Nick Garbutt / Male American Bison (Bison bison) walking through snow. Firehole River Valley, Yellowstone National Park, Wyoming, USA. January

Jim Manthorpe / American bison (Bison bison) feeding, Yellowstone National Park, Wyoming, USA, August.

Danny Green / RF - Bison (Bison bison) breathing in the cold air, Yellowstone National Park, USA, February (This image may be licensed either as rights managed or royalty free.)

Danny Green / Bison (Bison bison) herd walking in line in snow, Yellowstone National Park, Wyoming, USA, February

Klein & Hubert / American bison (Bison bison) bull, North Dakota, USA.

George Sanker / American buffalo (Bison bison) with group of calves, Yellowstone National Park, Wyoming, USA. May.

Peter Cairns / Bison (Bison bison) pair standing on road in winter, Yellowstone National Park, Wyoming, USA, March.

George Sanker / American Buffalo or Bison (Bison bison) group with calves, Yellowstone National Park, Wyoming, USA, May

Staffan Widstrand / American Bison (Bison bison) in habitat, Grand Teton National Park, Wyoming, USA, October.

Staffan Widstrand / American Bison (Bison bison) herd in habitat, Yellowstone National Park, Wyoming, USA, October.

Ben Cranke / American Bison (Bison bison) foraging in the snow, Yellowstone National Park, Wyoming, USA. January.

Peter Cairns / American bison (Bison bison) walking through snow using tracks made by other animals, Yellowstone National Park, Wyoming, USA, February.

Peter Cairns / Tourists watching American bison (Bison bison) Yellowstone National Park, Wyoming, USA, February 2013.

Peter Cairns / American Bison (Bison bison) walking through deep snow, Yellowstone National Park, Wyoming, USA, February.

Peter Cairns / American Bison (Bison bison) walking through snow field, Hayden Valley,Yellowstone National Park, Wyoming, USA, January.

Peter Cairns / American Bison (Bison bison) lying in snow field, Hayden Valley,Yellowstone National Park, Wyoming, USA, January.

Peter Cairns / Bison (Bison bison) standing in geothermal run-off in winter, Yellowstone National Park, Wyoming, USA, February.

George Sanker / American Buffalo / Bison (Bison bison) grazing in open plains. Yellowstone National Park, Wyoming, October.

Danny Green / Bison (Bison bison) seen through blizzard snow. Yellowstone National Park, USA, February.

Danny Green / Bison (Bison bison) in snow. Yellowstone National Park, USA, February.

Danny Green / Bison (Bison bison) in snowstorm. Yellowstone National Park, USA, February.

Danny Green / Bison (Bison bison) in snowstorm. Yellowstone National Park, USA, February.

Danny Green / Bison (Bison bison) in snowstorm. Yellowstone National Park, USA, February.

George Sanker / Bison (Bison bison) two males head to head, Yellowstone NP, Wyoming, USA

Kirkendall-Spring / American buffalo / Bison (Bison bison) in deep snow, Lamar Valley, Yellowstone National Park, Wyoming, USA, January

Chadden Hunter / Cameraman Jeff Turner and Producer Chadden Hunter exposed at -40 degrees in Arctic circle in Northern Canada while filming wolves and bison hunting sequence. Taken on location for BBC Frozen Planet se...

B&S Draker / American Bison / Buffalo (Bison bison) adult in front of old wooden barn and Grand Teton range. Antelope Flats, Grand Teton National Park, Wyoming, USA, June.

Charlie Summers / Cowboy working with herd of American Bison (Bison Bison) during the 2010 buffalo roundup in Custer State Park; South Dakota, September 2010

Charlie Summers / American bison (Bison bison) herd roaming the plains, Custer State Park, South Dakota, USA, September

Ingo Arndt / American Bison (Bison bison) herd stampeding, South Dakota, USA

Ingo Arndt / American Bison (Bison bison) herd stampeding, South Dakota, USA

Ingo Arndt / American Bison (Bison bison) herd grazing on a hillside, South Dakota, USA

Ingo Arndt / American Bison (Bison bison) head portrait of male scenting air, South Dakota, USA

Charlie Summers / American bison (Bison Bison) shaking its head to remove excess water, Yellowstone river, Hayden Valley, Wyoming, USA

Jeff Vanuga / Bison herd grazing in spring with Grand Teton mountains in background, Grand Teton National Park, Wyoming, USA, April 2009

Jeff Vanuga / Bison {Bison bison} cow and calf amongst flowering Arrowroot balsamroot {Balsamorhiza sagittata} with the Teton mountains in the background, Grand Teton National Park, Wyoming, USA, June 2008

Jeff Vanuga / Bison (Bison bison) in deep snow in the Upper Geyser Basin, Yellowstone National Park, Wyoming, USA, January 2008

Jeff Vanuga / Bull Bison (Bison bison) warming himself in the steam from the Firehole River, Upper Geyser Basin, Yellowstone National Park, Wyoming, USA, January

Shattil & Rozinski / Bison {Bison bison} on sand dunes, Medano Zapata Ranch, San Luis Valley, Colorado, USA

Pete Cairns / Bison {Bison bison} bull silhouetted in dawn mist, Yellowstone National Park, USA.

Pete Cairns / Line of Bison {Bison bison} geysers steaming, Yellowstone National Park, USA.

Jeff Vanuga / Bull bison {Bison bison} Grand Teton NP, Wyoming, USA.

Pete Cairns / Bison {Bison bison} in snow blizzard. Yellowstone National Park, Wyoming, USA

Pete Cairns / Landscape with Bison {Bison bison} and steam from geysers, Yellowstone NP, Wyoming, USA

Ingo Arndt / Bison bull {Bison bison} Yellowstone NP, Wyoming, USA

Suzi Eszterhas / American Bison (Bison bison) cow and calf playing, Yellowstone National Park, Montana

Donald M. Jones / American Bison (Bison bison) dust bathing, National Bison Range, Montana

Sebastian Kennerknecht / American Bison (Bison bison) herd grazing in winter, Gardiner, Yellowstone National Park, Montana