Bachelor and Harem Stallion Behavior and Endocrinology

Sue M. McDonnell and Samantha C. Murray
University of Pennsylvania School of Veterinary Medicine
New Bolton Center, Kennett Square, Pennsylvania 19348


    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 relatively 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 stallion 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-follower behavior. Emergence from bachelor to harem status consistently resulted in a sudden sharp rise in testosterone concentration, which remained high for the duration of harem status. Displacement from harem status back to the bachelor band was consistently associated with a sharp decrease in testosterone concentration. Testosterone concentrations with harem status were significantly higher than those with bachelor status for all months of the year. These changes in behavior and testosterone concentrations of sexually experienced mature stallions suggest social modulation of behavior and endocrinology, with enhancement of reproductive function as a harem stallion and/or suppression as a bachelor. Extremely wide variation in testosterone concentrations due to sociosexual conditions raises important questions about basic neuroendocrinology of stallions, including methodological concerns. Equally significant are questions raised regarding management of domestic breeding stallions.