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I have read about how they can change color, but is there literature about the chemical signaling process they use to do so? I read that it could be some combination of hormones and neurotransmitters, but I couldn't find specific information on the receptors or chemical mechanisms.
As said by @dblyons, there has not been a lot of research (biochemical) on chameleons. So, the exact part of mechanism that you're looking for is still not understood. However, we have recently caught the broad end of the stick. Chameleons don't have special cells for color, their complete skin has a layer of pigments (dermal iridophores) which helps them in changing color. See this (or this) article:
Chameleon skin has a superficial layer which contains pigments, and under the layer are cells with guanine crystals. Chameleons change color by changing the space between the guanine crystals, which changes the wavelength of light reflected off the crystals which changes the color of the skin.
These guanine crystals are actually like colorless mirrors, but by changing the distance between these crystals, wavelength of light absorbed (and reflected) can be changed. It is quite similar to how the color of ozone changes from pale blue in gas to violet-black in solid form. However, how chameleons trigger this change in distance of crystals, is not yet known.
This is how these crystal lattice works: sourcesource
This is what a guanine crystal looks like ((a) is guanine):
You can also see a nice real-time video of guanine crystals changing color of chameleon here.
The color changing portion of skin has 3 main layers:
superficial s-iridophores which help in changing color in visible region rapdily.
deep d-iridophores which reflect light in infrared refion and are thought to provide thermal protection to chameleon.
layer of melanin which is actually yellow in color, making chameleons naturally yellow(!) in color. When the guanine crystals come closer, they reflect the blue portion of visible spectrum. This blue light, along with the natural yellow color of melanin, becomes green which gives green color to chameleon. So, the actual color visible to us is a combination of color reflected by guanine crystal and yellow color of melanin.
As I said already, the exact biochemical signalling process behind this process is not known yet. It could be complex hormonal or neurotransmitter signals. However, hormones are considered as the main triggers because chameleons have been shown to change color due to changes in mood instead of environmental protection.
Cephalopods such as the octopus have complex chromatophore organs controlled by muscles to achieve this, whereas vertebrates such as chameleons generate a similar effect by cell signalling. Such signals can be hormones or neurotransmitters and may be initiated by changes in mood, temperature, stress or visible changes in the local environment.
This is a good question and… kind of remarkable because I don't think there is a specific body of literature regarding chameleons.
Generally though, vertebrate skin cells responsible for color (these cells are called chromatophores and arise from the neural crest during development) are responsive to the hormones melatonin, MSH, and others. These hormones, which are released from the pineal (melatonin) or pituitary (MSH), bind GPCRs on the surface of the chromatophores which elicit intracellular activation of PKA, for instance. This activity can cause mobilization of pigment within the cells. This article about fish coloration may help guide you somewhat in your studies, but chameleons in particular seem like an understudied clade of color-changing animals!
The secrets behind chameleon’s skin coloration change
Color patterns are important features of lots of animals, having key functions in protection against UV irradiation, camouflage, shoaling or sexual selection. Color patterns in birds and mammals are generated by a particular type of cell called melanocyte which produces melanin and transfers it to the tissues of fur or plumage. In fish, amphibia and reptiles, skin coloring is generated by a particular cell type called chromatophore. These cells are able to retain a pigment and the distribution of different chromatophores with different pigments in the skin is what determines the final pattern.
Chameleons are also colored creatures and their patterns are also determined by the distribution of different chromatophores. However, chameleons have the fascinating ability of changing their skin color in different situations. Despite what most people think, they don’t change to camouflage themselves trying to match the color of their environment but they do it mainly during their social behavior. For example, chameleons tend to show red-like colors when trying to intimidate others, while males show lighter and multicolored patterns when courting females. But how is that amazing process possible? How is the animal able to change its color in demand?
Monitoring color change in a Panther Chameleon adult male under excitation upon presentation of another adult male in its vision field. The first frame of the movie is shown in the lower-right for a better visualization of the extend of color change.
For a long time, we have thought that chameleons achieved their color morphing through the expansion of colored pigments in their skin cells. However, very recently scientists from the University of Geneva have shown that the mechanism behind this process is surprisingly and unexpectedly different from what we though 1 . Using different techniques and working in close collaboration with a team of physicists, Prof Michel Milinkovitch has discovered a new layer of cells beneath the pigmented skin cells. These new cells are quite different from regular cells since they contain in their interior lots of small close-packed nanocrystals made of guanine (one of the four key components of DNA). These nanocrystals are arranged inside the cell in a lattice (evenly spaced) and have the potential of behaving as the so-called photonic crystals, which can be sources of exceptionally bright and brilliant reflected colors that are generated from optical diffraction (i.e, these crystals are able to reflect different wavelengths depending on their relative position) 2 .
After finding the presence of this unusual, crystal-containing cell type (called iridophore), the authors measured if the structure of the crystals inside these cells was changing when chameleons where changing their color. To do so, they did excite a male chameleon just by showing it another male competitor, and just that could shift the background color of its skin from green to yellow/orange, whereas blue patches turned whitish and red became brighter. Then they took samples of the skin from both states and studied by electron microscopy (very high resolution) the structure of the crystals. Amazingly, they found that crystal size inside iridophores does not vary but the distance among crystals does!
When the chameleon was calm, the crystals were arranged in a packed network which mostly reflected blue light. That blue light, in combination with the yellow light reflected from the pigment-based upper layer, showed a final reflection of green light (blue plus yellow). When the animal was excited by the presence of a competitor, the crystal pattern in the second layer became 30% less dense, which changed its reflection from the original blue to yellows and reds. Now the combination of the upper yellow and the new yellow/red from the second layer changes dramatically the final appearance of the animal to a new red/orange skin color which will helps the animal to compete with the other male. This process is completely reversible, the animal just has to rearrange the crystals inside the cells of the second layer to be really dense again and reflect blue light to go back to the original green color!
Figure 1. Upper part of the figure shows the model by which final appearance of the chameleon is controlled by a second layer of cells called iridophores. When the chameleon is in the relaxed state, guanine crystals inside iridophores are densely packed, reflecting blue light. When the chameleon is excited, unknown cellular mechanisms increase the distance between the crystals which in turn changes the reflection from blue to red light. In the second part of the figure, original electron microscopy images of the article are shown.
These new discoveries about the dynamism and organization in the chameleon skin are a great example of how biological scientists need more and more the help of other scientific disciplines, such as physics or mathematics, for solving current puzzles in biology. Physics can offer biology fundamental and theoretical explanations, new techniques to solve old problems, computational methods and models to predict molecular behavior and, for example, explanations like on how photonic crystals can work in the skin of chameleons to change animal appearance. Luckily, more and more physicists are becoming interested in biological problems (Janelia Farms is a great example) and we are starting to hear for the first time that “Biology is the new physics” 3 . Optimistically, the gap between both disciplines will be increasingly less big in the future and this probably will help to solve difficult biological problems faster, hopefully including the cure of the some diseases of our society. Stay tuned!
Chameleons' Color-Changing Secret Revealed
The chameleon's uncanny ability to change color has long mystified people, but now the lizard's secret is out: Chameleons can rapidly change color by adjusting a layer of special cells nestled within their skin, a new study finds.
Unlike other animals that change color, such as the squid and octopus, chameleons do not modify their hues by accumulating or dispersing pigments within their skin cells, the researchers found. Instead, the lizards rely on structural changes that affect how light reflects off their skin, the researchers said.
To investigate how the reptiles change color, researchers studied five adult male, four adult female and four juvenile panther chameleons (Furcifer pardalis), a type of lizard that lives in Madagascar. The scientists found that the chameleons had two superposed thick layers of iridophore cells &mdash iridescent cells that have pigment and reflect light. [See photos of color-changing chameleons]
The iridophore cells contain nanocrystalsof different sizes, shapes and organizations, which are key to the chameleons' dramatic color shifts, the researchers said. The chameleons can change the structural arrangement of the upper cell layer by relaxing or exciting the skin, which leads to a change in color, they found. For instance, a male chameleon might be in a relaxed state when it's hanging out on a branch, and in an excited state when it sees a rival male.
"When the skin is in the relaxed state, the nanocrystals in the iridophore cells are very close to each other &mdash hence, the cells specifically reflect short wavelengths, such as blue," said study senior author Michel Milinkovitch, a professor of genetics and evolution at the University of Geneva in Switzerland.
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 Live Science in an email.But chameleons aren't always blue. The lizards' skin also contains yellow pigments, and blue mixed with yellow makes green, a "cryptic" color that camouflages them among trees and plants, Milinkovitch said.
The "red skin hue does not change dramatically during excitation, but its brightness increases," the researchers wrote in the study.
Furthermore, the researchers 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 color, it's possible that they help the chameleons reflect heat and stay cool, the researchers said.
The researchers used a number of methods to study the iridophore cells. They filmed the chameleons' color changes using high-resolution videography and made numerical models that predict how the nanocrystals should reflect light.
"The results are a perfect match with what we observe [in real life]," Milinkovitch said.
The researchers also manipulated the cells by subjecting them to solutions of varying concentrations, which caused the cells to swell or shrink. These modifications changed the distances between the nanocrystals, and altered their visible colors, just as the researchers predicted they would, Milinkovitch said.
However, only adult male chameleons change color, especially when they see a rival male chameleon they want to chase away, or a female to attract, Milinkovitch said. Females and young chameleons are dull-colored and have a very reduced upper layer of iridophore cells, he said.
The findings may help engineers and physicists replicate the chameleon's color-changing capacities in new technology, such as appliances that eliminate reflection, Milinkovitch said.
The study was published online today (March 10) in the journal Nature Communications.
How do chameleons and other creatures change colour?
Rapid colour change may occur due to various “triggers” – but what are they? Credit: Today is a good day
When most people think of colour change, they think of octopuses or chameleons – but the ability to rapidly change colour is surprisingly widespread.
Many species of crustaceans, insects, cephalopods (squid, cuttlefish, octopuses and their relatives), frogs, lizards and fish can change colour.
They all have one thing in common: they are ectotherms (animals that cannot generate their own body heat in the same way as mammals and birds) and only ectotherms have the specialised cells that enable colour change.
Watch the first 20 seconds of the video below – it will blow your mind:
Colouration in animals is produced by reflection and scattering of light by cells and tissues, and by absorption of light by chemical pigments within cells of the skin.
In ectotherms, cells containing pigments are called chromatophores and are largely responsible for generating skin and eye colour.
Vertebrate colour changers
In vertebrate ectotherms (such as frogs, lizards and fish), there are three main types of chromatophore:
- xanthophores, which contain yellow-red pigments
- iridophores containing colourless stacks of crystals or platelets that reflect and scatter light to generate hues such as blues, white and ultra-violet
- melanophores, which contain black melanin pigment
The melanophores play a crucial role in colour change.Cape dwarf chameleon (Bradypodion pumilum). Credit: Adnan Moussalli and Devi Stuart-Fox
They are large, star-like cells with long "arms" (dendrites) that extend towards the skin's surface.
Colour change occurs due to the movement of "packets" of melanin pigment (melanosomes) within the melanophores.
When melanin pigment is aggregated within the centre of the cell, the skin appears very pale, whereas when it is dispersed through the arms of the melanophores towards the skin's surface, the animal appears dark.Credit: Wikimedia Commons
Because the arms of the melanophores extend between and over the other types of chromatophore (generating yellows, reds, blues, etc.), varying the degree of dispersion of the melanin can conceal or reveal those chromatophores, thereby varying the animal's colour.
Colour change may also occur due to changes in the spacing of the stacks of platelets or crystals within the iridophores, which changes the way they reflect and scatter light, and therefore their colour.
In cephalopods, the structures known as chromatophores are very different to those of vertebrates.
Cephalopod chromatophores contain a pigment-filled sac, surrounded by radial muscle fibres.
These muscles contract to change the size and shape of the pigment-filled sac (e.g. thin, flat disc vs small sphere), resulting in the near-instantaneous and dramatic colour change.Cuttlefish can completely change colour in less than a second. Credit: Wikimedia Commons
Underlying the chromatophores in cephalopods are two other types of cells:
- iridophores, which are much the same as iridophores in vertebrates
- leucophores, which appear white
When the pigment sacs are contracted, these other cells are revealed, changing the colours we see.
So although colour change in cephalopods and chameleons both involve chromatophores, the chromatophores are very different structures, as is the mechanism of colour change.
In chameleons, colour change occurs due to the movement of pigments within chromatophores, whereas in cephalopods, colour change occurs due to muscle-controlled "chromatophore organs" changing the shape of pigment sacs.
Rapid colour change may occur due to various "triggers" including temperature or light (a reflexive response via light-sensitive receptors in skin).
That's why chameleons are very pale at night when asleep but darken as soon as a torch is shone on them (and only on the side with the light shining on it).
Most importantly, animals change colour in response to their surroundings (including variations in background colour, presence of predators, mates or rivals).
They need to assess their surroundings so that they know what colour to change to.
Information about an animal's surroundings (from the senses) is processed by the brain and the brain sends signals directly, or via hormones, to chromatophores.
Colour change is a very useful ability.
Given that colour-changing animals cannot generate their own body heat, colour change can help animals to regulate their body temperature.
So, when cold, a lizard may be dark because dark colours absorb more heat, whereas when hot, a lizard may become very pale because light colours reflect heat.
But perhaps the two most important functions of colour change are camouflage and communication.
Colour change allows animals to flash bright colours to warn rivals or attract mates, while remaining camouflaged at other times.
Male giant cuttlefish use moving waves of black and white stripes in aggressive and courtship displays (see video above), while chameleons show an impressive range of conspicuous colour patterns.
Yet, when they are not communicating to each other, they are superbly camouflaged.
Colour change allows unparalleled flexibility, which is perhaps why we find it so fascinating.
This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).
Conspicuous Social Signaling Drives Evolution Of Chameleon Color Change
What drove the evolution of color change in chameleons? Chameleons can use color change to camouflage and to signal to other chameleons, but a new paper shows that the need to rapidly signal to other chameleons, and not the need to camouflage from predators, has driven the evolution of this characteristic trait.
The research, conducted by Devi Stuart-Fox and Adnan Moussalli, shows that the dramatic color changes of chameleons are tailored to aggressively display to conspecific competitors and to seduce potential mates. Because these signals are quick--chameleons can change color in a matter of milliseconds--the animal can afford to make it obvious, as the risk that a predator will notice is limited.
This finding means that the evolution of color change serves to make chameleons more noticeable, the complete opposite of the camouflage hypothesis. The amount of color change possible varies between species, and the authors cleverly capitalise on this in their experiments.
Stuart-Fox and Moussalli measured color change by setting up chameleon "duels": sitting two males on a branch opposite each other and measuring the color variation.
By comparing species that can change color dramatically to those that only change slightly, and considering the evolutionary interrelationships of the species, the researchers showed that dramatic color change is consistently associated with the use of color change as a social signal to other chameleons. The degree of change is not predicted by the amount of color variation in the chameleons' habitat, as would be expected if chameleons had evolved such remarkable color changing abilities in order to camouflage.
Citation: Stuart-Fox D, Moussalli A (2008) Selection for social signalling drives the evolution of chameleon colour change. PLoS Biol 6(1): e25. doi:10.1371/journal.pbio. 0060025
Materials provided by PLoS Biology. Note: Content may be edited for style and length.
Chameleon Colors Reflect Their Emotions
When light hits a chameleon's skin, the cells appear different colors depending on the mood of the animal.
Scientists have long thought that chameleons change color when skin cell pigments spread out along veinlike cell extensions.
But Michel Milinkovitch, an evolutionary geneticist and biophysicist, says that theory didn’t wash—there are many green chameleons but no green pigments in their skin cells.
So Milinkovitch and his University of Geneva colleagues began “doing physics and biology together,” he says.
Beneath a layer of pigmentary skin cells, they found another layer of skin cells containing nanoscale crystals arranged in a triangular lattice. (Also see "Amazing Pictures: Baby Chameleon Doesn't Know It Hatched.")
By exposing samples of chameleon skin to pressure and chemicals, the researchers 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—a kaleidoscopic display that’s common among some panther chameleons as they progress from relaxed to agitated or amorous.
How do chameleons change color?
1. How do reptiles like chameleons change color?
2. Do other animals do this? If so do they do it the same way?
In answer to question 2 first, yes, many other animals change colour, some in very short periods of time (cuttlefish, squid etc. are brilliant at changing colours in rapid succession) and some that take a bit longer (plaice, flounders and other bottom dwelling flatfish often match their surroundings very accurately after a couple of minutes or so).
The pigment and light reflecting cells in animals (mainly cephalopods like the squid, fish, reptiles like the chameleon and amphibians) are called chromatophores. For animals like cuttlefish and squid that change colour rapidly, either to hide / sneak up on prey (known as cryptic (this simply means ‘hidden’) camouflage) or to signal to a mate / rival etc, this is known as physiological colour change and is controlled by muscles around each chromatophore squeezing and distorting the shape of the chromatophore (a bit like the muscles around the eye squeeze the lens to enable us to focus).
Chameleons change colour through a rather different mechanism, which has far more to do with cell signalling (this is part of a complex system of communication that governs basic cellular activities and coordinates cell actions), so chameleons change colour as a result of mood (anger, mate attraction etc.), rather than to blend into their background.
Our results show that the coloration of chameleon prawns provides effective visual camouflage to predator vision against their main seaweed substrates. We first quantified the level of in situ camouflage between prawns and seaweed using vision models of two fish predators, showing that prawn concealment was closer and more effective against the algal substrate against which they would reside. Next, we show that green and red prawns change colour over time when placed on mismatched seaweed and improve their camouflage on the new substrate. Prawn coloration is therefore highly plastic, with prawns able to switch from red to green and vice versa over a period of weeks. Finally, we show that individuals actively choose a background based on their current coloration that improves their level of camouflage to predator vision.
The capacity to change appearance across species has likely evolved to cope with both spatial and temporal uncertainty over a short time frame, or with predictable changes over a longer time period 11 . In both cases, it enables animals to change their appearance as they move between patches within their environment (e.g. spatial heterogeneity) or as the composition of environment changes around them over time (e.g. temporal heterogeneity) 11 . In the case of chameleon prawns, our results indicate that colour change is unlikely to have evolved as response to the spatial heterogeneity of their habitat, as is the case for rapidly changing animals such as chameleons 16 and cuttlefish 32 . Instead, the slower colour change of chameleon prawns likely allows individuals to maintain their camouflage in response to seasonal variation in the abundance of seaweed species, in accordance with more predictable patterns of environmental variation 11,13 . In contrast to slow colour change, our results from the behavioural experiment show that the ability of chameleon prawns to select appropriate backgrounds is likely a key strategy for maintaining camouflage in the short term and to cope with the considerable spatial variation in the habitat where individuals live 27 . Oriented choices will also be important to help prawns dealing with some unique challenges of the intertidal environment, such as wave action dislodging individuals from preferred substrates and tidal changes influencing habitat availability over the day.
Our results also indicate that the effectiveness of colour change for camouflage was higher for small green prawns compared to larger individuals. This relationship needs to be properly investigated in future studies, but speculatively could indicate that larger green prawns have less selection acting on them due to more effective escape behaviours or by achieving a size refuge from predators, or due to physiological limitations. Why this occurs only for green prawns is difficult to explain but may be related to the fact that red prawns when changing to green always exhibit lower JNDs compared to the opposite (Fig. 3). This seems to be a physiological constraint, since the red coloration is probably defined by the presence of red-yellow pigments within chromatophore cells, while the green tone is provided by the presence of only the yellow pigment (similar to that observed in the prawns Heptacarpus pictus and H. paludicola 37 ). Therefore, changing from red to green may be easier and faster than the opposite since both pigments (i.e. red and yellow) are already present within the colour cells of red prawns. On the other hand, green prawns changing to red would need to metabolise red pigments (probably by food ingestion 41 ), which would take more time, especially for larger individuals, potentially explaining the higher JNDs during the colour change process and the size effects we observed.
Seasonal variation in animal appearance in response to changes in substrate availability is frequently observed in nature 11 . Many birds and mammals, for example, change their coat colour from brown in the summer to completely white in the winter as response to the appearance of snow 11 . In addition, populations of the polymorphic pacific tree frog (Hyla regilla) are characterized by both fixed and colour changing morphs and the maintenance of such colour variation in the population is associated with changes in microhabitat use of individuals due to seasonal changes in substrate availability 50 . The assemblage of seaweed species within the intertidal zone varies through the year as a function of both the species’ life history and some environmental conditions. The red dulse (Palmaria palmata) is a perennial species and, while it undergoes a seasonal burst in growth over the summer months, its holdfasts and fronds provide a ‘fixed’ habitat over a period of several years 51 . On the other hand, the green sea lettuce (Ulva lactuca) has a pseudo-perennial life cycle in which the basal portion but not the fronds survive over the years. In this case, seaweed biomass and therefore the habitat availability for algal-dwelling species fluctuates over the year 51,52 . Sea lettuce exhibits a marked period of rapid growth during the warmer months and, although it may be found throughout the year, it is more susceptible to the effects of harsher winter weather (e.g. lower temperatures, storms and currents) in shallow regions such as rock pools 52 . As such, the combination of slower colour changes and behavioural habitat preferences may enable chameleon prawns to maintain the benefits of cryptic coloration, meanwhile allowing the species to take advantages of seasonal abundances of algal habitats throughout the year. Camouflage through colour change in the carnival prawn (Hippolyte obliquimanus), which associates with different seaweeds along the Brazilian coast, is also related to seasonal fluctuations in the cover of its main habitat, the brown algae (Sargassum furcatum) 41 . During the summer, this seaweed dominates the shallow rocky areas in southeast Brazil and brown prawns attain the largest densities 53 . However, in winter, Sargassum cover decreases and the density of the different colour types in the population changes considerably, following the fluctuation of seaweed habitats (Duarte and Flores, unpublished data). In addition to seasonal changes in substrate, intertidal species may undergo seasonal shifts in predation pressures as fish species move inshore and as juveniles develop, and future work could quantify how the level of crypsis may vary with these predator shifts.
Phenotype–environment matching is assumed to be a common outcome of selection for cryptic traits, yet most research to date has shown indirect associations between animal phenotypes and habitats (i.e. has not quantified camouflage itself, but see ref. 35 ). Our work demonstrates that camouflage in chameleon prawns is enhanced on substrates where they live, and that this close association between phenotype and habitat is predicted to be effective to predator vision. Studies investigating associations between the appearance of juvenile shore crabs (Carcinus maenas) and that of their habitat substrate composition over a range of spatial scales have demonstrated the strongest associations at the micro-scale (<1 m 2 ) 54 . While camouflage is dependent on the appearance match between individuals and their local habitat, an animal may improve this by either reorienting its body relative to its background or by selecting a more appropriate substrate 27 . Indeed, many individuals from the same or different species have evolved preferences for habitat patches that enable increased levels of camouflage 27 . For highly mobile species, it is likely that these behavioural preferences allow for the active maintenance of phenotype–environment associations within heterogeneous habitats. For species capable of colour change, we would expect that behavioural preferences for substrates would change in tandem with a change in body coloration to maintain the selective advantages conveyed by visual camouflage 27 . In addition, we might also expect other processes to come into play for maintaining colour variation, including multiple morph types acting to hinder predator search image formation, and frequency-dependent selection 55 .
Changes in behavioural preferences mediated by modifications of body coloration have also been demonstrated in guppies (Poecilia reticulata), in which individuals spent significantly more time in black and white habitat zones after being induced to change colour in corresponding black and white tanks 56 . Future work should further consider coloration and camouflage with regards to predator vision and measured attack rates. Another research avenue is to understand how predator cues may affect colour change and cryptic behaviours. For example, in the presence of a perceived predation threat, animals may improve their capacity to change colour and select concealing backgrounds, the latter approach especially in slow colour change species. In salamander larvae, the addition of predator cues in experimental tanks increases larval preference for dark backgrounds followed by a corresponding change in individual coloration 34 . However, in the absence of predator cues, larvae spend equal time in light and dark habitat zones, adopting a more intermediate colour form 34 . In the case of chameleon prawns, we would expect that the addition of predator cues may speed up the colour change process and lead to an increase in the proportion of prawns making a choice for concealing substrates.
In the case of polyphenic species, intra-specific variation in coloration and behaviour may allow different individuals to utilise distinct aspects of visual camouflage to adopt alternative life-histories 57 . Duarte et al 41 demonstrated differences between colour morphs of the ‘carnival’ prawn (Hippolyte obliquimanus) in algal preference, showing morph-specific differences in morphology and mobility, which indicate contrasting benthic/pelagic lifestyles 41 . Homogenous coloured morphs showed greater habitat fidelity and a stouter body shape, whereas transparent morphs displayed a more streamlined body shape and increased levels of swimming activity 41 . In the case of chameleon prawns, besides the homogenous coloured forms we studied here (Fig. 1), there exists an assortment of alternative forms that combine colour patterns (spots or stripes) with some degree of transparency 13,43 , and these may also reflect different camouflage, behaviour and life history strategies. In our study, the visual models used are based on colour perception and the spectral sensitivities of ecologically relevant predators available in the literature 45,46 . However, we do not model the spatial acuity of the predators, which is relevant to pattern matching and something that may be especially relevant to transparent prawn types with their intricate markings.
Although our results clearly show that prawns choose backgrounds that improve camouflage, there are many questions regarding what cues control preferences for certain substrates. There is limited information about the existence of colour vision in similar crustaceans, which limits our understanding of whether chameleon prawns are able to identify different seaweeds based on colour cues. Alternatively, caridean prawns living on the pelagic seaweed Sargassum natans select appropriate backgrounds based on their shape, with individuals preferring habitats containing structures that best matched their body shape 24 . Therefore, in some cases, the structure of the habitat allied with a range of behavioural adaptations in the use of that structure may provide better protection from predators than concealing coloration 58 . Since our knowledge of chameleon prawn visual capabilities is limited, it may be that individuals depend upon identifying the structural form of their preferred habitat when making a choice either independently of, or in conjunction with, its coloration. Finally, there is growing evidence about the importance of chemical cues and habitat complexity regulating habitat choices in a wide array of marine organisms, especially for those living on biological substrates (e.g. seaweeds, corals) 59 . Future work could aim to quantify the importance of visual components and other sensory cues that species use when identifying suitable substrates for crypsis.
A wide range of mechanisms and evolutionary pressures control appearance and the adaptive benefits of colour forms in polymorphic/polyphenic populations, and the fine-tuning of a species’ cryptic stratagem may depend on the integration of morphology, behaviour and the environment itself 13,29 . Here, we show that chameleon prawns are able to alter body coloration to improve camouflage against new substrates, potentially allowing prawns to exploit seasonal changes in resource abundance (e.g. food and shelter 43 ). This would allow the exploitation of a wider range of resources within structurally complex habitats, potentially reducing intra-specific competition 60 and predation risk 61 . Concurrently, behavioural preferences facilitate camouflage over time-scales when colour change is too slow. The growing number of studies testing how combinations of chromatic (particularly colour change) and behavioural traits influence crypsis, and the fact that the above-mentioned traits are displayed by a range of phylogenetically and ecologically distinct systems, is indicative of the convergent evolution of these cryptic strategies and the importance of adaptive benefits conveyed to species in order to maintain crypsis in heterogeneous habitats in wild systems.
Published in the journal Nature Communications, the study was a collaboration between quantum physicists and evolutionary biologists at the University of Geneva.
First of all, the team noticed there were no big, spidery cells containing yellow or red pigment that could explain the shifts in hue.
They hit upon the importance of the crystals when they looked inside a type of cell called an "iridophore" using an electron microscope. Whichever angle they looked at them from, the crystals formed an incredibly neat, regular pattern - just the sort of arrangement that creates structural colours.
"When you see this with the eye of a physicist, you know it will have an effect on light," said senior author Prof Michel Milinkovitch.
So Prof Milinkovitch and his colleagues set out to establish whether these crystals might explain not just the chameleon's bright colours, but its changes to those colours as well.
Looking closely at video footage of the colour changes, they saw a pattern (from blue, through green, into yellow and orange) that could not be explained by the pigments available in the chameleon's skin. But when they modelled what changes might be produced by shifting the spacing of the crystals, they found a very close match.
And, crucially, when they compared a tiny piece of "relaxed" chameleon skin with a sample from the same animal when it was "excited" (showing off in front of another male), there was an obvious change in the crystal pattern.
"The net effect is that it will work as a selective mirror," Prof Milinkovitch told the BBC.
"Light will go through except for very specific wavelengths. If the distance between the layers is small, it reflects small wavelengths, like blue if the distance is large it reflects larger wavelengths - for example, red."
The researchers also took a sample of skin and showed that if they altered the crystal packing themselves, by putting the cells in salty water to suck the fluid out of them, they could reproduce a colour change just like the one seen on the animals.
This is the first time that reptile skin has been shown to change colour thanks to this kind of geometrical shift, Prof Milinkovitch said.
Chameleons have some of the most unusual feet in the world. Chameleons are the only animals with completely horizontal feet with toes that stick straight out to either side of the sole. Chameleon feet are sometimes referred to as being zygodactyl, like the feet of birds, but that is not an accurate description, since chameleon's toes are positioned very differently from the toes of birds. No animal on earth has feet like a chameleon.
These one-of-a-kind feet developed for one purpose: gripping. All chameleons live in trees or large bushes, where one slip could mean a nasty fall. But a chameleon's horizontal feet allow it to wrap its toes completely around branches and hold tight. A chameleon's feet aids in protecting the reptile from predators. Birds -- a chameleon's main predators -- hunt by swooping upon their prey and carrying it off in their talons. But a chameleon's grip makes it very difficult to pry it from a branch, even for large birds.
When it comes to forest survival, few animals are better equipped than the chameleon with its literal head-to-toe arsenal of specialized adaptations.