Article curated by Joshua Fleming
There are over two hundred species of Chameleon; a lizard that has lived on the ‘Old World’, when there was supposedly only three continents. These ancient animals hold a fascinating blend of traits that scientists have been studying for many years. Recent breakthroughs in several areas of chameleon research have built the beginnings of new conclusions about the animals behaviour, but fresh questions have also arisen…
Colour Changing Conundrum
Probably the most defining characteristic of the chameleon is the changing of their colour. These transformations can be both rapid and incredibly complex - exactly how the camouflaged creatures do this is not fully understood. Chameleons change colour for several reasons. It acts as a social signal; male chameleons show light, bright multicoloured patterns when attempting to court females, but turn much darker shades if they are angry or trying to appear intimidating. Some chameleons also use their ability for camouflage, and others for absorbing and reflecting heat depending on the time of day.
Chameleons have structural colours and pigments. The latter, now found to be mostly yellow, is like a skin ‘stain’, which scientists used to think was dispersed in different ways to change the chameleons’ appearance. However it is now apparent that chameleons have two layers of cells called iridophores within their skin, with the top layer specifically being responsible for the colour changes. Iridophores contain layers of nanocrystals which reflect light and produce the mesmerizing colours we see on their skin. To change from one colour to another the chameleon has to change the structure of these nanocrystals. They will then reflect light differently and new hues become visible.
Researchers discovered in 2015 that a calm chameleon has a tight packing of nanocrystals. This structure reflects blue light and so mixed with the yellow skin the chameleon looks green. But when its mood becomes more active the crystal lattice loosens and yellow and red colours are then reflected and seen. The second layer of iridophores is less organised, but the mess of larger nanocrystals works remarkably well in blocking the sun's damaging rays, preventing the chameleon overheating. For now we don’t know how the chameleons control their nanocrystal networks, or how they develop in the first place, but that is the direction research is now headed in.
A chameleon’s tongue is one of the most bizarre in nature. It has a cylindrical end, literally engulfing their prey before it’s even been pulled back into the mouth and eaten. Firing out and accelerating over 41 g’s (the maximum a human has reached on a rocket sled is 46.2g), reaching nearly twice its body length and hitting prey in less than a second - it is a fearsome weapon, and the fastest tongues on Earth. But because that sheer amount of force and power can’t actually be produced by a muscle, scientists aren’t certain as to how it works.
The prevailing theory is the use of an ‘elastic recoil’ mechanism. A study over 10 years ago was one of the first to provide evidence for the mechanism. But since then none have been carried out to improve on a paper that lacked experimental data for some of the physiological details. Elastic recoil has been studied more recently in several types of fish, but it remains unclear whether this mechanism is responsible for the movement of the chameleon tongue.
Chameleons have long been thought to have independent eyes; unlike humans they can move them in different directions separately and seemingly randomly compared to each other. This ability, called ‘voluntary strabismus’, was tested by getting chameleons to play a computer game…
The game involved showing the creature a double image of an insect. Each replicated insect would be moving away in opposite directions and the chameleon would try to catch one of them with it’s tongue. They found that the chameleon would focus with one eye at first on one image, before deciding to go for the insect. If the eyes were truly independent with regard to co-operation, the other eye wouldn’t be expected to play a role. Instead, the second eye snapped round to focus on the same insect as the first eye. The difference in how scientists now perceive the chameleon eye independence is similar to how we consider our hands work. Prior to the research, scientists believed that each eye had no influence on the other; imagine our hands moving without regard for the other, unaware what it’s counterpart was doing, making tasks like typing with both hands impossible. But after assessing these results it is clear that some level of cooperation is present; we can type with one hand at a time in what would appear to be an entirely independent and non-cooperating fashion, but we can also type with two, working together.
It may be that some sort of ‘cross talk’ occurs, where each eye knows what the other is doing and movements are coordinated, but individual. Then, when a point of focus is chosen they co-operate. This is one of the first examples of this behaviour to be studied comprehensively; whether it is the correct theory and can be applied to other species of chameleon remains to be seen. A greater understanding could also help in the study of other animals with eyes that work on different levels of independence; for example the frogmouth bird is capable of binocular vision (the same as humans) but if it feels threatened it’s eyes then move separately to cover more of it’s surroundings.
This article was written by the Things We Don’t Know editorial team, with contributions from Joshua Fleming.
This article was first published on 2018-11-10 and was last updated on 2018-11-10.
why don’t all references have links?
 Teyssier, J., et al., (2015) Photonic crystals cause active colour change in chameleons Nature Communications 6(1) DOI: 10.1038/ncomms7368
 de Groot, J,H., van Leeuwen, J,L., (2004) Evidence for an elastic projection mechanism in the chameleon tongue Proceedings of the Royal Society B: Biological Sciences 271(1540):761-770 DOI: 10.1098/rspb.2003.2637
 Katz, H,K., et al., (2015) Eye movements in chameleons are not truly independent - evidence from simultaneous monocular tracking of two targets Journal of Experimental Biology 218(13):2097-2105 DOI: 10.1242/jeb.113084
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