Our Window on Nature

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Stalking The Bighorn

Filed under: Mammals — Lowell Christie -- June 10, 2008 @ 3:05 pm

Desert Bighorn

With vision so sharp it equals that of a man with binoculars, a bighorn ram gazed down the mountain. In the open vistas favored by this species, keen eyesight far overshadows the need for acute hearing or sense of smell, so we were certain the animal eyed us long before we spotted him. Even so, he seemed more curious than alarmed.

We hiked up Truchas Peak in New Mexico’s Sangre de Cristo Mountains especially to see bighorn. As in any designated wilderness, if you want to visit the critters, you walk. Gradually, our eyes picked out several more bighorn, less visible against the rocks. Oh, for their surefootedness as they ambled down the almost-vertical slope. We couldn’t believe they were actually approaching us. Could it be that there were enough hikers along here to turn these wild, free creatures into panhandlers? Surely not.

Bighorn once ranged widely throughout the West. Today they’re found mostly in steep canyons and higher mountain slopes. But this time of year travelers may get lucky and see them from the road. Twice we’ve spotted bighorn in the open desert between the Colorado River and Las Vegas. They spend their summers in cooler country, but as winter approaches, they move farther down the slopes.

The bighorn herds that now roam in several states are the result of restocking programs. Overgrazing of public lands has been the main reason for bighorn loss, but competition with domestic cattle and lack of immunity to their diseases have also lowered bighorn numbers. Unfortunately, these animals aren’t easy to reintroduce; some projects succeed, but many others fail.

Actually, bighorn have lived in North America longer than man. Paleontologists believe that man and bighorn followed the same land route from Eurasia, but the sheep got an earlier start. By the time non-Indian settlement began, the sheep had spread across the western half of the continent from Alaska to northern Mexico.

It’s amazing that an animal constructed as strangely as a bighorn would survive as long as it has. The head of a fully grown ram can weigh 50 pounds – perhaps a quarter of the animal’s total mass – and most of that weight is centered in the horns. The core of the horn is bony but porous; otherwise, the horns would be even heavier to carry. Just outside lies a layer of blood vessels that nourish the horns. The sheath, or covering, of the horn consists of keratin, which is the wrinkled, or ridged, part.

Experts insist that every set of bighorn sheep’s horns is as individual as a human fingerprint. The horns stop growing during the rutting season, when the blood supply retreats to the skull; this produces an annual growth ring. Every animal experiences slightly different conditions and states of health, resulting in a unique growth pattern to the horns.

Unlike antlers, horns aren’t shed, so what you see has been growing since the animal was a few months old. A bighorn sheep may live 10, 12, or even 16 years in the wild, so even though the tips of the horns wear off, they may still reach full circle by that time.

To some extent, a bighorn’s life span is measured by the wear on its front teeth. Bighorn are browsers, and they prefer shrubs to grass; they bite off twigs and branches, but this is tough on the teeth. Old age and abrasion can cause the front teeth to fall out, and if you can’t eat, you die.

Female bighorn share the same dental problems as their male counterparts, but only the males carry massive horns. Since the horns are not designed to be good weapons, biologists conclude that they serve mainly for dominance displays. We’ve all seen movie footage of headbashing contests between males, but even the females joust a bit with their much smaller horns. Also, as do the males, the females travel in groups and appear to have a pecking order.

You won’t see bighorn roaming in large herds as you do pronghorn and elk. A half-dozen or so females, yearlings, and bachelors typically move around together. During the breeding season, most of the bachelors form groups of their own. The strongest rams battle for breeding rights, but they don’t gather harems as bull elk do.

After a five-month to six-month pregnancy, a bighorn ewe climbs to a high, inaccessible niche that is safe from predators, where she gives birth to a five-pound lamb. By the time it’s a few days old, the lamb is sure-footed enough to caper over the rocks, but it’s so small that it really has to stretch to reach the milk supply.

Bighorn hooves are designed for traction. The ability to run 25 mph over loose rocks and steep slopes is made possible by hooves that are rigid around the outside edges, with soles that are rubberlike and shock resistant. One writer describes the hooves as having the stick-to-itiveness of Velcro. Only the most reckless sheep end up careening to the bottom of a canyon.

Their numbers may be small, but given a decent habitat and lack of competition from man and his domestic animals, the bighorn sheep will do just fine, thank you very much.

FMC293


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