Juan Carlos Munoz / Horseman herding spanish fighting bulls (Toros bravos) Sevilla, Andalucía, Spain, March 2008

Juan Carlos Munoz / Herd of Spanish fighting bulls (Toros bravos) running, Sevilla, Andalucía, Spain, March 2008

Juan Carlos Munoz / Herd of Spanish fighting bulls (Toros bravos) running, Sevilla, Andalucía, Spain, March 2008

Ingo Arndt / Cape Fur Seal (Arctocephalus pusillus) bulls fighting, Cape Cross, Namibia

Wild Wonders of Europe / Geslin / Grey seal (Halichoerus grypus) bulls fighting, covered in mud, Donna Nook, Lincolnshire, UK, November 2008

Wild Wonders of Europe / Geslin / Grey seal (Halichoerus grypus) bulls calling surrounded by resting mothers and pups, Donna Nook, Lincolnshire, UK, November 2008

Tony Heald / Two African elephant (Loxodonta africana) bulls sparring gently, Etosha National Park, Namibia, January

Donald M. Jones / American Elk (Cervus elaphus nelsoni) bulls sparring in winter, western Montana

Donald M. Jones / American Elk (Cervus elaphus nelsoni) bulls fighting, western Alberta, Canada

Donald M. Jones / American Elk (Cervus elaphus nelsoni) calf rubbing mature bulls' rub, western Montana

Andy Rouse / African elephant (Loxodonta africana) two bulls walking to waterhole, Savute, Botswana (non-ex)

Andy Rouse / African elephant (Loxodonta africana) two bulls at mud wallow, Savute, Botswana (non-ex)

Andy Rouse / African elephant (Loxodonta africana) herd of bulls drinking at waterhole, Savute, Botswana (non-ex)

Andy Rouse / Silhouette of two African elephant (Loxodonta africana) bulls drinking at sunset, Savute, Botswana (non-ex)

Andy Rouse / African elephant (Loxodonta africana) two bulls walking to water, Savute, Botswana (non-ex)

Andy Rouse / African elephant (Loxodonta africana) two bulls, Savute, Botswana (non-ex)

Andy Rouse / African elephant (Loxodonta africana) bulls walking to water, front one showing aggression, Savute, Botswana (non-ex)

Richard Du Toit / African Elephant (Loxodonta africana) bulls at sunset, Chobe National Park, Botswana

Richard Du Toit / African Elephant (Loxodonta africana) bulls fighting, Chobe National Park, Botswana

Suzi Eszterhas / Zebu cattle {Bos indicus} bulls fighting, at Ol Pejata Conservancy, Northern Kenya

Suzi Eszterhas / Northern Elephant Seal (Mirounga angustirostris) bulls fighting, San Benito Island, Baja California, Mexico

Matthias Breiter / Northern Fur Seal (Callorhinus ursinus) bulls displaying, Pribilof Islands, Alaska

Matthias Breiter / Northern Fur Seal (Callorhinus ursinus) bulls displaying, Pribilof Islands, Alaska

Pete Oxford / South American Sea Lion (Otaria flavescens) bulls fighting, Kidney Island, Falkland Islands

Colin Monteath / Southern Elephant Seal (Mirounga leonina) bulls, Macquarie Island, Subantarctic, Australia

Yva Momatiuk & John Eastcott / Alaska Moose (Alces alces gigas) bulls sparring in tundra during fall rut, Denali National Park, Alaska

Yva Momatiuk & John Eastcott / Alaska Moose (Alces alces gigas) bulls sparring in tundra during fall rut, Denali National Park, Alaska

Yva Momatiuk & John Eastcott / Alaska Moose (Alces alces gigas) bulls on fall tundra, Denali National Park, Alaska

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Antarctic Fur Seal (Arctocephalus gazella) bulls with one blond morph sitting on gravel beach, Antarctic Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for access to females, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bull roaring to drive away other bulls on beach during breeding season, Gold Harbour, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bull roaring to drive away other bulls on beach, Gold Harbor, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting for females on beach during breeding season in spring, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fighting in surf during breeding season, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls fight for access to females during breeding season, St. Andrews Bay, South Georgia Island

Yva Momatiuk & John Eastcott / Southern Elephant Seal (Mirounga leonina) bulls challenge each other on beach during breeding season, St. Andrews Bay; South Georgia Island

Ingo Arndt / Pacific Walrus (Odobenus rosmarus divergens) bulls resting, Bristol Bay, Round Island, Alaska

Ingo Arndt / Pacific Walrus (Odobenus rosmarus divergens) bulls resting, Bristol Bay, Round Island, Alaska

Ingo Arndt / Pacific Walrus (Odobenus rosmarus divergens) bulls, Bristol Bay, Round Island, Alaska

Suzi Eszterhas / African Elephant (Loxodonta africana) bulls fighting, Masai Mara, Kenya

Suzi Eszterhas / African Elephant (Loxodonta africana) bulls fighting, Masai Mara, Kenya

Suzi Eszterhas / African Elephant (Loxodonta africana) bulls fighting, Masai Mara, Kenya

Richard Du Toit / African Elephant (Loxodonta africana) bulls, Chobe National Park, Botswana

Richard Du Toit / African Elephant (Loxodonta africana) bulls, Chobe National Park, Botswana

Richard Du Toit / African Elephant (Loxodonta africana) bulls walking along Chobe River, Chobe National Park, Botswana

Matthias Breiter / Pacific Walrus (Odobenus rosmarus divergens) bulls silhouetted against setting sun, Round Island, Alaska

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

Konrad Wothe / Elk (Cervus elaphus) bulls fighting in the snow, Yellowstone National Park, Wyoming

Anup Shah / Wildebeest {Connochaetes taurinus} bulls fighting in rutting season, Serengeti NP, Tanzania

Tim Fitzharris / Northern Elephant Seal (Mirounga angustirostris) bulls fighting in the surf for dominance, California

Tim Fitzharris / Northern Elephant Seal (Mirounga angustirostris) two bulls fighting in the shallows, California

Tim Fitzharris / Northern Elephant Seal (Mirounga angustirostris) two bulls fighting in the surf, California

Tim Fitzharris / Northern Elephant Seal (Mirounga angustirostris) two bulls fighting in the surf, California

Pete Oxford / Lions (Panthera leo) feeding on a sable carcass caught at a waterhole - buffalo bulls approaching to show their dislike at having the pride at the waterhole. Makalolo Plains, Hwange National Park, Z...

Nick Turner / Bull in field warning sign on a public footpath in the countryside, Gloucestershire, England

Phil Savoie / Acacia ant {Pseudomyrmex sp} on Bulls horn acacia. Belize

Richard Du Toit / Cape Buffalo (Syncerus caffer) bulls sparring, winter, Malamala Game Reserve, South Africa

Richard Du Toit / Hippopotamus (Hippopotamus amphibius) two bulls fighting, Moremi Wildlife Reserve, Botswana

Yva Momatiuk & John Eastcott / Alaska Moose (Alces alces gigas) two young bulls standing in colorful autumn tundra, Denali National Park, Alaska

Pete Oxford / Bull fighting during the Quito festival, Ecuador

Pete Oxford / Bull fight Quito Festival, 6th December, Ecuador

Steven David Miller / Egg mass of Apple / Bulls-eye snail (Pomacea calignosa) laid on the underside of aquatic leaf, Florida, USA

George McCarthy / Bull Elk (Cervus canadensis) with antlers in rutting season, Yellowstone National Park, Montana, USA

George McCarthy / Bull Elk (Cervus canadensis) roaring during rut, Yellowstone National Park, Montana, USA

George McCarthy / Male Elk (Cervus canadensis) with large antlers during the autumn rut, Yellowstone National Park, Montana, USA

George McCarthy / Bull Elk (Cervus canadensis) at dawn, Yellowstone National Park, Montana, USA

George McCarthy / Bull Elk (Cervus canadensis) crossing Madison River at dawn, Yellowstone National Park, Montana, USA

Jean E. Roche / Three Aubrac cattle (Bos taurus) in grazed mountaine landscape. Bull in background, Cézallier, Cantal, France

Jean E. Roche / Aubrac bull in grazed mountain upland pasture landscape, Cézallier, Cantal, France

Jean E. Roche / Aubrac bull in grazed mountain upland landscape, Cézallier, Cantal, France

Jean E. Roche / Domestic cattle, Salers breed (brown) and Charollais breed (white bull) in pasture, Cantal, France

Jean E. Roche / Herding bulls during "Abrivado", the traditional way of purchasing cattle in the Camargue, France 2006

Jean E. Roche / Herding bull during "Abrivado", the traditional way of purchasing cattle in the Camargue, France 2006

Jean E. Roche / Herding bull during "Abrivado", the traditional way of purchasing cattle in the Camargue, France 2006

Ingo Arndt / Northern Elephant Seal (Mirounga angustirostris) two fighting bulls, Ano Nuevo State Reserve, California

Ingo Arndt / Northern Elephant Seal (Mirounga angustirostris) two fighting bulls, Ano Nuevo State Reserve, California

Ingo Arndt / American Bison (Bison bison) bulls fighting, Yellowstone National Park, Wyoming

Ingo Arndt / American Bison (Bison bison) bulls fighting, Yellowstone National Park, Wyoming

Suzi Eszterhas / Northern Elephant Seal (Mirounga angustirostris) bulls sparring, Ano Nuevo, California

Mark Raycroft / Moose (Alces alces shirasi) two large bulls fighting

Mark Raycroft / Moose (Alces alces shirasi) two large bulls fighting

Tui De Roy / Galapagos Sea Lion (Zalophus wollebaeki) juvenile bulls greeting, Santa Fe Island, Galapagos Islands, Ecuador

Tony Heald / African elephant {Loxodonta africana} bulls fighting, Etosha national park, Namibia.

Tony Heald / African elephant {Loxodonta africana} bulls fighting, Etosha national park, Namibia.

Tony Heald / African elephant {Loxodonta africana} bulls fighting at waterhole, zebra in background, Etosha national park, Namibia.

Tony Heald / African elephant {Loxodonta africana} bulls fighting at waterhole, zebra in background, Etosha national park, Namibia.

Tony Heald / African elephant {Loxodonta africana} bulls fighting at waterhole, zebra in background Etosha national park, Namibia.

Sharon Heald / African elephant {Loxodonta africana} bulls fighting in waterhole, Etosha national park, Namibia.