The American Bison (Bison bison) is the largest terrestrial mammal in North America, a massive herbivore whose life revolves around consuming vast quantities of forage. Their activity profile is a dynamic rhythm dictated by biological needs, environmental conditions, and seasonal reproductive cycles. Understanding when these animals are most active requires examining the interplay between their physiology and their grassland environment. This article explores the distinct patterns governing the bison’s 24-hour cycle and the dramatic shifts that occur across the calendar year.
Daily Activity Patterns
The bison’s 24-hour cycle is characterized by alternating periods of grazing, resting, and ruminating, with the most concentrated movement occurring near dawn and dusk. This pattern is a physiological adaptation allowing them to graze during cooler periods and retreat from intense midday heat. During the summer, movement can decrease by as much as 48% when air temperatures climb above 28°C, a reduction necessary to avoid overheating due to their large body mass and insulating coat. During the hottest hours, they prioritize lying down to chew their cud, a resting period vital for maximizing nutrient absorption from their fibrous diet.
Seasonal Shifts in Behavior
The bison’s overall behavior shifts significantly throughout the year in response to resource availability and temperature extremes. In the summer, activity involves continuous grazing and daily travel, with herds typically moving about three kilometers each day between foraging sites. Biting insects prevalent in summer also influence activity, driving the bison to seek relief through wallowing and rubbing against objects. Winter demands an energy-conserving strategy to survive on lower-quality forage; bison slow their metabolism and reduce foraging time to minimize energy expenditure. When snow covers the ground, they use their massive heads and powerful neck muscles to swing aside snow, accessing buried grasses up to four feet deep.
The Intensity of the Rut
The single most intense and visibly active period for the American Bison is the rut, or mating season, which typically peaks across July and August. This period is characterized by dramatic displays of dominance and competition among the bulls, severely disrupting the herd’s normal pattern of grazing and rest. Bulls engage in loud, guttural bellowing and aggressive sparring with head-to-head impacts to establish a dominance hierarchy.
Rut Behaviors
The high hormonal drive during the rut triggers a significant increase in wallowing. Bulls roll violently in dirt depressions, often after urinating in the spot, coating themselves in dust and scent. This practice acts as a display of aggression and helps attract females. The sheer exhaustion of these activities means bulls may forgo eating entirely to focus on mating, resulting in a loss of up to 10% of their body mass by the end of the season.