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Dynamics of a Feral Horse Population in Montana

Robert A. Garrott and Lynne Taylor
The Journal of Wildlife Management
Vol. 54, No. 4 (Oct., 1990), pp. 603-612
Published by: Wiley on behalf of the Wildlife Society
DOI: 10.2307/3809357
Stable URL: http://www.jstor.org/stable/3809357
Page Count: 10
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Dynamics of a Feral Horse Population in Montana
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Abstract

We studied the dynamics of an isolated feral horse (Equus caballus) population by analyzing data collected by the Bureau of Land Management from 1970 to 1986. All animals in the population were individually identifiable by 1976, and all but 4 animals had been captured, sexed, and aged by 1978. Annual survival was usually high, ranging from 0.93 to 0.99 during most years; however, 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 (P = 0.05) than adult male with most mortality occurring in animals ≥15 years old. Females did not begin to reproduce until 3 years old. Annual foaling rates of reproductive age females ranged from 36 to 65%. The 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; however, primiparous females were less likely to reproduce the following year. The adult sex ratio was consistently skewed approximately 2 to 1 in favor of females and was attributed primarily to the removal of disproportionate numbers of males during herd reduction programs. The apparent mean growth rate of the population was 18% annually; however, by removing animals, the population was maintained at approximately 120-150.

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