The American wild mustang is a feral horse, descended from once-domesticated animals brought to the Americas by Spanish explorers. Today, these free-roaming horses are primarily managed by the Bureau of Land Management (BLM) across the western United States. While domesticated horses often live into their late 20s or 30s, the life of a wild mustang is significantly shorter. The general lifespan range for a mustang on the range is typically between 15 and 20 years, reflecting the harsh realities of surviving in a challenging natural environment.
Lifespan Expectations for Wild Mustangs
A wild mustang’s average lifespan is markedly lower than its maximum potential lifespan, largely due to high mortality rates early in life. While some mustangs may live as long as 25 years, the average lifespan on the range is closer to 15 years.
This lowered average is influenced by the high rate of death among the youngest horses. Foal mortality rates in the first year often range from 10% to 16%, and sometimes higher. If a mustang survives its first year, its chances of reaching its late teens are significantly improved.
Adult mustangs have a much higher annual survival rate, often exceeding 90% in their prime years. However, challenges like dental wear, chronic injury, and the difficulty of keeping up with the herd eventually take their toll. Without human intervention and veterinary care, a wild mustang reaching 20 years old is a rare accomplishment.
Natural Environmental Factors Limiting Longevity
The primary factors limiting the longevity of a wild mustang are resource scarcity and predation. The arid and semi-arid public lands where mustangs roam are frequently subjected to drought, which directly impacts the availability of food and water. Extended dry periods cause vegetation to wither, leading to poor forage quality and reduced body condition in the horses. This increases the likelihood of death, particularly for pregnant mares and foals.
The constant need to search for nourishment and water requires extensive travel, putting immense strain on the musculoskeletal system. Environmental injuries, such as a broken leg, are often death sentences in the wild due to the inability to keep pace with the herd. Illnesses, such as severe dental disease that prevents effective grazing, also reduce the horse’s ability to survive.
Predation acts as a natural control on the population, primarily targeting the most vulnerable age groups. Mountain lions are the most significant natural predator for mustangs in the American West. They overwhelmingly focus on foals and juveniles, though they may occasionally take an older or weakened adult.
The Role of Management in Mustang Survival
The Bureau of Land Management (BLM) intervenes in the lives of mustangs, altering their life trajectory and potential longevity. The most dramatic intervention is the periodic roundup, where horses are removed from the range, ending their wild lifespan. Horses removed from the range are moved into government holding facilities for adoption or lifetime placement in off-range pastures.
While the initial gather process carries a small risk of injury and death, once in a controlled environment, the horses are protected from the natural mortality factors of the range. Although holding facilities provide protection from predators, drought, and starvation, some facilities have experienced high mortality rates, sometimes exceeding 10% annually, due to traumatic injuries and unknown causes.
Another management technique is the use of fertility control vaccines, such as Porcine Zona Pellucida (PZP), administered to mares to prevent pregnancy. Mares treated with PZP often show improved body condition because they are not expending energy on continuous reproduction and lactation. This reduction in physical stress can lead to increased longevity, indirectly extending the mare’s life on the range.