The head hair, beard and pantaloons of mature male American bison (Bison bison) are greatly reduced after each breeding season, primarily by internal physiological events, rather than external, mechanical forces. The consequent reduction in sexual dimorphism may function to reduce post-breeding season aggression between ♂♂. Minimization of aggression at that time would facilitate adaptation to the seasonal resource scarcity characteristic of temperate zones by permitting the ♂♂ to maximize energy storage prior to the winter food shortage. This adaptive value of a disposable display may help to explain some other instances of seasonal loss of sexually dimorphic features, such as antlers in deer and horn sheaths in pronghorns.