A family history: Effects of parental age on offspring life-history traits in bighorn sheep

Long-term fitness effects of parental age could affect population dynamics, as age structure can vary considerably over time. Few studies of wild animals, however, have examined the possible long-term fitness consequences of parental age. Based upon 50 years of data on wild bighorn sheep, we investigated how parental age affected three offspring traits – mass at three years, longevity, and female lifetime reproductive success. We also tested for a survival filter which could mask or increase fitness effects of parental age. Our results showed a significant, quadratic negative association between maternal age, offspring longevity and female lifetime reproductive success. Offspring born to mothers aged 5-6 years lived about 2 years longer and weaned about 1.5 lambs more than siblings born to mothers aged 12-13 years. The effect of paternal age was not significant. There was also a positive association between the longevity of mothers and offspring. We did not detect any effects of parental age on mass at three years for offspring of either sex. These results demonstrate the presence of persistent maternal age effects in a long-lived species. The sex-specific effects support the importance of analyzing maternal and paternal age effects separately, as well as effects on female and male offspring. This study advances our understanding of evolutionary processes and population dynamics in wild long-lived mammals.