Colour-changing chameleons evolved to stand out, not blend in

Blogging on Peer-Reviewed ResearchChameleons aren’t exactly known for being showy. Indeed, they are so synonymous with blending in that we use the term ‘social chameleon’ to refer to people who are at home in any social setting. But new research suggests that this reputation needs a rethink. The chameleon’s ability to change colour evolved not to blend in, but to stand out.

Chameleon headChameleons are a group of small lizards that are almost synonymous with camouflage. Common folklore has it that their vaunted ability to change their skin colour allows them to go undetected in a variety of environments.

Certainly, their default colours match their surroundings well. But Devi Stuart-Fox and Adnan Moussalli from South Africa have found that the changing hues they are best known for evolved for communication not disguise. They allow chameleons to make themselves incredibly but temporarily noticeable to mates and rivals, while remaining inconspicuous for the rest of the time.

Flashy colours

Colourful signals are far from rare in the animal kingdom but in most cases they are constant. Many birds and butterflies, to name a few, have permanently colourful coats and the mates these attract help to offset their increased visibility to predators. But for small, slow-moving animals like chameleons, it pays to spend most of their time concealed and instead, use signals that can be flashed briefly and detected easily.

The small lizards can certainly do that. Like octopuses and squid, their skins contain special pigment cells called chromatophores that can be expanded and contracted at will to produce a rainbow of colours in a matter of milliseconds.

Chameleon on branchStuart-Fox and Moussalli studied the evolutionary forces that produced this quick-change ability by comparing all 14 species and 7 sub-species of southern African dwarf chameleon (Bradypodion spp.). These 21 lineages vary greatly in terms of the colours they use, how dramatic the changes are and the habitats they call home.

Fight! Fight! Fight!

To measure these differences, the scientists staged duels by placing two males from the same species on a single branch and recording their shifting skin-tones as they argued over dominance. Each species has their own set of dominant colours and a wildly different set of submissive colours, used upon defeat or rejection. It’s in these social contests that chameleons show their full repertoire of hues.

The duo took a colour record of these duels with a technique called ‘reflectance spectrometry’ that measures the proportion of different wavelengths of light that bounce off different parts of the rivals’ skins. They also took readings from the environment where they originally caught the chameleons, from grassland to dense forest understory.

Taking into account the way chameleon eyes see colour, Stuart-Fox and Moussalli found that the species which show the most dramatic colour changes are also those with signals that contrasted most strongly with other parts of their bodies and the surrounding vegetation. The greater the species’ capacity of colour change, the more visible they were to other chameleons and the more they stood out from their environment.

That’s exactly the opposite of what you would expect if the ability evolved for better camouflage. Moreover, the lineages showed no link between their capacity for colour change and the nature of the backgrounds they would have had to match.

ChameleonEvolving palettes

Stuart-Fox and Moussalli believe that chameleons, and certainly the dwarf species, evolved their varying palettes for social purposes rather than secretive ones.

Related lizards like agamas and iguanas can also change colours but to a much less dramatic extent. They mainly alter their appearances by becoming brighter or darker and they use these changes for camouflage and to control how much heat they absorb from their surroundings.

Stuart-Fox and Moussalli theorise that early chameleons originally developed colour-changing skins for the same reasons. But the later development of their ability to its present spectacular levels was more strongly driven by arguments and seductions.

Reference: Stuart-Fox, D., Moussalli, A. (2008). Selection for Social Signalling Drives the Evolution of Chameleon Colour Change. PLoS Biology, 6(1), e25. DOI: 10.1371/journal.pbio.0060025

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