Espen Bergersen / Aerial shot of Killer whales (Orcinus orca) near the surface, showing mating behaviour including a male Killer whale displaying penis, Troms, Norway, August.

Franco Banfi / Sperm whale (Physeter macrocephalus) young male trying to copulate with a female. The calf of the female was in the way so the male is trying to chase it away. Dominica.

Dave Bevan / Female Giant wood wasp (Uroceras gigas) inserting ovipositor into tree stump to lay eggs, Carmarthenshire, Wales, UK, September.

Francois Savigny / Tiger male carrying Chital deer kill, Bandhavgarh NP, Madhya Pradesh, India

Tony Wu / Humpback whale (Megaptera novaeangliae) male with his penis extended. Vava'u, Tonga. South Pacific.

Yukihiro Fukuda / Japanese macaque (Macaca fuscata fuscata) rare white furred baby jumping, Jigokudani Valley, Nagano Prefecture, Japan. June.

Fiona Rogers / Wild mature male mandrill (Mandrillus sphinx) in gallery forest during dry season. Lope National Park, Gabon. June 2008.

Wild Wonders of Europe / E Haarberg / Chamois (Rupicapra rupicapra) male with erect penis in rutting season, Gran Paradiso National Park, Italy, November 2008

Karl Ammann / Male Bonobo {Pan paniscus} sitting, looking thoughtful. Lola Ya Bonobo Sanctuary, Kinshasa, DR of Congo, 2007

Doug Perrine / Humpback whale {Megaptera novaeangliae} with penis extended, Pacific (taken under NMFS research permit 882 issued by Hawai Whale Research Foundation)

Anup Shah / Hamadryas baboons display red bottoms {Papio hamadryas} C

Anup Shah / Male Chimpanzee sitting on tree buttress, Gombe NP Tanzania. Apollo

Hermann Brehm / African elephant male in musth charging {Loxodonta africana} Botswana

Natasha Breed / Gelada baboons males grooming and inspecting each other's genitals {Theropithecus gelada} Ethiopia

Charlie Summers / Lion (Panthera leo) ready to mount female, Ngorongoro Crater, Tanzania.

Andy Rouse / Bengal tiger (Panthera tigris) male rolling on ground after mating. Ranthambhore National Park, India.

Doug Gimesy / Grey-headed Flying-fox (Pteropus poliocephalus) hangs from one foot from a branch. Yarra Bend Park, Kew, Victoria, Australia. December, 2017.

Heather Angel / Chalcid wasp (Gasteruption jaculator) nectaring on Yarrow (Achillea millefolium) cultivar, picking up pollen on legs and head. Long ovipositor used to lays eggs inside solitary bee nests. In garden, S...

Andy Rouse / Indian Flying Fox , (Pteropus giganteus), roosting, India

Anup Shah / Olive baboon (Papio anubis) male, taken with remote camera. Masai Mara National Reserve, Kenya.

Anup Shah / Vervet monkey (Cercopithecus aethiops) male approaching with curiosity - remote camera. Masai Mara National Reserve, Kenya..

David Perpinan / Necrosis / tissue dying in hemipenis of captive garter snake (Thamnophis sirtalis) small repro only

David Perpinan / Phallus/penile prolapse in a captive Yellow bellied slider (Trachemys scripta). Small repro only

Shane Gross / Nurse sharks (Ginglymostoma cirratum) ready to mate. Showing male claspers, which are used to channel semen into the female during mating, Eleuthera, Bahamas.

Shane Gross / Nurse sharks (Ginglymostoma cirratum) ready to mate. Showing male claspers, which are used to channel semen into the female during mating, Eleuthera, Bahamas.

Franco Banfi / Zebu bull, Bonito, Mato Grosso do Sul, Brazil

Staffan Widstrand / Wild Wonders of China / Portrait of a male Golden snub-nosed monkey (Rhinopithecus roxellana) Foping Nature Reserve, Shaanxi, China. Endangered species

MYN / Lily Kumpe / Balloon-winged katydid (Tympanophora diminuta) female, William Bay National Park, Western Australia. Meetyourneighbours.net project.

MYN / Lily Kumpe / Giant ichneumon wasp (Megarhyssa) female with very long ovipositor, Dugana Khad, Tuv Province, Mongolia. Meetyourneighbours.net project.

Daniel Heuclin / Potto (Perodicticus potto) male in tree, Togo. Captive.

Tony Wu / Humpback whale (Megaptera novaeangliae) male blowing bubbles, likely a display of dominance during heat run. The male had a small portion of his penis protruding. Vava'u, Tonga. Pacific Ocean.

Claudio Contreras / Olive Ridley Sea Turtle (Lepidochelys olivacea) nesting, egg in nest, Arribada (mass nesting event), Playa Morro Ayuta, Oaxaca state, southern Mexico, IUCN Vulnerable, August

Kim Taylor / Rose sawfly (Arge ochropus) female cutting a groove in a rose stem in which to lay her eggs. Surrey, England, UK, June.

Kim Taylor / Parasitic wasps (Braconidae) approaching caterpillar of Lesser willow sawfly (Nematus pavidus) to lay eggs on, Surrey, England, UK. June.

Kim Taylor / Parasitic wasps (Braconidae) approaching caterpillar of Lesser willow sawfly (Nematus pavidus) to lay eggs on, Surrey, England, UK. June.

Kim Taylor / Parasitic wasp (Braconidae) with ovipositor extended approaching caterpillar of Lesser Willow Sawfly (Nematus pavidus). Surrey, England, June.

Kim Taylor / Parasitic wasp (Braconidae) with ovipositor extended approaching caterpillar of Lesser Willow Sawfly (Nematus pavidus). Surrey, England, June.

Dave Bevan / Female Giant wood wasp (Uroceras gigas) withdrawing ovipositor from tree stump after laying eggs, Carmarthenshire, Wales, UK, September.

Sandesh Kadur / Arunachal macaque (Macaca munzala) male, Arunchal Pradesh, Himalayas, India.

Tony Wu / Spinner dolphin (Stenella longirostris) male leaping out of the water. The protruding tip of the dolphin's penis is just visible. Sri Lanka. Indian Ocean.

Claudio Contreras / Gentoo penguin (Pygoscelis papua) pair mating, Sea Lion Island, Falkland Islands, October

Tony Wu / Whitespotted bamboo shark (Chiloscyllium plagiosum) working his way into a crevice to approach female to mate. The males two claspers (the organs used for inseminating the female shark) are clearly vi...

Philippe Clement / Red deer (Cervus elaphus) stag with erect penis urinating on itself, rubbing antlers in grass and flicking tongue during the rut, Jaegersborg Dyrehaven, Denmark, October.

Philippe Clement / Common teal (Anas crecca) preening feathers and showing corkscrew-shaped penis, Bavarian Forest National Park, Germany, May. Captive.

Ingo Arndt / Leopard slug (Limax maximus) climbing and eating at the same time a rope of mucus after mating. These slugs are hermaphrodites and can be seen here transferring sperm to one another through their male...

Ingo Arndt / Leopard slug (Limax maximus) mating, hanging from a rope of mucus. These slugs are hermaphrodites and can be seen here transferring sperm to one another through their male organs.Switzerland. Sequence...

Ingo Arndt / Leopard slug (Limax maximus) mating, hanging from a rope of mucus. These slugs are hermaphrodites and can be seen here transferring sperm to one another through their male organs.Switzerland. Sequence...

Ingo Arndt / Leopard slug (Limax maximus) mating, hanging from a rope of mucus. These slugs are hermaphrodites and can be seen here transferring sperm to one another through their male organs.Switzerland. Sequence...

Steve Downer / Close up of a Black slug (Arion ater agg.), showing penis, England, UK, October.

Steve Downer / Close up of a penis of a Black slug (Arion ater agg.), England, UK, October.

Kim Taylor / Rose-hip chalcid (Megastigmus aculeatus) female on flowering grass. Surrey, England, May.

Kim Taylor / Rose-hip chalcid (Megastigmus aculeatus) female on flowering grass. Surrey, England, May.

Kim Taylor / Braconid wasp (Dinocampus coccinellae) using its slender ovipositor to insert an egg into the abdomen of an unsuspecting ladybird (Coccinella 7-punctata). Surrey, England.

Tony Phelps / Alpha male Chacma baboon (Papio ursinus) with female and infant, grooming, DeHoop Nature Reserve, Western Cape, South Africa, September.

Ingo Arndt / Carpet python (Morelia spilota) showing protruding hemipenis, Australia.

Ingo Arndt / Tulip flower (cultivated variety) close-up of reproductive parts showing stamens, style and stigma

Francois Savigny / Old African elephant bull in 'must', period of sexual dominance when he will mate with receptive females in his home range. Etosha National Park

SCOTLAND: The Big Picture / Scottish wildcat (Felis silvestris grampia) male, on back to show underside for pelage scoring. Inside The Royal Zoological Society Scotland / RZSS mobile vet unit, Strathsprey, Cairngorms National P...

SCOTLAND: The Big Picture / Conservationist from The Royal Zoological Society Scotland / RZSS measuring the size of a Scottish wildcat's (Felis silvestris grampia) testes. Inside RZSS mobile vet unit, Strathsprey, Cairngorms Nat...

Tony Wu / Indo-Pacific bottlenose dolphin (Tursiops aduncus) with penis extended. Ogasawara / Bonin Islands, Japan.

Rick Price / Penis of a Southern Elephant Seal (Mirounga leonina), Salisbury Plain, South Georgia. January.

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) female 'Eliza' aged 21 years swollen sexual skin. Gombe National Park, Tanzania. October 2012.

Luke Massey / Proboscis monkey (Nasalis larvatus), Tarakan, Indonesia

Luke Massey / Proboscis monkey (Nasalis larvatus) male, Tarakan, Indonesia

Nick Upton / Giant wood wasp (Urocerus gigas) ovipositing / laying eggs in Cedar log, Wiltshire garden, UK, May.

Andy Sands / Braconid (Agathidinae sp.) Ovipositing into fleabane seedhead, Oxfordshire, England, UK, September

Inaki Relanzon / Geladas (Theropithecus gelada) grooming and resting, one inspecting penis, Simien Mountains National Park, Ethiopia.

Ingo Arndt / Leopard slug (Limax maximus) mating, hanging from a rope of mucus. These slugs are hermaphrodites and can be seen here transferring sperm to one another through their male organs.Switzerland. Sequence...

Franco Banfi / Young Sperm whale (Physeter macrocephalus) with erect penis Dominica, Caribbean Sea, Atlantic Ocean. Vulnerable species.

Franco Banfi / Young Sperm whale (Physeter macrocephalus) with erect penis Dominica, Caribbean Sea, Atlantic Ocean. Vulnerable species.

Doug Perrine / Northern elephant seals (Mirounga angustirostris) bask on the beach during their annual moult - one sleeping seal has his penis extended, Piedras Blancas, California, USA, June

Pascal Kobeh / Nudibranchs (Nembrotha lineolata) mating, they are hermaphodites but need another individual for reproduction, Sulu Sea, Philippines

Pascal Kobeh / Nudibranchs (Nembrotha lineolata) mating, they are hermaphodites buty need another individual for reproduction, Sulu Sea, Philippines

Pascal Kobeh / Anna's nudibranchs (Chromodoris annae) two looking about to mate, Sulu Sea, Philippines

Tony Wu / Genital slit of a dead male Blue whale (Balaenoptera musculus) found floating south of Mirissa, Sri Lanka. Indian Ocean. The tip of the whale's penis appears to be protruding from the animal. The whal...

Yashpal Rathore / Indian rhinoceros (Rhinoceros unicornis), young male urinating. Kaziranga National Park, India

Cyril Ruoso / Red-shanked Douc langur (Pygathrix nemaeus) adult male standing portrait, Vietnam

Juan Carlos Munoz / Grevy's zebra (Equus grevyi) close up of hind quarters of male with penis extended from sheath, captive.

Tony Wu / Group of three Sperm whales (Physeter macrocephalus) with male at the top with protruding penis visible. Caribbean.

Claudio Contreras / Lacandonia (Lacandonia schismatica) parasitic flower, with inverted sex organs, Lacandon Rainforest, Mexico, May. Endangered and endemic species.

Aflo / Ginger and white tom cat, rolling in sunny spot on pavement near to boat, Yamaguchi, Chatra, Japan.

Alex Mustard / Black hamlets (Hypoplectrus nigricans) spawning, backlit to make the eggs visible. Oro Verde Wreck, Grand Cayman, Cayman Islands. Caribbean Sea, January.

Bruno D'Amicis / Silvery-cheeked hornbill (Bycanistes brevis) male perched, Jimba, Lake Tana Biosphere Reserve, Ethiopia.

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) female 'Fanni' aged 30 years rubbing genitals with female 'Gaia' aged 22 years to comfort her after a disagreement with another chimpanzee. Gombe N...

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) male 'Faustino' aged 22 years, rear view showing testicles. Gombe National Park, Tanzania.

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) infant male 'Gizmo' aged 2 years inspecting the estrus swelling of his adolescent sister 'Glitter' aged 13 years. Gombe National Park, Tanzania.

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) male 'Wilkie' aged 38 years resting amongst grass. Gombe National Park, Tanzania.

Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) male 'Wilkie' aged 38 years resting amongst grass. Gombe National Park, Tanzania.

Dave Watts / Platypus (Ornithorhynchus anatinus) spurs and cloaca, from dead specimen, Tasmania, Australia

Cyril Ruoso / Sumatran rhino (Dicerorhinus sumatrensis) penis, captive male in breeding program, Way Kambas National Park, Sumatra, Indonesia.

Cyril Ruoso / Close up of Sumatran rhinoceros (Dicerorhinus sumatrensis) adult male, captive male in a breeding program, Way Kambas National Park, Sumatra, Indonesia.

Yukihiro Fukuda / Japanese macaque (Macaca fuscata fuscata) mother with rare white furred male baby, Jigokudani Valley, Nagano Prefecture, Japan. June.

MYN / Paul van Hoof / Great green bush-cricket (Tettigonia viridissima) female, The Netherlands, July. Meetyourneighbours.net project

MYN / Paul van Hoof / Oak bush-cricket (Meconema thalassinum) female, France, July. Meetyourneighbours.net project

Alex Hyde / Ovipositor of Scaly Cricket (Pseudomogoplistes vicentae). England, UK. April. Captive.