The image of the chameleon as a shrinking violet, using camouflage to conceal itself, has been challenged by new research showing that colour change evolved to attract the attention of other chameleons.

The research by Dr Devi Stuart-Fox, from the University of Melbourne and collaborator Adnan Moussalli, turns the popular idea that chameleon colour change evolved for camouflage on its head.

"Our research suggests that chameleons are outgoing show offs and evolved colour change for signalling, to intimidate rivals or attract a mate, and not so they could match a greater variety of backgrounds," said Dr Stuart-Fox.

Not all chameleons have the same ability to change colour - of the 150 species some only exhibit shades of brown while others change between striking combinations of blues, greens and oranges, even ultraviolet colours invisible to humans - yet the reason for this enormous variation in colour change ability was previously unclear.

Dr Stuart-Fox and Dr Moussalli tested two theories; the first that colour change evolved via natural selection for the ability to camouflage against a variety of backgrounds to hide from predators (crypsis), and the second that it evolved to enable social signalling where the chameleons flash conspicuous colour patterns that are highly detectable to other chameleons, while appearing camouflaged at other times.

“To determine which selective pressures may have driven the evolution of colour change, we compared the coloration and colour-change capacities of 21 lineages of southern African dwarf chameleons (Bradypodion spp.) in a series of field-based behavioural trials,” said Dr Moussalli.

The first step was to determine the extent of colour change for each species, which the researchers did by measuring colour in a range of circumstances in the field. The team looked at how chameleons change colour in response to other chameleons as well as to predators, by presenting them with a model bird or snake.

“We found that the greatest range of colours exhibited by a species was during male contests. If a male is challenged by another male they both show their brightest colours - until one figures out the other is going to win and changes to a submissive, dark, 'don't beat me up colour'. The difference between dominant and submissive colours effectively provides a measure of a species’ capacity for colour change.” said Dr Stuart-Fox.

“If colour change evolved to enable camouflage”, explained Dr Moussalli, “we would expect species with the greatest capacity for colour change to have the greatest range of backgrounds colours that they need to match in their environments – but we found no evidence for this."

"Instead, we found that chameleon species that changed colour the most had displays that were most conspicuous to other chameleons. “

In order to test how colour is used for camouflage or to communicate, the scientists had to determine what chameleon colours look like to other chameleons and to their predators. This is because both chameleons and their predators have a different visual system to humans; they have a type of cone which is ultra-violet (UV) sensitive.

"We measured colour with a spectrometer, which measures both the UV and visual colour range, and combined this with information on the chameleon visual system to model chameleon colour perception. We also used information on bird visual systems to model what chameleon colours look like to birds." explained Dr Stuart-Fox.

By combining this data with information about the evolutionary relationships among the 21 species, the researchers demonstrated that chameleon species that have evolved the greatest colour change also possess display signals that are most conspicuous to other chameleons – because they show a highly contrasting colour pattern and each colour contrasts markedly with surrounding vegetation.

These results support the hypothesis that dramatic colour change evolved in dwarf chameleons largely as a strategy to facilitate social signalling rather than camouflage.

“Our study shows that examining animal signals from the point of view of other animals (members of the same species – or predators), can tell us much about the evolution of animal communication and the diversity of colours and ornaments in nature”.

The study is reported in February's edition of PloS Biology.