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

Daniel J. Cox / Gray Wolf (Canis lupus) predating female Elk (Cervus elaphus) crossing river, Alum Creek, Yellowstone National Park, Wyoming, sequence 4 of 4

Daniel J. Cox / Gray Wolf (Canis lupus) killing female Elk (Cervus elaphus) crossing river, Alum Creek, Yellowstone National Park, Wyoming, sequence 3 of 4

Daniel J. Cox / Gray Wolf (Canis lupus) attacking female Elk (Cervus elaphus) crossing river, Alum Creek, Yellowstone National Park, Wyoming, sequence 2 of 4

Daniel J. Cox / Gray Wolf (Canis lupus) pair predating female Elk (Cervus elaphus) crossing river, Alum Creek, Yellowstone National Park, Wyoming, sequence 1 of 4

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

Nick Garbutt / North American bobcat (Lynx rufus) striding through deep snow. Madison River Valley, Yellowstone National Park, Wyoming, USA. January

George Sanker / Golden-mantled ground squirrel (Callospermophilus lateralis) feeding on flowers, Yellowstone National Park, Wyoming, USA, June.

Kirkendall-Spring / Lower Yellowstone Falls on the Yellowstone River viewed from Artist Point, Yellowstone National Park, Wyoming, USA, June 2013.

Kirkendall-Spring / Lower Yellowstone Falls with rainbow, on the Yellowstone River viewed from Lookout Point Trail, Yellowstone National Park, Wyoming, USA, June 2013.

Kirkendall-Spring / Base of Lower Yellowstone Falls on the Yellowstone River in Yellowstone National Park, Wyoming, USA, July 2013.

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