BOZEMAN, Mont. — When dozens of wild American bison wandered out of Yellowstone National Park in search of greener grass and wound up five years later sheltered on a giant ranch owned by Ted Turner, media mogul and bison meat kingpin, the species reached what many believe could be a turning point.

Mr. Turner, under an unusual custodial contract with the state of Montana, offered to shepherd the animals for the next five years as part of an experimental program. It will grant him a sizable portion of their offspring in exchange, much to the chagrin of environmentalists who sued the state, saying the bison belong to the public. Mr. Turner is not restrained from using the bison for commercial breeding or sale.

The “Yellowstone 87” are a kind of Noah’s ark of their kind. Genetically, these bison still carry the shaggy swagger of their Ice Age forebears that lived alongside saber-toothed cats and woolly mammoths.

Montana wildlife managers hope they will be the fount for establishing new free-roaming populations elsewhere in the state or around the West — if the animals prove, through the five years of testing, to be free of diseases that can infect cattle, especially brucellosis.