Alex Mustard / 2020VISION / Compass jellyfish (Chrysaora hysoscella) swimming over a rocky reef, Plymouth, Devon, UK, August. Did you know? Compass jellyfish change gender throughout their lives, starting off male, and later bec...

Pete Oxford / Hijra performer, balancing bicycle wheel on teeth. Hijra are transgender people, legally recognized as a third gender in India. Uttar Pradesh, India, October 2014.

Pete Oxford / Hirja performer, balancing bicycle wheel on head, Hijra are transgender people, legally recognized as a third gender in India. Uttar Pradesh, India, October 2014.

Bryan and Cherry Alexander / The men and children of the family place specially prepared food on the ground for the spirits during the Chukchi 'Festival of the Young Reindeer' during which young reindeer are sacrificed. Iultinsk...

Terry Whittaker / 2020VISION / Researcher blowing on Myotis bat probably a Daubenton's bat (Myotis daubentoni) to check it's gender, near Faversham Kent, UK, September 2010

Pete Oxford / Lioness (Panthera leo) with mane, a rare mutation that causes her to resemble a male lion. Okavango Delta, Botswana.

Roland Seitre / Cauca Poison Frog (Ranitomeya bombetes) being sexed by examination of the mouth. Endangered. Valle Dagua, Cali, Colombia, January 2010.

Roland Seitre / Cauca Poison Frog (Ranitomeya bombetes) being sexed by examination of the mouth. Endangered. Valle Dagua, Cali, Colombia, January 2010.

Charlie Summers / Navajo hogan for a man. The gender of a hogan is determined by its construction. Monument Valley, Arizona, September 2011.

Charlie Summers / Navajo hogan constructed for a woman. The type of construction, specifically how the entrance is made, determines the gender of a hogan. Monument Valley, Arizona, September 2011.

Michael D. Kern / Northern elephant seals (Mirounga angustirostris) mating pair and calf, San Simion, California, USA

Jane Burton / Sexing a male Chinchilla, UK

Jane Burton / Sexing a female Chinchilla, UK

Jane Burton / Sexing a female Guinea pig

Jane Burton / Sexing a male Guinea pig, penis extruded

Adam White / Stick insects mating {Phasmids}, NB sexual dimorphism, Seychelles

LYNN M. STONE / Eclectus parrots {Eclectus roratus}, captive; female (left) male (right) USA

Roger Powell / Eurasian wigeon (Anas penelope) mixed gender flock in flight. Holy Island / Lindisfarne, Northumberland, England, UK. October

David Woodfall / Researcher blowing on Little tern (Sterna albifrons) to move feathers to check gender. County Wicklow, Republic of Ireland, June.