off young horses of both sexes may also be an instinctive behavior that minimizes the risk of inbreeding within the herd, as most young are the offspring of the dominant stallion in the group. In some cases, a single younger mature male may be tolerated on the fringes of the herd. One theory is that this young male is considered a potential successor, as in time the younger stallion will eventually drive out the older herd stallion. Fillies usually soon join a different band with a dominant stallion different from the one that sired them. Colts or young stallions without mares of their own usually form small, all-male, "bachelor bands" in the wild. Living in a group gives these stallions the social and protective benefits of living in a herd. A bachelor herd may also contain older stallions who have lost their herd in a challenge. Other stallions may directly challenge a herd stallion, or may simply attempt to "steal" mares and form a new, smaller herd. In either case, if the two stallions meet, there rarely is a true fight; more often there will be bluffing behavior and the weaker horse will back off. Even if a fight for dominance occurs, rarely do opponents hurt each other in the wild because the weaker combatant has a chance to flee. Fights between stallions in captivity may result in serious injuries; fences and other forms of confinement make it more difficult for the losing animal to safely escape. In the wild, feral stallions have been known to steal or mate with domesticated mares. Reproductive anatomy The stallion's reproductive system is responsible for his sexual behavior and secondary sex characteristics (such as a large crest). The external genitalia comprise: the testes, which are suspended horizontally within the scrotum. The testes of an average stallion are ovoids long, high by wide; the penis, within the "penile sheath". Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the sheath. The retractor penis muscle is relatively underdeveloped. Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis. When not erect, the penis is housed within the prepuce, long and in diameter with the distal end . The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. When erect,