive senescence and of trade-offs between longevi ty and early reproductive success in ewes, which have an observed maximum l ife-span of 19 yr. Lamb production decreased in older ewes, while weaning s uccess for parous ewes was independent of age in one population and decreas ed with age in the other. The age-related decrease in lamb production follo wed a slight decrease in body mass, which began at similar to 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-summ er body mass at 6 yr. Contrary to reproductive cost theories, there was no negative relationship between early and late reproductive success or betwee n early reproductive success and longevity; instead, those relationships te nded to be weakly positive. Longevity had a strong positive influence on li fetime reproductive success in both populations. Since the longest lived ew es were among the heaviest as young adults and had high reproductive succes s throughout their lifetime, longevity appears to be state-dependent; only ewes of higher phenotypic quality survived long enough to reach reproductiv e senescence. Models that assume that survival to old age is random with re spect to phenotype are therefore not applicable to reproductive senescence in bighorn sheep.