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.

Eric Baccega / Giant panda (Ailuropoda melanoleuca) pair mating, female Huan Huan and male Yuan Zi, watched by zookeeper, and Chinese scientist, 23rd March 2017. Female pandas enter estrous for only 48 hours in a...

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.

Charlie Summers / Rocky Mountain Bighorn (Ovis canadensis) portrait of ram displaying Flehman grimace to tell if a Ewe has come into estrous, McMinn Plateau,Yellowstone National Park, Wyoming, USA

Charlie Summers / Pronghorn Antelope (Antilocapra americana) Buck (right) chasing Doe (left) to guard her until she comes into estrous. Image taken in Custer State Park, South Dakota, USA

Chris O'Reilly / European lynx (Lynx lynx) adult male and female, female warning the male off during estrous, boreal birch forest, Nord-Trondelag, Norway, Captive

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