Morley Read / Slow motion clip of a Forest whiptail lizard (Kentropyx pelviceps) blinking its eye, showing nictitating membrane crossing the eye, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Leaf frog (Agalychnis hulli) blinking its eyes, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Leaf frog (Agalychnis hulli) blinking its eyes, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Map treefrog (Boana geographica) blinking, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Clown treefrog (Dendropsophus reticulatus) opening its eyes, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Leaf frog (Agalychnis hulli) blinking its eyes and turning around, Orellana Province, Ecuador. (non-ex)

Ingo Arndt / Red-eyed tree frog (Agalychnis callidryas) eye closed with semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica.

Ingo Arndt / Red-eyed Tree Frog (Agalychnis callidryas) close- up of eye half open, the golden webbed semi-transparent eyelid allows to see its surrounding even while resting, Costa Rica.

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Bence Mate / Dalmatian pelicans (Pelecanus crispus) underwater view of group feeding, Lake Kirkini, Greece. Highly commended in the GDT competition 2013. Commended in the Wildlife Photographer of the Year Awards c...

Morley Read / Leaf frog (Agalychnis hulli) blinking its eyes, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Map treefrog (Boana geographica) blinking, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Map treefrog (Boana geographica) blinking, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Clown treefrog (Dendropsophus reticulatus) closing its eyes, Amazon rainforest, Orellana Province, Ecuador. (non-ex)

Morley Read / Leaf frog (Cruziohyla craspedopus) blinking its eyes, Amazon rainforest, Napo Province, Ecuador. (non-ex)

Jack Dykinga / Raven (Corvus corax) with nictitating membrane over eye, in a blizzard on Hurricane Ridge, Olympic National Park, Washington, USA. Sequence 2/2

John Waters / Common European toad (Bufo bufo) at surface, retreats underwater, Priddy, Somerset, England, UK, April.

Eric Baccega / Shoebill stork (Balaeniceps rex) portrait with a closed eyelid. Swamps of Mabamba, Lake Victoria, Uganda.

Jordi Chias / Whale shark (Rhincodon typus), closing nictitating membrane, South Ari Atoll, Maldives Islands, Indian Ocean.

Nick Upton / Rook (Corvus frugilegus) calling while perched in a Willow tree and blinking to show its blue nictitating membrane, Gloucestershire, UK, February.

Dave Bevan / European common frog (Rana temporaria) in a pond, closing its nictating membranes to lubricate its eyes, Carmarthenshire, Wales, UK, February.

Ingo Arndt / Spurell's Flying Frog (Agalychnis spurelli) eye closed with semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica.

Juan Carlos Munoz / Bateleur eagle (Terathopius ecaudatus) portrait with nictitating membrane closed, Captive, occurs in Africa.

Richard Robinson / Blue shark (Prionace glauca) with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / Blue shark (Prionace glauca) with mouth open and nictitating membrane almost completely shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

Richard Robinson / A Blue shark (Prionace glauca) just below the surface, with nictitating membrane partially shut to protect its eye, Hauraki Gulf, Auckland, New Zealand, June.

David Tipling / Rook (Corvus frugilegus) calling, Norfolk, February.

Chris Mattison / Red-eyed leaf frog (Agalychnis callidryas) close-up of eye showing patterned lower eyelid.

Roland Seitre / Bocourt's Eyelid Skink (Phoboscincus garnieri) captive, Parc zoologique et forestier / Zoological and Forest Park, Noumea, South Province, New Caledonia

Dave Watts / Bateleur (Terathopius ecaudatus) showing nictitating membrane. Kgalagadi National Park, South Africa

Dave Watts / Bateleur (Terathopius ecaudatus) showing nictitating membrane. Kgalagadi National Park, South Africa

Ingo Arndt / Gliding Leaf Frog (Agalychnis spurrelli) eye closed with semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica

Ingo Arndt / Red-eyed Tree Frog (Agalychnis callidryas) eye closed with semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica

Ingo Arndt / Red-eyed Tree Frog (Agalychnis callidryas) semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica

Thomas Marent / Red-eyed Tree Frog (Agalychnis callidryas) with half closed eyelid, Tortuguero National Park, Costa Rica

Jeff Rotman / Eye of Tiger shark (Galeocerdo cuvier), nictitating membrane closing to protect eye, Great Barrier Reef, Australia, Coral Sea.

Mark Bowler / Red eyed tree frog (Agalychnis callidryas) showing two of its three eyelids, patterned eyelid completely closed over left eye Captive

Wild Wonders of Europe / Varesvuo / European Roller (Coracias garrulus) preening, Pusztaszer, Hungary, May 2008

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

Philippe Clement / Arizona toad (Bufo microscaphus) burrowing in mud and showing the transparent inner eyelid or nictating membrane, Arizona, USA. Captive

Philippe Clement / Arizona toad (Bufo microscaphus) burrowing in mud and showing the transparent inner eyelid or nictating membrane, Arizona, USA. Captive

Jane Burton / Border Collie wearing Elizabethan collar to prevent scratching, after operation to stitch lower eyelid

Jane Burton / Border Collie, 12-year, in Elizabethan collar to prevent scratching of sutured lower eyelid

Piotr Naskrecki / Fantastic Leaf-tail Gecko (Uroplatus phantasticus) cleaning eyelid's transparent shield with its large, muscular tongue, Madagascar

Piotr Naskrecki / Red-eyed Tree Frog (Agalychnis callidryas) semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica

John Cancalosi / Regal horned lizard discharging blood from eyelid in defense {Phrynosoma solare}

Ingo Arndt / Red-eyed Tree Frog (Agalychnis callidryas) eye half covered with semi-transparent eyelid that allows it to see its surroundings even while resting, Costa Rica

Joel Sartore / Photo Ark / Violet-necked lory (Eos squamata obiensis) looking at the camera and vocalising, Anak Burung Tropikana, Bali. Captive.

Joel Sartore / Photo Ark / Violet-necked lory (Eos squamata obiensis) looking at the camera and blinking, Anak Burung Tropikana, Bali. Captive.

Joel Sartore / Photo Ark / Oriental honey-buzzard (Pernis ptilorhynchus orientalis) looking around and blinking, Bali Bird Park, Bali. Captive.

Klaus Nigge / Philippine eagle (Pithecophaga jefferyi) blinking with crest raised, showing nictitating membrane, Philippine Eagle Centre, Malagos, Davao, Mindanao, Philippines, August 2006. Critically endangered....

Andy Rouse / Brown fish owl (Ketoupa zeylonensis) fishing in shallow water, with nictitating membrane visible on one eye, Ranthambhore, India.

Andy Rouse / Brown fish owl (Ketoupa zeylonensis) spreading wings whilst fishing in shallow water, Ranthambhore, India.

John Cancalosi / Timelapse of Common snapping turtle (Chelydra serpentina) lifting head out of muddy water, blinking and then submerging again, Maryland, USA.

Eric Baccega / Buzzard (Buteo buteo) with nictitating membrane visible, perched on tree stump, Lorraine, France. March.

Joel Sartore / Photo Ark / Iraqi eyelid gecko (Eublepharis angramainyu) head portrait, Saint Louis Zoo. Captive, occurs in Middle East.

Joel Sartore / Photo Ark / Iraqi eyelid gecko (Eublepharis angramainyu) portrait, Saint Louis Zoo. Captive, occurs in Middle East.

Joel Sartore / Photo Ark / Iraqi eyelid gecko (Eublepharis angramainyu) portrait, Saint Louis Zoo. Captive, occurs in Middle East.

Daniel Nunez / Red eyed tree frog (Agalychnis callidryas) pair in amplexus at night, Calakmul Biosphere Reserve, Campeche, Mexico. Cropped.

Daniel Nunez / Black-eyed tree frogs (Agalychnis moreleti) pair in amplexus, male with nictitating membrane visible, Finca El Amate, Escuintla, Guatemala.

Daniel Nunez / Black-eyed tree frogs (Agalychnis moreleti) pair in amplexus, male with nictitating membrane visible, Finca El Amate, Escuintla, Guatemala.

Manuel Gracia / Booted eagle (Hieraaetus pennatus) looking around and calling. The animal blinks showing its nictitating membrane. Seville, Spain.

Manuel Gracia / Booted eagle (Hieraaetus pennatus) head-shot. The animal blinks showing its nictitating membrane. Seville, Spain.

Manuel Gracia / Tracking shot of a Booted eagle (Hieraaetus pennatus) standing on a log and cleaning its beak. The animal blinks repeatedly showing its nictitating membrane. Seville, Spain.

Joel Sartore / Photo Ark / Starrett's frog (Isthmohyla tica) profile. The animal seated, breathing. The animal then closes its eye, showing its nictitating membrane. La Libia, Costa Rica. Critically Endangered. Controlled condi...

Joel Sartore / Photo Ark / A close up of a Brolga crane's (Grus rubicunda) head. The animal blinks repeatedly and looks around. International Crane Foundation, Baraboo, Wisconsin, USA. Captive.

Joel Sartore / Photo Ark / Mountain yellow-legged frog (Rana muscosa) lying down, breathing. The animal opens its eye, showing its nictitating membrane. Then the frog jumps out of the frame. Oakland Zoo, California, USA. Captiv...

Joel Sartore / Photo Ark / Mountain yellow-legged frog (Rana muscosa) lying down and opening its eye, showing its nictitating membrane. The animal is showing respiration. The frog then jumps out of the frame. Oakland Zoo, Calif...

Joel Sartore / Photo Ark / Foothill yellow-legged frog (Rana boylii), sitting upright, breathing. The animal is opening and closing its eye, showing its nictitating membrane. Oakland Zoo, California, USA. Captive.

Joel Sartore / Photo Ark / Egyptian fruit bat (Rousettus aegyptiacus) lying down and looking at the camera. The animal blinks. Omaha?s Henry Doorly Zoo and Aquarium, Nebraska, USA. Captive.

Joel Sartore / Photo Ark / A close up of a Yellow-bellied marmot (Marmota flaviventris) juvenile male's head looking at the camera. The animal blinks. Calgary Wildlife, Canada. Captive.

Joel Sartore / Photo Ark / A close up of a Bulwer?s pheasant's (Lophura bulweri) head. The animal repeatedly blinks and then looks at the camera. This animal is the last known female in captivity. Al Bustan Zoological Centre, S...

Joel Sartore / Photo Ark / Senegal dove (Streptopelia senegalensis) looking around and blinking, Lincoln Children's Zoo, Nebraska, USA. Captive.

Franco Banfi / Alpine newt (Ichthyosaura alpestris) swimming in lake, nictitating membrane visible, portrait, upper Verzasca valley, Ticino, Switzerland. August.

Harry Collins / Belted kingfisher (Megaceryle alcyon) perched overlooking a small pond. Animal is showing nictitating membrane. Florida, USA.

Manuel Gracia / Ocellated lizard (Timon lepidus) sitting on a rock and basking in the sun, Sevilla, Sierra Morena, Spain.

Harry Collins / African leopard (Panthera pardus pardus) lying in the shade. Animal blinks and its ears move as it listens. Mara Triangle, Maasai Mara National Reserve, Kenya, Africa, November.

Neil Aldridge / Long-nosed horned frog (Megophrys nasuta) close up of eye and elongated upper eyelid (horns), Danum Valley, Sabah, Borneo.

Neil Aldridge / Common toad (Bufo bufo) in a meadow closing and opening eye showing nictitating membrane and eyelid, Bunloit Estate, Scotland, summer.

Harry Collins / Peregrine falcon (Falco peregrinus) perched and closing eye showing nictitating membrane and eyelid, New Jersey, USA, June.

Joel Sartore / Photo Ark / White-tailed mongoose (Ichneumia albicauda) looking around, Arabia's Wildlife Centre. Captive.

Joel Sartore / Photo Ark / Razor-billed curassow (Mitu tuberosum) looking around, Urku Center. Captive.

Joel Sartore / Photo Ark / Black francolin (Francolinus henrici) male close up of head looking around, Wadi Al Safa Wildlife Center. Captive.

Joel Sartore / Photo Ark / Egyptian turtle dove (Streptopelia turtur rufescens) looking around, Sheikh Butti Maktoum Wildlife. Captive.

Joel Sartore / Photo Ark / Arabian eagle owl (Bubo milesi) female close up of head looking around, Kalba Bird of Prey Center. Captive.

Joel Sartore / Photo Ark / Arabian eagle owl (Bubo milesi) female close up of head looking around, Kalba Bird of Prey Center. Captive.

Joel Sartore / Photo Ark / Mottled owl (Ciccaba virgata centralis) close up of head looking around, Toucan Rescue Ranch. Captive.

Joel Sartore / Photo Ark / Mottled owl (Ciccaba virgata centralis) close up of head looking around, Toucan Rescue Ranch. Captive.

Joel Sartore / Photo Ark / Black-and-white owl (Ciccaba nigrolineata) close up of head looking around, Toucan Rescue Ranch. Captive.

Joel Sartore / Photo Ark / Northern blue jay (Cyanocitta cristata bromia) looking around, profile. Captive.

Roland Seitre / King vulture (Sarcoramphus papa) yawning, showing its protective nictitating membrane or "third eyelid", head portrait, Parque Jaime Duque, Bogota, Colombia. Captive.

David Fleetham / Galapagos shark (Carcharhinus galapagensis), listed as potentially dangerous, at sea surface, with nictitating membrane covering its eye, Hawaii, Pacific Ocean.

David Weiller / Long-nosed horned frog (Pelobatrachus nasutus) juvenile close up portrait, closing its eyes, showing the nictitating membrane, Crocker Range, Sabah, Borneo, July.

Joel Sartore / Photo Ark / European Tengmalm's owl (Aegolius funereus) close up of face, International Centre for Birds of Prey. Captive.

Joel Sartore / Photo Ark / Elf owl (Micrathene whitneyi) mid shot portrait of head, blinking. Captive.