Solvin Zankl / Fire Ant (Solenopsis geminata) head of a major worker with mandibles used for opening seeds. Specimen photographed using digital focus stacking

Solvin Zankl / Just three hours after they hatch from the water and moult into mature adults, the male Long-tailed / Tisza mayfly (Palingenia longicauda) perish. Adult mayflies have no mouth opening and no guts. The...

Mitsuaki Iwago / African Lion (Panthera leo) female roaring, Serengeti National Park, Tanzania

Mark Raycroft / Brittany Spaniel (Canis familiaris) with training bumper

Mark Moffett / Tokay Gecko (Gecko gecko) showing underside of feet

Mitsuaki Iwago / Spotted Hyena (Crocuta crocuta) laughing, Serengeti

Shin Yoshino / African Lion (Panthera leo) male roaring, Kenya

Mitsuaki Iwago / African Wild Dog (Lycaon pictus) calling, Serengeti National Park, Tanzania

Gerry Ellis / Ostrich (Struthio camelus), east Africa

Jim Brandenburg / Nile Crocodile (Crocodylus niloticus) mouth defense displaying, South Africa

Mitsuaki Iwago / African Lion (Panthera leo) pride with one female snarling, Serengeti National Park, Tanzania

Mike Parry / Saltwater Crocodile (Crocodylus porosus) underwater, South Australia.

Yva Momatiuk & John Eastcott / Red Fox (Vulpes vulpes) kit yawning on ridge near den, North Slope, Alaska

Norbert Wu / Great White Shark (Carcharodon carcharias) near divers in shark cage, Australia

Matthias Breiter / Kodiak Bear (Ursus arctos middendorffi) with mouth agape, native to Kodiak Island, Alaska

Konrad Wothe / Orangutan (Pongo pygmaeus) juvenile swinging in tree, Tanjung Puting National Park, Borneo

Tui De Roy / Waved Albatross (Phoebastria irrorata) courtship display sequence, Punta Cevallos, Espanola Island, Galapagos Islands, Ecuador

Fred Bavendam / Great White Shark (Carcharodon carcharias) attacking with mouth open as he launches from the water, Neptune Islands, South Australia

Mitsuaki Iwago / Japanese Macaque (Macaca fuscata) group grooming with one calling, Japan

Flip Nicklin / Bottlenose Dolphin (Tursiops truncatus) playing in shallow water, Hawaii

Luiz Claudio Marigo / Yacare Caiman (Caiman yacare) opening its mouth. The Pantanal wetlands of Mato Grosso State, Brazil, October.

Suzi Eszterhas / Gray Whale (Eschrichtius robustus) opening mouth and showing baleen plates, San Ignacio Lagoon, Baja California, Mexico

Hiroya Minakuchi / Orca (Orcinus orca) opening mouth for food in aquarium, native to the Pacific

Andy Rouse / Saltwater crocodile (Crocodylus porosus) opening mouth, Northern Territories, Australia

Luciano Candisani / Jacare Caiman (Caiman yacare) group fishing by opening mouths and waiting for prey, Pantanal Ecosystem, Brazil

Mitsuaki Iwago / African Lion (Panthera leo) female roaring, Serengeti National Park, Tanzania

Gerry Ellis / Hippopotamus (Hippopotamus amphibius) male displaying, Linyanti Swamp, Botswana

Mitsuaki Iwago / Domestic Cat (Felis catus) yawning, Japan

Hiroya Minakuchi / Sperm Whale (Physeter macrocephalus) opening mouth near surface, Ogasawara Island, Japan

Mark Raycroft / Keeshond (Canis familiaris) standing, portrait

Mark Raycroft / Flat-coated Retriever (Canis familiaris) black female portrait on rock at beach

Hiroya Minakuchi / Gray Whale (Eschrichtius robustus) at ocean's surface with mouth open showing baleen, San Ignacio Lagoon, Baja California, Mexico

Shin Yoshino / Hippopotamus (Hippopotamus amphibius) pair fighting, Masai Mara, Kenya

Winfried Wisniewski / Waved Albatross (Phoebastria irrorata) courting pair engaged in bill clappering

Pete Oxford / Mountain Sipo (Chironius monticola) defensive posture with mouth open, Mindo cloud forest ecosystem, Ecuador

Mitsuaki Iwago / African Lion (Panthera leo) female yawning, Serengeti National Park, Tanzania

Mitsuaki Iwago / African Lion (Panthera leo) male roaring after rainstorm, Serengeti National Park, Tanzania

Mitsuaki Iwago / African Lion (Panthera leo) male approaching female to mate, Serengeti National Park, Tanzania

Mitsuaki Iwago / African Lion (Panthera leo) male growling, Serengeti National Park, Tanzania

Mitsuaki Iwago / Cheetah (Acinonyx jubatus) growling, Serengeti, Tanzania

Mitsuaki Iwago / Spotted Hyena (Crocuta crocuta) calling, Serengeti, east Africa

Yva Momatiuk & John Eastcott / Blue Monkey (Cercopithecus mitis) in forest with mouth agape, Masai Mara National Reserve, Kenya

Hiroya Minakuchi / Southern Elephant Seal (Mirounga leonina) male roaring, Falkland Islands

Sumio Harada / Black-footed Ferret (Mustela nigripes) calling from burrow in ground, recently released from captive breeding program, Fort Belknap Indian Reservation, Montana

Claus Meyer / Jacare Caiman (Caiman yacare) resting in shallow water, thermoregulating by opening jaws, Pantanal ecosystem, Brazil

Claus Meyer / Green Iguana (Iguana iguana) portrait, Caatinga ecosystem, Brazil

Duncan Usher / Eurasian Wren (Troglodytes troglodytes) parent at nest with four hungry chicks, Europe

Mike Parry / Great White Shark (Carcharodon carcharias) breaking surface of water to feed, Neptune Islands, south Australia

Mike Parry / Great White Shark (Carcharodon carcharias) feeding from baited line, Neptune Islands, South Australia

Mike Parry / Great White Shark (Carcharodon carcharias) feeding from baited line, Neptune Islands, South Australia

Heidi & Hans-Juergen Koch / Egg-eating Snake (Dasypeltis sp) opens it's mouth to swallow a bird egg, specially adapted to eat bird eggs, native to Africa

Sumio Harada / Red Squirrel (Tamiasciurus hudsonicus) exhausted mother rests outside of her nest full of babies in a tree trunk, Rocky Mountains, North America

Konrad Wothe / Siberian Tiger (Panthera tigris altaica) yawning in snow storm, Siberian Tiger Park, Harbin, China

Hiroya Minakuchi / Gray Whale (Eschrichtius robustus) pet by tourists in boat, San Ignacio Lagoon, Baja California, Mexico

Hiroya Minakuchi / Gray Whale (Eschrichtius robustus) pet by tourists in boat, San Ignacio Lagoon, Baja California, Mexico

Konrad Wothe / Southern Elephant Seal (Mirounga leonina) pair calling, Macquarie Island, Australia

Katherine Feng / Giant Panda (Ailuropoda melanoleuca) baby sitting on ground in bamboo forest at the China Conservation and Research Center for the Giant Panda, Wolong Nature Reserve, China

Thomas Mangelsen / Bohemian Waxwing (Bombycilla garrulus) eating berries on tree, southwest Montana

Mike Parry / Saltwater Crocodile (Crocodylus porosus) aggressive posture, Oro Bay, Papua New Guinea

Tim Fitzharris / Grizzly Bear (Ursus arctos horribilis) calling, North America

Gerry Ellis / Orangutan (Pongo pygmaeus) hanging on tree, Sepilok Forest Reserve, Sabah, Borneo

Duncan Usher / Eurasian Wren (Troglodytes troglodytes) chicks in nest, Europe

Heidi & Hans-Juergen Koch / Colubrid Snake (Elaphe sp) two making initial contact, using tongue (Nasovomeral sense) to identify friend, enemy, or prey

Tim Fitzharris / Grizzly Bear (Ursus arctos horribilis) calling, North America

Tim Fitzharris / Grizzly Bear (Ursus arctos horribilis) calling, North America

Konrad Wothe / Jacare Caiman (Caiman yacare) thermoregulating by opening jaws, Pantanal, Brazil

Konrad Wothe / Lion-tailed Macaque (Macaca silenus) calling, India

Michael Quinton / Common Raven (Corvus corax) using its beak to open a snowmobile's storage compartment in search of food, Yellowstone National Park, Wyoming

Tui De Roy / Galapagos Penguin (Spheniscus mendiculus) braying to attract mate, Cape Douglas, Fernandina Island, Galapagos Islands, Ecuador

Tui De Roy / Masked Booby (Sula dactylatra) calling at nest site, Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Masked Booby (Sula dactylatra) calling, Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Masked Booby (Sula dactylatra) yawning, Hood Island, Galapagos Islands, Ecuador

Tui De Roy / Komodo Dragon (Varanus komodoensis) yawning juvenile, Komodo National Park, Komodo Island, Indonesia

Tui De Roy / Southern Royal Albatross (Diomedea epomophora) courtship display, Campbell Island, New Zealand

Flip Nicklin / Humpback Whale (Megaptera novaeangliae) gulp feeding, showing baleen, Alaska

Tui De Roy / Waved Albatross (Phoebastria irrorata) couple in courtship display, Punta Suarez, Galapagos Islands, Ecuador

Shin Yoshino / Bengal Tiger (Panthera tigris tigris) yawning, India

Mitsuaki Iwago / Bengal Tiger (Panthera tigris tigris) growling, India

Flip Nicklin / Long-finned Pilot Whale (Globicephala melas) portrait, North America

Flip Nicklin / Long-finned Pilot Whale (Globicephala melas) portrait, North America

Mitsuaki Iwago / Domestic Cat (Felis catus) adult Tabby cat yawning

Mitsuaki Iwago / Domestic Cat (Felis catus) close up of fluffy cat yawning

Mitsuaki Iwago / Domestic Cat (Felis catus) two kittens in a basket, one yawning

Mitsuaki Iwago / Domestic Cat (Felis catus) vocalizing white kitten with one blue eye and one green eye, a condition called heterochromia that does not affect vision

Pete Oxford / Blue and Yellow Macaw (Ara ararauna) portrait, native to Amazon rainforest, South America

Mitsuaki Iwago / Black-backed Jackal (Canis mesomelas) calling, Serengeti

Mitsuaki Iwago / African Lion (Panthera leo) male yawning, Serengeti National Park, Tanzania

Matthias Breiter / Kodiak Bear (Ursus arctos middendorffi) with mouth agape, native to Kodiak Island, Alaska

Claus Meyer / Blacktail Piranha (Pygocentrus piraya) vivsection showing teeth, Amazon Basin, Brazil

Gerry Ellis / African Elephant (Loxodonta africana) orphan baby checking Malaika's mouth for coconut meal, David Sheldrick Wildlife Trust, Tsavo East National Park, Kenya

Tim Fitzharris / Northern Elephant Seal (Mirounga angustirostris) female calling, Isla Benito, Baja California, Mexico

Shin Yoshino / Jaguar (Panthera onca) black panther color morph, growling, Central America

Flip Nicklin / Humpback Whale (Megaptera novaeangliae) gulp feeding, Southeast Alaska