Reproduction in feral horses: an eight-year study.

The study explores the reproduction rates and foal survival in free-ranging ponies on Assateague Island for a span of eight years (1975-1982). It reveals the significant factors that influence these statistics such as the months of foal births, age of mares, and conditions on the island.

Birth Pattern of Foals

• The research records that the highest birth rate for foals (52%) occurs in May. Following that, 22.6% are born in June, 13% in April, 10.4% in July, and less than 1% in August and September.

Foaling and Survival Rates

• The mean foaling rate was found to be 57.1 +/- 3.9% and survival rate was 88.3 +/- 3.6%.
• Out of 103 foals born, 48 were colts (male) and 55 were fillies (female). Thus, the sex ratio was 53% female.

Age of Mares and Foaling Rates

• The mares under the age of 3 did not have any offspring.
• The foaling rate was only 23% for 3-year-old mares, which increased with their age: 46% at 4 years old, 53% at 5 years old, and 69% at 6 years old.

Impact of Lactating Stress and Forage Quality

• The less than optimal reproduction rate was attributed to the stress faced by mares while lactating and carrying a foal simultaneously, especially when the forage quality on the island was low.
• This inference was supported by higher reproduction rate (74.4 +/- 2.4%) observed with ponies in the Chincoteague National Wildlife Refuge located on the southern part of the island, where foals are weaned and sold every July.

Growth in the Pony Population

• Despite the comparatively low reproduction rate, the pony population on the Assateague Island National Seashore saw an increase from 43 to 80 during the study period. This indicates a growth of over 10% per year.
• The study also accounted for losses in the population, with 24 deaths, as well as a loss of 8 due to dispersals from the study area.