Chameleons are famed for changing colour to blend in with their surroundings and hide from predators - but new research on chameleons in their native habitat shows some of their colour changes evolved for exactly the opposite purpose - attracting attention.

African dwarf chameleons live in habitats in southern Africa ranging from grassland to rainforest.

They engage in complex social signalling, with bright colour changes along their flanks used by females to signal interest or rejection to males, and by males to signal aggression or submission to other males, and interest towards females. Males even square off in rapid-fire, colourful signalling duels.

©Adnan Moussalli and Devi Stuart-Fox
Chameleons can signal others in a matter of milliseconds by brightening their flanks. This keeps down the risk of being seen by a predator.

"Chameleons use colour change for camouflage and communication, but we don't know why some species change colour much more than others", says Devi Stuart-Fox of the University of Melbourne in Australia.

She and colleague Adnan Moussalli reasoned that if these differences evolved solely to enable the chameleon to match itself to its surroundings, chameleons living in backgrounds that vary a lot in colour should produce a wider palette, whereas chameleons in less colourful environments should not.

Dramatic signals

The pair collected males from all 21 genetic groups of dwarf chameleon that live across South Africa and faced them off in duels within their normal habitats. They also measured the colours in their habitats and on the chameleons' flanks.

"We used a spectrometer because unlike humans, chameleons can see in the ultraviolet," says Stuart-Fox. "We wanted to work out how conspicuous a chameleon appeared to another chameleon - get a 'chameleons eye's view'."

They also assessed how the colours would look to the visual systems of the shrikes and hawks that eat chameleons.

They found that the chameleons' colour palette did not broaden with more colorful surroundings, as it would if the driving force on its evolution had been its ability to blend in. "Species that changed colours the most were the ones with the most conspicuous displays," Stuart-Fox says, meaning the capacity for colour change evolved mainly to produce conspicuous social signals.

The chameleons do change their overall colours and brightness to blend in with their surroundings. But for social signalling, they do not have to worry about predators seeing their dramatic flank displays, as these change in a few milliseconds.

Journal reference: PLoS Biology (DOI: 10.1371/journal.pbio.0060021)