A new study has found that wolf clans often fight and kill rival members over territory and that limited space could lead to the end of wolves in the Yellowstone National Park.

The research was conducted by Utah State University ecologist Dan MacNulty and colleagues from the University of Oxford.

The US Fish and Wildlife Service listed Rocky Mountain wolf (Canis lupus) as an endangered species in 1973. Greater Yellowstone is one of three areas designated as recovery regions for the wolves. The Park has not only helped conserve the species, but has also helped researchers monitor behavior of the wolves.

Wolves are a highly-social species and live in packs. The size of the pack varies according to food availability. The social structure of the Rocky Mountain wolf is elaborate with certain members being given a specific role such as a leader or a sub-ordinate. Adult wolves are known to kill each other when food is limited.

The present study shows that fights among rival wolves could occur even in prey-abundant areas.

"Many think wolf populations are limited only by prey numbers," said MacNulty, assistant professor in USU's Department of Wildland Resources and the USU Ecology Center, according to a news release. "That's not the case. A vital resource for wolves is enemy-free space to safely raise pups."

The study was based on data collected from 280 wolves over a 13-year period. The wolves live in the Yellowstone National Park and are radio-collared. The research team looked at the abundance of prey, wolf density and population structure. Researchers also accounted for wolf-survival in prey-rich and prey-poor areas of the Park.

"It's difficult to study territorial behavior outside the park where wolves are more widely dispersed," he says. "Inside the park, we could watch fights between packs."

The team found that attack by rival wolf packs was the leading cause of death, even in areas where prey was easily accessible. "More wolves meant more fighting and killing," MacNulty says. "As a result, survival rates declined as wolf density increased."

The research could help conservationists plan wolf-management strategies, study authors said

The study is published in the Journal of Animal Ecology and was supported by National Science Foundation, Yellowstone National Park and the Yellowstone Park Foundation.