Bison are getting smaller. It’s another sign of climate change — and poses challenges for herds
Bison that lived 3,000 years ago were 37% larger than those living today because of a warming climate — a trend that will accelerate, with bison projected to become 46% smaller by the end of the century. Bison are shaggy sentinels of climate change on the prairie.
RAPID CITY, S.D. — Jeff Martin noticed a striking trend in winners picked in competitions at the annual National Bison Association conventions that has persisted throughout the more than two decades he's attended.
The winning bison, invariably the largest and most impressive specimens, typically came from the north, while those passed over for recognition usually came from the south.
He observed a similar, although more subtle, contrast in size between the bison he helped raise on his family operation in western Wisconsin and those he encountered working as a ranch hand at a South Dakota bison ranch while in college.
It gradually dawned on Martin that the bison at the 777 Bison Ranch in southwestern South Dakota were visibly smaller than those on his family’s ranch in Wisconsin, where the climate is cooler, especially in winter.
Intensely curious, he began searching for explanations. For more than a decade, starting as an undergraduate and later while earning his doctorate, Martin has extensively researched the puzzle of why bison are larger in the north — and has compiled a body of evidence that the answer lies in warming temperatures.
Quite simply, as temperatures rise, bison become smaller, a correlation Martin found played out over thousands of years and continues.
“I’ve seen bones of ancient bison that were much, much larger than the bones of bison today,” said Martin, research director at the Rapid City-based Center of Excellence for Bison Studies at South Dakota State University.
As an undergraduate student studying paleontology at the South Dakota School of Mines & Technology in Rapid City, Martin first was drawn to the skulls and horns of ancient bison, which were much larger than today’s bison.
He later realized that skulls are not a good indicator of body size and instead focused on the “hock” bone, or calcaneum tuber, in the heel. He has measured and examined almost 1,000 of the fossilized bones.
“That’s where my eyes go when I look at a bison today,” he said.
The hock bone of ancient bison living 3,000 years ago was 37% larger than that of contemporary bison. Ancient bison, in fact, are genetically identical to bison today. “The only difference is their size,” Martin said.
The trend, now thousands of years old, of bison getting smaller as temperatures get warmer, will accelerate — Martin projects bison will be 46% smaller than they are today by the end of the century, assuming temperatures rise 4 degrees Celsius, or 7.2 degrees Fahrenheit.
“It’s significant,” he said. “It’s a substantial and statistically significant change."
Using chemical analysis of air bubbles in an ice core of a Greenland ice sheet, providing a record of climatic conditions over thousands of years, Martin was able to correlate the bison’s shrinking size with rising temperatures.
Today, a mature bison bull weighs about 2,000 pounds. By the end of the century, Martin predicts a bull on the southern Plains will weigh about 825 pounds.
It isn’t clear, Martin said, whether bison will be able to adapt body size to temperature by the year 2100, a span of 10 generations.
Bison, in other words, are shaggy sentinels of climate change, the prairie’s equivalent of the canary in the coal mine.
Side effects of shrinking
Martin’s fascination with bison and their declining size took him to Texas A&M University, where he earned a doctorate in wildlife biology.
For his dissertation project, Martin examined 19 bison herds on the Great Plains of the United States and Canada. He used a thermal-imaging camera to obtain indirect but accurate size measurements.
The thermal imaging technique had another advantage: enabling an analysis of how bison disperse heat from fermenting the grass they eat.
Smaller bison require less time to digest, and the resulting heat dissipates more quickly. Also, in warmer conditions, the quality and nutritional value of the grasses diminish, meaning less energy is available to the bison, and their energy intake decreases.
Bison require energy to grow or to stay cool; more energy diverted to cooling the body inhibits the bison’s ability to grow larger.
Besides collecting information from the thermal-imaging study, Martin also examined detailed records kept on the Wind Cave National Park bison herd in the Black Hills.
"They’re dead-center in the historic bison range, so that’s fortuitous," he said.
Once again, he found a pattern of bison getting smaller over time as temperatures climbed.
Over a 50-year period, mature females in the Wind Cave herd have decreased 11% in body mass — equivalent to 21 pounds per decade — and males have declined 23%, or a loss of 82 pounds per decade.
“The part that was eye-popping to me was how quickly they respond,” Martin said, referring to the rate of adaptation to hotter temperatures. “Every single generation, it’s responding in real time. The rate at which they’re declining is also alarming.”
Another alarming aspect is the shortening lifespans of the female bison at Wind Cave. In the 1970s, the maximum age reached by 90% of females was 21.5 years, decreasing to 17.5 years in the 1980s and 16.5 years in the 1990s.
The same trend of smaller sizes was evident across the 19 Great Plains bison herds Martin studied, including South Dakota’s Custer State Park and Badlands National Park herds but no herds in North Dakota.
“Body mass declines were associated with warming and with increasing drought severity,” Martin and two colleagues wrote in a paper outlining his findings.
Any changes in body size resulting from migration, disease or contact with people likely would have been temporary, they concluded.
“The strong correspondence between body size of bison and air temperature is more likely the result of persistent effects on the ability to grow and the consequences of sustaining a large body mass in a warming environment,” Martin and his colleagues wrote.
“Continuing rises in global temperature will likely depress body sizes of bison, and perhaps other large grazers, without human intervention,” the researchers wrote.
Among large mammals, the relationship between higher temperatures and smaller sizes has been known by scientists for many decades, so the progressively smaller size of bison fits a larger picture. Martin has a shelf of books in his office examining the phenomenon.
Although northern Plains herds are favored by a cooler climate, they face a more rapidly warming climate, according to projections from climate scientists. They predict the southern Plains average temperature will increase 3 to 5 degrees Celsius, while the northern Plains average temperature will climb 5 to 7 degrees Celsius, or 9 to 12.6 degrees Fahrenheit.
“So the rate of warming is much higher up here,” Martin said, “which is more alarming to me.”
Because of the warming climate, he added, “I think there will be places we just won’t be able to sustain bison any more.”
Many regard the comeback of the bison as one of conservation’s greatest success stories. Their immense herds once roamed the Great Plains by the millions and were found throughout most of today’s United States.
By 1893, however, after being hunted relentlessly for their hides and being exposed to novel diseases from encroaching cattle herds, their numbers plunged to fewer than 1,000.
Despite the genetic bottleneck caused by the population plunge, bison today have recovered impressively, now numbering almost 400,000, with more than 30,000 in public herds and the rest owned by private ranchers.
The International Union for Conservation of Nature, a large group of scientists, has been monitoring the bison. In every evaluation since the 1990s, it has moved the species further from danger of extinction thanks to conservation efforts and the bison’s ability to adapt.
“We need to reconcile bison still are evolving,” Martin said. “They are responding to the climate just as much as we are. And as it changes, they, too, will change, and so keeping them in places that are particularly harsh and austere is a driver of evolution.
”To help the bison evolve to adapt to a warming climate, herds should be maintained on the southern Plains, where the conditions will be the harshest. That might not be feasible for private ranchers, so public herds and their conservation partners should make that possible," he said.
“It’s critically important to keep bison in these places,” he added. “We can’t rely on private industry.”
In a hopeful sign, Martin said, the government of Mexico has been introducing bison to Chihuahua and a few other locations in the north of that country.
“Those bison will become critically important in the future,” he said.
Martin’s research into the effects of warming and the shrinking size of bison continues.
Warmer conditions favor parasites, which proliferate as temperature increases. He believes that could account for some of the decrease in bison’s body size.
To test that theory, he plans to study six herds in five states managed by The Nature Conservancy that were entirely stocked by bison from Wind Cave National Park, providing a good comparison of conditions, including the parasite load of animals in each herd.
“As you get hotter and more humid, you get a much larger parasite burden,” he said. Blood-sucking worms called nematodes live on grasses in their larval stages and could rob more nutrients and tax the immune systems of bison hosts as they proliferate, he believes.
“This is a compounding offense” of climate change, Martin said. “If you’re already resource-limited, and we add climate change to it, you now become resource-scarce.”
Parasites and pathogens “really can bring populations to their knees,” he said. “They’re an antagonist to sustainability.”
Climate change poses the greatest challenge to public and private bison herd managers, in Martin’s view. Lowered immune function, for instance, could mean higher vet bills. On the other hand, smaller animals could be an advantage, since bigger animals will be more stressed.
Proper management is vital. “I feel like I’m beating a dead bison here, but it is critically important for their survival.”
Awareness of the growing challenges is beginning to take hold, as Martin has engaged with ranchers and public herd managers on the subject.
“There’s a lot of buy-in to what I’m doing,” he said. “A lot of ranchers want to know more about this and what they can do.”
Mimi Hillenbrand, who runs her family's 777 Bison Ranch and is familiar with Martin's research, said the ranch has observed warming temperatures in recent years, but is uncertain how long it will take the bison to adapt.
"I think as climate change continues to heat up, animals will have to adapt or they will end up like the dinosaurs," she said.
Bison ranchers are aware of and support Martin's work, but she's not sure they'll observe the changes in their lifetime.
"As for us changing our management? Probably not," she said. "We manage for the whole and focus on regeneration and restoring grasslands. The bison are helping us achieve this. ... The 777 is creating a partnership with nature.' Bison are part of our team."