Foaling and mortality rates of known-age feral horses equipped with radio transmitters were obtained in 1981 and 1982 for 2 areas in Nevada. These data showed considerable annual variation. In one area, 45% of the mares produced foals in both years while only 10% produced foals in the other area. Foal mortality ranged from 2% to 33%. Both the highest mortality (33% in 1981) and the lowest (2% in 1982) occurred in the same area. The cause for this apparent inconsistency was unknown. The highest age-specific reproductive rates occurred in 5 to 10 year-old mares.
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Siniff et al. (1986) studied this question.