Many animals go to considerable lengths to make it obvious whether they are male or female, but in some species, such as the Australian long-tailed finch Poephila acuticauda, the sexes are completely indistinguishable - a male has to perform a courtship display to show a female that he is of the opposite sex. This would seem to be a waste of time when he could simply rely on permanent signals such as coloured plumage - so why be so secretive?
Strawberry poison frogs from neighbouring islands in the Bocas del Toro Archipelago in Panama, Central America, look strikingly different. Some are vivid blue, some blinding yellow, others lurid green or orange, even though they are all the same species.