Reproductive success in female mountain goats

Abstract

Kid production by marked females, age ratios from a 24-year study, and the literature are used to evaluate hypotheses that three intrinsic and three extrinsic factors affect reproduction by female Rocky Mountain goats (Oreamnos americanus). On Sheep Mountain – Gladstone Ridge, Colorado, mountain goat age ratios declined while the herd grew during 1966–1979, and continued to decline with population stability during 1980–1989, suggesting a continued increase in ecological density of goats 39 years after they were introduced in 1950. Among intrinsic factors, age and persisting individual characteristics have influenced reproductive success of females. Reproductive success in any year has not limited success in the following year, except in 4-year-olds. Among extrinsic factors, reproductive success of female mountain goats has been negatively influenced by density or ecological density and by snowpack during gestation. Reproductive success of females may have been positively influenced by snowpack that enhanced forage conditions prior to breeding. The relative importance of these six factors in determining reproductive success of females may vary among and (temporally) within herds. Most hypotheses regarding reproductive success in female mountain goats remain poorly tested. Short-term observational studies hold little promise for testing hypotheses, owing to large among-years variation in reproductive success. Long-term, intensive observational studies, or manipulative experiments, are suggested.