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."