Top: A bull bugles a challenge to his rivals in stormswept meadow in northern Yellowstone during an early season snow storm.
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.

Below: An angry bull, brimming over with testosterone and ill temper, charges towards me in Mammoth during the fall rut. Clashes between the elk and humans are common at that time of year, with many of the roads and walkways at Park Headquarters being staked out as prime territory by some of the biggest bulls in the Park.