Chameleons can famously change colour to blend in with their surroundings, but they apparently also use colour as a kind of war paint to intimidate rivals.
When two males fight, the one with the more brightly coloured head usually wins, according to researchers from Arizona State University (ASU) in the United States.
The speed of head colour change is also an important predictor of the outcome of a contest, they said.
The researchers, Russell Ligon, a PhD candidate at ASU's School of Life Sciences, and Kevin McGraw, an assistant professor of evolutionary and systems biology at the school, presented their findings in the British scientific journal Biology Letters, published by The Royal Society.
To examine the poorly understood communication function of rapidly changing colours in animals such as chameleons, the two said they had obtained 10 male veiled chameleons (Chamaeleo calyptratus) caught in the wild.
Males of the species, native to the Arabian Peninsula, display "intense antagonistic behaviour" towards each other, they noted, rocking back and forth during interactions, hissing, curling and uncurling their tails and rapidly changing colour throughout their bodies.
The researchers said they had staged one-on-one encounters among the chameleons "in a tournament format," recording the results with two video cameras - one focused on each chameleon - that also took still pictures.
Using recently developed photographic and mathematical modelling tools, they then analysed the recordings, measuring the colour of 28 different body regions on each chameleon.
The analyses focused on colour brightness, colour movement and speed of colour change.
At the beginning of an encounter, male veiled chameleons will turn sideways and show their stripes. The brighter its stripes, the greater the chance the chameleon will approach its rival to fight, the researchers said.
Side-stripe brightness predicted approaches 71 per cent of the time, while the brightness of head colouring was a somewhat less accurate predictor.
If neither chameleon backs off and it comes to physical combat with head-butting, lunging, and biting, the researchers found that the one with brighter head colouration won 83 per cent of the time. The one with faster head colour changes was also considerably more likely to beat the rival into retreat.
Ligon and McGraw could only speculate on the purpose of the colour changes. They suggested the colouring might be linked to hormone levels or energy reserves, having evolved to communicate to rivals the motivation and ability to fight.
