Cow residue lingering in modern buffalo

A bull American plains bison grazing in Theodore Roosevelt National Park, North Dakota. / Flickr: Furryscaley

By John Upton

Look an American plains bison in the eye. Stare deep into those dark, gaping peepers and you may catch a glimpse of something foreign, something slightly un-bison-like: A dash of contemporary cow.

The ancestors of domestic cows and American plains bison split apart 1 to 2 million years ago. But their genes washed together again 100 to 150 years ago, at a time when North America’s bison population had been hunted down from tens of millions of specimens to fewer than 100. During that time, ranchers bred surviving buffalo with their cows in a bid to toughen up the domesticated creatures and improve meat yields and flavor.

But genes flowed in both directions and cow DNA ended up in the tiny buffalo herds from which all of today’s buffaloes were descended. New research shows that those cow genes are still swirling around in the buffalo DNA, reducing individuals’ height and weight – and perhaps even their ability to survive.

Researchers studied the DNA of two very different buffalo populations. One of the populations was introduced by a filmmaker in the 1920s to an island off California, where the animals have prospered, wild but hungry, well outside the species’ normal range. The other population sampled was a well-fed private herd in Montana.

According to the results of the analysis, published this month in the journal Conservation Biology, cow genes were present in about half of the Catalina Island population and in five percent of the Montana population. In both populations, buffaloes with cow genes were smaller than the pure buffalo.

I asked lead researcher James Derr, a professor at Texas A&M University’s veterinary medicine college, whether the presence of cow genes reduces a buffalo’s fitness and makes it less likely to survive.

“Fitness is a tricky issue and not easy to prove,” Derr emailed back. “What we do know is that bison with domestic cattle mtDNA are, on average, smaller than bison with bison mtDNA. Runts are seldom at an advantage, I would guess.”

If it’s true that cow genes make a bison less likely to survive in the wild, evolution would be expected to run its course and continue to weed those genes back out of the native American ungulates. It may take a while, but the lingering cow bits will probably fade out of the almighty buffaloes eventually.

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