SCIENTIFIC AMERICAN (Washington, DC) 10 December 13 So Skinny, So Bright: How Colour Change Predicts the Odds of a Chameleon Battle (Becky Crew)
When it comes to male-on-male chameleon battles, sometimes it’s not all about who’s the biggest or the strongest. Sometimes it’s about mastering what chameleons do best – changing colours.
With some males reaching more than 60 cm long, the veiled chameleon (Chamaeleo calyptratus) from the southwestern coastal regions of the Arabian Peninsula is one of the largest species of chameleon in the world. While the females tend to be mostly green all over, with muted splotches of yellow and white, the males boast a wide range of colours, including green, yellow, blue, orange and black, which are displayed in distinct patterns along their sides, head and tail. It’s been assumed that the males somehow use these colour patterns to communicate among themselves during their elaborate courting and territorial behaviours, but just what they’re saying has remained a mystery.
Now a new study published in Biology Letters today has investigated the role of colour change in aggressive male-on-male veiled chameleon interactions – and not through our eyes or what a typical camera can process, but through the eyes of a chameleon.
Researchers have long-suspected that colour change in lizards is more about communication than camouflage, but quantifying this phenomenon has proven to be a real challenge. Because how do you measure colour in an organism that is constantly changing, and only in very specific circumstances, without interfering?
“You certainly can’t do it with a spectrophotometer, which is the tool that most biologists use to measure colour,” says lead researcher Russell Ligon from Arizona State University. “When you hold the light probe up to a chameleon’s skin, the probe itself changes the colour of the chameleon’s skin, because their skin is sensitive to both light and temperature. So you can actually be inducing colour change with the thing that you’re trying to measure colour change with.”
Not to mention the fact that you’d also have to pick up the chameleon to get a proper reading from the spectrophotometer, which would further mess with the meaning and function of its colour change.
And then there’s the issue of analysing the colours as they’d be uniquely perceived through those large, independently rotating chameleon eyes.
While human eyes have three kinds of cone cells to help us perceive colour, chameleons, like many birds, have four kinds, which extends the range of their colour vision into the ultraviolet. So they likely have awesome vision, and very different from our own, which Ligon had to account for in his analysis.
“Fortunately there are people who are a lot smarter than me who have figured out a really neat way to use cameras to do that for us,” he says of the work done by Martin Stevens from the Centre for Ecology and Conservation at the University of Exeter and Thomas Pike at the University of Lincoln. “Cameras capture what we think of as reality, based on the sensitivities of the different photoreceptors that they have within them. And if you know the sensitivities of those different sensors, and you know the sensitivities of the photoreceptors of your animal – so in this case a chameleon – you can use a mathematical equation to convert between the two, so that the colours you measure from your picture correspond to how that same colour would stimulate the photoreceptor of a chameleon.”
Devi Stuart-Fox, a Melbourne-based zoologist who’s been studying colour change in southern African dwarf chameleons over the past decade, predicts that this method will set a very important standard for future colour change analysis. “What they’ve shown is that quantifying colour during behaviours that are rapid and dynamic can be done using high definition video footage and quantifying colour from that. The methods they’ve used, that they didn’t invent but that they’ve applied in a really rigorous way, I think will set a really good standard, and a lot of people will try to replicate their approach.”
In about half the 45 trials Ligon conducted, both males displayed overt aggression towards each other. When this happened, the first thing a male would do is face his opponent side-on and undergo the most unbelievable, ridiculous physical transformation. Srsly wtf I can’t even:
“They basically turn their whole body into a billboard,” says Ligon, estimating that in this moment they can get about as skinny as your pinky finger. “They get unbelievably narrow. And in conjunction with that, while they’re turning their bodies into billboards, they’re undergoing this colour change.”
Ligon suggests that they’re able to achieve this incredible skinniness using muscles attached to highly specialised, hinged ribs that slot into each, and they might also be able to somehow stack their elongated organs to make everything fit.
If a male has decided that his rival isn’t backing down fast enough in response to his billboard display, he’ll approach him, and when the pair get close enough to each other, it’s time to trade blows.
Holding onto the branch with their fused, ‘zygodactyl’ feet, the males will rear up and head-butt each other with open mouths. Sometimes, if they manage to land their head-butts properly, that can get a bite or two in as well. The longest battles Ligon observed lasted only about 10 seconds, which is probably about as long it takes for one of the males to realise he’s been outdone.
“One of the things that was most surprising was that mass was not a very good predictor of fighting ability relative to colour,” says Ligon. “You’d think bigger guys would be harder to displace, but that was not as important as the colour signals.”
What was important, he found, was how bright a chameleon’s large side stripes could get, and how fast its head colours could change. The analysis revealed that the brighter the side stripes – which play a crucial role in communication from a distance – the more likely a male was to escalate an encounter to the point of a physical fight. And the brighter the head colour patterns – which are important in face-to-face communication – the more likely a male was to win the fight. The speed at which these colours changed was also a good predictor of how a particular male would fare.
As to what this all means, Ligon isn’t sure yet, but guesses that males who can change colour more quickly are better able to signal their willingness to escalate an encounter with another male, and also their ability to win it. Which means they’re not wasting energy on getting stuck in a bunch of battles they can’t win, like other males who can’t communicate using colour change quickly or effectively enough. And that may over the long term give them more energy for foraging and finding females. “All of which could translate to some increased fitness and provide the selection pressure that really favours this ability to communicate efficiently,” he suggests.
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