The effects of immunocontraception on harem fidelity in a feral horse (Equus caballus) population

Feral horses on Shackleford Banks Island, North Carolina, are managed by the National Park Service in order to reduce their impact on the fragile barrier island ecosystem. Management techniques include removal of young horses and immunocontraception of many of the mares using Porcine Zona Pellucida immunocontraceptive. This immunocontraceptive reduces the number of horses that need to be physically removed from the island, but there is concern that the contraception may be influencing the social behavior of the contracepted mares. We investigated the effect of immunocontraception on harem stability by tracking the number of harem changes of each adult mare through the breeding season over two seasons. In both seasons the mares that had been treated with the immunocontraceptive changed harems significantly more than mares never treated (2007 P=0.037 and 2008 P=0.016) and visited significantly more harems (2007 P=0.021 and 2008 P=0.011). The number of years treated did not have a significant effect on the number of harem changes (2007 P=0.145 and 2008 P=0.848), nor did the number of years a mare had been off contraceptive once the contraceptive was discontinued (2007 P=0.443 and 2008 P=0.826). Additionally, there was no significant difference in harem changes between mares that were actively contracepted and mares that had been treated in the past but were not currently actively contracepted (2007 P=0.336 and 2008 P=0.533). These results indicate that the PZP immunocontraceptive has a significant effect on harem stability and that once a mare has been contracepted the behavioral effect of the contraceptive treatment may not be readily reversed.