By Paul Donovan
Why that colour?
Colouration is used for many purposes in the animal world. For example, birds use it for courtship, where males are often vibrantly coloured, and when combined with often elaborate dances, is used to court females. In a similar approach, in some species of lizards, say Agamas, it can also be used for the same purpose. During the breeding season, males may develop brightly coloured heads and bodies, and when included with various behaviours such as head bobbing or ‘push ups’, signals to females in the area he is ready to mate. It can also be used to counter unwanted rivalry from other males. In snakes, colouration is used primarily for camouflage as, unlike many species of lizards, they do not engage in courtship rituals associated with colouration.
Not all as it seems
When we look at reptiles or amphibians, one of the features which sticks out are the multitude of colours they exhibit. But colouration in the animal world is a strange thing. Let’s take as an example, a green snake. As we look at it, to our eyes it’s a green snake. But the strange thing is, green (and blue) pigment does not exist in the reptile world. But if we see them as green or blue, what are we actually seeing, then?
Blue and green is not a pigment per say in reptiles, but is created by reflection of blue light coming through an over-layer of yellow pigmentation due to Iridophores, sometimes also called guanophores. These are coloured cells located in the upper part of the dermis which contain a semi-crystalline substance called guanine.
Think of guanine as millions of tiny mirrors in the skin. As light hits the guanine, they refract it by dividing it at its surface. As light moves at different speeds through the crystals, they cause it to be separated into different wavelengths; a phenomena known as Tyndall, or Rayleigh scattering. By joining with the cells containing melanin (melanophores), guanophores produce blue colouration which, when combined with the yellow lipohores, then give rise to green colouration.
It is the guanophores which are responsible for the iridescence we see in many snakes such as the Rainbow boa Epicrates cenchria. By refracting light at its various wavelengths, the snake shimmers all colours of the rainbow.
We can see how pigment is not a molecular component of the reptilian skin, by the fact that when the skin is shed, it is colourless. If the pigment was integrated into the skin structure at a molecular level, the shed skin would also be colourful.
Pigment cells
As well as guanophores, reptile and amphibian skin also gets its colour from pigmentation, and within pigmentation, there are several factors which influence the skin’s colour. Possibly the one having the greatest impact is melanin, for this is responsible for the dark colours we see, such as black and brown.
Most dark pigmentation is derived from melanin which is synthesised through the oxidation of the amino acid tyrosine, and is the most common pigmentation in the animal world. Melanin evolves from several forms, the three most common being:
Eumelanin. This is the most widespread type as it forms the foundation for black, brown and grey pigmentation. Eumelanin is contained in vesicles called melanophores which are distributed throughout the cell. Eumelanin is generated from tyrosine, the key element of which to enable its synthesis is Tyrosinase. Where this is not present, no melanin can be produced, thus leading to varying degrees of albinism.
Pheomelanin. This pigment is what gives rise to a reddish or beige tinge.
Neuromelanin. A pigment found in the brain of higher life forms (humans and primates) but is less seen in lower animals.
Although there may be well defined differences in terms of colour variation between these forms of melanin, all are usually present in dark individuals. It may seem strange, but although melanin is the most widely known pigment in the animal world, and much studied, we still know very little about it. The main reason for this is that when it is removed from the tissue, it basically turns into a ‘blob’ and reacts differently, making analysis of how it functions extremely difficult.
Chromatophores
Within the reptile and amphibians’ skin, as well as Melanophores are colour cells called chromatophores. These are arranged in three layers and are responsible for generating skin and eye colour in cold-blooded animals. The upper layer of skin contains Xanthophores; iridophores below that, and melanophores at the very bottom. These chromatophores have subdivisions according to the colour they are based on under a neutral white light; Xanthophores (yellow), erythrophores (red/orange [carotenoids]), leucophores (white), melanophores (black/brown), cyanophores (blue) and iridophores (iridescence).
Xanthophores and erythrophores
A word about xanthophores and erythrophores. Although xanthophores contain yellow pteridine pigments, and erythrophores red/orange carotenoids, both of pigments can sometimes be found within the same cell, and the ratio of yellow to red/orange will influence the skin’s colour. Because of this, many scientists consider the distinction between the two somewhat subjective.
While pteridine is synthesised from guanosine triphosphate, carotenoids are metabolised from the animal’s diet and transferred to the erythrophores. As carotenoids are directly metabolised from what many lizards and frogs eat, if this is lacking in their diet, green colour will be suppressed in favour of blue. In other words, if you bought a wild caught frog which was green, and fed it a diet of crickets which had not been fed any form of food which contained carotenoid pigmentation, eventually you will end up with a frog which has turned blue, because cyanophora will become the dominant pigment.
This is one reason why feeder crickets should also be fed a varied diet prior to being offered as food. Much emphasis is placed on sprinkling, or gut loading feeder insects with a calcium/mineral supplement to ensure that the lizard/amphibian gets an adequate intake to ensure good bone density, but an acceptable intake of metabolised pigment is also important for some species if colour is to be retained.
Background adaptation
Colouration, being so important to reptiles and amphibians as a means of courtship and camouflage, has also seen a number of species evolve the ability to change their colour in response to changes in their background. This type of response is called ‘background adaptation’ and there are some prime species which employ this technique. It is not so widely practiced in amphibians as it is in reptiles, but I have experience of one frog which practices it here in Botswana. The Grey Foam-nest frog Chiromantis xerampelina is, in its normal guise, a grey colour, but when it ventures onto a white surface, turns pure white.
In the reptile world, although a number of lizard species have the ability to practice background adaptation, such as Anoles, by far the most famous exponent of this are the chameleons. Chameleons have the ability to generate a lot of different colours within seconds in response to background demands. The difference between chameleons changing colour and other background adaptives, is that chameleons can change colour due to other stimuli such as stress levels, moods, and temperature, whereas other background adaptives use it more for camouflage purposes, and it is therefore a visually based need. But what is the process which enables such rapid colour changes? Well, it is all down to those Iridophore cells.
Unlike other animals such as squid, cuttlefish and octopus which change colour by dispersing pigments within the skin cells, colour changing lizards have adopted a different approach. They rely on physical changes, either through a relaxed state or excited state that shape how light reflects off their skin. Their skin encompasses two layers of iridophores cells which contain nanocrystals of varying shapes, sizes and grouping in the cells. By relaxing the upper layer of skin, the arrangement of cells in that layer changes structurally. The nanocrystals in iridophores cells in that layer suddenly gather close together which causes them to reflect specific short wavelengths of light, particularly blue.
In an excited state, these nanocrystals begin to space themselves out, so that each iridophore reflects longer wavelengths of light such as red, orange and yellows. As the skin also contains yellow pigment, mix these different wavelengths of light and pigment together, and we get the varying array of colours which chameleons are so renowned for.
Throwbacks
So, it is with the amalgamation of all these different types of pigmentation that results in the colours displayed by a reptile or amphibian, and these are pretty uniform across the range of species. There are, of course, throwbacks in which pigments can manifest in ‘abnormalities’. By this I mean an individual which does not exhibit its ‘normal’ range of pigmentation, but something quite different. These in the reptile world are known as ‘colour morphs’. Such individuals show colouration or patterns of colouration which some people consider more desirable than the typical colour, and are prepared to pay handsomely for. These morphs are often specifically bred for these colour strains.
Albinism: Typically, albinism is the total lack of any pigmentation in the skin and eyes giving the individual a white or yellow appearance. Take the humble corn snake as an example. Its natural colour is an amalgamation of reds, browns, oranges and black. In an individual lacking melanin, the skin takes on a pinkish-white colour. This is because the three primary pigment cells (chromatophores) in the skin are affected; these being melanophores which contain the brown/black pigment melanin; xanthophores which contain red and yellow pigment, and iridophores containing light reflecting crystals. As these three primary pigments can also be affected by genetic influence, numerous colour morph throwbacks are possible.
True albinos also have complete lack of pigmentation in the eyes, giving the eyes a pink colour. A pure white individual, but with normal coloured eyes, is not a true albino. Generally, in the wild, albinos would not survive for very long, as they become more conspicuous to predators. Also, the loss of eye pigmentation makes the eyes much more sensitive to light, and therefore can affect the individuals’ vision. In captivity, it is wise to reduce the amount of light an albino is exposed to, so as to avoid eye damage.
By way of interest, eye colouration in animals is also influenced by the amount of melanin present. Those with black or brown eyes have higher amounts of melanin than those with lighter coloured eyes such as blue. In albinos, melanin is absent from the retinal pigmented epithelium (RPE), or the iris, and the redness of the eyes is due to the underlying blood vessels showing through. Albinism is hereditary; the gene which causes it, inhibits the body from making sufficient amounts of melanin to give it colouration.
Leucism: Leucism is the partial loss of pigmentation resulting in a white, or pale individual. It may occur over the entire body, or in patches. Often confused with albinism, leucistic snakes retain normal eye colouration, and not the pink as seen in true albinos. As with albino snakes, in the wild, leucistic snakes are more vulnerable to predators.
Melanism: Melanism in some species is a natural colour form, but can be a throwback in others. Classically, an excess of brown or black pigmentation results is a pure black or brown individual. The eyes are also dark due to the presence of melanocytes in the RPE. In the wild, such individuals would have the same survival rate as the atypically coloured individuals in terms of predation, though may suffer from heat related problems, as black absorbs heat, whereas lighter colours reflect it.
Tail–end
Pigmentation in both the reptile and amphibian world is far from being a simple case of having a set pigment to define a colour. As we have seen, a green snake is not green through pigmentation, but light refraction. Furthermore, the absence of certain pigments through genetic abnormalities, or selective breeding, can lead to colour morphs. Still further, the lack of certain pigments in prey animals can affect a frog’s colour. Colour, then, in all its wonderful entirety, is not as straightforward as we first assume. Irrespective of how it is generated, though, few could dispute reptiles and amphibians are some of the most colourful of all living animals.
