Bison once numbered in the hundreds of millions and roamed in vast herds throughout North America. But after Europeans colonized and began developing the continent, the newcomers set about killing the shaggy herbivores en masse. Their numbers declined rapidly, and, by the 1870s, American bison—also called buffalo—had nearly gone extinct, dropping to a total population of around 500.
But bison are survivors. They hung on in a small herd in what’s now Yellowstone National Park, as well as another tiny population in Canada. A handful of ranchers also kept some alive, most of whom also experimented with breeding buffalo and cattle in a failed attempt to create a better beef-producing animal.
More than a century’s worth of dedicated conservation has helped the animals rebound tremendously, to a total population around 500,000.
But hidden in their DNA, these iconic species bear marks of their scrape with humanity: A recent study published in the journal Scientific Reports finds that all American bison tested have low but significant quantities of domestic cattle DNA.
The results came as a surprise to study co-author James Derr, a biologist at Texas A&M University. “I was absolutely in denial,” he says. “It was like being punched in the mouth.”
The result is upsetting, Derr says, because bison in Yellowstone and a few other populations were once thought to be free of cattle genes. But there may also be an upside: Some government restrictions on breeding bison—such as preventing gene flow into certain populations, for fear of ruining the group’s genetic “purity”—might be unnecessary, he adds.
“It opens up opportunities to manage [herds] based on the best available resources,” Derr says.
Mark Kossler, vice president of ranch operations for Turner Enterprises Inc., which owns more than 50,000 bison raised for meat production, agrees.
“The bison industry has been in a turmoil over the introgression of cattle genes, some not worried about it and others obsessed with the ‘purity’ of the genetics in their bison, either for production or conservation,” he says by email.
“Everyone needs to take a deep breath and relax since [apparently] all bison contain some cattle genes.”
Cattlemen to the rescue?
Bison and domestic cattle diverged from a common ancestor somewhere around three million years ago. But like many related mammals, they can still interbreed and produce fertile hybrids. (Read more about hybrids and how they happen.)
Recent research has shown that evidence of past hybridization is far more common than previously thought. Humans, for example, have significant quantities of genes from Neanderthals and Denisovans, two separate species of human-like ancestors. Many Europeans and Asian have genomes that are 2 percent Neanderthal, and some Melanesians are genetically 6 percent Denisovan.
“We’re increasingly finding out that many species have evidence of gene flow into them recently or in the distant past when there were separate lineages,” says Oliver Ryder, director of conservation genetics with the San Diego Zoo Wildlife Alliance, who wasn’t involved in the paper.
Though people controlled much of the hybridization between bison and cattle, some was likely accidental, or the result of cattle escaping to the wild and breeding. Generally, though the vast majority of the time the two species avoid each other, it is not unheard of for male bison to mate with female cattle.
In fact, a handful of ranchers played a vital role in saving bison following European settlers’ attempt to kill off the species, in part to starve Native Americans of their main food source. In the late 1800s, Charles Goodnight of Texas, Fred Dupree of Montana, Charles Jones of Kansas, and Walking Coyote of Montana—a member of the Pend d'Oreille tribe—raised herds of the animals sourced from wild calves. Without their efforts, the species likely would have vanished.
All of these cattlemen, except for Walking Coyote, is known to have encouraged some degree of hybridization with domestics. (Coyote’s herd was acquired by other ranchers who intermixed these animals with bison that had a history of cattle interbreeding.)
In Yellowstone, where the bison population hovered as low as 30 animals by 1900, wildlife officials brought in bison from private herds in Texas and Montana to boost that population. Though these imported animals had a past history of hybridization with cattle, it was unknown how much cattle DNA they carried, if any.
For the study, researchers sequenced whole genomes from 19 modern bison and eight historical bison specimens taken from a variety of locations in the United States and Canada, collectively chosen to cover all known bison lineages. Assistant professor Brian Davis and Ph.D. student Sam Stroupe, both co-authors, used a sensitive tool to compare these complete bison genomes, focusing on nuclear DNA, with other bison and domestic cattle. Stroupe also combed through historical records to show which bison came from what populations and where they ended up.
The results revealed the presence of cattle DNA in all samples, although the amount was generally low, representing between 0.5 and 2.5 percent of the total DNA. The Yellowstone bison had the smallest fraction; one individual from the park had only 0.24 percent cattle DNA.
The study also found cattle DNA in two historical bison samples from the late 1800s the predate the widespread hybridization experiments of the early 1900s, suggesting that even earlier breeding must have occurred to a certain extent between the two species, perhaps caused by cattle that escaped captivity.
It’s unknown how big of an impact this addition of cattle DNA is having on wild bison, but it’s unlikely to play a huge role, Derr says. Some wild buffalo were already known to possess cattle mitochondrial DNA—which is passed from mothers to their offspring—and which can have negative effects on growth and overall size, but such genes have not been found in Yellowstone and Wind Cave and a few other wild populations.
Ryder cautions against viewing the results in terms of purity, which is ultimately a human concept that is difficult to square with the complex development of many species.
Davis agrees, pointing out that hybridization is an essential component of evolution and plays a role in the development of many species. “Within mammals alone, we see recent and ongoing hybridization repeatedly in natural populations of rabbits, bears, numerous rodents, South American cats and even North American wolves.” Moreover, since the bison population fell to such a low number, having outside genes from cattle could possibly boost the species’ diversity and overall health.
Rurik List, an ecologist at the Metropolitan Autonomous University in Mexico who wasn’t involved in the paper, says the results are sad. But in a way, they’re not as important as the ongoing effort to restore bison to the landscape, where they have a variety of beneficial effects, such as helping grasslands thrive by greatly boosting promoting plant and animal diversity. Even bison with traces of cattle genes do this, and are still ecologically functioning as bison, which is key, he says.
Holding out hope
This study’s result also intrigue ranchers that raise bison throughout North America. Most of the half-million American bison alive today are privately owned for meat production.
“Bison producers seek diversified bison genetics ideally free of cattle DNA to raise healthy herds, but this study shows that even with small amounts of cattle DNA, the species still look and act like buffalo,” says Jim Matheson, executive director of the National Bison Association, a non-profit association of bison producers, processors, and enthusiasts.
The study authors think it is unlikely there are any bison left free of cattle genes.
“There are no major herds that don’t have cattle introgression—[though] there could be one or two or a few individuals,” Davis says.
Kossler, the Montana rancher, holds out hope. But his philosophy is: “Stop worrying about what you can’t change, select the best animals available for conservation work, and get on with it,” he says.