Hiroya Minakuchi / Minden / Humpback Whale (Megaptera novaeangliae) trio gulp feeding, Inside Passage, southeast Alaska

Klein & Hubert / Nile crocodile (Crocodylus niloticus) close up of jaw and teeth, Chobe River, Botswana, Africa.

Andy Murch / Lemon shark (Negaprion brevirostris) surfacing with open mouth. Tiger Beach, Little Bahama Bank, Bahamas.

Andy Murch / Great hammerhead shark (Sphyrna mokarran) mouth wide open, feeding in shallow water, surrounded by fish. South Bimini Island, Bahamas.

Shane Gross / Cunner (Tautogolabrus adspersus) pair in Eelgrass (Zostera marina) bed. Newfoundland, Canada. September.

Pal Hermansen / Oystercatcher (Haematopus ostralegus) landing on coast in midnight sun. Lofoten, Norway. July.

Brandon Cole / Amazon river dolphin (Inia geoffrensis) split level view showing playful dolphin with mouth open. Rio Negro, Brazil.

Danny Green / Polar bear (Ursus maritimus) male walking across frozen landscape. Svalbard, Norway, April 2019.

Danny Green / Polar bear (Ursus maritimus) male in snow. Svalbard, Norway, April.

Danny Green / RF - Polar bear (Ursus maritimus) male walking through snow covered landscape, mountains in background. Svalbard, Norway, April (This image may be licensed either as rights managed or royalty free.)

Andres M. Dominguez / Hawfinch (Coccothraustes coccothraustes) perched on branch amongst Hawthorn (Crataegus sp) berries. Sierra de Grazalema Natural Park, Cadiz, Spain. February.

Jordi Chias / Leopard seal (Hydrurga leptonyx) with mouth open, Antarctic Peninsula, Antarctica.

Shane Gross / American crocodile (Crocodylus acutus) with open mouth amongst Seagrass (Alismatales). Gardens of the Queen National Park, Cuba.

Franco Banfi / Striped marlin (Tetrapturus audax) feeding on Sardine (Sardinops sagax). Magdalena Bay, Baja California Sur, Pacific Ocean, Mexico.

Eric Baccega / Chimpanzee juvenile (Pan troglodytes schweinfurthii) in a tree. Kibale National Park, Uganda. January.

Tony Wu / View down the throat of a male Zoarchias major eelpout (Zoarchias major) that has opened its mouth during a confrontation with another male. Yamaguchi Prefecture, Japan. Pacific Ocean.

Tony Wu / Two male Zoarchias major eelpouts (Zoarchias major) engaged in a mouth-to-mouth confrontation. Yamaguchi Prefecture, Japan, Pacific Ocean.

Tony Wu / Zoarchias major eelpout (Zoarchias major) with her mouth wide open. Yamaguchi Prefecture, Japan, Pacific Ocean.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) male feeding chick aged a few days. Atka Bay, Antarctica. August. Sequence 1/2.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) male feeding chick aged a few days. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) chick begging for food, on parent's feet. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two chicks aged six to eight weeks brooding on parents' feet, one vocalising. Atka Bay, Antarctica. September.

Mark Carwardine / Aerial view of Fin whale (Balaenoptera physalus) lunge-feeding, with mouth open and throat pouch distended, southern Sea of Cortez (Gulf of California), Baja California, Mexico.

Paul Hobson / Parrot snake (Leptophis ahaetulla) in aggressive pose with mouth open. Costa Rica.

Paul Hobson / Keel-billed toucan (Ramphastos sulfuratus) perched on branch with beak open. Laguna del Lagarto, Alajuela, Costa Rica.

Paul Hobson / Red fox (Vulpes vulpes), two cubs play fighting. Sheffield, England, UK. May.

Paul Hobson / Wood warbler (Phylloscopus sibilatrix) singing whilst perched on branch, silhouetted at dawn. Sheffield, England, UK. May.

Pal Hermansen / Brown bear (Ursus arctos), two showing aggression at night. Finland. August.

Tui De Roy / Nazca booby (Sula granti) in flight at dusk. Punta Suarez, Espanola Island, Galapagos.

Suzi Eszterhas / Giant panda (Ailuropoda melanoleuca) sitting with Bamboo in background. Shenshuping Panda Base, Wolong Nature Reserve, Wenchuan, Sichuan Province, China. Captive.

David Fleetham / Great white shark (Carcharodon carcharias), two below surface. Guadalupe Island, Mexico.

David Fleetham / Great white shark (Carcharodon carcharias) breaking surface with mouth open. Guadalupe Island, Mexico.

David Fleetham / Clark's anemonefish (Amphiprion clarkii), pair tending to egg mass placed beside the protection of an anemone. Yap, Micronesia.

David Fleetham / Smooth trunkfish (Lactophrys triqueter), portrait. Bonaire, Dutch Caribbean.

David Fleetham / Great barracuda (Sphyraena barracuda) with three parasitic Copepods at end of upper jaw. Hawaii.

Loic Poidevin / Golden jackal (Canis aureus) portrait. Danube Delta, Romania, May.

Tony Heald / Hippopotamus (Hippopotamus amphibius) splashing in pool in evening light. Mana Pools National Park, Zimbabwe.

Steven David Miller / Emu (Dromaius novaehollandiae) juvenile with beak open to catch a leaf, Cleland Wildlife Park, South Australia, March. Captive.

Guy Edwardes / Hippopotamus (Hippopotamus amphibius) in Chobe River, Botswana.

Guy Edwardes / Little bee-eater (Merops pusillus) vocalising whilst perched on branch, Khwai, Botswana.

Alex Mustard / Striped mackerel (Rastrelliger kanagurta) mouth wide open as it swims through the water, filtering zoopankton with its gill rakers. Marsa Shouna, Port Ghalib, Marsa Alam, Egypt. Red Sea

Mark Carwardine / Grey whale (Eschrichtius robustus) at water surface with mouth open showing baleen plates, San Ignacio Lagoon, Baja California, Mexico

Claudio Contreras / American crocodile (Crocodylus acutus) extreme close up with jaws open, Banco Chinchorro Biosphere Reserve, Caribbean region, Mexico

Ernie Janes / RF- Lesser black-backed gulls (Larus fuscus) calling from shoreline, beaks open, England, UK, May. (This image may be licensed either as rights managed or royalty free.)

Neil Aldridge / Vervet monkey (Chlorocebus pygerythrus) yawning while sitting on a fallen tree in forests, Limpopo River, Kruger National Park, South Africa.

Neil Aldridge / Lioness (Panthera leo) yawning in early morning light on Kariega Game Reserve, South Africa.

Ingo Arndt / American Alligator (Alligator mississippiensis) split level, Everglades, USA, January.