Absence of effects from immunocontraception on seasonal birth patterns and foal survival among barrier island wild horses.

This research article investigates whether immunocontraception, a form of birth control, in wild horses causes births outside of the usual season, which could potentially lead to foal mortality. The study, conducted on wild horses from Assateague Island, Maryland, found no significant effects of immunocontraception on the season of birth or foal survival rate.

Study Methodology and Population

• The research was carried out from 1990 to 2002 to investigate the effects of a specific immunocontraception method known as porcine zonae pellucidae (PZP) on the breeding patterns and foal survival of wild horses in Assateague, Maryland.
• The study focused on a total of 168 mares, with 91 mares that had never been treated with immunocontraception serving as the control group and 77 that had been treated.

Study Findings

• In the untreated mares, the study found that 75.8% of foals were born within the usual season (April, May, and June).
• In the treated mares, 64.9% of foals were born on season. Further subdivisions within the treated group include: 68.9% of mares that gave birth within one year of treatment (classed as contraceptive “failures”), and 62.5% of mares that had been treated for over two years then withdrawn from treatment had foals born in season.
• Statistical analysis of these proportions found no significant difference between the treated and untreated groups. Therefore, there was no evidence that the use of PZP contraception affected the season of birth.

Impact on Foal Survival

• The survival rate of the foals, regardless of whether they were born in or out of season, was not significantly affected by whether or not their mothers had been treated with PZP contraception.
• These results indicate immunocontraception does not impact foal survival in barrier island habitats.

Study Implications

• This research is the affirmation of the safety of using PZP immunocontraception in the management of wild horse populations.
• These findings dispute concerns about increased out-of-season births and ensuing foal mortality due to immunocontraception use.
• It provides an evidence-base that PZP contraception does not affect the natural breeding season or the survival rate of foals which can support its continued use in the management of wild horse populations.