Age-related reproductive effort in bighorn sheep ewes

Abstract

The "terminal investment" hypothesis predicts that reproductive effort should increase with age as life expectancy, and therefore residual reproductive potential, decreases. Several studies, however, have found that reproductive success decreases with age because of senescence. We monitored the age-specific reproductive success of individually marked bighorn sheep ewes at Sheep River, Alberta, from 1982 to 2006. Lamb production increased from 2 to 4 y of age, then remained at approximately 88% until age 13 y, when it began to decline. Lamb survival was higher for ewes aged 4 to 12 y than for ewes aged 2 or 3 y. Reproductive senescence began at 13 y of age but was restricted to a decrease in lamb production. There were no differences in lamb survival between prime-aged and older ewes. The onset of reproductive senescence is about 5 y later than the onset of survival senescence. Positive correlations between reproductive success before 9 y of age and longevity, and between reproductive success before and after 9 y of age, suggest that ewes that survived to the onset of reproductive senescence were mostly of high phenotypic quality. Our data provide no clear support for the terminal investment hypothesis. Future tests of this hypothesis should account for individual heterogeneity in phenotypic quality. Show more