Robert Valentic / Eastern tiger snake (Notechis scutatus) female, Yarra River corridor in suburban Melbourne, Victoria, Australia, in mid-summer. She is in a defensive pre-strike position with hood expanded, accompanie...

Nick Garbutt / Yacare Caiman (Caiman yacare) gaping to regulate its body temperature, with attendant hoverflies, the Paraguay River, Taiama Reserve, western Pantanal, Brazil, South America.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) low angle perspective of open bills, Lake Kerkini, Greece, February. Highly Commended, 2011 Wildlife Photographer of the Year competition.

Juergen Freund / Yellow ribbon eel / Ribbon moray eel (Rhinomuraena quaesita). Lembeh Strait, North Sulawesi, Indonesia.

Juergen Freund / Yellow ribbon eel / Ribbon moray eel (Rhinomuraena quaesita). Lembeh Strait, North Sulawesi, Indonesia.

Juergen Freund / Blue ribbon eel / Ribbon moray eel (Rhinomuraena quaesita). Lembeh Strait, North Sulawesi, Indonesia.

Mark Taylor / Baby Jackdaw (Corvus monedula) gaping to be fed

Colin Varndell / Blue tit brings caterpillar to young in nestbox {Parus caeruleus} UK

Charlie Summers / Arctic grayling (Thymallus arcticus) male gaping at the approach of another male in the background. North Park, Colorado, USA. June.

Nick Garbutt / Gathering of sunbathing Yacare caiman (Caiman yacare) at the edge of a lagoon. Some gaping to regulate its body temperature +thermoregulation). Northern Pantanal, Mato Grosso, Brazil,

Charlie Summers / Arctic grayling (Thymallus arcticus) male gaping which is a sign of aggression, North Park, Colorado, USA.

Loic Poidevin / Eurasian hoopoe (Upupa epops) at nest in stone wall with hungry chicks gaping, Vendee, France, June

Sylvain Cordier / Nest of Great Tit (Parus major) with six chicks gaping begging to be fed, Alsace, France

Aflo / Barn swallow (Hirundo rustica) chicks at nest, gaping mouths begging for food. Kamikawa, Hokkaido, Japan, September.

Nick Upton / Two Herring gull chicks (Larus argentatus) on a rooftop, gaping in hot weather, St.Ives, Cornwall, UK, June.

Nick Upton / Two Herring gull chicks (Larus argentatus) on a rooftop, gaping in hot weather, St.Ives, Cornwall, UK, June.

Nick Upton / Head on view of a male Tenerife lizard / Western Canaries lizard (Gallotia galloti) raising all four feet and gaping after getting hot, Teide National Park, Tenerife, May.

Nick Garbutt / Yacare Caiman (Caiman yacare) basking / gaping to regulate its body temperature at the edge of the Piquiri River, northern Pantanal, Brazil.

Jabruson / Hippopotamus (Hippopotamus amphibius) low-angle view into the gaping mouth of a yawn, Rwindi sector of the Virunga National Park. This was prior to the decimation of the hippo population numbers at th...

Tony Heald / Nile Crocodile (Crocodylus niloticus) on riverbank with gaping mouth, most likely to cool down, Hwange National Park, Zimbabwe October 2012

Yukihiro Fukuda / Masked booby (Sula dactylatra) parent and young with beaks gaping to cool down in harsh sunlight, Christmas Island, Indian Ocean, July

Colin Seddon / Person syringe feeding Chaffinch (fringilla coelebs) nestling in care, UK

Edwin Giesbers / Pygmy hippopotamus (Choeropsis liberiensis) with mouth wide open, captive

David Shale / Gaping mouth of Angler Fish (Chaenophryne draco). Near Cape Verde, Atlantic Ocean.

Fabrice Cahez / Great Cormorant (Phalacrocorax carbo) gaping on water. River Allier, France, December.

Diane McAllister / A Coyote pup (Canis latrans)wwith sleeping siblings. Suburban southwest Reno, Nevada, USA, April

Juergen Freund / Fimbriated moray eel (Gymnothorax fimbriatus) with a cleaner shrimp. Lembeh Strait, North Sulawesi, Indonesia.

Juergen Freund / Rockmover wrasse (Novaculichthys taeniourus) with its mouth open. Misool, Raja Ampat, West Papua, Indonesia.

Juergen Freund / Fimbriated moray eel (Gymnothorax fimbriatus) with a cleaner shrimp. Lembeh Strait, North Sulawesi, Indonesia.

Nick Upton / 2020VISION / Common snipe (Gallinago gallinago) stretching its flexible beak wide open as it stands in marsh with hoar-frosted vegetation and frozen water in the background. Greylake, Somerset Levels, UK, January

Andrew Parkinson / Shag (Phalacrocorax aristotelis) sub-adult head portrait, with gaping beak, Shetland Islands, Scotland, UK, July. (non-ex)

Doug Perrine / Juvenile Morelet's / Mexican crocodile {Crocodylus moreletii} gaping to keep cool while basking on the banks of a cenote (freshwater spring) near Tulum, Yucatan Peninsula, Mexico. Endangered

Chris Newbert / Ribbon Eel (Rhinomuraena quaesita) female gaping in defensive behavior, Indonesia

GEORGETTE DOUWMA / Darkspotted / fimbriated moray (Gymnothorax fimbriatus) with gaping mouth. Banda Island, Moluccas, Indonesia

Michael Hutchinson / Dipper (Cinclus cinclus), yawning / gaping with nictitating membrane over eye, Derbyshire, UK

Chris O'Reilly / Shag {Phalacrocorax aristotelis} keeping cool by panting / gaping its bill while on nest, Farne Island, Northumbria, UK

Bengt Lundberg / Great tit taking butterfly prey to 6-day-old chicks inside nest box {Parus major} Sweden

Yva Momatiuk & John Eastcott / Atlantic Puffin (Fratercula arctica) gaping, a behavior used to defend their personal space, summer breeding season, Newfoundland, Canada

Flip Nicklin / Needlefish (Ablennes hians) on water surface, Bonaire, Netherland Antilles

Bengt Lundberg / Great tit pair inside nest box with 3-day-old chicks {Parus major} Sweden

GEORGETTE DOUWMA / Coral hind with mouth wide open {Cephalopholis miniata} Andaman sea, Thailand

Paul D Stewart / Red Fox (Vulpes vulpes) portrait detail from a woodcut with old colouring. Conrad Gesner 'Icones Animalium' publ. Christof Froschover, Zurich, 1560. Gesner's artist catches the perceived intelligent a...

Paul D Stewart / Red Fox (Vulpes vulpes) portrait detail from a woodcut with old colouring. Conrad Gesner 'Icones Animalium' publ. Christof Froschover, Zurich, 1560. Gesner's artist catches the perceived intelligent a...

Martin Willis / Tawny Frogmouth (Podargus strigoides) chick with gaping mouth, Townsville, Queensland, Australia

Robin Chittenden / Juvenile Laughing gull (Leucophaeus atricilla) gaping, first winter, New Jersey, USA, September

Nick Upton / White stork (Ciconia ciconia) with three chicks in nest at top of an Oak tree, gaping in hot sunshine with one exercising its wings, Knepp Estate, Sussex, UK, June.

Nick Upton / Three White stork (Ciconia ciconia) chicks in nest at top of an Oak tree, preening and gaping in hot sunshine, Knepp Estate, Sussex, UK, June.

Dave Watts / Gaping leek orchid (Prasophyllum incorrectum). Tasmania, Australia. October.

Dave Watts / Gaping leek orchid (Prasophyllum incorrectum). Tasmania, Australia. October.

Hermann Brehm / Red-backed shrike (Lanius collurio), female with chicks gaping, begging for food, Germany

Robert Valentic / Blotched blue-tongue skink (Tiliqua nigrolutea) female gaping with tongue out, in defense against perceived threat. Lake Tantangara, Snowy Mountains, New South Wales, Australia.

Robert Valentic / Speckled brownsnake (Pseudonaja guttata) female gaping and flicking tongue in defense, orange phase on cracked earth. Blacksoil Plains, Queensland, Australia.

Nick Garbutt / Yacare Caiman (Caiman yacare) at the edge of the Piquiri River, northern Pantanal, Brazil. Gaping to regulate its body temperature (thermoregulation).

Nick Garbutt / Komodo dragon (Varanus komodoensis) male gaping to regulate temperature. Rinca Island, Komodo National Park, Indonesia. Endangered.

Nick Garbutt / Komodo dragon (Varanus komodoensis) male gaping to regulate temperature. Rinca Island, Komodo National Park, Indonesia. Endangered.