A bison bull.

Wild bison have continuously inhabited Yellowstone National Park since prehistoric days and easily rule as one of the park’s most popular wildlife attractions. Retaining an ancient look, bison average six feet tall at the shoulder, sport curved horns, and occasionally carry up to 2,000 pounds of pure strength under their shaggy brown coats. Distinguished as North America’s largest land mammal, bison bulls (males) weigh in at a hefty 1,800 pounds while the female cows average 1,000 pounds. Despite such massive proportions, bison’s legs are unusually thin and delicate, and the creatures are amazingly agile.


A bison herd grazes in a meadow. NPS Photo

Distinguished from males by their slender horns and thinner beards, bison cows mate with their vegetarian grazing counterparts every mid-July to mid-August. The cows give birth to a single calf each May and ensure the longevity of the herd. However, other factors also affect the bison’s lifespan.

At the dawn of the nineteenth century, bison numbers in the park were as low as fifty in comparison to today’s average population of 2,000 to 2,500. Policies were immediately enacted to prevent the extinction of the distinguished animal, and twenty-one bison were transplanted from private ranches to Yellowstone and the Lamar Valley Buffalo Ranch. Within no time, the wild and domestic bison had intermingled, bison hunting and poaching was outlawed, and population numbers were allowed to increase to an all-time high of 3,500 in 1996. At the same time that record bison numbers were recorded, more than half the bison were reported as carriers of the brucellosis virus (a virus that causes cattle to miscarry and can prevent ranchers from shipping livestock out of state). With much discussion, it was determined that ranchers, state officials, and federal agents could defy natural law and shoot any bison that wandered outside the park’s boundaries in an effort to contain the spread of brucellosis. Along with this policy, annual winterkill and wolf predation has lowered bison populations to their relatively stable current level. Those bison that escape all of the above factors may live as long as forty years, but most average a lifespan of twelve to fifteen years. 

Although bison may appear docile as they graze along the Firehole River and in the Hayden and Lamar Valleys, these creatures are anything but gentle. As wild animals, bison are highly unpredictable, are easily agitated, can move at speeds up to thirty miles per hour, and have been known to gore or even kill individuals who approach too closely. Maintain your distance, and watch these magnificent animals from the safety of your vehicle!

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