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

Jeff Vanuga / Aerial view of Grand Prismatic Springs Midway Geyser Basin. Yellowstone National Park, Wyoming, July 2011.

Floris van Breugel / A skier glides past an erupting geyser in the Old Faithful geyser basin in Yellowstone National Park. The cold temperatures in the winter cause the geysers to release more steam than usual making the...

Mark Taylor / Forest fire at night with silhouetted trees, Yellowstone National Park, Wyoming, USA

Paul D Stewart / Aerial view of Grand Prismatic thermal spring in Yellowstone National Park, USA. The bright colours are natural, coming from thermophile bacteria in the water. People can be seen walking on the raised...

George Sanker / Black bear (Ursus americanus) young cub portrait, Yellowstone National Park, Wyoming, USA

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

George Sanker / Black Bear (Ursus americanus) cub balancing on branch in tree, Yellowstone NP, Wyoming, USA

George Sanker / Coyote (Canis latrans) standing on rocks at water's edge, sniffing, Yellowstone NP, Wyoming, USA, October

George Sanker / Grizzly bear (Ursus arctos horribilis) walking through long grass, Yellowstone NP, Wyoming, USA, October

George Sanker / Grizzly bear (Ursus arctos horribilis) walking, Yellowstone National Park, Wyoming, USA, October

George Sanker / Long-tailed weasel (Mustela frenata) standing on hind legs, Yellowstone National Park, Wyoming, USA, June

George Sanker / Two Least Chipmunks (Tamias minimus) sitting on tree stump feeding, Yellowstone National Park, Wyoming, USA, October, crop of 1373882

George Sanker / Newborn Elk (Cervus canadensis) calf looks up to its mother, Yellowstone National Park, USA, June

George Sanker / Coyote (Canis latrans) climbing over rocks carrying pup in mouth, Yellowstone National Park, Wyoming, USA, June

George Sanker / Black bear (ursus americanus) cub standing on hind legs, Yellowstone National Park, USA, June

George Sanker / Baby Black Bear (Ursus americanus) up a tree, Yellowstone National Park, USA, May

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

George Sanker / Portrait of Grey Wolf (Canis lupus). Yellowstone National Park, Wyoming, USA, October.

George Sanker / Portrait of Golden Mantled Ground Squirrel (Spermophilus lateralis) with a blade of grass in its mouth. Yellowstone National Park, Wyoming, USA, May.

George Sanker / Portrait of a Grizzly Bear (Ursus arctos horribilis) against rock. Yellowstone National Park, Wyoming, USA, May.

George Sanker / Lesser Scaup (Aythya affinis) female walking across ice. Yellowstone National Park, Wyoming, USA, June.

George Sanker / Yellow-bellied Marmots (Marmota flaviventris) mating. Yellowstone National Park, Wyoming, USA, June.

Jeff Vanuga / Thermal hot spring at Mud Volcano, Hayden Valley, Yellowstone National Park, Wyoming, USA.

Jeff Vanuga / Silhouette of Gray Wolf (Canis lupus) standing on a rock by the Yellowstone River. Yellowstone National Park, USA,

Jeff Vanuga / Grand Canyon of the Yellowstone and the Yellowstone River. Yellowstone National Park, Wyoming, USA, August 2009.

Diane McAllister / Golden manteled ground squirrel (Spermophilus lateralis) holding its tail. Yellowstone National Park, Wyoming, USA, May

Charlie Summers / Elk / Wapiti (Cervus canadensis) stag Bull standing on snowy ridge, Yellowstone National Park, Wyoming, USA

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

Patricio Robles Gil / Coyote (Canis latrans) walking over snow, Yellowstone NP, Wyoming, USA, October

George Sanker / Black bear (Ursus americanus) cub, cinnamon phase, climbing fir tree, Yellowstone National Park, Wyoming, USA, June

George Sanker / Black bear (Ursus americanus) cubs, male and female, one black the other brown, playing roughly, Yellowstone National Park, Wyoming, USA, May

George Sanker / Male North American red squirrel (Tamiasciurus hudsonicus) alarm calling, Yellowstone National Park, Wyoming, USA, June

George Sanker / Black bear (Ursus americanus) cub, brown phase, looking out from behind tree trunk, Yellowstone National Park, Wyoming, USA, May

George Sanker / Young Black bear (Ursus americanus) cub climbing mature fir tree, Yellowstone National Park, Wyoming, USA, May

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

Jeff Vanuga / Grizzly bear {Ursus arctos horribilis} beside water, long exposure image, Yellowstone National Park, Wyoming, USA, October

Jeff Vanuga / Dead Thermal Spider encased in thermophilic bacteria in hot Spring, Yellowstone National Park, Wyoming, USA, October

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

Jeff Vanuga / Silhouette of male Elk {Cervus canadensis} at dawn, crossing meadow in Yellowstone National Park, Wyoming, USA, September

Jeff Vanuga / Boiling mud in Pocket Basin, Yellowstone National Park, Wyoming, USA, June 2006

Jeff Vanuga / Female Elk {Cervus canadensis} amongst wildflowers in spring, Mammoth area of Yellowstone National Park, Wyoming, USA, June

Jeff Vanuga / Two male Elk {Cervus canadensis} in velvet, resting in foggy meadow at dawn, Yellowstone National Park, Wyoming, USA, July

Jeff Vanuga / Lone Star Geyser erupting in Lone Star Geysyer Basin at night, Yellowstone National Park, Wyoming, USA, October 2007

Yva Momatiuk & John Eastcott / Great Fountain Geyser erupting, Yellowstone National Park, Wyoming

Yva Momatiuk & John Eastcott / Saplings amidst coniferous trees burned by forest fire, Yellowstone National Park, Wyoming

Yva Momatiuk & John Eastcott / Sapling amid coniferous trees burned by forest fire, Yellowstone National Park, Wyoming

Jeff Vanuga / Blue Funnel Spring, West Thumb Geyser Basin, Yellowstone Nationa Park, Wyoming, USA

Pete Oxford / Dead trees in Mammoth Hot Springs travertine terraces, Yellowstone National Park, Wyoming

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

Jack Dykinga / Morning Glory pool with Lodgepole pines {Pinus contorta} in background, Yellowstone NP, Wyoming, USA

David Welling / Bull Moose {Alces alces} walking at waters edge. Yellowstone National Park, USA.

Sue Flood / Morning Glory Pool, Yellowstone National Park, Wyoming, 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.

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

Shin Yoshino / American Bison (Bison bison) silhouetted at dawn, Yellowstone National Park, Wyoming

Tim Fitzharris / Absaroka Range reflected in lake, Yellowstone National Park, Wyoming

Pete Cairns / Morning glory pool, Old Faithful geyser, Yellowstone NP, Wyoming, USA

Thomas Mangelsen / Sunset over Antler Creek, Yellowstone National Park, Wyoming

Thomas Mangelsen / Clouds over Yellowstone River, Hayden Valley, Yellowstone National Park, Wyoming

Tim Fitzharris / Yellowstone Falls and Grand Canyon of Yellowstone National Park, Wyoming

Sumio Harada / Old Faithful Geyser spouting, Yellowstone National Park, Wyoming

Thomas Mangelsen / Elk (Cervus elaphus) in misty waters, Yellowstone National Park, Wyoming

Tim Fitzharris / Castle Geyser, Yellowstone National Park, Wyoming

Tim Fitzharris / Winter in Yellowstone National Park, Wyoming

Thomas Mangelsen / Hayden Valley in late winter, Yellowstone National Park, Wyoming

Thomas Mangelsen / Coyote (Canis latrans) standing in grass along stream, Yellowstone National Park, Wyoming

Tim Fitzharris / Tourists viewing Grand Prismatic Pool, Midway Geyser Basin, Yellowstone National Park, Wyoming

Sumio Harada / American Bison (Bison bison) free roaming wild herd along river, Yellowstone National Park, North America

Tim Fitzharris / American Bison (Bison bison) in winter, Yellowstone National Park, Wyoming

Tim Fitzharris / American Bison (Bison bison) herd grazing near Fountain Paint Pots, Yellowstone National Park, Wyoming

Konrad Wothe / Elk (Cervus elaphus) two males fighting, Yellowstone National Park, Wyoming

Jim Brandenburg / Aerial view of Grand Prismatic Pool, Yellowstone National Park, Wyoming

Michael Quinton / Common Raven (Corvus corax) checking out tourist, Yellowstone National Park, Wyoming

Michael Quinton / Common Raven (Corvus corax) using its beak to open a snowmobile's storage compartment in search of food, Yellowstone National Park, Wyoming

Jim Brandenburg / American Bison (Bison bison) and snowmobiles, Yellowstone National Park, Wyoming

Jim Brandenburg / Morning Glory pool in Upper Geyser Basin, Yellowstone National Park, Wyoming

Shin Yoshino / Moose (Alces alces shirasi) standing along a misty riverbank, Yellowstone National Park, Wyoming

Shin Yoshino / Early morning on the Yellowstone River, Yellowstone National Park, Wyoming

Michael Quinton / Elk (Cervus elaphus) bull bugling during rut in the fall, Yellowstone National Park, Wyoming

Michael Quinton / Yellowstone fire, burning forest, Yellowstone National Park, Wyoming

Michael Quinton / Yellowstone fire, sprouting wildflower near burnt Lodgepole pine cone, Yellowstone National Park, Wyoming

Michael Quinton / Elk (Cervus elaphus) two males sparring in the fall, Yellowstone National Park, Wyoming

Michael Quinton / Common Porcupine (Erethizon dorsatum) sitting in a tree in winter, Yellowstone National Park, Wyoming

Michael Quinton / North American River Otter (Lontra canadensis) on iced over river, Yellowstone National Park, Wyoming

Michael Quinton / Steaming pool in winter, Mammoth Hot Springs, Yellowstone National Park, Wyoming

Michael Quinton / Snowshoe Hare (Lepus americanus) in snowfall, Yellowstone National Park, Wyoming

Shin Yoshino / American Bison (Bison bison) herd drinking from lake, Yellowstone National Park, Wyoming

Shin Yoshino / American Bison (Bison bison) two males fighting, Yellowstone National Park, Wyoming

Shin Yoshino / Coyote (Canis latrans) portrait, Yellowstone National Park, Wyoming