In Lake Tanganyika in the heart of Africa, there lives an unusual species of fish, Haplochromis burtoni of the cichlid family. It is peculiar for having two varieties of male. One group of males is bright yellow or blue with black stripes around the eyes, while the other is a dull grey, similar in appearance to females of the species.

The main difference between them is that the colourful males are dominant, aggressive and territorial. They belong, as it were, to a kind of fish aristocracy an...