Juergen & Christine Sohns / African Wild Dog (Lycaon pictus) group attacking Cape Warthog (Phacochoerus aethiopicus), Tswalu Game Reserve, South Africa

Alex Mustard / School of White collar butterflyfish (Chaetodon collare) pack together above a coral reef. North Male Atoll, Maldives. Indian Ocean

Daniel J. Cox / Gray Wolf (Canis lupus) pack on ridge in winter, Lamar Valley, Yellowstone National Park, Wyoming

Frederique Olivier / Group of Adelie penguins (Pygoscelis adeliae) diving from the edge of the sea ice, Antarctica.

Frederique Olivier / Adelie penguin (Pygoscelis adeliae) colony, with icebergs and pack ice in the background, Antarctica.

Ingo Arndt / Harp Seal (Phoca groenlandicus) pup on ice, Magdalen Islands, Gulf of Saint Lawrence, Quebec, Canada

Tim Fitzharris / Gray Wolf (Canis lupus) pack walking in snow, Montana

Steven Kazlowski / Polar bear (Ursus maritimus) 3-year-olds play fighting on newly formed pack ice, Beaufort Sea, off the 1002 area of the Arctic National Wildlife Refuge, North Slope, Alaska

Steven Kazlowski / Polar bear (Ursus maritimus) sow and cub sliding on their backs over the pack ice, much like ice skating, 1002 coastal plain of the Arctic National Wildlife Refuge, Alaska, USA

Chadden Hunter / Aerial view of hunting pack of Timber wolves (Canis lupus) in Arctic circle in Northern Canada, Taken on location for BBC Frozen Planet series, 2009

Suzi Eszterhas / African Wild Dog (Lycaon pictus) pack attacking Warthog (Phacochoerus africanus), northern Botswana

Suzi Eszterhas / African Wild Dog (Lycaon pictus) pack attacking Warthog (Phacochoerus africanus), northern Botswana