American Bison

The Bison, also known as the American Bison, is a species of large bovine mammal scattered throughout their long geographical range in almost entire North America in two distinct subspecies. The population of the species went down dramatically, to only 541 in 1889, with 500,000 animals presently, after extensive recovery efforts. Currently, they are restricted to only a few reserves and national parks.

Physical Description

Size: The range length is 3.6 m to 3.8 m in males, whereas 2.13 m to 3.18 m in females.

Weight: The males of these tall animals stand at 3.6 m to 3.8 m, and females at 2.13 m to 3.18 m.

Body & coat: The body is covered with shaggy brown fur, mane and beard on the chin. The head is large enough while there is a hump on the shoulder.

Eyes: The eyesight is relatively poor.

Tail: Short and tasseled, with the tip having a tuft of dark hair, and can be up to 65 cm (26 in) in length.

Horns: Short black horns are present in both the sexes.

Sexual Dimorphism: Not conspicuously visible, except that the males are larger than the females.

European Bison vs. American Bison: The Main Differences

1. The American bison tends to be heavier than their European counterpart.
2. They also graze more than those found in Europe.
3. European bison have less hair on the body, compared to the American bovids.
4. The European bison have their horns pointing forward, while the American bison’s horns are somewhat upward.
5. American bison can be tamed more easily, and bred with domestic cattle, compared to the European bison.

Lifespan

The longevity status of the bison in the wild is 15-20 years, and in captivity, it is up to 40.

Distribution

Their long geographic range extends from northern Mexico to Alaska, with central Alberta being the divider between the wood bison to the north and the plains bison to the south.

Habitat

The American bison are found on the plains, prairies and river valleys, often close to a source of water.

Classification of Species

Based on their northern and southern ranges, the American bison population has been divided into two populations – the wood and the plains bison (Bison bison athabascae & Bison bison bison, respectively).

Behavior

The American bison are gregarious bovines. They live in typical groups depending upon age, season, habitat, and sex. For example, the groups of cows comprises of females, and the male calves under the age of around three years along with their mothers. However, when the season of the beginning of the estrus cycle in the females arrives, more male bison begin to enter this group.

The males, on the other hand, can live either in groups of up to 30 individuals, or else, even individually. In a group, linear dominance between the bulls can be noticed, with the bulls having a higher social rank mating more often compared to those with a lower. However, even the females can live in linear dominance hierarchy, but that is decided earlier in their lives.

They move out for foraging and grazing in loose groups during the day, and in several periods. The groups travel in a single line, with the infants and young ones traveling ahead, and the adults following and protecting them from behind. However, the entire team is headed by an adult cow. Interestingly, unlike most bovine species, the American bison are good swimmers, excellent runners, with a speed reaching up to 62 kmph.

During the period of rut, the frequencies of tree horning and wallowing increase. These large animals communicate with each other vocally using sounds – giving out grunting and snorting calls.

Homosexual behaviors have also been observed in the animals, especially in the males. This is thought to be not backed by dominance, but rather, for the sake of social bonding, as also, for gaining sexual experience.

Diet

These herbivorous bovine live on various kinds of plant matters including leaves, roots, tubers, wood, bark, and stems. They also need water every day.

Reproduction & Life Cycle

The mating season of the American bison lasts for a short time – between August and September. Opposite-sex herds usually do not mingle until the arrival of the breeding season. After a successful act of copulation, the female American bison gets impregnated usually with one calf (twins are rare) with the gestation period lasting for about 285 days.

When the season of rut begins, the bulls engage in fighting among themselves in order to win over a mate, often injuring each other. Copulation is initiated by the male, and the act is usually quick. The calf is normally reddish-brown in color and is nursed by the mother and other female members of the group until the next calf is born, or until it is 18 months old. But the average time of nursing is minimum 7 or 8 months.

Most young baby bison are weaned a little before they are a year old. When they attain the age of three years of age, the female juveniles can produce a calf.

The young female offspring continue to live in maternal herds along with other females and their offspring, while the young males leave the maternal herd at around three years of age. At this stage, they go on to live alone, or else, join other male individuals in bachelor herds.

Adaptations

1. Their poor vision has been compensated by a very strong olfactory sense, crucial in detecting danger.
2. They also has a very keen sense of hearing and can differentiate between large and small objects from a distance as long as 1 km, and hear moving objects from 2 km away.
3. In the forest studded by wild animals, the large size of the animal helps them stay relatively safe from predators.
4. During fall and winter, they rub their horns against trees, saplings, and even utility poles. This act is thought to be for defense against insects, while the aroma released by cedar and pines (their preferred trees for this act) probably act as a deterrent for insects.

Predators

While the adult bison are almost safe from predators, the sick and the young ones often fall prey to mountain lions, wolves, and even humans.

Conservation Status

Considering the decrease in their population, the IUCN 3.1 has marked them as a ‘NT’ (Near Threatened) species.

Interesting Facts

• The American bison had the widest natural range of any North American herbivore.
• Mistakenly, though they are also known as ‘American buffalo’ or simply ‘buffalo’, biologically they do not belong to the ‘true buffalo’ group.
• The American bison is the national mammal of the United States of America.