Evaluation of Porcine Zona Pellucida (PZP) Immunocontraception for Behavioral Effects in Feral Horses (Equus caballus)

Powell, David

With the assistance of Earthwatch volunteers, I conducted behavioral observations on a population of feral horses (Equus caballus). These horses live on Assateague Island National Seashore (A.S.I.S.), a 33 mile long barrier island running along the coasts of Maryland and Virginia. Observations were restricted to the Maryland herd which contains approximately 173 individuals and is essentially unmanaged except for population control. Though equid social organization appears to be closely tied to ecology (Rubenstein, 1981), the typical social group in feral horses is the harem band which contains one breeding male, several reproductive mares, and their dependent offspring. Harem bands on Assateague occupy overlapping home ranges and are not characterized by territoriality. A dominance hierarchy exists within each harem band, and the stallion may not be the alpha animal (Houpt & Keiper, 1982). Among mares, rank correlates most highly with tenure (Seligsohn, 1987) but also very highly with age (Rutberg & Greenberg, 1990; Keiper & Sambraus, 1986). Female hierarchies tend to be stable with few reversals (Keiper & Sambraus, 1986 but see Berger, 1986). Nine bands of horseswere observed June I-September 5, 1997. These bands contained 43 sexually mature mares.