DYNAMICS OF A FERAL HORSE POPULATION IN MONTANA

Annual feral Equus caballus survival was usually high, ranging from 0.93-0.99 during most years, but, increased mortality due to severe winter conditions in 1977 and 1983 resulted in annual survival rates of 0.49 and 0.87, respectively. Adult female survival was significantly higher than adult male with most mortality occurring in animals ≥15 yr old. Females did not begin to reproduce until 3 yr old. Annual foaling rates of reproductive age females ranged from 36-65%. Highest foaling rates occurred after a 51% reduction in the population, suggesting a density-dependent response. The reproductive history of multiparous females the previous year had no effect on probability of foaling, but primiparous females were less likely to reproduce the following year. The adult sex ratio was consistently skewed 2 to 1 in favor of females, attributed primarily to the removal of disproportionate numbers of males during herd reduction programs. Apparent mean growth rate of the population was 18% annually, but by removing animals, the population was maintained at c120-150. -from Authors