Why Do Chameleons Change Colors?
Chameleons have a reputation for their ability to change color. But, why do chameleons change colors? How does color change work? And what can we learn from this amazing ability?
Where Does The Basic Coloration In The Skin Of Chameleons Come From?
The skin of a chameleon contains yellow pigments, which combined with the blue reflected by a relaxed crystal lattice results in the characteristic green color which is common of many chameleons in their relaxed state.
How Does The Mechanism Of Color Change Operate?
Chameleons have two superimposed layers within their skin. The top layer contains a lattice of guanine nanocrystals.
Milinkovitch and his University of Geneva colleagues – by combining microscopy, photometric videography, and photonic band-gap modeling, and by exposing samples of chameleon skin to pressure and chemicals, discovered that these crystals can be “tuned” to alter the spacing between them.
That in turn affects the color of light that the lattice of crystals reflects. As the distance between the crystals increases, the reflected colors shift from blue to green to yellow to orange to red.
In other words, exciting the lattice increases the distance between the nanocrystals, and the skin reflects longer wavelengths of light. Thus, in a relaxed state the crystals reflect blue and green, but in an excited state the longer wavelengths such as yellow, orange, green, and red are reflected.
Since Milinkovitch and his colleagues made public their conclusions, we know that unlike other animals that change colors such as the squid and octopus, chameleons do not modify their hues by accumulating or dispersing pigments within their skin cells, but instead, rely on the above described structural changes that affect how light reflects off their skin.
Videos On How Chameleons Change Color
3 Main Reasons Chameleons Change Skin Coloration
Milinkovitch and colleagues have also studied the reasons why chameleons change their coloration.
Everybody believes that chameleons change their skin coloration to avoid predators. Of course, chameleons can change the color of their skin to match the environment.
But thanks to Milinkovitch we know this is not the main reason why. Besides this, chameleons change their color as a means of social signaling and as a reaction to temperature changes.
The more elaborate displays, such as when multiple, bright colors appear at once, are saved for these purposes.
1. Camouflage
Much of the “blending in” chameleons don’t require a color change at all, Milinkovitch says. In their natural state, some chameleon species already look a lot like leaves or branches.
Camouflage abilities of chameleons do not only mean to help them camouflage with their surroundings. In a study conducted with Smith’s Dwarf Chameleons, it was proved that chameleons can adjust the degree of their color shifts to the visual capacities of their predators (bird or snake). And there’s a good reason for it: these lizards are utterly defenseless.
They don’t have a dangerous bite, their skin isn’t packed with poison, and they can’t move quickly. Staying hidden is pretty much their only tactic to evade predators.
And even if birds and snakes both feed on chameleons, while the former has a great perception of shapes and colors, the latter doesn’t have such a sharp vision. It’s seen that Smith’s Dwarf Chameleons show more convincing color changes when faced with a predator bird than they do when faced with a snake.
2. Social Signaling
Milinkovitch and his colleagues proved that color changes signal a chameleon’s physiological condition and intentions to other chameleons. Chameleons tend to show brighter colors when displaying aggression to other chameleons and darker colors when they submit or “give up”.
Chameleons are highly territorial. When two males encounter each other, there’s a fierce show-off—in this case, of color.
“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,” found Milinkovitch.
“On the other hand, when the skin becomes excited, the distance between neighboring nanocrystals increases, and each iridophore cell (which contains these nanocrystals) selectively reflects longer wavelengths, such as yellow, orange, or red”, Milinkovitch told.
Male Chameleons would not just use color to impersonate other males but to dazzle females during courtship.
But no matter how brilliant the display, some female lizards won’t be interested—and they’ll use color to let the men know. In many species, females present more conspicuous and contrasted patterns when they are in heat, while they show a darker coloration after mating.
When seeing these signals, males know which females are available. “The female will react, depending on whether or not she’s available,” Milinkovitch says. “If she already has the sperm of another male in her reproductive tracks”, he says, “then she’s going to become very dark, and very aggressive.”
Males can be violent, he says, so females must avoid them if they do not need for insemination. If the female is available she won’t show much color and instead remains a greenish-brown, Milinkovitch says, indicating submission.
A Chameleon Changes Color Out In The Wild Of Madagascar
Another Interesting Fact:
Some 31 different species of Calumma chameleons, all native to Madagascar, have bones that glow when under ultraviolet light. This phenomenon is known as biogenic fluorescence.
The glow results from proteins, pigments, chitin, and other materials that make up a chameleon’s skeleton. This possibly gives chameleons a secondary signaling system that does not interfere with their color-changing ability and may have evolved from sexual selection.
3. Thermoregulation
Because chameleons are ectothermic, another reason why they change color is to regulate their body temperatures. They change either to a darker color to absorb light and heat to raise their temperature, or to a lighter color to reflect light and heat, thereby either stabilizing or lowering their body temperature.
A sample of this thermoregulation use of color-changing is shown by the desert-dwelling Namaqua Chameleon. Namaqua becomes black in the cooler morning to absorb heat more efficiently, and then adopt a lighter grey color to reflect light during the heat of the day.
The Evolutionary History Of Color Change In Chameleons
The color-changing ability evolved several times independently in reptiles, neon tetra fish, butterflies, and cephalopods like octopus and squid.
While the exact evolutionary history of color change in chameleons is still unknown, there is one aspect of the evolutionary history of chameleon color change that has already been conclusively studied: the effects of signal efficacy. Signal efficacy, or how well the signal can be seen against its background, has been shown to correlate directly to spectral qualities of chameleon displays.
It was demonstrated that chameleons in brighter areas tended to present brighter signals, but chameleons in darker areas tended to present relatively more contrasting signals to their backgrounds.
This finding suggests that signal efficacy (and thus habitat) has affected the evolution of chameleon signaling.
Do All Chameleons Have The Same Ability To Change Color?
Not according to Milinkovitch’s findings. Only adult male chameleons change color, especially when they see a rival male chameleon they want to chase away or a female to attract. Females and young chameleons are dull-colored and have a very reduced upper layer of iridophore cells, he said.
Practical Implications of Chameleons Changing Color
The findings of chameleons’ color-changing abilities may help engineers and physicists replicate the chameleon’s color-changing capacities in new technology, such as appliances that eliminate reflection.
For example, scientists have already yielded new smart skin that changes color when exposed to the sun. The material could be used to make everything from camouflage clothing and coatings to chemical and environmental sensors.
Yi Xiao Dong, a doctoral student in Salaita’s lab, suggested creating a hydrogel with two layers, just like the chameleon skin. He created a small, thin flexible structure, not unlike a silicone bracelet, that contained one layer embedded with photonic crystals of iron oxide mixed with silicon dioxide. The other contained a colorless polymer.
In one experiment, Dong shaped a yellow smart skin into a leaf. After five minutes in the sun, the leaf had turned green, which made it blend into a group of leaves he had snipped from a tree outside the lab. Thus he demonstrated the smart skin’s camouflage potential. He performed a similar color change with a fish-shaped polymer.
Dong then created even faster shifts using laser light.
One of the biggest challenges will be making smart skins large enough for clothing, panels, and other human uses.
For Additional Information Check: Chameleons inspire ‘smart skin’ that changes color in sun.
Conclusion
By changing colors chameleons either try to become invisible to predators, which subtle color shifts help them achieve. Or they try to be noticed -again by changing their color, but this time much more explosively to repel another male or attract a female.
Some chameleon species use their color-changing abilities for thermoregulation purposes.
Evolution and environmental conditions have a lot to do with color-changing abilities in chameleons.
Because of their practical implications, research is being conducted to emulate a chameleon’s abilities to change their skin color in special fabrics and appliances.
Source: Milinkovitch’s Original Article: https://www.nature.com/articles/ncomms7368 (Teyssier, J., Saenko, S., van der Marel, D. et al. Photonic crystals cause an active color change in chameleons. Nat Commun 6, 6368 (2015). https://doi.org/10.1038/ncomms7368)
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