Gerrit Vyn / Dall sheep (Ovis dalli) rams head butting, one behaviour by which males establish dominance during the Autumn mating season. Denali National Park, Alaska, USA

Donald M. Jones / Bighorn Sheep (Ovis canadensis) rams butting heads, western Montana

Sumio Harada / Bighorn Sheep (Ovis canadensis) males butting heads to compete for females, Rocky Mountains, North America

Sumio Harada / Bighorn Sheep (Ovis canadensis) males butting heads while a third looks on, Rocky Mountains, North America

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

Michio Hoshino / Muskox (Ovibos moschatus) pair butting heads, Nunivak Island, Alaska

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

Ernie Janes / Kerry Hill domestic sheep, two spring lambs head butting, England, UK.

Nick Upton / Two polled Wiltshire horn sheep (Wiltipoll breed) ewes standing and grazing before butting one another, Corsham Court, Wiltshire, England, UK, April.

Nick Upton / Two Polled Wiltshire horn sheep (Wiltipoll breed) ewes butting one another, Corsham Court, Wiltshire, England, UK, April.

SCOTLAND: The Big Picture / Black grouse (Tetrao tetrix) males fighting at lek, Cairngorms National Park, Scotland, UK, April 2015.

Lynn M Stone / Alpine Goats (a dairy breed) nanny goats butting heads in pasture, Poplar Grove, Illinois, USA