Bison, North America’s largest land mammals, are magnificent creatures that once roamed vast plains in the millions. Their impressive physical characteristics allow them to thrive in diverse environments, from grasslands to semi-desert habitats and even lightly wooded areas. The unique anatomy of these powerful animals plays a significant role in their ability to survive and influence the ecosystems they inhabit. Understanding their structure provides insight into their remarkable resilience.
Distinctive External Features
One of the most recognizable features of the bison is the prominent hump situated over its front shoulders. This large, muscular mass gives the animal a distinctive silhouette. Bison also possess a massive head, which is heavily insulated with long, dark hair, particularly on the forehead and neck.
Both male and female bison are equipped with short, sharply pointed, hollow horns that curve outward and upward from the sides of their heads. The shaggy coat of the bison is thick and dark brown, with longer hair on the head, neck, and forelegs, and much shorter hair towards the rear of the body. Adult male bison, or bulls, also display a black beard that can be about one foot long.
Bison possess strong, sturdy legs and cloven hooves. A mature bull can stand between 1.67 to 1.9 meters (5.5 to 6.5 feet) high at the hump and measure 2.7 to 3.8 meters (9 to 12.5 feet) in length, weighing between 816 to 1,088 kilograms (1,800 to 2,400 pounds). Females are somewhat smaller, reaching 1.5 meters (5 feet) high at the hump and 2.1 to 3.2 meters (7 to 10 feet) in length, with weights ranging from 318 to 545 kilograms (700 to 1,200 pounds).
Skeletal and Muscular Structure
Their skeletal system features robust bones, particularly in the skull, spine, and limbs, which are built to support their heavy mass and absorb impact. The skull alone can weigh around 17 kg (38 pounds), with the entire head reaching 35 kg (77 pounds) [5, 1.2].
The prominent hump on a bison’s back is supported by elongated dorsal spinous processes of the thoracic vertebrae. These flattened and thickened vertebral tips provide firm attachments for substantial muscle mass. The cervical vertebrae in the neck allow for head movement [1.4].
The powerful musculature is especially concentrated in the neck, shoulders, and hindquarters. The hump itself is primarily composed of muscle, not fat, and is anchored to these extended vertebrae [1.2, 1.3, 5]. The front legs are not directly attached to the skeleton but are held in a muscular sling, while the back legs are directly connected at the hip joint [1.4]. This arrangement contributes to their powerful build, with the front quarters weighing more than the hindquarters [1.1].
Specialized Internal Systems
Bison possess specialized internal systems adapted to their herbivorous lifestyle. As ruminants, they have a four-chambered stomach, a complex digestive system that allows for efficient breakdown and absorption of nutrients from fibrous plant materials like cellulose [1.1, 1.2, 1.3, 1.5]. The first and largest chamber, the rumen, holds food for extended periods, where microbes begin the breakdown process [1.3].
Food then moves through the reticulum, omasum, and finally the abomasum. This multi-chambered system enables bison to extract maximum nutrition from tough grasses and vegetation, even from food lower in nutritional quality, as digestion can take approximately 80 hours [1.2]. Their large lung capacity supports their endurance, allowing for efficient oxygen uptake.
The robust circulatory system, including a strong heart, supports their large body mass and the physical demands of their movements [1.5]. Studies on the European bison show that the heart’s blood supply is provided by well-developed left and right coronary arteries, with the left coronary artery typically being more developed [2.2, 2.3]. Blood from the heart walls drains into the great cardiac vein, middle cardiac vein, right coronary veins, and numerous small cardiac veins [2.1, 2.4].
Anatomical Adaptations for Survival
The prominent hump supports the massive neck and shoulder muscles, enabling bison to plow through deep snow to access forage during winter [2.1]. This muscular mass also contributes to the force behind their head-butting behaviors.
Their large heads and short, sharp horns serve multiple purposes. These structures are used for defense against predators and for establishing dominance within the herd [1.3]. Bison also use their heads and horns to clear snow for foraging, sweeping their heads from side to side to expose grasses beneath the snow cover [1.1, 2.2].
The bison’s dense, shaggy coat provides excellent insulation, protecting them from extreme cold and helping to regulate their body temperature in harsh weather conditions [1.5]. The strong, sturdy legs and cloven hooves allow bison to navigate varied terrain, break through snow, and achieve speeds of up to 60 km/hr (37 mph) [1.1].
Their ruminant digestive system allows them to thrive on a diet primarily consisting of grasses and other fibrous plants [1.1, 1.2, 1.3]. The combined strength of their skeletal and muscular systems, along with their specialized internal functions, contributes to their overall robustness and resilience in challenging environments.