Alex Mustard / Western clown anemonefish (Amphiprion ocellaris) poses with a large, dominant female behind. The clownfish in the foreground is probably a juvenile. Bitung, North Sulawesi, Indonesia. Lembeh Strait, M...

Terry Whittaker / Mandarin duck (Aix galericulata) male female pair, Reddish Vale Country Park, Greater Manchester, UK. December

Steven David Miller / Eclectus parrot (Eclectus roratus) female perched in a tree, The Wildlife Habitat Zoo, Port Douglas, Queensland, Australia. August. Captive.

Theo Bosboom / Variable damselfly (Coenagrion pulchellum) pair mating, De Peel National Park, the Netherlands, July.

Jussi Murtosaari / Blackcap (Sylvia atricapilla) female and two males huddling together for warmth while sleeping, Finland, May.

Lou Coetzer / Pintailed whydah (Vidua macroura) male displaying to female, Masai Mara, Kenya.

Andres M. Dominguez / Red deer (Cervus elaphus) stag and two hinds, Los Alcornocales Natural Park, southern Spain, September.

Bence Mate / Flock of Mallard ducks (Anas platyrhynchos) taking off, Lake Csaj, Kiskunsagi National Park, Pusztaszer, Hungary. December.

Bence Mate / Red footed falcon (Falco vespertinus) pair mating, Lake Csaj, Kiskunsagi National Park, Pusztaszer, Hungary. February.

Bence Mate / Red footed falcon (Falco vespertinus) pair mating, Lake Csaj, Kiskunsagi National Park, Pusztaszer, Hungary. February.

Wim van den Heever / African lion (Panthera leo) male female pair reflected in water, Okavango Delta, Botswana

Markus Varesvuo / Female Capercaillie (Tetrao urogallus) ready to mate, Vaala, Finland, May.

Brandon Cole / Sockeye salmon (Oncorhynchus nerka) interacting in shallow river, males chasing each other while female waits over gravel suitable for depositing her eggs after fertilization. Adams River, British Col...

Markus Varesvuo / Black grouse (Lyrurus tetrix) mating in snow, Utajarvi, Finland, May.

Nick Garbutt / Bare-faced Curassow (Crax fasciolata) male and female, Near Araras Lodge, Mato Grosso, Northern Pantanal, Brazil, South America.

Dong Lei / Asian paradise-flycatcher (Terpsiphone paradisi) pair perched on branch, Shanyang town, Gutian County, Hubei province, China.

Markus Varesvuo / Capercaillie (Tetrao urogallus) male calling with female, Jalasjarvi, Finland, April

Juan Carlos Munoz / Black howler monkeys (Alouatta caraya) male and two females calling from tree, captive, Apenheul Park, Netherlands.

Paul Harcourt Davies / Short-horned grasshoppers (Calliptamus italicus) mating, showing sexual dimorphism, Pescia Romana, Tarquinia, Umbria, Italy, September

Ross Hoddinott / 2020VISION / Common blue damselfly {Enallagma cyathigerum}, mating pair, Little Bradley Ponds, Devon, UK. July. 2020VISION Book Plate.

Ross Hoddinott / 2020VISION / Common blue damselfly {Enallagma cyathigerum}, mating pair, Little Bradley Ponds, Devon, UK. July

Konrad Wothe / Eclectus Parrot (Eclectus roratus) courtship behaviour in rainforest, Cape York Peninsula, North Queensland, Australia, captive

Wild Wonders of Europe / Varesvuo / Red-footed Falcon (Falco vespertinus) mating pair, Hortobagy NP, Hungary

Wild Wonders of Europe / Varesvuo / Red-footed Falcon (Falco vespertinus) mating pair, Hortobagy NP, Hungary, May 2008

Wild Wonders of Europe / Varesvuo / Red-footed Falcon (Falco vespertinus) male brings vole prey to female in nestbox, Hortobagy NP, Hungary, May 2008

Wild Wonders of Europe / Varesvuo / Red-footed Falcon (Falco vespertinus) pair perched, male on left, Hortobagy NP, Hungary, May 2008

Wild Wonders of Europe / Varesvuo / Red-footed falcon (Falco vespertinus) mating pair, Hortobagy NP, Hungary, May 2008

Wild Wonders of Europe / Varesvuo / Red-footed falcon (Falco vespertinus) mating pair, Hortobagy NP, Hungary, May 2008

Wild Wonders of Europe / Varesvuo / Red-footed falcon (Falco vespertinus) mating pair, Hortobagy NP, Hungary, May 2008

Anup Shah / Fur Seal pair mating (Arctocephalus pusillus pusillus) Cape Cross Seal Reserve, Namibia

Anup Shah / Fur Seal pair mating (Arctocephalus pusillus pusillus) Cape Cross Seal Reserve, Namibia

Anup Shah / Fur Seal pair mating (Arctocephalus pusillus pusillus) Cape Cross Seal Reserve, Namibia

Gabriel Rojo / South American / Patagonian sealion pair mating {Otaria flavescens} Valdez peninsula, Argentina. Note large difference in size between male and female

Gabriel Rojo / South American / Patagonian sealion pair {Otaria flavescens} on beach, Valdez peninsula, Argentina

Gabriel Rojo / South American / Patagonian sealion pair {Otaria flavescens} Valdez peninsula, Argentina

Doug Allan / Southern elephant seal pair courtship {Mirounga leonina} Antarctica

Doug Allan / Southern elephant seal pair courtship {Mirounga leonina} Antarctica. NB Large difference in size of male and female.

David Shale / Deep sea Anglerfish male. Male is ten times smaller than female and has well developed olfactory system to help locate females in the darkness, Atlantic ocean

Ron O'Connor / Nephilia spider pair on web, Kruger NP, South Africa. Note huge size difference - female is larger.

Thomas Marent / Banana Spider (Nephila clavipes) female and smaller male, Bocas del Toro, Panama

Mark Raycroft / Woodland Caribou (Rangifer tarandus caribou) bull displaying to female, Newfoundland, Canada

Donald M. Jones / Northern Pintail (Anas acuta) males flying with female in spring courtship display, Montana

Donald M. Jones / Bighorn Sheep (Ovis canadensis) female and male in winter, Jasper National Park, Alberta, Canada

Chien Lee / Stick Insect (Monticomorpha flavolimbata) pair mating in mountains, Bengoh Range, Ecuador

Donald M. Jones / Bighorn Sheep (Ovis canadensis) ewe and ram, western Montana

Michael Quinton / Dall's Sheep (Ovis dalli) ram, lamb, and ewe, Yukon Territory, Canada

Luciano Candisani / Green Anaconda (Eunectes murinus) female constricting male after copulation, after which she will consume him, Brazil

Alan Murphy/ BIA / Northern Bobwhite (Colinus virginianus) female and male, Texas