Independence Day in the United States was this week, on July 4th. We’ve already discussed two US bird icons on this blog, the bald eagle and the turkey, which means it’s time to look at a mammal. Say hello the American bison America’s national mammal since 2016.
American bison (Bison bison) are North America’s largest terrestrial mammals, with adult males growing significantly larger than females. An adult male can get up to six feet tall (1.8 meters) at his shoulder and be nine to 12.5 feet long (3.8 meters). Females, while smaller, are still large, reaching seven to ten feet long (3 meters) and five feet tall (1.5 meters) at their shoulders. This large size has an equally impressive weight: adult males can be 1,000-2,200 pounds (453-998 kilograms), and adult females can be 790-1,200 pounds (358-544 kilograms). In addition to being large, bison have a very noticeable shoulder hump. They also grow longer, dark hair on the front halves of their body, and males will grow beards up to a foot long. Both males and females have horns that can grow two feet (0.6 meters) long.
Bison are the only native wild cattle species in North America. Their predecessors crossed the Bering Strait from Asia into North America around 300,000 years ago. These prehistoric bison were giants: one prehistoric bison, Bison latiforns, had horns 9 feet long (2.75 meters)! Fossils of the modern American bison appear in the fossil record about 5,000 years ago.
At one point, you could find bison throughout North America, from Alaska to northern Mexico. Before Europeans arrived, an estimated 30 million bison were on the continent. Unfortunately, their numbers started to decline once the Europeans arrived, turning into a mass slaughter by 1870. People were encouraged to hunt bison for both economic reasons (new tanning technology made bison hides more profitable) and political ones (the US Army actively encouraged bison hunting as a way to destroy Indigenous people’s way of life and force them onto reservations). The population of bison went from eight million in 1870 to fewer than 1,000 in just 20 years. Thanks to hunting laws and other environmental protections, the population has rebounded to around 500,000 animals, with only a small fraction roaming wild on public lands in conservation herds.
Bison, not buffalo
There are two subspecies of American bison: plains bison and wood bison. In addition to habitat differences (plains bison live on the Great Plains, and wood bison live in the boreal forest), there are slight physical differences between the two. While plains bison have short noses and shaggy “capes” of hair on their shoulders, wood bison have triangular heads, less defined shoulder and head hair, and bigger shoulder humps.
Neither plains bison nor wood bison, however, are buffalo! Although people may colloquially call bison “buffalo,” the reality is that bison aren’t closely related to the Cape buffalo of Africa or the water buffalo of Asia. You can quickly tell the difference between bison and true buffalo by looking at their horns: bison typically have shorter and sharper horns than buffalo. While bison horns quickly curve up, Cape buffalo horns look like a handlebar mustache and water buffalo horns are long and curved in a crescent shape.
Herd of bison
American bison are social animals that like to live and travel in groups called herds. These herds are separated by sex for most of the year: females and their calves make up one herd, while mature males form bachelor herds of up to 30 individuals. Dominance hierarchies within each herd are often decided by birth date, as animals born earlier usually grow larger by adulthood.
Males and females come together during the breeding season. Males show their dominance and fitness by butting heads with other males. Once who’s on top is settled, dominant males spend almost all of their time protecting their harem of females from other males. Males will spend so much time guarding these females that they may lose over 200 pounds (90 kilograms) over the breeding season. Subordinate males will mate with any leftover females. Once the breeding season is done, the males leave the females again.
Bison calves are born mid-spring, weighing about 50 pounds (23 kg) and covered in reddish fur. Bison are precocial animals; calves can stand up within an hour after birth. Females care for their calves for about a year. By the time they’re two to three years old, females are ready to breed; males begin breeding after they’re six and large enough to compete with other males for females.
Changing the landscape
Bison play an essential role in shaping the landscape around them. For instance, bison commonly engage in “wallowing,” rolling around on the dry ground. Wallowing can help bison shed their winter coat, stop insects from biting them, and form a dirt shield against parasites like ticks and lice. Laying in the cool dirt can also help cool bison down during summer.
Yellowstone National Park/Jim Peaco; Flickr; Public Domain
The aftermath of a wallowing session is a “wallow,” or bowl-like depression in the ground. As bison roll in wallows, they disturb the soil and can provide a habitat for different plant species to move in. Wallows also serve as water reservoirs and make small ponds that serve as habitats for vertebrates and invertebrates. Plants that require moist environments also thrive in these small ponds created by bison.
Bison can move a lot of snow around in the winter. They graze on grasses and other plants year-round, eating around 1.6% of their body mass in dry vegetation daily. Finding this much grass after a winter blizzard when snow covers everything is considerably more challenging! Bison solve this problem by sweeping their massive heads from side to side, clearing snow down to the plants underneath. Because bison primarily eat grasses, other animals, like pronghorn, can eat the vegetation they uncovered but left behind.
Bison play a huge role in shaping the prairies they live on. Their nomadic movements searching for food and water help increase diversity in plant species. A bison’s shaggy coat picks up seeds that fall off in new places, spreading plants across the prairie. Bison keep the places where they live healthy, and we should continue to protect them.
Resources
Leave a Reply