Wild horses live year round in bands composed of many family members. One stallion leads a band of mares and provides it with protection. He is usually more than 6 years old and has established his dominance though numerous battles with other stallions. The stallion breeds with the mares in his band to establish his bloodline through the foals.
The band usually has a dominate mare who leads the group to select grazing land, water and shelter. When viewing a herd of wild horses in motion, the dominate female will usually be leading the group. The stallion often brings up the rear to provide protection from predatory attack. Most mares drive their female offspring away when they reach their first oestrus to avoid inbreeding in the group. The young females join a new band led by another stallion, which helps ensure genetic diversity and maintain the health of the horses. Stallions rarely breed with their own offspring, even if females are allowed to remain in the group.
Lead stallions generally allow immature males to remain within the band until they reach approximately two or three years old, at which point they are driven away. A young stallion driven from the herd will usually join a group of bachelor stallions until he can establish his own band. The adolescent bachelor stallions spend their time fighting with each other to learn combat skills, and roaming around looking for unattached mares to claim. Also young stallions occasionally manage to breed with mares in an established band behind the lead stallion's back, according to the Journal of Animal Science. Young males may challenge an older dominate stallion in an attempt to steal his entire band, or they may focus on one female only and to try to entice her away from the group.
A mare will usually allow a knowledgeable stallion to mount her at approximately 2 or 3 years old. Wild horses can breed year round but the prime breeding season occurs from April to May. During oestrus the mare will exhibit loving behaviour towards the stallion by nuzzling or licking him. She may also stand in front of the stallion and raise her tail in invitation. The stallion will raise his head, sniff the air and often curl his upper lip back as he gauges her oestrus. The libido of the stallion differs between individual males. Some males have an exceptionally high sex drive and others appear uninterested in mating excessively, according to the Oregon State University.
The mare's gestation period averages 340 days after breeding. Foals often perish during their first year of life. Mortality in first year foals can average 40 to 50 per cent depending on the region the wild horse herd inhabits. The Mojave Desert Gazette states that in areas with ample grazing land a band of 100 females will average approximately 40 foals in a season but in depleted areas a band of 100 mares may only produce 20 foals.