The Plains bison, a majestic symbol of the Great Plains and the national mammal of the United States, is the largest mammal on the continent. This impressive creature weighs around 2,000 pounds. It is easily identified by its shaggy, winter-resistant coat and formidable horns that serve as both warning and weapon.
The bison’s scientific name, Bison bison bison, seems to echo its grandeur. However, the formidable appearance of this iconic animal belies a vulnerability in its history. Its population plummet from tens of millions to just a few hundred within a few colonial centuries.
Thankfully, conservation efforts have helped to increase the bison population to roughly 20,000. Experts have upgraded its status from endangered to near-threatened. However, a recent study led by Nic McMillan from the University of Nebraska–Lincoln indicates that the ongoing conservation of the Plains bison will require a greater focus on climate factors, particularly the number of scorching days and drought-affected landscapes it faces in the future.
In collaboration with colleagues from Oklahoma State University, McMillan has found GPS-backed evidence that temperature and extreme drought can drive movement among herds of Plains bison. Continued increases in both of these factors, combined with the reality that most bison herds are now confined to significantly smaller areas than they once roamed, could present challenges in managing this iconic species.
“When we think about reintroducing bison or any large animal to a landscape, the landscape that the animal is inhabiting is potentially a lot smaller than it was historically,” said McMillan. “In 1491, if there was a drought in northeastern Montana, the bison had the entire Great Plains to escape that drought. They could move as far as they needed to.”
As we experience more extreme temperatures and droughts, McMillan suggests that rethinking how we structure these landscapes is necessary. Experts should consider whether or not they’re actually meeting the fundamental physiological needs of these animals.
The research team arrived at these conclusions after analyzing the movement data of 33 Plains bison from two different sites in Oklahoma. These sites were the Wichita Mountains Wildlife Refuge in the southwestern part of the state, and the Tallgrass Prairie Preserve near its northern border.
The researchers equipped these iconic animals with GPS collars to track their location every 12 minutes over multiple years. The data collected amounted to a total of 715,344 measurements. This was paired with temperature, rainfall, wind, and other variables recorded by nearby weather stations.
Additionally, the team analyzed moisture readings of the soils at both study sites to assess the presence of drought. This comprehensive data set allowed the researchers to identify links between bison movement and weather patterns.
The experts found that air temperature explained variations in bison movement better than any other factor analyzed. Bison movement increased 92.5 percent for every 18-degree rise in temperature. This ranged from a few degrees below zero to 83 degrees Fahrenheit.
Movement nearly doubled when the temperature rose from, for example, 65 degrees to 83 degrees. However, above the 83-degree threshold, an 18-degree increase corresponded to a 48.5 percent decrease in movement.
The temperature-related increase in movement suggests that the Plains bison were searching for grasses that grew better in the higher heat. This is especially noteworthy given that bison obtain the majority of their water from foraging.
On the other hand, the reduced movement in excessive heat indicates that the Plains bison likely rested and cooled themselves in places with standing water and shade from nearby trees to prevent heat stroke.
McMillan highlighted the significance of the findings: “When you consider (that) this is the first study of Plains bison across multiple herds – and then we find the same relationship across herds, in two very different landscapes – that’s a big deal.” He also noted that the trend paralleled a study of wood bison in Canada.
To assess the potential effects of drought, the team examined sensors that measured soil moisture at two depths. They chose depths of five centimeters, indicating moderate drought, and 25 centimeters, signifying severe drought. While bison appeared mostly unaffected by moderate drought, they moved substantially more during severe drought. One such drought hit Oklahoma in the early 2010s.
“There’s a lot of (prior) research that suggests that bison are basically drought-proof,” said McMillan. “They’re like these tanks out on the prairie that don’t need anything. They can just take whatever comes, and no big deal. At least, that’s the dogma in the reintroduction world. So I think this is really interesting, because we show that, hey, they still are not immune to drought. They have this potential threshold where they can’t handle it.”
The Plains bison, once abundant on the Great Plains, has played a crucial role in American history. Indigenous peoples used it as a vital food source and cultural touchstone. Bison were also a keystone species for many other plants and animals. However, their population has dwindled significantly. Scientists have relocated them to protected areas. This raises questions about whether they still serve the same ecological purposes as they once did.
Regardless, McMillan emphasizes the importance of preserving these animals due to their cultural significance and their role in North American identity.
“They’re incredibly culturally important and represent the cultural identity for all of the Plains tribes, at a minimum,” McMillan said. “But they also represent one of the most charismatic animals that we have in North America. So they’re really important for our identity as a country and all the people who live here.”
For McMillan, his interest in bison is deeply personal. As the son of a plant biologist and ecologist who hosted a national TV show on PBS, McMillan accompanied his father on a visit to Nebraska when he was 15. The experience left him captivated by the Great Plains and kindled his “profound attachment” to bison, ultimately shaping his career.
“We came to the Great Plains, and I was just hooked,” he said. “I never thought about anything else after that. I never necessarily originally envisioned myself being a scientist or studying bison as a scientist, but I was just always fascinated by how they exist on the landscape, bringing them back – that really emotional story. So for me, it’s very personal.”
McMillan hopes that the findings of his research can help inform the management of Plains bison in areas such as Yellowstone National Park. Yellowstone houses the largest wild herd of the species. The study’s results, published in the journal Ecology and Evolution, make a compelling case for paying attention to the behavior of these animals and what they are communicating to ecologists and conservationists.
Although national parks and other protected areas may appear to offer everything the Plains bison need, the fact that the animals still attempt to leave suggests that this may not be the case.
As heat and extreme drought, which encourage Plains bison to move, become more commonplace, the space previously considered sufficient for their habitat may no longer be adequate. McMillan explained that ensuring the diversity of their habitat – including access to grasslands, trees, and standing water – becomes even more critical as the available space shrinks.
“Then it’s an ethical question for us,” McMillan said. “Are we really being ethical if we’re forcing these animals to live in a landscape that may not actually be suited for them into the future? Whether bison were in Yellowstone historically is irrelevant to the likelihood that they can persist there into the future. Because today is completely and fundamentally different than yesterday.”
This thought-provoking study highlights the importance of understanding the relationship between the changing climate and the ecological needs of the Plains bison. It raises questions about the ethics of our conservation efforts and how we can best protect these iconic creatures in a rapidly changing world.
Bison, often referred to as buffalo in North America, are large herbivores native to the continent. They are divided into two subspecies: the Plains bison (Bison bison bison) and the Wood bison (Bison bison athabascae). Here are some additional facts about bison:
Bison are the largest terrestrial mammals in North America. Adult males can weigh up to 2,000 pounds (900 kg) and stand 5 to 6.5 feet (1.5 to 2 meters) tall at the shoulder. Their distinctive features include a shaggy winter coat, a massive head, a hump at the shoulders, and sharp, curved horns.
Bison are social animals, typically forming herds with separate groups for males and females. Female herds consist of cows, their calves, and young males, while adult males form smaller bachelor herds. During the mating season, also known as the rut, the two groups come together, and males compete for access to females.
Bison are herbivores, primarily grazing on grasses and sedges. They have a unique digestive system with a large rumen. This allows them to break down and extract nutrients from fibrous plant material efficiently.
Historically, bison roamed across much of North America, from the eastern United States to the Rocky Mountains and from Canada to Mexico. Their range has significantly decreased due to overhunting and habitat loss. Today, people mostly find them in protected areas, such as national parks and wildlife reserves.
Bison play a crucial role in maintaining healthy grassland ecosystems. Their grazing patterns promote the growth of various plant species, which in turn support a diverse range of wildlife. Bison also create wallows—depressions in the ground—by rolling in the dirt, which can provide habitat for other species and help retain water in the landscape.
During the 19th century, bison populations plummeted due to overhunting, habitat loss, and the spread of diseases from domestic cattle. Their numbers dropped from an estimated 30-60 million to fewer than 1,000 individuals. Conservation efforts, including breeding programs and habitat restoration, have helped bison populations rebound to around 500,000 today, with most residing on private ranches and public lands.
Bison have been an essential part of the lives of Indigenous peoples in North America for thousands of years. They provided food, clothing, tools, and shelter and held spiritual significance for many tribes. Today, bison continue to be an important symbol of the American West and North American heritage.