Elk are the most abundant large mammal found in Yellowstone; paleontological evidence confirms their continuous presence for at least 1,000 years. Yellowstone National Park was established in 1872, when market hunting of all large grazing animals was rampant. Not until after 1886, when the U.S. Army was called in to protect the park and wildlife slaughter was brought under control, did the large animals increase in number.
More than 30,000 elk from 7-8 different herds summer in Yellowstone and approximately 15,000 to 22,000 winter in the park. The subspecies of elk that lives here are found from Arizona to northern Canada along the Rocky Mountain chain; other species of elk were historically distributed from coast to coast, but disappeared from the eastern United States in the early 1800s. Some other subspecies of elk still occupy coastal regions of California, Washington, and Oregon. Elk are the second largest member of the deer family (moose are larger). Adult males, or bulls, range upwards of 700 pounds while females, or cows, average 500-525 pounds. Their coats are reddish brown with heavy, darker-colored manes and a distinct yellowish rump patch.
Bulls grow antlers annually from the time they are nearly one year old. When mature, a bull's rack may have 6 to 8 points or tines on each side and weigh more than 30 pounds. The antlers are usually shed in March or April, and begin regrowing in May, when the bony growth is nourished by blood vessels and covered by furry-looking velvet. Antler growth ceases each year by August, when the velvet dries up and bulls begin to scrape it off by rubbing against trees, in preparation for the autumn mating season or rut. A bull may gather 20-30 cows into his harem during the mating season, often clashing or locking antlers with another mature male for the privilege of dominating the herd group. By November, mating season ends and elk generally move to their winter ranges. Calves weighing 25-40 pounds are born in late May or early June.
Climate is the most important factor affecting the size and distribution of elk herds here. Nearly the whole park - approximately 2.2 million acres - provides summer range for elk. However, winter snowfalls force elk and other ungulates to leave the greater part of the park. Only the northern, lower-elevation portion of Yellowstone, where temperatures are more moderate and snowfall less than in the park interior, can support large numbers of wintering elk. Annual precipitation, which occurs mostly as snow, averages as high as 75 in the southern, high-mountain plateaus of the park; minimum temperatures there are often well below 0 F, and have been as low as -66 F. In contrast, most of the northern range averages less than 30 of precipitation annually, and winter temperatures are considerably warmer.