The Consequences of Porcine Zona Pellucida Immunocontraception on the Behavior and Health of the Feral Horses (Equus Caballus) of Shackleford Banks

Abstract

Shackleford Banks is a barrier island off the coast of North Carolina that is home to a population of feral horses. Since 2000 the National Park Service has maintained a program of porcine zona pellucida immunocontraception in order to ensure the management of the population to a size of 100-110 horses, based on the Shackleford Banks Wild Horses Protection Act of 1997. In recent years the number of mares contracepted each season has decreased, due to stabilization in population growth . In my research I examined various aspects of health and the behavior of the mares in order to determine if intermittent use of PZP immunocontraception continues to have behavioral and physiological effects on the population as was the case when immunocontraception was applied on a yearly basis (Madosky, Rubenstein, Howard, and Stuska 2010; Nunez et. al, 2014). There was a negative correlation between parasite load and body condition, providing evidence that extent of parasitic infection quantified in this manner can be used as an indicator of health. Additionally, mare health is significantly affected by both PZP contraception and foaling history. Body condition score was elevated and parasite egg count was lower in those mares that had never had a foal, and there was a negative correlation between body condition and the number of foals over the past 15 years. There is a strong legacy effect in that even after continual administration has ceased there remains a strong negative correlation between body condition score and number of vaccinations. While mares that received PZP were in better overall health, they were also more likely to switch harems. Switching harems correlated with increased harassment, but elevated body condition from years without foals as a result of PZP likely served as a buffer to the negative effects of the harassment. Mares may suffer from increased stress from harem switching as a result of PZP, but the reduction in the physical costs associated with repeated reproduction are large and appear to offset likely costs of increased harassment. Contracepted mares are less likely to produce more foals over their lifetime, and are thus more often released from the stress of procreation.