We kept our distance. It made sense to not get between a bison cow and her wandering calf or a bull pursuing a mate or one chasing away a young competitor. A full-grown bull bison can weigh nearly a ton and stand more than 6 feet tall at the shoulder: the largest land animal in North America. It's hard to believe that these enormous wild animals can also be part cattle, a genetic mishap that Blue Mounds bison have avoided.

It was August, and calves born that spring were shedding their reddish coats to become brown like the adults. Cows were coming into heat and followed by bulls eager to mate. The herd moved slowly through the grass, grazing and softly grunting to each other. Above, swallows swooped in the bright sunlight, chasing flying insects. I gazed off to the horizon and imagined when the scene of grazing bison unfurled for mile after mile across the northern plains, before bison were almost exterminated—and then later mixed up with cattle.

Beefalo and Cattalo

The destruction of the American plains bison is well-known: In the late 1800s, market hunters and pioneers hunted the animals ruthlessly to near extinction. By the end of the 19th century, the tens of millions of plains bison had been reduced to fewer than 1,000. In 1905 the American Bison Society was established to conserve the species. It created additional preserves and stocked them with animals from Yellowstone as well as with bison bred at the Bronx Zoo.

What is less well-known is that enterprising ranchers had also captured and bred bison, crossing them with cattle to create livestock that would be winter-hardy like bison yet docile like cattle. They created "beefalo" and "cattalo" with varying success, and their crosses caused cattle genes to become widespread in herds of bison raised on ranches. These commercial herds now represent the majority of plains bison in existence, more than 400,000 animals. A tiny fraction of today's plains bison, approximately 24,000 animals, roam in conservation herds on public lands. Conservationists worry that many of these animals also contain cattle genes.

But that is not the case with the bison at Blue Mounds State Park. Genetic tests conducted on hair and blood samples collected from the park's bison have revealed that few show any cattle ancestry. Twenty-six animals were tested in 2011 and 35 in 2012; only one bison each year showed any evidence of cattle genes. The plains bison at Blue Mounds appear to be among the most genetically pure in the United States.

Herd of Genuine Genes

"We thought they'd be lousy with cattle genes," says Courtland Nelson, Department of Natural Resources director of Parks and Trails. "We never expected such good results. We were jumping for joy!"

Given the mixed origins of the park's bison, Nelson had reason to worry. Bison were first brought to Blue Mounds in 1961 to complement the park's tallgrass prairie landscape. The herd began with one bison cow and two bulls from Fort Niobrara National Wildlife Refuge in Nebraska. Since then the herd has grown through calving and the addition of animals, mostly bulls, from conservation herds in South and North Dakota, Montana, and Oklahoma. New bulls help reduce inbreeding, but until recently none were genetically tested—a relatively new practice in bison management.

Genetic testing of bison is done during the park's fall roundup. The 533-acre prairie pasture at Blue Mounds can ideally accommodate only 75 adult animals. Come fall, the herd with its growing calves can number nearly 100 animals and must be reduced. A good solution is a roundup, gathering the entire herd and auctioning many of the yearlings to ranchers. This reduces the potential for inbreeding and replicates the loss the herd would have experienced historically to predators in the wild. The roundup is also an opportunity to take hair and blood samples when bison are temporarily held in a chute. After the DNA testing, any animal found to contain cattle genes may be auctioned.

Why do cattle genes in the herd matter? If a bison with cattle genes still looks like a bison, isn't that good enough? According to James Derr, a geneticist at Texas A&M University, those looks are only skin-deep. Derr recently compared bison with cattle genes to those without and discovered that, on average, the bison with cattle genes are shorter and weigh less. That difference is not merely cosmetic. The hybrids, Derr says, are smaller because they are likely less efficient in converting grass to energy—a serious concern for a grass-feeding species renowned for its ability to survive on the windswept winter prairie when grazing is meager.

Derr has studied the genetics of bison herds all across North America, including the herd at Blue Mounds. Texas A&M's collection of bison DNA is the largest and most comprehensive assembled for any wildlife species. Derr argues that a species is its genome. Change a genome enough through hybridization or inbreeding and the end result is a different animal, less able to survive in the wild. It is our responsibility, Derr insists, to manage bison to prevent that outcome. Derr's team determined that Blue Mounds' bison are a valuable genetic basis for future herds.

Preserve the Genome

The Minnesota Zoo in Apple Valley shares Derr's concern about preserving genomes—it is a big part of the zoologists' work with endangered species. Animals that are rare in the wild and sustained through zoo-bred populations require close management to prevent the inbreeding that can alter genomes over time.

The zoo has exhibited plains bison for years, but discontinued breeding the animals when it could no longer find suitable homes for their offspring—the exhibit's 6 acres would soon be trampled if the zoo kept too many. Prior to the Blue Mounds roundup in 2011, the zoo approached the DNR. What if Blue Mounds bison became the basis for new animals in the exhibit and their calves were returned to the park?

"We want to contribute our space and expertise to conserving Minnesota species," explains Tony Fisher, animal collections manager at the zoo. "Bison are a species that people know and love. And although there are a lot of bison, there aren't a lot of pure bison. We can help conserve those bison, just as we help conserve tigers and other rare animals."

The DNR had planned to begin collecting genetic samples from the Blue Mounds bison, and the zoo agreed to help. The zoo also signed an agreement with the DNR to cooperatively manage a genetically pure bison herd at Minnesota state parks and at a zoo exhibition. As a result, four animals from the 2012 roundup moved to Apple Valley—two calves and two pregnant cows. This spring the cows gave birth to two calves, now part of the zoo's herd.

The zoo also brought in a young bull, chosen for its diverse genome and lack of cattle genes, from Badlands National Park in South Dakota. That animal is expected to start breeding this year, contributing its genetic wealth to Blue Mounds via calves that will return to the park.

Two yearling bulls from Wichita Mountains Wildlife Refuge in Oklahoma arrived at Blue Mounds in October 2012. They'll breed in 2014, adding more wild bison genes to the park herd.

Ambassadors for Prairie

Sustaining the genetic health of Blue Mounds bison will require regular introduction of animals with proven genetics. Genetic studies conducted by the National Park Service and others have indicated that a population of fewer than 400 bison will lose genetic diversity over time. Because no single place in Minnesota can accommodate that many bison, satellite herds must be established at various locations and managed to make a much larger metapopulation. Moving animals between these herds, continuing "arranged marriages," can prevent the inbreeding that would erode the genetic resiliency of Minnesota's wild bison.

Where will these herds live? Ed Quinn, natural resource program coordinator for DNR Parks and Trails, leads a team that is evaluating other state parks as possible homes for a future bison herd to be established by late 2014. The park must be within the historical range of bison. It must have at least a few hundred contiguous acres of grassland, adequate water sources to support a herd of 50 or more animals, and enough space for the handling facilities needed to round up and move bison.

"It also has to be a park near people," Quinn says, "because people want to see bison." The popular animals can help increase awareness of the dwindling prairie ecosystem. "They are ambassadors for the prairie," Quinn summarizes. "The next park with bison must be an outstanding center for interpretation."

Bison are a keystone species that shapes prairie. They graze some areas more than others. After a prescribed burn, ash-blackened land heats up and new plants sprout. Less-grazed patches burn more intensely, and bison are partial to the tender new growth there. Grazing and prescribed burns work together to increase the diversity of plant life, benefiting other creatures including rare butterflies and ground-nesting birds.

Park visitors can see bison as a part of the grassland landscape and experience a connection to Minnesota's past. Says Beckman, "Our bison are direct descendants of those animals long ago when the plains were black with bison."

Return to Roundup

In September I return to Blue Mounds, invited by Beckman to see the bison during the fall roundup. It's an impressive event. Bison are safely moved into a corral, smaller pens, and eventually, one by one, into a squeeze chute. The chute is similar to those used at cattle roundups but more massive—bison are stronger, more agile, and less cooperative than cattle.

"No hollering or jumping," Beckman reminds everyone, because those actions make bison anxious. Working from catwalks above and stations beside the holding pens, wranglers encourage the bison to move forward and then shut gates behind the animals. I watch this orchestrated dance between bison and workers, culminating when metal panels press against the sides and neck of the animal in the chute. Another team works quickly to draw blood and collect tail hair of any bison of an unknown genetic makeup.

Then the chute's front gate opens and, unless it is being sent for auction, the bison trots off to rejoin the herd. I watch the herd wander out into the tall grasses and begin to graze. Once again I imagine seeing the scene repeated across the prairies of Minnesota. This time, though, I'm envisioning not the past but the future of bona fide bison.