Porcine zona pellucida (PZP) immunocontraception was developed to provide a more humane, eff ective, and inexpensive method of population regulation for wildlife species. It has been used to regulate populations of several species including white tailed deer (Odocoileus virginianus), elk (Cervus elaphus), black bear (Ursus americanus), and the feral horse (Equus ferus caballus) with varying levels of success. Early studies on Assateague Island National Seashore, Maryland, USA, suggested PZP was as an ideal form of fertility control because it reduced the likelihood of conception to <10%, could be delivered remotely, was thought to be reversible, lacked debilitating physiological side eff ects, could not pass through the food chain, and showed minimal eff ects to social behaviors in a closed population of feral horses. However, research on Shackleford Banks, North Carolina, USA and on 3 western populations located in Little Brook Cliff s (Grand Junction, Colorado, USA), McCollough Peaks (east of Cody, Wyoming, USA), and the Pryor Mountains (Lovell, Wyoming, USA) has revealed behavioral and physiological side eff ects of long-term PZP use. When compared to untreated mares (those that have never received treatment), treated mares demonstrated decreased fi delity to the band stallion, increased and prolonged reproductive behavior, and an increased likelihood of extending reproductive cycling into the nonbreeding season. These eff ects were more pronounced in animals receiving more total and/or consecutive contraception treatments and can persist even after several years of treatment cessation. Finally, new data indicate that these changes to previously treated mares can aff ect the behavior and stress physiology of their band stallions, demonstrating the potential for the contraception of individuals to have population-level eff ects. These results are important to consider if we are to achieve both the eff ective management of feral horse populations in addition to the maintenance of their overall health and well-being.