Anup Shah / Eastern chimpanzee (Pan troglodytes schweinfurtheii) female 'Nasa' aged 25 years close-up of estrus swelling . Gombe National Park, Tanzania. September 2014.

Tony Phelps / Female Chacma baboon (Papio ursinus) in oestrus, DeHoop Nature Reserve, Western Cape, South Africa, September.

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.

Tony Wu / Pair of Japanese dwarf flying squirrels (Pteromys volans orii) shortly before mating (the female is in front). Hokkaido, Japan.