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Wolves kill one another and the pups of competing packs in battles over territory even if there is plenty to eat, according to a new study from Yellowstone National Park.

The research is a rare glimpse into the way wolves behave when humans are generally out of the picture, said Utah State University ecologist Dan MacNulty.

"At the end of the day, the success of a wolf from an evolutionary perspective is based on how many pups it leaves behind," said MacNulty, who worked with scientists from the University of Oxford and the Yellowstone Wolf Project on a new paper published online in the Journal of Animal Ecology. "If they're packed close together, they have the opportunity to raid each other and kill pups and eliminate the competition."

For a wolf, closeness is relative — as in 65 wolves per 1,000 square miles, the point at which adult survival rates drop below 70 percent.

The study, which will also appear in a print edition of the British Ecological Society publication, is based on 13 years of data from radio-collared wolves at Yellowstone. Until now, it's been hard to say how a large population of the animals interact with one another in the wild because their numbers were tightly controlled.

The animals were eliminated from Yellowstone by the National Park Service in the 1920s. They were reintroduced starting in 1995 and grew to something unique in the country — a group of wolves protected from human development and hunting.

The population peaked in 2004, though, and has declined since — but not for lack of food. The canines had plenty of their main prey: elk, as well as bison, bighorn sheep and mule deer.

Rather, the No. 1 cause of death during the study period was other wolves.

"They need more than simply food," MacNulty said. "That's sort of an unappreciated aspect of their biology."

If wolves leave the park looking for more elbow room, they can be hunted, hit by cars or otherwise affected by people, though they occasionally survive to establish new packs with Wyoming wolves.

Researchers, though, generally don't follow the predators after they leave Yellowstone.

The research suggests wolf populations are self-limiting, MacNulty said.

"There's a perception that if wolves come into a new area, there will be no holding them back," he said, "but ultimately what will be holding them back, if humans don't, is themselves."