A stallion is an adult male horse that has not been gelded (castrated). Stallions follow the conformation and phenotype of their breed, but within that standard, the presence of hormones such as testosterone may give stallions a thicker, "cresty" neck, as well as a somewhat more muscular physique as compared to female horses, known as mares, and castrated males, called geldings. Temperament varies widely based on genetics and training, but because of their instincts as herd animals, they may be prone to aggressive behavior, particularly toward other stallions, and thus require careful management by knowledgeable handlers. With proper training and management, stallions are effective equine athletes at the highest levels of many disciplines, including horse racing, horse shows, and international Olympic competition. "Stallion" is also used to refer to males of other equids, including zebras and donkeys.
Herd behavior
Young female horses usually leave their band and join one with a different stallion from the one that sired them. Young male horses 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.
Reproductive anatomy
The external genitalia comprise the testes and the penis. The testes are suspended horizontally within the scrotum. The testes of an average stallion are ovoid, 8 to 12 cm (3.1 to 4.7 in) long, 6 to 7 cm (2.4 to 2.8 in) high, and 5 cm (2.0 in) wide. Stallions have a musculocavernosus penis with a well-developed glans. 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 or penile sheath, 50 cm (20 in) long and 2.5 to 6 cm (0.98 to 2.36 in) in diameter with the distal end 15 to 20 cm (5.9 to 7.9 in) free in the prepuce. The retractor muscle contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. When erect, the penis increases in length and thickness by 50% while the thickness of the glans increases by four to five times. The urethra opens within the urethral fossa, a small pouch at the distal end of the glans. A structure called the urethral process projects beyond the glans. The internal genitalia comprise the accessory sex glands, which include the vesicular glands, the prostate gland and the bulbourethral glands. These contribute fluid to the semen at ejaculation, but are not strictly necessary for fertility.
Management and handling of domesticated stallions
Stallions are trained and managed in a variety of ways, depending on the region of the world, the owner's philosophy, and the individual stallion's temperament. In all cases, however, stallions have an inborn tendency to attempt to dominate both other horses and human handlers, and will be affected to some degree by proximity to other horses, especially mares in heat. They must be trained to behave with respect toward humans at all times or else their natural aggressiveness, particularly a tendency to bite, may pose a danger of serious injury. The advantage of natural types of management is that the stallion is allowed to behave "like a horse" and may exhibit fewer stable vices. In a harem model, the mares may "cycle" or achieve estrus more readily. Proponents of natural management also assert that mares are more likely to become pregnant in a natural herd setting. Some stallion managers keep a stallion with a mare herd year-round, others will only turn a stallion out with mares during the breeding season. In some places, young domesticated stallions are allowed to live separately in a "bachelor herd" while growing up, kept out of sight, sound or smell of mares. A Swiss study demonstrated that even mature breeding stallions kept well away from other horses could live peacefully together in a herd setting if proper precautions were taken while the initial herd hierarchy was established. As an example, in the New Forest, England, breeding stallions run out on the open Forest for about two to three months each year with the mares and youngstock. On being taken off the Forest, many of them stay together in bachelor herds for most of the rest of the year. New Forest stallions, when not in their breeding work, take part on the annual round-ups, working alongside mares and geldings, and compete successfully in many disciplines. There are drawbacks to natural management, however. One is that the breeding date, and hence foaling date, of any given mare will be uncertain. Another problem is the risk of injury to the stallion or mare in the process of natural breeding, or the risk of injury while a hierarchy is established within an all-male herd. Some stallions become very anxious or temperamental in a herd setting and may lose considerable weight, sometimes to the point of a health risk. Some may become highly protective of their mares and thus more aggressive and dangerous to handle. There is also a greater risk that the stallion may escape from a pasture or be stolen. Stallions may break down fences between adjoining fields to fight another stallion or mate with the "wrong" herd of mares, thus putting the pedigree of ensuing foals in question.
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