Desert Bighorn Sheep
Desert bighorn sheep are the wild sheep of the American Southwest. They live on steep rocky desert mountains, where their incredibly grippy hooves and powerful legs let them run up vertical cliffs that no predator can follow. Adult males have huge spiral horns that they use to fight other males during the autumn mating season, in head-on clashes so violent that the booms can be heard over a kilometre away.
- Where it livesMojave, Sonoran, Chihuahuan desertsPlus Death Valley and Joshua Tree
- WeightMales 90 to 120 kg, females 35 to 90 kg
- Horn weight (male)Up to 14 kgBigger than a bowling ball
- Climbing abilityAlmost any rock faceHooves like climbing shoes
- Lifespan10 to 15 yearsIn the wild
- StatusVulnerablePopulation recovering from near-extinction
How heavy are bighorn sheep horns?
The horns of a fully-grown male bighorn ram can weigh as much as 14 kg, more than two bowling balls strapped to its head.
What is a desert bighorn?
The desert bighorn (Ovis canadensis nelsoni) is a subspecies of the wider bighorn sheep family. It lives in the deserts of the southwestern United States and northern Mexico, especially in the rocky mountain ranges that rise above the desert valleys. The desert bighorn is adapted to drier and hotter conditions than its mountain cousins further north; it can go for up to a week without drinking and tolerates body temperatures up to 42 °C without harm.
Born to climb
The desert bighorn's most remarkable feature is its ability to climb. The sheep have:
- Concave hooves. The bottom of each hoof is cupped, like a suction cup, providing extra grip on rock.
- Soft hoof pads that conform to the rock surface, like grippy rubber soles.
- Split hooves that can spread apart to grip on uneven surfaces or steep angles.
- Powerful legs that can launch them up to 3 metres in a single leap.
- Excellent balance that lets them stand on ledges only a few centimetres wide.
Their ability to climb almost any rock face is essential survival; mountain cliffs are the bighorn's main defence against predators like pumas and coyotes.
The mating season
From late summer to early winter, male bighorns fight each other to win the right to mate with females. The fights are settled by the dramatic head-on clashes described above. Bigger horns generally win, which is why males have evolved such enormous horns despite the obvious cost in weight. After the males have established their rank, the dominant males get to mate with most of the females.
The female bighorn (called a ewe) carries her single lamb for approx. 180 days. Lambs are usually born in spring on isolated cliff ledges, safe from predators. Within a few hours of birth a lamb can climb almost as well as its mother, and within days it can keep up with the herd.
Conservation
Desert bighorn sheep almost went extinct in the early 20th century. Hunting (for sport and food), competition from introduced livestock, and diseases caught from domestic sheep cut populations from hundreds of thousands to around 20,000 by the 1960s. Decades of careful conservation has brought numbers back to around 70,000 today. The animals are protected in many national parks (Death Valley, Joshua Tree, Big Bend) and on Native American reservations.
The biggest remaining threat is pneumonia from domestic sheep. Bighorns have no resistance to many of the diseases that domestic sheep tolerate; a single sick domestic sheep contacting a herd of wild bighorns can trigger a die-off that kills 80 to 90% of the herd. Conservation managers work hard to keep wild bighorns and domestic sheep separated.
Deeper dive: skull mechanics, pneumonia die-offs and reintroduction programmes
The skull of a male bighorn ram is one of the most spectacular pieces of natural engineering in the animal kingdom. To withstand the repeated head-on collisions of the mating season, the skull is reinforced with a thick bony cap on the forehead and has spongy bone-tissue between the inner and outer skull walls that absorbs and distributes impact forces. The horns themselves are made of keratin (the same material as human fingernails) over a bony core, with internal structures that act like crumple zones. The neck is supported by powerful muscles and tendons that can take the shock of impact. Slow-motion video has shown that the rams' brains barely move inside their skulls during impacts that would knock out almost any other mammal. Researchers are studying bighorn skull biomechanics to design better human helmets, especially for sports like American football and cycling.
Pneumonia outbreaks are the biggest single threat to desert bighorn populations. Domestic sheep carry several species of bacteria (especially Mycoplasma ovipneumoniae) that cause mild illness in domestic sheep but devastating pneumonia in bighorns. When a bighorn herd is exposed, the resulting die-off can kill 80% or more of the adults, plus most lambs born to surviving females for several years afterwards. The remaining herd may take decades to recover. Conservation managers work to keep wild bighorns and domestic sheep separated by buffer zones, but this is difficult because the same mountain ranges are often grazed by both. Recent research into vaccinating wild bighorns against the most dangerous strains is showing promise but is logistically difficult to scale up.
The recovery of the desert bighorn from near-extinction has involved a major programme of reintroductions over the past 50 years. Wildlife managers in Arizona, Nevada, California, Utah, New Mexico and Mexico have repeatedly captured small groups of wild bighorns from healthy populations, transported them by helicopter to mountain ranges where bighorns had been hunted out, and released them to start new populations. Sometimes captive-bred animals are added to boost numbers. The success rate is around 60 to 70%: about two thirds of the releases produce self-sustaining populations within 10 to 20 years. The cumulative result has been the restoration of bighorn herds to dozens of mountain ranges across the southwestern US and northern Mexico, one of the major conservation success stories in modern North America.
For other desert animals, see desert animals. The home deserts include the Mojave and the Sonoran.