Bison is a massive, bulky animal, as large or larger than Domestic Cattle. Its size, together with the obvious shoulder hump and disproportionately smaller hindquarters in lateral profile, distinguish it from the almost horizontal profile of the back of Cattle. Bison can also be identified by their dark brown shaggy coat in winter, and distinct cape of long hairs over the forequarters in summer. Other unique characteristics of Bison are the beard and the mop of hair on the top of the head between the horns. Both are especially noticeable in males, particularly the large dark broad head of adults. These characteristics make it difficult to confuse Bison with any other ungulate in the province.
The forehead of the skull is wide between the orbits, and the somewhat tubular orbits project from the sides of the head in adult males. The pointed bone cores, present in both sexes project laterally from behind the orbits in a gentle upward curve, and usually have a strongly ridged surface in adult males. These characteristics make the skull readily distinguishable from all but Domestic Cattle. Bison skulls differ from those of Cattle in the occipital region and skull shape. The Bison skull is much more obviously tapered from rear to front than that of Cattle. The forehead is also broader and more convex, while the orbits are more tubular and often protrude more than those of Cattle. In dorsal view, the bone cores of Bison leave the frontal bones anterior to the para-occipital crest, whereas the cores of Cattle leave the frontal bones in line with this crest. The shape of the para-occipital crest is mildly convex in Bison, but has two bulges on either side of the centre line in Cattle. In lateral profile, the occipital region above the condyles is concave in Cattle and convex or flat in Bison.
Tracks of Bison are similar to those of Cattle, and for most of us probably indistinguishable in the field. But in areas where there are no Cattle, the large size and rounded shape of the hoof prints are characteristic of Bison. The only other species with similarly round, semi-circular-shaped hooves is Caribou, but their tracks are smaller and distinctly sausage-shaped, with clear indications of dew claws, which in Bison are not seen unless the ground is soft and the animals have sunk in to some depth. Moose and Elk have similar-sized tracks to Bison, but Moose tracks are pointed and much narrower, and Elk tracks are less rounded and narrower. Bison faeces are almost identical to those of Cattle, varying in shape and consistency with seasonal changes in food.
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The gestation period is between 277 and 293 days, and births take place from late April to June, sometimes into July. Females usually give birth within the herd, although some may leave for a few days. A single calf is born, weighing between 14 and 18 kg; twins are extremely rare. Calves under three months of age, are readily distinguished by their bright reddish brown coat; the reason for this distinct colour is unknown. Female Bison are protective mothers and their young are followers, staying with them until at least their first winter, although the bond between them begins to weaken noticeably in the mating season. In one instance in the Northwest Territories, herd members were seen to protect a calf from Wolves, but how frequently this cooperative behaviour happens is unknown.
The growth rate of female Bison slows around 3 to 4 years of age, but in males continues until about 5 or 6 years of age. Game ranchers report that males in captivity can continue growing even longer. Most healthy female Bison bear their first young on their third birthday, but some give first birth a year sooner or later. Under favourable conditions, most females produce one young each year throughout their life; in poorer environments, they may only bear two calves every three years. Male Bison, like many wild ungulates, are sexually mature around 18 months old, but do not usually begin to be fully active in mating until at least 6 or 7 years of age.
| Bison are primarily grazers, eating sedges, grasses, rushes and forbs, but they also eat some browse species. A study of Wood Bison in the Slave River lowlands, Northwest Territories, identified 29 different plant categories in the diet and that the main plants eaten year round were Slough Sedge and reedgrass. Bison in other areas of North America feed mainly on grasses in summer and sedges in winter, and others feed mainly on grasses year round. It appears that the range of digestible forages available to Bison determines the array of plant species they eat. In comparison to Domestic Cattle, Bison appear to digest forages more efficiently, especially those of low quality. In winter, rather than using their front feet to paw through the snow to forage, as many ungulates do, Bison sweep their heads from side to side to clear snow from vegetation. |
In the field, horn shape and size can be used to determine the approximate age of a Bison up to about 6 or 7 years in males and 3 to 4 years in females. Bison of both sexes can live more than 20 years, but the average life span is probably between 10 and 15 years.
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Wolves are the main predators of Bison; Grizzly Bears prey on them occasionally. Other species such as Coyote and Wolverine may only scavenge on carcasses of animals killed by larger predators, disease or old age. The diseases important in the management of Bison include brucellosis, tuberculosis and anthrax, all probably introduced to North America by Domestic Cattle. None have been diagnosed in free-ranging Wood or Plains bison populations in British Columbia. Other Domestic Cattle diseases, such as respiratory viruses, Bovine Virus Diarrhoea virus and gastrointestinal parasites, have been found in captive Bison. Disease transmission to and from Domestic Cattle is potential wherever free-ranging Bison herds contact ranched Bison or Cattle herds. Another mortality factor, both historical and current, is death by drowning. This happens when Bison herds break through thin ice while crossing large rivers or lakes in late fall and early spring.
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Bison are a social species, living in groups year round. Typically, adult males live apart from females and younger animals, so there are all-male groups and much larger maternal groups. Lone adult males of all ages are also not uncommon. Young males usually leave their maternal group when about four years old. Group size varies greatly depending on population size and density, habitat, and season. The largest groups form during the rut. When Europeans first came to North America, they reported seeing Bison herds containing thousands of individuals. Today, in large populations, maternal and rutting groups of more than 100 are common; the maximum group size is often limited by the habitat. The more open and large the meadow, the larger the group; groups in forested habitats are small. Bison constantly vocalize in a group, uttering short grunts as they walk and feed. Grunting is typical of many large, group-living ungulates residing in open habitats, and it may help maintain group cohesion.
A notable behaviour of both sexes is wallowing. Bison paw shallow depressions in the ground and roll in them. Wallows are usually located in drier areas, and wallowing creates a dusty soil that the Bison seem to like. Wallows are scattered throughout their range and many are used over and over again until they are several metres in diameter. Outside the rutting season, wallowing probably has some grooming function, helping to shed the winter coat, and with the dust, helping to control biting flies.
Rutting males often utter challenging roars that can be heard from long distances. The roars are quite unlike the bellowing of male Domestic Cattle. Opponents approach each other head on, usually walking slowly, with tails raised (the raised tail is a sign of aggression in Bison and can be given towards humans). Two evenly matched males will stand facing each other performing nod threats, first with the heads turned to one side, and then quickly dropping and raising their heads in unison. Nod threats are sometimes repeated for several minutes. Lateral or broadside displays are also common: the displaying male stands four to eight metres away, at a right angle to his opponent. Males often roar during displays, and if the opponent moves, the displaying animal will walk in a slow, stiff-legged gait and stand in front of him again. Occasionally, both males will display laterally, standing parallel to each other. Male Bison also paw the ground, wallow briefly and rub nearby trees with their horns as preludes to further aggression. An animal signals submission by turning his head away, then moving away slowly.
If threats do not work, a fight usually results. The males face each other a few metres apart, then suddenly charge, butting heads with considerable force. The large mat of hair on the head probably helps cushion the impact, as does some minor pneumation of the skull. Males will also try to hook and gore each other along the sides with their short, sharp horns, occasionally with lethal results.
Bison are polygynous, and mating takes place usually between July and September. During the rut, a male seeks out a single female to court within a group. The mating unit is often referred to as a tending pair, because an adult male defends and courts only one female at a time. Male courtship is quite simple, the male stands close alongside the female while she grazes. Periodically, he checks her oestrous state by testing her urine and lip-curling. When she comes into oestrus, he stands at her rear, steps towards her, swings his chin towards the top of her rump, often making a soft panting sound. The female usually moves away from the male at this stage, as she does when he tries to mount her. Only when in full oestrus will the female allow the male to mount her, before immediately moving away. The male must usually copulate while walking or running on his hind legs. After a successful copulation, the female often arches her back, urinates and then holds her tail horizontally for four or more hours. Most females accept only a single copulation during each mating season.
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