A common male homosexual vice in feedlot cattle is an activity where some steers, \"bullers\", sexually attract other steers, \"riders\". One theory of the cause is that bullers are excessively feminized and give off sexually stimulating pheromones. Because pheromones are detected in many mammals by the vomeronasal organ (VNO), we tested the role of this system by cauterizing duct openings which lead to the VNO. Cautery of non-bullers did statistically reduce bulling, but the magnitude of bulling was still high. Cautery of bullers revealed that they were indeed subjected to less mounting than were the non-cauterized bullers in the same herd. Bullers commonly participated in mounting and had aggression scores about twice that of riders. We believe that aggresssion is a key element in bulling behavior because: (1) most (over 80%) of the steers withdrew from the social contesting engaged in by both riders and bullers; (2) riders were challenged by bullers, in that bullers also mounted riders, as well as other bullers; (3) bullers were more aggressive than riders in the conventional measures of aggression; and (4) the amount of bulling was much greater during the periods of greatest social stress (such as just after the herds were formed and, in one pen, just after 50 new steers were added to the pen). Thus, we are led to advance a new hypothesis about bulling; namely, that it is a ritualized \"game\" based on social hierarchy contesting. © 1983.