Donald M. Jones / Black-tailed Prairie Dog (Cynomys ludovicianus) pair greeting, South Dakota

Will Burrard-Lucas / Black and white image of African elephant (Loxodonta africana) bulls males facing off against each other, Tsavo Conservation Area, Kenya. Editorial use only.

Ashley Cooper / Sun rays reflected onto solar tower at Ivanpah Solar Thermal Power Plant, the largest solar thermal plant in the world. It covers 4,000 acres of desert and produce 392 megawatts (MW) of electricity....

Alan Murphy/ BIA / Great Kiskadee (Pitangus sulphuratus) and Green Jay (Cyanocorax yncas) fighting, Texas

Thomas Hinsche/ BIA / European Bee-eater (Merops apiaster) pair squabbling, Saxony-Anhalt, Germany

Thomas Hinsche/ BIA / European Bee-eater (Merops apiaster) pair, Saxony-Anhalt, Germany

Tui De Roy / Volcan Alcedo Giant Tortoise (Chelonoidis nigra vandenburghi) pair in volcanic landscape, Alcedo Volcano, Isabela Island, Galapagos Islands, Ecuador

Tui De Roy / Blue-footed Booby (Sula nebouxii) males courting female, Punta Vicente Roca, Isabela Island, Galapagos Islands, Ecuador

Ingo Arndt / Baya weaver (Ploceus philippinus) subadult birds practising building 'play nest', Singapore.

Brais Seara Fernandez/ BIA / Atlantic Puffin (Fratercula arctica) pair preening, Farne Islands, United Kingdom

Richard Du Toit / Burchell's Zebra (Equus burchellii) pair nuzzling, Rietvlei Nature Reserve, South Africa

Bence Mate / Great egrets (Ardea alba) in breeding colours one with dark beak, facing each other with beaks raised towards the sky, Lake Csaj, Pusztaszer, Hungary, February.

Bence Mate / Black vulture (Coragyps atratus) face to face with King vulture (Sarcoramphus papa) Costa Rica.

Michael Milicia/ BIA / Piping Plover (Charadrius melodus) with chick, Massachusetts

Matthias Breiter / Polar Bear (Ursus maritimus) males fighting, Hudson Bay, Manitoba, Canada

Markus Varesvuo / Two male Black grouse (Tetrao / Lyrurus tetrix) fighting, Kuusamo, Finland, April.

Donald M. Jones / Dall's Sheep (Ovis dalli) rams posturing, central Alaska

Donald M. Jones / Dall's Sheep (Ovis dalli) rams posturing, central Alaska

Ben Hall / Galapagos sea lion (Zalophus wollebaeki) mother and young touching noses, Galapagos Islands, May

Edwin Giesbers / Barbary macaque (Macaca sylvanus) baring teeth as a sign of submission, Gibraltar Nature Reserve, Gibraltar,

Pedro Narra / Green turtle (Chelonia mydas) two interacting at surface, Bissagos Islands, Guinea Bissau Endangered species. 3rd Place in the SOS Especes Menacees / SOS Endangered Species Portfolio category of the...

Markus Varesvuo / Dippers (Cinclus cinclus) fighting in snow, Kuusamo, Finland, March. Second Place in the Telephoto Lens Category of the Terre Sauvage Nature Images Awards competitiion 2015.

Bence Mate / Great white egret (Ardea alba) silhouetted at sunset, Lake Csaj, Pusztaszer, Hungary, February. Winner of the Portfolio category of the Terre Sauvage Nature Images Awards competition 2015.

Bence Mate / Grey heron (Ardea cinerea) and Great egret (Ardea alba) fighting over fish, Lake Csaj, Pusztaszer, Hungary, December. Winner of the Portfolio category of the Terre Sauvage Nature Images Awards competi...

Bence Mate / Two Great egrets (Ardea alba) standing opposite each other with Grey heron (Ardea cinerea) in between. Lake Csaj, Pusztaszer, Hungary, January. Winner of the Portfolio category of the Terre Sauvage Na...

Bence Mate / Two Great egrets (Ardea alba) face to face against white sky, Lake Csaj, Pusztaszer, Hungary. Winner of the Portfolio category of the Terre Sauvage Nature Images Awards competition 2015.

Fabrice Cahez / Red fox (Vulpes vulpes) cubs playing, Vosges, France, May.

Fabrice Cahez / Red fox (Vulpes vulpes) cubs playing, Vosges, France, May.

Angelo Gandolfi / Gannets (Morus bassanus) courtship behavior on nest on abandoned boat, La Spezia Gulf, Italy. Mediterranean Sea. July.

Sven Zacek / Roe deer (capreolus capreolus) buck with velvet antlers and fawn, Tartumaa, Estonia, March.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) courtship weed dance, Cardiff, UK, March.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) courtship dance, Cardiff, UK, March.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) courtship dance Cardiff, UK, March.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) courtship dance at dawn, Cardiff, UK, March. Highly Commended in the Animal Behaviour Category of the BWPA Competition 2015.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) rival males fighting during mating season, Cardiff, UK, March.

Andy Rouse / Mute Swan (Cygnus olor) courtship behaviour, Wales, UK, April.

Andy Rouse / Great crested grebe (Podiceps cristatus cristatus) courtship dance, Cardiff, UK, March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) pair in 'weed dance' courtship display, The Netherlands, March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) two in territorial fight, The Netherlands, March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) two in territorial fight, The Netherlands, March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) pair in 'weed dance' courtship display, The Netherlands. March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) pair in courtship display, The Netherlands. March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) pair in courtship display, The Netherlands. March.

Krijn Trimbos / Great crested grebe (Podiceps cristatus) pair in courtship display, The Netherlands. March.

Espen Bergersen / Atlantic puffin (Fratercula arctica) two calling aggresively in snow, Hornya birdcliff, Finnmark, Norway.

Dong Lei / White eared-pheasant (Crossoptilon crossoptilon) two in front of Buddhist prayer flags, Sichuan Province, China, July.

Ole Jorgen Liodden / Walruses (Odobenus rosmarus) adult and young hauled on ice. Hinlopen, Svalbard, Norway, June.

Alan Murphy / Green Jay (Cyanocorax yncas) and Altamira Oriole (Icterus gularis) fighting in flight, Texas

Alan Murphy / Clay-colored Thrush (Turdus grayi) and Great Kiskadee (Pitangus sulphuratus) pair fighting, Texas

Alan Murphy / Red-winged Blackbird (Agelaius phoeniceus) males fighting, Texas

Greg Oakley / Crimson Rosella (Platycercus elegans) with juvenile, Victoria, Australia

Andy Sands / German wasp (Vespula germanica) worker carrying rubbish, flying out of nest in air brick, Hertfordshire, England, UK. July

Chris & Monique Fallows / Wandering albatross (Diomedea exulans), engaged in mating display. South Georgia Island, Southern Ocean.

Michael D. Kern / Two Dyeing dart frogs (Dendrobates tinctorius) against background, captive occurs in Guyana, Suriname, Brazil, and French Guiana. Available for on-line use only.

Kristel Richard / Two wild Przewalski / Takhi Horse (Equus ferus przewalskii) stallions play fighting Hustai National Park, Tuv Province, Mongolia. Endangered species. September.

Andy Rouse / Bengal tiger (Panthera tigris tigris) 11 month cubs play fighting, Ranthambhore National Park, India.

Markus Varesvuo / Black grouse (lyrurus tetrix) female and chick at nest, Vaala, Finland, June.

Andy Rouse / Mountain hares (Lepus timidus) boxing in snow, Scotland, UK, December.

Roy Mangersnes / Polar bears (Ursus maritimus) courting on ocean ice north of Spitsbergen, Svalbard, Norway, July.

Marie Read / Royal terns (Sterna maxima) pair crossing bills during courtship behaviour, Fort DeSoto Park, Florida, USA

Graeme Guy / Blue-throated Bee-eater (Merops viridis) pair, Penang, Malaysia

David Pattyn / Great crested grebe (Podiceps cristatus) pair performing their 'weed dance' in which they take a vertical position in front of each other and offer plant material to each other. The Netherlands. March...

David Pattyn / Great crested grebe (Podiceps cristatus) adult with a young chick on the back and another chick in the water. The Netherlands.June 2014

David Pattyn / Great crested grebe (Podiceps cristatus) courting pair. The Netherlands.May 2014

David Pattyn / Great crested grebe (Podiceps cristatus) courting pair. The Netherlands.May 2014

Fiona Rogers / Tana mangabey (Cercocebus galeritus) female grooming a juvenile. Tana River Forest, South eastern Kenya.

Nick Garbutt / Scarlet Ibis (Eudocimus ruber) flying into roost sight at dusk, Hato La Aurora Reserve, Los Llanos, Colombia, South America.

Nick Garbutt / Zebu Cattle (Bos primigenius indicus), Los Llanos, Colombia, South America.

Denis-Huot / Two male Thomson's gazelles (Eudorcas thomsonii) fighting, Masai-Mara game reserve, Kenya.

Sven Zacek / Roe deer (Capreolus capreolus) in the snow, two running away towards a third facing the camera, Vorumaa, Estonia, January. Winner of the Animal stories portfolio in the Melvita Nature Images Awards co...

Pal Hermansen / Wood ants (Formica rufa) with larvae on rusty car, Bastnas car graveyard, Varmland, Sweden, June.

Adri de Visser / African Lion (Panthera leo) female looking at Kob (Kobus kob) calf, Queen Elizabeth National Park, Uganda

Sergey Gorshkov / Musk ox (Ovibos moschatus) two in habitat, Wrangel Island, Far Eastern Russia, October.

Sergey Gorshkov / Musk ox (Ovibos moschatus) portrait, Wrangel Island, Far Eastern Russia, March.

Bence Mate / Great reed warbler (Arcocephalus arundinaceus) chick begging for food, with bokeh affect from water, Pusztaszer, Hungary, June.

Bence Mate / Common chaffinchs (Fringilla coelebs) fighting, Puszaszer, Hungary, May.

Bence Mate / Hooded crow (Corvus coronix) and Eurasian Magpie (Pica pica) waiting to scavenging White tailed eagle's (Haliaeetus albicilla) food, Pusztazer, Hungary.

Erlend Haarberg / Brambling (Fringilla montifringilla) fighting. Vauldalen, Sor-trondelag. Norway, May.

Erlend Haarberg / Brambling (Fringilla montifringilla) and Chaffinch (Fringilla coelebs) fighting. Vauldalen, Sor-trondelag. Norway, May.

Erlend Haarberg / Mountain hares (Lepus timidus) boxing, Vauldalen, Sor-Trondelag, Norway, May.

Juergen Freund / Stars facing north towards constellation Perseus with Perseid meteor shower, above farming fields in the Atherton Tablelands, Far North Queensland. August 14, 2013 4am, composite image.

Kristel Richard / Two Cambodian Pony stallions play fighting, rearing on back legs, Siem Reap, Cambodia. March 2013

Denis-Huot,Denis-Huot / Greater flamingos (Phoenicopterus ruber roseus) males displaying, lake Nakuru, Kenya

Andy Rouse / Cheetah (Acinonyx jubatus) cubs playing, Maasai Mara, Kenya, Africa

Ben Hall / Great crested grebe (Podiceps cristatus) pair performing courtship displaying at dawn, backlit and surrounded by mist, Cheshire, UK, April. Highly honoured in the Birds Category of Nature's Best Windl...

Diane McAllister / Japanese Macaque (Macaca fuscata) pair rest together on a rock near the river in Jigokudani, Japan.

Markus Varesvuo,Markus Varesvuo / Two male Black grouse (Lyrurus tetrix) displaying at lek, Utajarvi, Finland, April.

Markus Varesvuo,Markus Varesvuo / Two male Black grouse (Lyrurus tetrix) displaying at lek, Kuusamo, Finland, April.

Pal Hermansen / Willow warbler (Phylloscopus trochilus) feeding young on steering wheel of abandoned car in 'car graveyard' Varmland, Sweden, June

Bence Mate / Keel billed toucan (Ramphastos sulfuratus) in aggressive encounter with Montezuma Oropendola (Psarocolius montezuma) in rain, Laguna del Lagarto, Santa Rita, Costa Rica. Front cover bookplate of the H...

Steven Kazlowski / Polar bear (Ursus maritimus) 3-year-olds play fighting on newly formed pack ice, Beaufort Sea, off the 1002 area of the Arctic National Wildlife Refuge, North Slope, Alaska

John Downer / Bengal Tiger (Panthera tigris tigris) mother and cub, Pench National Park, Madhya Pradesh, India, taken on location for 'Tiger - Spy in the Jungle'

Andrew Parkinson / Gannet (Morus bassanus) breeding pair during part of their elaborate courtship ritual. Saltee Islands, Republic of Ireland, May.

Ben Hall,Ben Hall / Great crested grebes (Podiceps cristatus) pair performing courtship displaying, Cheshire, UK, March

Juergen Freund / Sarus Cranes (Grus antigone) Atherton Tablelands, Queensland, Australia

Andy Rouse,Andy Rouse / Ringtail Lemurs (Lemur catta) playing. Madagascar.