The animal on the right is no ordinary chicken. Its right half looks like a hen but its left half (with a larger wattle, bigger breast, whiter colour and leg spur) is that of a cockerel. The bird is a ‘gynandromorph‘, a rare sexual chimera. Thanks to three of these oddities, Debiao Zhao and Derek McBride from the University of Edinburgh have discovered a truly amazing secret about these most familiar of birds – every single cell in a chicken’s body is either male or female. Each one has its own sexual identity. It seems that becoming male or female is a very different process for birds than it is for mammals.

In mammals, it’s a question of testicles,