CUSTER STATE PARK, South Dakota: As part of an annual effort to maintain the health of the species, which has recovered from near-extinction, cowboys and cowgirls rounded up a herd of more than 1,500 bison in Custer State Park, South Dakota, on September 29.

"How many times can you get this close to a buffalo herd? You hear the grunts and the moans and see the calves coming and running alongside mamas," said Kevin Robling, South Dakota Game, Fish and Parks Secretary, who was among the 50 riders herding the animals.

Superintendent Matt Snyder said that Custer State Park conducts the only annual "Buffalo Roundup" in the US, to assess the health of animals and vaccinate calves.

As many as 60 million bison, sometimes called buffalo in the US, once roamed North America, but they were driven to the brink of extinction more than a century ago when hunters, U.S. troops, and tourists shot them in large numbers for commercial purposes. By 1889, only a few hundred animals remained.

"Now, after more than a century of conservation efforts, there are more than 500,000 bison in the U.S. The Custer State Park bison herd has contributed greatly to those efforts," said South Dakota Governor Kristi Noem, a horseback rider who participated in the roundup.

Snyder said that the park's herd began with 36 animals bought in 1914, but it can now sustain some 1,000 based on how snow and rain conditions affected the grasslands this past year.

"The other 500 or so will be auctioned off, and over the next week, officials will decide which bison will remain and which will go. About 400 calves are born in the park each year," he added.

"Each year, we sell some of these bison to intersperse their genetics with those of other herds to improve the health of the species' population across the nation," Noem said.