‘With a flick of its tail it’s got the attention of the other lizard,’ says zoologist Dr Peters, explaining the communication antics of one of his subjects, a Jacky lizard, also known as a tree dragon.
Captured on video, it then directs other movements to an intruder: a swipe of its foreleg, a little push up, a body-rock followed by a leap from its rock and some rap-like antics.
‘It’s quite a complex display, and you get variations upon that across a wide range of other lizards,’ he says.
While we may be a long way from understanding the full lexicon of lizard language, Dr Peters says the tail flick and other movements signal territory ownership to a rival male.
‘Basically it’s to indicate he’s there: “This is my territory, you best move on”. The intruder can then decide to move away, maybe give a signal to say he’s just passing, or perform his own signal to challenge the resident.’
This can result in a fight, with the winner retaining his territory or taking over, giving him access to females whose territories overlap.
Dr Peters says movement-based signalling is seen in most Agamid – or ‘dragon’ – lizards, like bearded dragons, water dragons and Jacky lizards as well as in smaller lizards like skinks.
‘Although with skinks it’s much simpler. They do tiny little head bobs, very simple motor patterns.’
Dr Peters says in other parts of the world, for example Anolis lizards of the Caribbean will often extend their throat fans, which usually are brightly coloured, and then retract them.
Such motion cues are used for long-range signalling, he says. ‘They then might draw attention to static visual cues like colour or patterning. We see a lot of lizards, even in Australia, that have motion signals but also bright-coloured patches.’
Dialects of different tail flicking
Thanks to pain-staking work by Dr Peters involving computer-generated 3D graphics and algorithms, we are also beginning to understand that, as among humans, lizard signal language also has ‘dialects’ which vary depending on where lizards live.
He explains: ‘We know animals vary their sound according to their habitat, depending on what other sounds are in the environment. If there’s lots of noise, maybe from other species or from humans, animals will adjust their audio signal to ensure it is heard either above the background noise, or at different frequencies.
‘I have been looking for the same kind of explanation for movement-based signals. It’s very technical work and difficult to unravel the factors involved.’
Clearly, he says, lizard signals have to be detectable against plant movement, so the signals have to differ from plant motion. His experiments use motion analysis techniques to determine the velocity, or movement, information in the signal.
‘I film lizards displaying and I film plants, and then I use motion analysis algorithms to identify the spatio-temporal information in a particular scene, whether it be from a lizard signal or plant movement.’
When there is lots of plant movement, lizards adjust their signal. So rather than fixed signal designs, Dr Peters has established that lizards adapt their signalling to their local environment.
One strategy is to signal faster, which Anolis lizards seem to do. However, Jacky lizards do not and this is probably because of the different habitats they live in. ‘If you do some calculations, you can explain mathematically why it doesn’t make sense to signal faster if you’re right up against plants.’
‘I’ve demonstrated that in Jacky lizards, where I manipulated wind conditions and therefore plant motion, there was clear evidence that all the animals tested changed their signal to longer-duration tail flicking and to intermittent movement, start-stop signalling, to which motion vision systems are particularly sensitive.’
(Apart from signals and sounds, Dr Peters says lizards also use olfactory communication. It occurs in skinks and a number of lizards. With the dragon lizards there is some use of chemical signals for territoriality.)