Yellowstone's iconic bison herds have merged into a single entity after 100 years of wandering the park

Bison in Yellowstone National Park spent over 100 years as two genetically distinct herds. But now they're roaming as one interbreeding population, a new study suggests.

Herds of wild bison (Bison bison) have meandered through Yellowstone's vibrant landscape since prehistoric times. Grazing freely in the expansive grasslands of the park, these are the last free-range bison in the United States.

Although these animals are now abundant in the national park, poaching pushed the local population near extinction by the turn of the 20th century. Park managers tried several methods to help conserve Yellowstone’s bison, halting poaching and even introducing a new herd to the region.

Thanks to conservation efforts, both the native herd and the introduced herd, which consisted of adults from Montana and Texas and calves from Yellowstone’s native herd, were able to thrive. Later genetic analyses showed that descendants of this introduced group remained genetically distinct from the native bison — until now.

In the past 20 years, Yellowstone's two bison subpopulations have become one large interbreeding herd, according to the new genetic study, published Sept. 13, 2024 in the Journal of Heredity.

"I think the kinds of questions that we ask about this population at Yellowstone can only be answered using genetic technology," study senior author Jim Derr, a professor at Texas A&M's College of Veterinary Medicine and Biomedical Sciences, told Live Science. "No observational information is going to help you because you don't know who's breeding with who."

To reveal the breeding dynamics of the droves of bison in Yellowstone, workers at the National Park Service (NPS) collected tissue samples biopsied from 282 individuals for analysis.

Closely analyzing the bison's genetic markers gave the researchers clues into their lineage and how the population has changed over time.

Comparing the genetic markers they found in Yellowstone's current bison population to those from samples taken in the early 2000s, the researchers concluded that the two groups roaming the land interbred frequently enough that they are no longer genetically distinct.

The exact cause of the change isn't clear, but the researchers say it was likely a gradual shift in behavior over the past 20 or more years. "Part of it is just the bison exploring different areas and figuring their way out and running into each other," study lead author Sam Stroupe, a postdoctoral research associate at Texas A&M University, told Live Science.

The researchers hope this new analysis will help with ongoing efforts to conserve and manage the herd at Yellowstone.

Staff at the national park have been managing the bison population as two groups since individuals were first brought in from out-of-state in 1907. Having only one herd to look after could make conservation and management of the species easier, the researchers said.

"I think everyone wants bison in Yellowstone National Park to be managed appropriately and to have good stewardship of that herd," Derr said. "Hopefully we can give them a little bit of insight with this genetic information."