How chameleons change colour

By Daniel Todd

Chameleons are perhaps the most intriguing reptiles on the planet. Living in warmer habitats ranging from rainforest to desert conditions, all different species can be found in Africa, Madagascar, southern Europe, and across southern Asia as far as Sri Lanka, while also being introduced to Hawaii, California, and Florida. Often kept as household pets, they arguably have the most interesting physical characteristics, distinguished by their zygodactylous feet with two toes pointing forward and two backward, suited for climbing trees, and their very long rapidly extrudable, are highly modified to their hunting methods. Not forgetting, however, their swaying gait which camouflages them as a leaf blowing in the wind when walking along a branch, and crests or horns on their brow and snout, as well as a prehensile tail, adapted for grasping branches. Nonetheless, their most mystical trick of all – their disappearing act of changing appearance - remains the most impressive of these little magicians’ abilities, and so it is the secret that will be revealed in this article.

A popular belief is that chameleons change colours to disguise themselves, to blend in with their surrounding environment in order to hide from predators. However, chameleons are very fast - many can run up to 21 miles per hour - and can avoid most predators quite easily. Camouflage is therefore a minor reason why most chameleons change their colour.

Since chameleons are ectothermic and thus cannot generate their own body heat, changing the colour of their skin is a way to maintain a favourable body temperature. Chameleons use the fact that darker colours absorb the sun’s heat better while lighter colours reflect the sun’s heat better to their advantage, in how they may become darker to absorb more heat or turn paler to reflect the sun’s heat.

Chameleons will also use bold colour changes to communicate with other chameleons. Males become bright to signal their dominance to other chameleons and turn dark in aggressive encounters. Females can also let males know if they are willing to mate by changing the colour of their skin. Some even speculate that chameleons made this adaptation over time as they became more popular house-hold pets as a way of communicating their moods and feelings more effectively to their owners. For many years, scientists have believed that chameleons have many layers of skin, each containing specialised cells called chromatophores, except the outermost layer, which is transparent. There are different types chromatophores in each layer with different names, as they contain sacks of different kinds of pigments, with the deepest layer containing melanophore cells, filled with brown melanin, the same pigment responsible for the variety of shades present in human skin. Other layers above the melanophore layer include the layer of iridophore cells containing blue pigment, the layer of xanthophore cells containing yellow pigment, and the layer of erythrophore cells containing red pigment.

When a chameleon experiences changes in body temperature or mood, its nervous system tells specific chromatophores to expand or contract, thus changing the colour of the cell. Through varying the activity of the different layers of chromatophores, the chameleon can produce a whole variety of colours and patterns. For instance, an excited chameleon might turn red by fully expanding all of its red erythrophores, blocking out the other colours beneath them. A calm chameleon, on the other hand, might turn green by contracting its red erythrophores and allowing some

of the reflected light from his blue iridophores to mix with his layer of somewhat contracted yellow xanthophores (Mary Bates, 2014).

However, recent research has revealed that, although this theory has some truth, it may not tell the whole story as to how chameleons change their colour. Research conducted in 2014 revealed that pigment movement only represents part of the mechanism chameleons use in performing this spectacle! There are cells, which contain pigment and reflect light, that are made up of hundreds of thousands of guanine crystals. Chameleons can relax or excite their skin, causing these special crystals to move and change structure. Researchers found that this enables these cells to reflect different wavelengths of light to create the variety of tones we see.

To investigate how the reptiles change colour, a study was done on a type of lizard from Madagascar called ‘Furcifer pardalis’, more simply known as panther chameleons, investigating the colour changing of five males, four females and four juveniles. The scientists found that the chameleons had two superposed thick layers of blue iridophore cells. The iridophore cells contain nanocrystals of different sizes, shapes, and organisations, which are key to the chameleons’ dramatic colour shifts.

The chameleons can change the structural arrangement of the upper cell layer by relaxing or exciting the skin, which leads to a change in colour: “when the skin is in the relaxed state, the nanocrystals in the iridophore cells are very close to each other — hence, the cells specifically reflect short wavelengths, such as blue,” says Milinkovitch, a professor of genetics and evolution at the University of Geneva in Switzerland. The yellow from the xanthophores, plus the blue light reflected from the iridophores, results in the colour green, which chameleons appear to be when calm. This is shown in the picture below – you can see how, when the nanocrystals are close together, all the colours in the spectrum are absorbed except for blue, which is reflected, and passes through the yellow xanthophore cells to create a green colour.

On the other hand, when the skin becomes excited, the distance between neighbouring nanocrystals increases, and the iridophore cells containing these nanocrystals selectively reflect longer wavelengths, such as yellow, orange or red. In the picture below, you can see how, when the nanocrystals are much further apart, all the colours in the spectrum are absorbed except for red, which is reflected, and passes through the yellow xanthophores to create a similar yellow, orange or red colour (Laura Geggel, 2015; Michelle Konstantinovsky, 2019).

To prove this new explanation of how chameleons changed colour, they tested their theory by looking at the nanocrystals in a male while it is calm, and then also when it is excited or angry. The results showed that the nanocrystals were closer together when the chameleon was calm, giving it its green colour, and further apart when it was excited or angry, giving it more yellow, orange, and red colours. This can be seen below:

Finally, the researchers tested this theory even further by physically putting pressure on the skin of one of their chameleons, which is shown below:

Once the pressure was released, the skin had turned blue where it had been put under pressure:

After a few seconds, the skin returned to a green colour as before. These results showed that, by putting pressure on the skin, the nanocrystals are forced closer together, which makes the skin reflect even shorter wavelengths such as purple, creating a blue colour as it passes through the yellow xanthophores. As the pressure is released, the nanocrystals can spread out and expand again, which makes it reflect the slightly longer wavelength of blue light, creating a green colour as it passes through the yellow xanthophores. By replicating what would happen naturally (as a response to temperature, or as a way of showing mood), the researchers managed to prove this theory by instead putting force on the surface of the chameleon’s skin.

However, the researchers also discovered that only adult male chameleons change colour, especially when they see a rival male chameleon they want to chase away, or a female to attract. Females and young chameleons are dull-coloured and have a very reduced upper layer of iridophore cells, the researchers found. Furthermore, they found a deeper and thicker layer of skin cells that reflect a large amount of near-infrared sunlight. While these cells do not appear to change colour, it is possible that they help the chameleons reflect heat and stay cool, the researchers said (Derek Muller, 2015).

This topic has become especially important in recent years, as a similar mechanism that chameleons use to change colour has been used on a material that changes colour when flexed. In previous attempts to create a material that could change colour, materials were designed to select particular wavelengths of light by using narrow slits, but this approach made the colours dim. Instead, researchers from the University of California, Berkeley, etched ridges onto the surface of a layer of silicon 1,000 times thinner than a human hair. Depending on the spacing between the ridges, the silicon, which reflected up to 83 percent of the incoming light, displayed brilliant green, yellow, orange, or red. The scientists then embedded a 1-centimeter square of etched silicon into flexible silicone and were able to obtain a range of colours by stretching the material.

This method is very similar to the method chameleons use: the material changes colour as the distance between the ridges is altered, making the material reflect shorter or longer wavelengths, in the same way that chameleons can alter the spacing of the nanocrystals in their skin, making the cells reflect shorter or longer wavelengths as well. Possible applications for this material include outdoor display technology and camouflage, among other things. “This is the first time anybody has made a flexible chameleon-like skin that can change colour simply by flexing it,” said Connie Chang-Hasnain in a paper published in Optica, The Optical Society’s (OSA) high-impact journal, in March 2015 (Jenny Rood, 2015). However, this chameleon-skin mechanism has also found applications in a laser able to change colours, which could ‘provide advances in responsive optical displays, wearable photonic devices, and ultra-sensitive strain sensors’ (Danqing Wang, Teri W. Odom, 2018). It is quite amazing to think that chameleons can achieve this naturally while scientists are still struggle to replicate it synthetically.

In conclusion, different sources seem to suggest that chameleons are able to pull off such a mesmerising spectacle in different ways. Some say they use the first theory of the specialised pigment-filled or contract to produce different hues, while others say they use the second theory of the nanocrystals which spread out or clump together to reflect different wavelengths and colours. The most popular theory currently, however, is that the nanocrystals reflect specific wavelengths of light, as explained in the second theory, which can be adapted as the light passes through the skin containing pigments, as described in the first theory, although the actual contraction and relaxation of the chromatophores containing these pigments in the first theory may not be as accurate as previously thought. References:

•Chameleons. (Last edited 2020, June 15/Accessed on 2020, July 3). Chameleons. [online] Wikipedia. Retrieved from https://en.wikipedia. org/wiki/Chameleon •Bates M. (2014, November 4). How Do Chameleons Change Colors? [online] WIRED. Retrieved from https://www.wired.com/2014/04/ how-do-chameleons-change-colors/#:~:text=Since%20chameleons%20can’t%20generate,maintain%20a%20favorable%20body%20 temperature.&text=The%20outermost%20layer%20of%20the,contain%20specialized%20cells%20called%20chromatophores. •“Why Do Chameleons Change Their Colors?” (Accessed 2020, July 1). Why Do Chameleons Change Their Colors? [online] Wonderopolis. Retrieved from https://www.wonderopolis.org/wonder/why-dochameleons-change-their-colors •Geggel L. (2015, March 10) Chameleons’ Color-Changing Secret Revealed. [online] Live Science. Retrieved from https://www.livescience. com/50096-chameleons-color-change.html •Konstantinovsy M. (2019, June 21) How Chameleons Change Color and Why They Do It. [online] How Stuff Works. Retrieved from https://animals.howstuffworks.com/animal-facts/chameleons-change-colors.htm •Muller D, Hat D. (2015, March 11). How Do Chameleons Change Color? [online] Veritasium. Retrieved from https://www.youtube.com/ watch?v=SQggDnScsvI&t=1s •Rood J. (2015, March 16). Chameleon Skin Mimic. [online] The Scientist. Retrieved from https://www.the-scientist.com/the-nutshell/ chameleon-skin-mimic-35779 •Wang D., Odom T. (2018, September 17). The chameleon and the crystal maze. [online] Laboratory news. Retrieved from https:// www.labnews.co.uk/article/2025064/the_chameleon_and_the_crystal_maze#:~:text=In%20nature%2C%20chameleons%20can%20 easily,that%20seen%20in%20chameleon%20skin. •Arnold C. (2019, September 11). New ‘smart’ skin changes color using a trick learned from chameleons. [online] National Geographic. Retrieved from https://www.nationalgeographic.com/animals/article/ chameleon-inspired-smart-skin-changes-color •All pictures were retrieved from https://www.youtube.com/ watch?v=SQggDnScsvI&t=1s. Muller D, Hat D. (2015, March 11). How Do Chameleons Change Color? [online] Veritasium.

acknowledgements

Ingenium only came together due to the driven, hardworking students that contributed to this. Each writer put their best effort in researching endlessly on their respective topics, to allow you all to expand your knowledge of science. They have honed their skills massively and the future of Ingenium seems extremely exciting with the prowess of our writers. Senior students including: George Ogden, Varun Ravikumar, Joe Greenway, Sam Greenway, Henry Bishop, Jack Byatt, Ben De Sousa, Joshua Todd, Surya Vijayanand, Aman De Silva and Yashvardhan Shetty have been an inspiration to the younger years through their contributions to Ingenium, (in particular George, Varun, Yash, Joe and Aman who have also contributed to this edition) over the years and we hope to carry on their legacy. The renewed board including: Jared Thompson, Franco Hillier, Daniel Todd, Adhiraiyan Sasikumar, Finlay Evans, Simeon Wren, Taisei Masumoto and Matthew Johnsen have proven to be extremely capable scientists and without them this edition of Ingenium would not be possible, whether it was reviewing an article or writing one, these students have all massively contributed to Ingenium. The future of Ingenium holds an abundance of potential. This goes without saying, but this all began because of the vision of a student: Yashvardhan Shetty, he has truly been inspiring to all of us and we thank him for creating Ingenium, providing opportunities to students to go beyond.

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