Doug Allan / Emperor penguin males incubating eggs huddle for warmth at -40 degrees centigrade, winter, Antarctica. Green is excrement on snow. {Aptenodytes forsteri}

Doug Allan / Emperor penguin {Aptenodytes forsteri} males huddle for warmth in minus 40 degrees centigrade while incubating eggs in winter, Antarctica Winter extreme cold survival birds seabirds nesting-behaviour

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) chick aged six to eight weeks sheltering on parent's feet, portrait. Atka Bay, Antarctica. September.

Jussi Murtosaari / Blackcap (Sylvia atricapilla) female and two males huddling together for warmth while sleeping, Finland, May.

Ingo Arndt / Monarch butterflies (Danaus plexippus) flying in warmth of midday sun, overwintering colony in Michoacan, Mexico

Thomas Marent / Gelada Baboon (Theropithecus gelada) male and females huddling for warmth, Simien Mountains National Park, Ethiopia

Thomas Marent / Gelada Baboon (Theropithecus gelada) group huddling for warmth, Simien Mountains National Park, Ethiopia

Ole Jorgen Liodden / Man resting by tree with small fire, with Northern Lights in the sky above, Valdres, Norway, January 2005.

Stefan Christmann / Emperor Penguin (Aptenodytes forsteri) huddling with chicks for warmth, Queen Maud Land, Antarctica

Stefan Christmann / Emperor Penguin (Aptenodytes forsteri) colony huddling for warmth, Queen Maud Land, Antarctica

Stefan Christmann / Emperor Penguin (Aptenodytes forsteri) colony huddling for warmth, Queen Maud Land, Antarctica

Thomas Marent / Japanese Macaque (Macaca fuscata) mother with young in tree, huddling for warmth, Jigokudani, Nagano, Japan