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

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

Will Watson / Female Great diving beetle (Dytiscus circumflexus) view of underside with cloacal sperm plug from previous mating, UK, captive.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) female with swelling of sexual skin to indicate she is in estrus, Tangkoko National Park, Sulawesi, Indonesia.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) close-up of rears, one female in estrus and one male, Tangkoko National Park, Sulawesi, Indonesia.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) female showing swelling of sexual skin in estrus, Tangkoko National Park, Sulawesi, Indonesia.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) close-up of female rear showing swelling of sexual skin in estrus, Tangkoko National Park, Sulawesi, Indonesia.

Anup Shah / Celebes / Black crested macaque (Macaca nigra) group grooming, showing female bottoms, one on right in estrus the other not,Tangkoko National Park, Sulawesi, Indonesia.

Suzi Eszterhas / Proboscis Monkey (Nasalis larvatus) dominant male sitting in tree, Sabah, Borneo, Malaysia

Juergen Freund / Spectacled flying fox (Pteropus conspicillatus) male grooming sexual organs, Tolga Bat Hospital, Atherton, North Queensland, Australia. January 2008.

Juergen Freund / Spectacled flying fox (Pteropus conspicillatus) male hanging upside down, Tolga Bat Hospital, Atherton, North Queensland, Australia. January 2008.

Juergen Freund / Spectacled flying fox (Pteropus conspicillatus) male hanging upside down and vocalising, Tolga Bat Hospital, Atherton, North Queensland, Australia. January 2008.

Ben Lascelles / Mantled howler monkey (Alouatta palliata aequatorialis) male and female mating, Punta Bejuco, Gulfo de Chiriqui, Panama

Florian Möllers / Quinling Golden snub nosed monkey (Rhinopitecus roxellana qinlingensis) adult female and infant grooming adult male. Zhouzhi Nature Reserve, Qinling Mountains, Shaanxi, China

Tony Phelps / Boomslang (Dispholidus typus) male showing hemipenes, deHoop Nature reserve, Western Cape, South Africa

Elio Della Ferrera / Male Hamadryas Baboon (Papio hamadryas) with erect penis, Ethiopia 1992