The “secrets” of how chameleons change their colour
The ability of a chameleon to change its colour to camouflage into the environment is something that truly captivates the imagination. A recent article was submitted to the journal Nature Communications that has claimed to explain some of the secrets to how these creatures can create changes in their skin colour. Significant media attention has been focused on this story, a selection of which are linked below. However, we wanted to dig a little deeper to find out what is so significant in this study.
For well-over one hundred years, it has been understood that animals can create colour in two distinct ways, pigment colours and structural colours. Pigment colours, as you might imagine, are analogous to having different colours of ink in your pen. Pigments absorb certain colours (or wavelengths) of light and reflect others – the reflected colours of light will define the colour that is observed. Structural colours are quite different, with the most commonly cited example being oil on water, where rainbow colours can be seen. In this case, colours are created by utilising small structures of materials (or thin layers, like the oil on water) that interact with light, causing interference and refection that is colour (or wavelength) specific. In 1913, a German researcher called Ballowitz studied the structural colour in the scales of several fish, including the seabass. Other researchers, such as Suffert (Germany) and Mason (UK), detailed how many colourful insects also obtained their colours through similar mechanisms (1920s and 30s). In these studies, small nanoscale crystals, called iridophores, were observed to be responsible.
Many years later, in 1990, Copper published work on how squid can change their colour rapidly by changing the alignment and spacing of these iridophores – the first demonstration of how biological systems can actively and quickly change their structural colour. It will be little surprise that lizards would soon be investigated in a similar way, and in 1996 Morrison detailed how one particular lizard could change its belly to blue through structural colour change.
So what is novel in this article? Well it is the first paper demonstrating this phenomenon in chameleons. However, the authors discovered something that no one has observed before in nature – which is arguably far more exciting – that an underlying second layer of iridophores exist with a larger and somewhat random spacing. The larger spacing in the structural arrangement means that longer wavelengths of light (in this case, infrared) will be reflected. Further studies are required to investigate what is happening here, however the authors of the paper propose, quite logically, that the chameleons use these to reflect the heat from the Sun and possibly allow them to have a competitive advantage to other animals when exposed to significant warmth.
A fascinating study, however, possibly less so regarding how chameleons change colour and more so regarding how chameleons have developed quite sophisticated technology for remaining cool in the blazing heat. All the articles we stumbled across had chosen to focus on the ability of chameleons to change appearance using structural colour, which was probably already known (or at least assumed) by chameleon experts. The ability of chameleons to stay cool in the heat, through finely tuned infrared reflectors built into their skin, is truly remarkable – although perhaps the novelty of this takes a little longer to capture the imagination than the ability of these animals to dramatically change colour!
Teyssier, J. et al. (2015). Photonic crystals cause active colour change in chameleons. Nature Communications. DOI:10.1038/ncomms7368