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— Six years ago, inbreeding threatened to destroy the last herd of southern Plains bison. Only 53 were left, and breeders were having trouble getting females to carry their calves to term. Tests showed that unless something was done to increase the diversity of genes in the historic herd, all the animals would be gone within 50 years.
Researchers now say a donation of a few bulls from media mogul Ted Turner seems to have done the trick. The herd has increased to 75 bison, and while more work to preserve the animals remains, there's no longer an immediate risk of extinction.
"It has made a significant difference," said James Derr, a professor of veterinary pathobiology at Texas A&M. "We have definitely improved the genetic diversity and reduced the inbreeding in the herd."
Bison are the largest land animals in North America, and as many as 60 million once roamed the Great Plains. When the Transcontinental Railroad was built across the United States in the 1800s, the bison were split into what was known as the Northern and the Southern herds. The Southern herd included animals from Texas, eastern New Mexico, eastern Colorado, Kansas, Oklahoma and southern Nebraska.
While there are no visible physical differences between the animals, those in the Southern herd have some unique genetic traits, Derr said.
"That herd is the last remaining vestige of the southern Plains bison herd," he said. "That is an incredibly important historical and genetic heritage for their conservation. That's why the state of Texas has an absolute jewel."
The herd that exists today was started in the 1880s by Charles Goodnight, one of the most prosperous cattlemen in the American West. His wife urged him to save five calves he had captured at a time when hunters were killing bison by the hundreds of thousands for their hides and meat and to crush American Indian tribes who depended on the animals for food and clothing.
At its peak, the herd numbered 250. It was donated to the state in 1997 and moved to Caprock Canyons State Park, which was once part of Goodnight's JA Ranch in the Texas Panhandle.
The rescue effort to save the herd began after pregnancy tests showed that 15 of the 18 adult females were pregnant in the fall of 2001. By spring, only five calves survived. The rest were either miscarried or died shortly after birth.
Disease and genetic problems, such as chromosomal defects, were ruled out. But as part of another project, Derr and another researcher were already sampling DNA from the federal bison herds throughout the United States. They knew how much genetic variation there should be in a typical herd, and they found the Texas animals had significantly less.
With no new genes entering the herd in 120 years, the researchers concluded it suffered from inbreeding and would probably die out in 50 years if nothing was done to save it.
The researchers turned to Turner, who owns the largest private bison herd in the world with about 55,000 head on 14 ranches in seven states. Nationwide, there were about 223,000 bison in 2007.
Turner's animals were important because they had plenty of genetic diversity but weren't hybrids of cattle and bison or infected with contagious diseases, such as such the brucellosis afflicting many of Yellowstone National Park's bison.
Also, some of Turner's animals have ties to the herd in West Texas. In 1902, Goodnight sold three of his bison bulls to the U.S. government, which was working to re-establish the animals in Yellowstone National Park. Some bulls in a herd Turner has in New Mexico are descendants of the Yellowstone herd, and in 2005, he donated three to Texas.
"This helps complete the circle," Derr said.
One of Turner's bulls never mated, but the two others provided the genetic diversity the Texas herd desperately needed. Together, they have sired 21 calves.
Today, the Texas herd has 38 cows and 37 bulls, and more are on the way. Eleven females are pregnant and will give birth this spring, the result of crossing Turner-Goodnight offspring with Goodnight animals. (Some older Goodnight animals have died.)
Russ Miller, general manager of Turner Enterprises, which oversees Turner's herd, said his boss is unequivocal in his support of what Texas is doing to conserve the iconic animals.
"It is gratifying to know that the animals we were able to provide to the state parks had such a great impact on bison conservation," he said.
Donald Beard, the Texas park superintendent who manages the herd, said it's in better shape but not yet out of the woods.
"We can't set by and let nature run its course yet," he said. "We still have to actively manage the breeding program."
Derr and officials with Texas Parks and Wildlife Department said they plan to approach Turner again for more animals to further diversify the herd. But they are pleased with the progress, noting that the Texas herd's average age, which scientists use to gauge its ability to survive, was 6.5 years in 2005. Now, it's 5.5 years.
The Turner bulls "put so many calves in the herd ... that it dramatically stopped that aging of the herd," Derr said. "And the herd's age has now continued to go down, which is a healthy sign."
