Lizard Wizardry: Gecko Popcorn and Chameleon Crystals
The gecko just revealed another trick: turning water into popcorn. We’ve reported about geckos’ amazing ability to climb smooth vertical surfaces with atomic adhesion, using microscopic hairs on their toes that cling with van der Waals forces. That’s been inspiring biomimetic engineers for years. Now, another trick has been discovered that could lead to improved waterproof surfaces. An eye-catching video at New Scientist, accompanied by a popping-cool soundtrack by Jimmy Fontanez, shows how the lizards make water literally fly off their skin like popcorn.
This "explosive self-cleaning trick" works because of a skin structure "thought to be unique" to these animals (emphasis added). The water beads up and then, when the droplets reach a sufficient size, they jump off like popcorn. This is "caused by a dense array of minuscule, hair-like structures called spinules, which cover the bodies of many geckos."
Most drops are launched far enough away for them to be removed from the vicinity of the skin by external forces, such as gravity or wind. Any drops that do not initially escape return to the skin and can combine with other drops to promote the jumping process. Jolanta Watson reckons that this phenomenon might have evolved to prevent moisture-loving microbes from thriving on the lizard’s skin, and it might also work as a self-cleaning mechanism.
Isn’t evolution wonderful? It thought of everything. The health and materials industries could apply this trick to create bacteria-resisting, unwettable surfaces. Let’s hope they don’t try to do it with blind, unguided processes.
Another kind of lizard is in the news. Science Magazine posted a video clip (sped up 8x) of a chameleon changing from lime green to bright yellow. The secret to the amazing color change, Robert F. Service writes, is "tiny crystals" in their skin.
Scientists had suggested they may do so by moving pigments around in their skin cells. Today, however, researchers report online in Nature Communications that chameleons change colors by rearranging a lattice of nanocrystals in one of the top layers of skin cells. These cells, called iridophores, contain tiny crystals made from guanine, one of the nucleic acid building blocks of DNA. The nanocrystals have a highly ordered arrangement, which normally causes them to strongly reflect one color of light, such as green. But as shown in this video, when another male enters their surroundings, the animals stretch their skin cells, broadening the nanocrystalline lattice, thereby causing it to reflect a longer wavelength of light, such as yellow. In addition, the team found that chameleons contain a second, deeper layer of iridophores that reflect heat-producing infrared light, which presumably helps the animals stay cool.
At the BBC News, Jonathan Webb notes that this makes the explanation more complex than previously thought:
It was previously suggested that the reptiles’ famous ability came from gathering or dispersing coloured pigments inside different cells.
But the new results put it down to a "selective mirror" made of crystals.
An unusual team made these discoveries: evolutionary biologists and quantum physicists. The physicists looked at the iridophores under an electron microscope, and found "an incredibly neat, regular pattern" of crystals — just the sort of arrangement that can play tricks with light. The structures are called "photonic crystals" in the paper; similar methods of producing "structural color" are known in other unrelated animals, including butterflies and birds.
The electron micrographs of the iridophores are reproduced in the BBC News article. When the chameleon stretches its skin, the pattern also stretches, affecting the wavelength of the refracted light. The second layer underneath, with its chaotic arrangement of crystals, is better at reflecting infrared light (heat). That’s why they believe it functions to keep the animal cool.
And the lizards might need to keep their cool. Only the males have the color-changing ability. They use it to attract females or scare off rivals. Females and young chameleons are dull colored and have a reduced upper layer of iridophores.
The evolutionary biologists speculate about how these traits came to be. For one, they had to acknowledge, "The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection." Co-author Michel Milinkovitch of the University of Geneva says, "Chameleons invented something completely new in evolution." This hardly constitutes an explanation. In fact, none of the articles did better than to call this an "evolutionary novelty."
These are just two of the amazing traits found in the lizard families (class reptilia, order squamata). The humble vertebrates that scurry around our feet deserve a closer look. They may seem more "primitive" than the birds and mammals, but the specialized adaptive traits in lizards are really astonishing when you look closely at these two spectacular examples, then consider all the others: iguanas, horned lizards, gila monsters, Komodo dragons, and the many colorful species on Caribbean islands, to name a few.
Many entries at ENV about the Cambrian explosion have documented the sudden appearance of body plans. Darwin’s dilemma only begins with the Cambrian explosion, but that explosion is just one of many explosions. As Casey Luskin reported, the fossil record shows a fish explosion, an angiosperm explosion, a mammal explosion, and a bird explosion. It appears there was a lizard explosion as well. How could mutation and selection produce a lizard from a Cambrian swimming vertebrate? How could that lizard produce skin with "evolutionary novelties" like structural colors, or toe pads that can walk up glass? Evolution can’t work by subtraction. At each stage, there must be new genetic information added to the genome.
William Dembski makes the case in Being as Communion that the fundamental entity of the universe is information. This is a top-down view. The evidence from chameleons, geckos, and countless other superbly crafted animals illustrates that, from the top, it’s information all the way down.