Most elk, bison and trumpeter swans are unfazed by the presence of snowmobiles in Yellowstone National Park, a new study suggests.

Researchers from the park and Montana State University observed more than 2,100 interactions between snowmobiles and wildlife last winter and found the animals can become habituated to the machines over time.

In 81 percent of the interactions, the animals either had no apparent response, or they looked and then resumed what they were doing, according to the study commissioned by the National Park Service.

The Park Service is in the midst of its third in-depth study of the snowmobiles in Yellowstone to develop a long-range plan for winter use in the park.

Currently, Yellowstone is operating under temporary winter use rules, which allow up to 720 guided snowmobiles into the park each day.

The study recommended the machines be required to stay more than 100 yards from groups of wildlife. It also urged reductions in the number of riders in groups and the time they can stop to watch wildlife.

Amy McNamara, national parks' program director for the Greater Yellowstone Coalition, said winters in Yellowstone are hard enough on wildlife without the added pressures of snowmobiles.

She said her conservation group has advocated mass-transit snowcoaches, which have previously been shown to be a better solution for the park than snowmobiles that often carry a single rider.

"If there's going to be oversnow travel, it has to be done in a way that minimizes the impact on wildlife to the greatest extent possible," she said.