A couple of years ago, I was walking in a city park with a friend from school. During our walk, I occasionally pointed out any living thing I spotted. A green stick-insect lying on a branch, some damselflies hovering over the little stream, or just a cool-colored beetle that mixed with the gravel. “I’ve been here countless times, I didn’t even know I could find these here. How did you even distinguish them?” – she said. I had no explanation, but it got me thinking: I was not able to some years ago. Things like this take practice, for sure. I would not even say that I am that good at it. I have friends that would have their hands full of awesome insects and lizards during the first five minutes of our field trips just while setting up camp!

Anyway, after years of herping, I guess your eyesight sharpens up. But there’s always those dudes that you cannot find. Or at least those ones that I cannot see as I walk by them until some superhuman points them to me. How do they do that?!

Camouflage

Camouflage can take many forms and colors (even sounds!). There are several ways for animals to go by without being noticed by their predators or to sneak behind their prey. Maybe what first comes to mind are octopuses, zebras or chameleons. But there are so many invertebrate and vertebrate animals that even have different means of camouflaging themselves. Sometimes, it’s not even for hiding purposes, but for sending signals between their own species or even to regulate their temperature.

But the question still is: HOW do they do it?  How can an animal know what color or pattern their environment will be like and be able to “match” it? What if they go somewhere else? What about the animals that can change their color back and forth? 

Crypsis and Mimicry

Let’s break down the terms a bit, as “camouflage” is too general. On this first entry on the topic, I’ll refer only to the ones I will explain below: masquerade mimicry (mimicry = to mimic) and background-matching crypsis (cryptic = hard to detect). Overall, the mechanisms for each type of camouflage are still not fully understood. Actually, even the terminology is still being debated, but let’s stick to those names for the time being.

Masquerade

Mimicry is when an organism resembles a non-related species or an inanimate object. For the latter, the most common term is “masquerade mimicry“, like the stick-insects or the leaf-like frog (Edalorhina perezi) in the above pictures. This type does not necessarily prevent being seen or detected, but it does prevent being recognized. 

An adaptation like this takes a looooong time to get established. For mimicry to evolve, usually a major change or mutation would have had to occur within a population. To be clear, we’re talking about mutations occurring during sexual recombination, and not a mutation developed on one individuals’ lifespan (as these are commonly detrimental and won’t make it through).

When mutations are beneficial, the reproductive success of the individuals possessing this mutation would pass on to the next generations. To put it in lay terms, individuals that have a characteristic that helps them survive until reproductive age will pass this characteristic on to the next generations. Eventually, the entire population would have this mutation(s). So masquerade mimicry is guided (over long periods of time) by sexual and natural selection.

Crypsis

Hiding is not the same as being cryptic.

Crypsis is basically about avoiding detection by ones’ own means. One of the forms of crypsis is background-matching. In the case of amphibians and reptiles, most species have some specific coloration and can only make themselves lighter or darker. In other cases, like chameleons, they can change the tones and colors a little bit more. In the former, the way to get to a population whose color matches their environment is similar to what we explained for the masquerade mimicry (natural selection). It takes time. But to understand how they can change color, also for the latter case, we need to first know how their skin is structured.

What lies beneath…

Most vertebrates’ skin is composed of very similar layers. Below the epidermis, some things might look different from species to species. Beneath the first layers, we find a complex referred to as the “dermal chromatophore unit”. This is composed of different types of cells.

Pigmentary chromatophores

Pigmentary chromatophores provide color by selectively absorbing certain wavelengths of light. These cells contain different pigments, which are the ones responsible for this absorption. In amphibians and reptiles, there are three main types:

1. Brown-Black-euMelanophores (containing melanin)
2. Red Erythrophores (carotenoids, pteridines or combination)
3. Yellow Xanthophores (carotenoids, pteridines or combination)

Structural chromatophores

Structural chromatophores aid in color by reflecting or scattering certain wavelengths with cellular nanostructures. There are two types of these:

1. Iridophores (colorless; reflect a specific spectrum of light including iridescence)
2. Leucophores (reflect only white light)

How does it work?

Inside the pigmentary chromatophores, we find the above-mentioned pigments within the cells’ cytoskeleton. Each pigment can absorb a different wavelength. For example, for us to see an animal green, all this has to happen at the same time: the Xanthophores absorb the short wavelengths (violet-blue tones), the Iridophores reflect medium wavelengths (yellow-green tones), and the Melanophores absorb the longer wavelengths (red tones). Each species has a specific chromatophore arrangement.

Structures in the cell’s cytoskeleton, such as microtubules and actin filaments, can push and pull these pigment granules inside the cell with the aid of motor proteins. In melanophores, for example, when the pigments are close together around the nucleus, the animal will appear lighter (since they are only absorbing light in a small region and the rest is reflected). On the other hand, if the pigment is spread all over the cell, they are absorbing light on a wider surface, so they will appear darker.

And how do the cells decide when to have the pigments together or spread? 

Neural and hormonal responses to their environment induce these reactions. When faced with a stressful situation (like being near predators or intraspecific rivalry) a neural or hormonal signal would send chemical messengers (e.g. alpha-Melanophore-Stimulating Hormone) that activate the cyclic-adenosine-monophosphate (cAMP)-synthesizing-enzyme inside the cells. The intracellular concentration of cAMP signals the cellular structures whether to disperse the melanosomes (too much cAMP) or to bring them back near the nucleus (decrease in cAMP). Whether this is neurally or hormonally controlled depends on the species (and can also be a combination).

In the case of the structural chromatophores, their reflectance will vary as they change position or direction.

The color changes can also be induced by temperature or light availability, so depending on the stimulus, different chromatophores will respond.

Overall, the amounts of each type of chromatophore and how they are arranged is what varies in each species (or even on different populations of the same species).

But how can chamaeleons CHANGE color?

Although chameleons can change their color to a certain extent, the green-to-purple-to-red-to-yellow to even mixed patterns shown in movies is NOT true. Sorry to disappoint you.

The truth is, in between the iridophores on chameleons’ skin there are tiny crystals (guanine nanocrystals). These crystals will reflect different wavelengths depending on how close together they are. The color reflected will act upon the individuals’ “base” color (created by the other chromatophores). So, for example, if the skin is relaxed, the crystals are clustered together reflecting short wavelengths. When this is combined with the underlying yellow tone, we see them as green. If it is excited, the skin would stretch, increasing the distance between the nanocrystals thus reflecting different, longer, wavelengths.

So, in contrast to the general reptile coloration described above, the structural iridophores and the nanocrystals are responsible for the rapid color changes; the other pigment-dependent chromatophores are more “stable” or slow and cannot provide this “chameleonic” change. This ability of iridophores to change rapidly also make them a good aid in thermal regulation!

Fun fact!

Since color changing is a costly process energetically speaking, chameleons would not do it unless it is really necessary. Not only do we now know that their visual responses can make them try to blend in with their environment, but Stuart-Fox & Moussali (2009) show that they even respond differently depending on the predator. When the threat of a snake nearby was the danger, they would get a little darker or lighter depending on the position; however, if the predator in sight was a bird, they would really change and blend in. Is as if they know that birds have a way better eye-sight than snakes! HOW COOL IS THAT?!

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References

1. Ligon, R.A. & K.L. McCartney. 2016. Biochemical Regulation of Pigment Motility in Vertebrate Chromatophores: A Review of Physiological Color Change Mechanisms. Current Zoology. 62(3):237-252.
2. Teyssier, J., S.V. Saenko, D. van der Marel & M.C. Milinkovitch. 2015. Photonic Crystals Cause Active Colour Change in Chameleons. Nature Communications. 6(6368)
3. Stevens, M. & S. Merilaita. 2009. Animal Camouflage: Current Issues and New Perspectives. Philosophical Transactions of the Royal Society B: Biological Sciences. 364(2009):423-427.
4. Stuart-Fox, D. & A. Moussalli. 2009. Review: Camouflage, Communication, and Thermoregulation: Lessons from Colour Changing Organisms.  Philosophical Transactions of the Royal Society B: Biological Sciences. 364(2009):463-470