Bachelor and Harem Stallion Behavior and Endocrinology1

Abstract

Horses naturally breed within a harem social unit that typically includes one mature breeding stallion with several mares and their immature offspring. Non-harem stallions comprise rela-tively stable social groups known as bachelor bands. When a harem position becomes vacant, a stallion emerges from a bachelor band to fill the vacancy. We studied harem and bachelor stallion phenomena using a model of pony stallions pastured together under semi-feral conditions. In this model, one stallion assumed the role of “harem stallion, ” vigilantly guarding an area within the stallion pasture along the fence line facing nearby mare pastures. The remaining stallions interacted as a bachelor band. If the harem stallion was removed from the pasture, a stallion from the bachelor group rapidly emerged to fill the position. If we replaced to the pasture a former harem stallion, the former harem stallion typically displaced the incumbent harem stal-lion back to the bachelor band. Order of emergence from the bachelor band to fill harem position vacancies, while highly repeatable, was not strongly associated with bachelor age, height, weight, testosterone concentrations, ranked level of aggressive behavior, or ranked leader-fol-lower behavior. Emergence from bachelor to harem status consistently resulted in a sudden