The ultimate goal of any wildlife contraceptive effort is some alteration of the target population, either through a slowing of growth, or stabilization or reduction of the population. Early population models suggested that short-term contraceptive agents applied to long-lived species would not achieve significant population changes.1-4 Native porcine zona pellucida vaccine (PZP) , a short-term contraceptive vaccine, was applied to a herd of wild horses inhabiting Assateague Island National Seashore, USA, over a 13-year period, with the immediate goal of achieving zero population growth, a secondary goal of reducing the population from 175 to 150, and a tertiary goal of reaching 120, all without the physical removal of animals. Contraceptive efficacy ranged from 92–100% on an annual basis, and the percent of adult females that was treated on any given year ranged from 42–76%. The first goal of achieving zero population growth was achieved in 2 years, and the initial decline of the population became apparent in 8 years, and by year 11 the population had declined to 137, a decrease of 21.7%. The lengthy period required for achieving a population decline was caused by increasing body condition scores, reduced mortality and significantly increased longevity among treated females.
This project was funded in part by Assateague Island National Seashore, The National Park Service, the National Institutes of Health, The Science and Conservation Center at ZooMontana, and carried out under the U.S. Investigational New Animal Drug file 8857–G0002 with the U.S. Food and drug Administration. Special thanks to the late Frances Velay and the Panaphil Foundation, for many years of support for this project.
1. Garrott RA. Feral horse fertility control: potential and limitations. Wildl Soc Bull. 1991;19:52–58.
2. Garrott RA. Effective management of free-ranging ungulate populations using contraception. Wildl Soc Bull. 1995;23:445–452.
3. Garrott RA. A preliminary investigation of the potential effects of reproductive inhibition as a means of controlling feral horse populations. In: Cohn PN, Plotka ED, Seal ES, eds. Contraception in Wildlife. Lewsiton, NY: Edward Mellon Press; 1997:243–254.
4. Hone J. Rate of increase and fertility control. J Appl Ecol. 1992;29:695–698.