We all tend to change our appearance to suit the occasion. Some of us might paint our faces to stalk deer in the woods or maybe you wear all black if you're in mourning or if you just want to look cool. Maybe you decorate yourself up all nice when you're trying to attract a mate. But by large you can't rely on the color of a person's outfit, let alone the color of their hair or their skin, to tell you how they're feeling or if they're interested in you or if they feel threatened. And that's also true with most animals, but some of them -- some of them have special powers.

Chameleons have this power, of course, but so do octopuses and cuttlefish as well as a few insects and even in a way some mammals and birds. The ability to change color is relatively uncommon in animals, but for those that have it, it's an incredibly useful evolutionary adaptation.  

Sure, blending in with a bunch of leaves, a chunk of coral, or a snowbank or something is super handy when you're hiding from predators or prey, but color changes also help animals communicate, signaling bad moods or the look of love. I won't tell you what "this" means though. It's a secret. 

Different kinds of animals use different feats of biology to change their coloration. Sometimes the process happens slowly and in keeping with the seasons, but other times it happens as quick as flipping a switch. So let's start with the animals that change color the most slowly: the seasonal changers.

Snowshoe hares, ermines, and arctic foxes all live at higher latitudes and undergo seasonal color changes. They and other animals like them typically do it by molting, which is the process of shedding their outer coverings, and in this case, their fur. As the days get shorter or longer, these cold-weather mammals start to molt, slowly morphing over a period of weeks mostly brown to pure white in the fall as they grow new fur and changing back to brown in the spring. Their winter coats also tend to be thicker, softer, and more densely-packed than their summer frocks. The ptarmigan also experiences seasonal changes to promote camouflage as they molt or grow new brown and white feathers in the spring or fall to better blend in with their environment. 

But just as we can't control the spreading of gray hair on our own heads, these animals have no say over the composition of their fur or feathers. In snowshoe hares for example, the cue to change color comes from the pineal gland of the brain, which senses seasonal changes and daylight length. Other animals have pineal glands, too, including humans. Theirs which produce melatonin, which is the hormone that regulates our natural circadian rhythm. Our pineal glands are also affected by seasonal changes, but luckily our hair doesn't turn white every fall. 

Some other species though can change colors more quickly and reverse them more easily. Amphibians, insects, fish, and cephalopods don't wear fur or feathers and thus don't really need time to grow a whole new outfit nor are they bound to seasonal changes. The title of Champion Color Changers though would probably have to go to cephalopods, which are members of the mollusk family like cuttlefish, squids, and octopuses. And for this distinction, most cephalopods can thank their collection of amazing, specialized chromatophore cells just under their skin. These special cells contain a stretchy sack full of pigment which is attached to a bunch of muscle fibers, and when those muscle fibers contract, that elastic sack stretches out and its pigment suddenly covers a larger area. And when those muscles relax, it shrinks. The effect is kind of like a spot of ink on a balloon: if the balloon is deflated, there's just a tiny concentrated dot of color, but if you blow it up, the color expands.

Octopuses and other cephalopods have pretty amazing control over these individual cells, and they can enlarge one sack while shrinking another to create any number of complex patterns. Cephalopods' chromatophore cells are also hooked up to nerve endings and are largely controlled by the brain. This allows the animals to change their colors and patterns really fast when faced with danger or depending on their mood.

But different chromatophore pigments can only produce particular shades of black, brown, yellow, red, and orange. To rock those flashy shades of blue and green or super-fly silvers, golds, and metallics like cuttlefish and some species of squid and octopus do, you need iridophore cells. These reflective cells are found beneath the chromatophores and are either protein based or are made of chitin, the stuff that many animals' exoskeletons are made of. Rather than producing pigment, iridophores work by reflecting light, like a mirror, which helps cephalopods blend in with their environment. And these cells aren't controlled by the nervous system like the others, although some new research suggests that they may actually respond to changes in hormone levels. And because they're found in a deeper layer of the skin, these reflective cells can either be covered up or revealed by the action of the chromatophores up above them. These layers of specialized cells are also what give chameleons their fancy technicolor-dream coats. Interestingly, it's a common misconception that they change color to camouflage themselves. It's not true. I mean, this guy would have to be sitting in front of a paintball-wielding clown to blend in. Instead, they change their skin color more to communicate their moods and to regulate their body temperature. Males of some species for example turn bright and bold when they want to show dominance and then turn dark and brooding when they're ticked-off. Ladies meanwhile change hues to show that they're up for mating.

So since reptiles are cold-blooded and can't generate their own a heat, a chameleon can also use its skin color to regulate its body temperature, turning pale to stay cool on a hot day or turning dark to absorb more sunlight when they get chilly. They're able to achieve these color fluctuations thanks to those same chromatophore cells. The different types of chromatophores in chameleons' skin specialize in different colors, and they're found at different layers within the skin. Their outermost layer of skin is actually transparent and just below that the chromatophores start. The first layer of chromatophores that start are called xanthophores, which contain yellow pigments. The next layer down has red-colored erythrophores, and beneath that is reflective, often bluish iridophores. The final layer down contains melanophores, which possess shades of brown pigment called melanin, just like what we humans have in our skin. 

So, just like in those color changing cephalopods, these layers in chameleons can interact with each other to create various effects with some cells activating to expand and others catering to contract, thereby showing a specific color or combination of colors while obscuring others. That's how chameleons can end up looking like "this," or "this," or "this." However, these changes are controlled by the chameleon's hormones and not its nervous system, so they occur a little more slowly than a  squid's. And finally chameleons, octopuses, and arctic foxes aren't the only ones with knack for canny wardrobe changes -- meet the golden tortoise beetle.  

Reversible color changes are extremely rare in the insect world, but the golden tortoise beetle is the first known insect species to demonstrate rapid and controlled color change. This tiny metallic insect is a member of the leaf beetle family found in North America, and it's quite lovely no matter what shade it's assuming. Like chameleons, they change color when agitated, but they do it most impressively while mating. One researcher observed that during calculation, which incidentally could last up to 9 hours, if these amorous little guys started out wearing a brilliant bling gold, they'd fade into a goldey orange and then eventually a sort of brownish orange with black spots, but the ones that started off more dull colored ended up bright gold. In either case, these changes started as quickly as 2 minutes into the "festivities."

Beetles make this color-changing magic happen using a kind of optical illusion. Using electron microscopes, a group of Belgian researchers were able to identify that the Panamanian tortoise beetle has a transparent shell containing a three-tiered structure that sits on top of a layer of red pigment. Each layer is full of tiny, tiny grooves and when they're flooded with this liquid pigment, they create a smooth surface that reflects light perfectly, giving the critter its metallic look. But say you come around and start poking that beetle and agitating it. It may respond to that stimulus by draining the red pigment from all those grooves, and suddenly you can see through those layers down to the pigment at the bottom. We still don't know exactly why tortoise beetles change color. It might be about signaling to mates or defensive camouflage, maybe the metallic gold provided glare that makes it hard for birds to see them. And some suggest that turning orange and spotted may be a type of Batesian mimicry as the critters try to fool predators by mimicking nasty tasting species like in this case the common lady beetle. Whether it's about expressing themselves, creeping around, staying warm, or just trying to get some lovin', animals changing their appearance is just another example of how freakin' cool nature is. 

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