Age-specific fertility rates of the Jeju pony (Equus caballus)

Age-specific fertility is an essential parameter of life history. Here we report age-specific fertility rates, measured as the number of foals per mare per year, for Jeju ponies aged 2-28 years. The total sample consisted of 545 foals produced by 178 mares from 1988 to 2002. The mean fertility rate across all ages of the mares was 0.65 foals/mare per year (+/-0.24 SD). The fertility rates were above average for the 7- to 8- and 19- to 20-year-old mares, whereas they were relatively low for mares under 4 years old. The fertility rates tended to increase with the age of the stallion, but the relationship was not significant (n =15, P =0.09). The incidence of inter-birth interval was not associated with the age that the mare first reproduced (n =64 mares, P =0.99). However, mares that reproduced later in life tended to have a reduced fertility rate due to an increase in the duration of inter-birth intervals relative to mares that reproduced earlier (n =4 years of first reproduction, P =0.068). The fertility rates of Jeju ponies were lower than for other horses, perhaps because only one stallion was introduced to a relatively large herd every year. We suggest that the introduction of more stallions to the herd each year would increase fertility rates.