Two bison (Bison bison) bulls facing off, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull and cow, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull in a spring snowstorm, Yellowstone National Park, UNESCO World Heritage Site, Wyoming, United States of America, North America

Bison (Bison bison) bull in the spring, Yellowstone National Park, UNESCO World Heritage Site, Wyoming, United States of America, North America

Bison (Bison bison) bull covered with frost in the winter, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull covered with snow in snowfall in the winter, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull covered with snow in the winter, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull in the winter, Yellowstone National Park, UNESCO World Heritage Site, Wyoming, United States of America, North America

Bison (Bison bison) bull, Yellowstone National Park, Wyoming, United States of America, North America

Two Bison (Bison bison) bulls sparring, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull in the snow, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull in the snow, Yellowstone National Park, Wyoming, United States of America, North America

Bison (Bison bison) bull among yellow wildflowers, Yellowstone National Park, Wyoming, United States of America, North America

Adult bull American Bison buffalo ( Bos bison ) with Brown-headed Cowbird ( Molothrus ater ) on back in prairie landscape at Riding Mountain National Park Manitoba Canada

American Bison (Bison bison), bull crossing a road in front of a car, Yellowstone National Park, Wyoming, USA

Bison (Bison bison), bull, Yellowstone National Park, Wyoming, USA, North America

Bison (Bison bison), bull, Yellowstone National Park, Wyoming, USA, North America

American Buffalo (Bison bison), portrait of a bull in heat, Yellowstone National Park, Wyoming, USA

A herd of bison moving along the Yellowstone National Park, Wyoming, USA.

American Bison bull (Bison bison) walking through winter snow in the Lamar Valley, Yellowstone National Park; Wyoming, United States of America

American Bison bull (Bison bison) on a snowy day in the North Fork of the Shoshone River valley near Yellowstone National Park; Wyoming, United States of America

Falling snow clings to an American Bison bull (Bison bison) standing in a meadow in the North Fork of the Shoshone River valley near Yellowstone National Park; Wyoming, United States of America

A large Wood bison bull (Bison bison athabascae) in snow, captive in Alaska Wildlife Conservation Center, Portage, Alaska, United States of America

Pair of American Bison bulls (Bison bison) walking through the 'Bobby Socks' trees in the Firehole River Valley of Yellowstone National Park; Wyoming, United States of America

Wood bison bull (Bison bison athabascae) portrait, Alaska Wildlife Conservation Center in South-central Alaska. The Alaska Wildlife Conservation Center played a big part in helping reintroduce these animals back into the Alaska wilds; Portage, Alaska, United States of America

American Bison bull (Bison bison) plowing through deep snow in the Firehole River Valley, Yellowstone National Park; Wyoming, United States of America