A mature bull can reach 5.5 to 6.5 feet (1.9 meters) high at the hump and nine to 12.5 feet (2.7 to 3.8 meters) in length. Females are normally smaller, at 7 to 10 feet in length (2.2 to 3.2 meters) and 5 feet high at the hump (1.5 meters). Bison can weigh 1,800 to 2,400 pounds (816 to 1,088 kilograms). Male (bull) weighs up to 2,000 pounds, female (cow) weighs up to 1,000 pounds. May live 12–15 years, a few live as long as 20 years. Feed primarily on grasses and sedges. Mate in late July through August; give birth to one calf in late April or May. Can be aggressive, are agile, and can run up to 30 miles per hour. Bison are mostly active during the day and at dusk, but may be active through the night. They are social animals that often form herds, which appear to be directed by older females. Group sizes average about 20 bison during winter, but increase in summer to an average of about 200, with a maximum of about 1,000 during the breeding season (known as the rut) in July and August. Bison are sexually mature at age two. Although female bison may breed at these younger ages, older males (>7 years) participate in most of the breeding. During the rut mature males display their dominance by bellowing, wallowing, and engaging in fights with other bulls. The winners earn the right to mate with receptive females. Once a bull has found a female who is close to estrus, he will stay by her side until she is ready to mate. Then he moves on to another female. Following courtship, mature males separate and spend the rest of the year alone or in small groups. Group sizes decrease through autumn and into winter, reaching their lowest level of the year during March and April. Bison are grazing animals that feed on native prairie grasses mostly. They have a strong herd instinct and tend to graze and travel in large groups. Their mob-like behavior creates a heavy hoof action and substantial manure spreading which is very good for the rejuvenation of prairie and pasture grasses and an integral part of the ecology of open grassland soils. They have a very thick winter coat that sheds away to a slick seal-like undercoat during the summer months. They manage their hair growth, shedding and conditioning by creating wallows, which are large dust baths that they will roll around in. Wallows usually then remain free of vegetation since they are used often and by each animal. All bison are born orange and turn brown just about the time that they are old enough to survive without nursing milk from their mother. All males are called bulls and all females are called cows, and there is no difference between a bison and an American buffalo. The first explorers called them buffalo, not really knowing exactly what they were with the word le boeuf meaning cow, ox or beef in French helped to further confuse the naming. The only true buffalo that exist in the world are African Cape Buffalo and Water Buffalo, and water buffalo number in the hundreds of millions worldwide and are recently being introduced into human and pet foods in the US as “buffalo” further complicating things. Bison are now the U.S. National Mammal as of 2016. Bison have humps in order to anchor their unusually large heads. The large head is used as a snow plow to move snow drifts away from grass that they can smell 3-4 feet down under the snow. Their hair coat can be so thick as to create a snow and ice insulation on their backs that does not melt. They have very large guts (called rumen) that are capable of holding a 3 day quantity of food while it slowly digests and crates massive amounts of heat (like a compost pile) that keeps the animal warm during the coldest northern winters in the upper reaches of Canada and Alaska. Bison do not need or use any shelter.