Humans have been fascinated by the colour-changing abilities of chameleons for a long time. Aristotle himself, the forefather of Western philosophy and also a keen zoologist, mentioned the lizard’s ability in his Historia Animālium, noting that the “change of colour takes place over the whole body,” suggesting the chameleon had a “timorous soul”.
The reasons chameleons change colour vary, including in response to temperature and light, and certainly the background-matching behaviour that comes to mind when most think of a chameleon. But a 2008 study of the South African dwarf chameleons provided compelling evidence that evolution has favoured the ability to stand out against one’s background rather than blend in – to impress potential mates, for example. This, coupled with behavioural descriptions of rapid colour change during social interactions, strongly suggests chameleons have evolved their dynamic colour palette as a means of communication.
To understand how an animal’s colours can serve as reliable signals to others requires an objective means of quantifying such colours and colour changes. Luckily there have been a number of recent advances in the photographic quantification of colours. Scientists can now use pictures to quantify animal colouration, and use mathematical and physiological models to get an impression of how colours and shades would stimulate an animal’s photoreceptors. In other words we can use photography and computing to measure the colours of animals in the way that animals see them.
Colour me angry
In a study published in the journal Biology Letters, we used these photographic techniques to quantify the colour and colour changes of chameleons – as seen by chameleons – during aggressive social interactions. Our research focused on contests between adult male veiled chameleons (Chamaeleo calyptratus), a species known for its pugnacious disposition.
By filming and photographing the encounters from behind a blind we examined the recordings of each chameleon throughout the trial. The aim was to measure the colour of 28 different body regions in order to uncover if and how the colours were linked to the chameleons' behaviour. Not knowing whether to focus on the speed of colour change, the amount of colour change, or the final colouration displayed, we measured all three.
We found that the lairy lizards most likely to approach their opponents, thereby escalating an encounter, were those that displayed the brightest stripes. This suggests that a male chameleon stepping into a fight may be able to assess just how keen his opponent is by evaluating the brightness of his opponent’s stripes.
What’s more, we also discovered chameleons with more brightly coloured heads whose colours changed more quickly were more likely to win aggressive encounters. So if you, as a chameleon, quarrel with another who reveals an intensely coloured, rapidly changing head – something that should be apparent as he marches directly towards you – then you may have messed with the wrong lizard.
But what benefit comes from signalling to your competitor information about your motivation or fighting ability? It seems that by flagging up very brightly its willingness to rumble in the jungle, the chameleon can ensure aggressive encounters are less costly. Considering a fight could potentially result in serious injury, it’s much better if opponents can be made to back down, without any physical contact being made.
This seems to be what is happening with chameleons, where we found that many contests between aggressive chameleons were resolved without any physical fisticuffs. If the information content of chameleon colour signals was perfect, no contests would actually need to escalate into the head-butting, lunging, biting fracas that we regularly observed. This suggests that chameleons can get information from one another based on their colour, but that this information isn’t always 100% reliable – or that they choose to ignore it in a display of rash, headstrong behaviour. Just like humans do, in other words.
True colours … or not
Humans have devised many and varied means to exchange information with one another, from words to morse code to email. However, as we can frequently influence the behaviour of others from what we communicate, there is sometimes a temptation to misrepresent ourselves, or the truth. Lying can sometimes provide short-term benefits to the liar, but there are frequently large costs in being identified as an unreliable person. These costs exist in the animal kingdom as well, albeit in different forms, and are thought to play a role in the evolution of stable signalling strategies.
While our recent work suggests chameleons possess the means to communicate information through their colour changes, we don’t yet understand how these colourful messages are interpreted, how they influence behaviour, and what mechanisms ensure that a chameleon can accurately assess competitors based on external appearance. Time to get back to the chameleons!