Colours are essential to animal life, and they greatly increase chances of survival. Very briefly, colours have functions that can be divided into three categories:
1. As communication between members of the same species.
2. As signals and other forms of interaction between members of different species, most notably between predators and their prey. Of this, camouflage is an important part, whether it is a predator sneaking upon its prey, or the prey trying to remain unnoticed by its predator(s).
3. As a part of adaptation to the physical environment. e.g. black pigments absorb harmful ultra violet light. (Compare that to human tanning.)
Most black, brown, red and yellow colours of the animal kingdom, as well as some green and blue, are pigment colours. Almost all species have melanin-like pigments, giving black, most browns and some of the dark red and yellow colours; like the red hair of humans, or the yellow of chickens. Melanin is the pigment that gives humans different skin colours, and also the one in "melanoma", pigment tumours. It comes in two forms: eumelanin and pheomelanin. Both are present in human hair, where the former gives black and dark brown; the latter gives reddish and yellow. Your hair colour (while young) depends on the ratio between them: which one of them that dominates, and how strongly it dominates.
Red, orange and yellow are mostly carotenoids or pterines (the latter mainly in some insects); or psittacins, which are found only in parrots. While animals, including humans, produce most of their pigments themselves, carotenoids are the exception to this. They must be assimilated through the food. Thus, especially birds can lose some of their beautiful colours if they, over a prolonged time, get too little carotenoid content in their food. The best-known example of this is the flamingo, which loses its colours in captivity if deprived of its natural food (normally containing carotenoids).
Humans eating very large amounts of carotenoids can get an orange or yellow skin tone. This is absolutely harmless and might even be beneficial. Carotenoids are powerful antioxidants, they block oxidation and prevent body fat from getting rancid. In the skin they protect from oxidation caused by sunlight.
Carotenoids Lutein and Zeaxanthin provide the colour of the "macula lutea", the yellow spot (in the retina) of the eye; and Rhodopsin, or visual purple, is essential for our vision.
Bound to a protein, carotenoids cause blue and green colours, e.g. the dark blue of the lobster. The red colour of a boiled lobster is a result of breaking down the protein while only the carotenoid remains.
The green colour of many larvae and grasshoppers is blue and yellow pigments mixed, while the green colours of many birds are a mixture of black and yellow.
Tyndall blue is what gives birds like parrots and kingfishers their shimmering blue colours, and it is the same phenomenon behind the blue of face and genitals of certain primates, like mandrills; and blue eyes. Very small proteins in the iris scatter the short waves of light and the reflected blue light is seen against a black layer of melanin further back in the iris. In brown eyes, the blue colour is concealed by occurrence of melanin among the scattering particles.
The varieties of green of many birds are caused by Tyndall blue combined with some yellow pigment, serving like a filter. For this to happen, there must be three layers; one of pigments, one of particles causing scattering, and an innermost layer of melanin.
White of animals is often caused by structural effects as well, depending on total reflection or scattering by particles too large to scatter only the short wavelengths of the light. In other cases it is simply due to albinism, or absence of pigment.
Many groups of animals can show off the effect of interference, but it is most striking among birds and beetles. If you want to see an example, watch the green head of a (male) mallard (a type of wild duck). It shifts in colour if you change the angle from which you watch it.
The thin layers that are necessary for interference occur as thin plates in the cells or in keratin, chitin, feathers, scales, etc. (Keratin is the albuminous substance forming the principal matter of nails, hair, horn, etc. Chitin is a hard amorphous compound, the chief constituent of the external covering or integument of crustaceans and insects.)
Luciferin is a pigment which emits light by oxidation without generating heat. It occurs in fireflies and other bioluminescent organisms.
Many animals have the ability to shift colour in one way or another. Some changes have to do with age or with the change of seasons. For mammals and birds, it is limited to that. If you do not count blushing. This, however, is a widely misunderstood phenomenon. Not long ago, I read an article in one of the world's most renowned scientific journals, by a scientist trying to derive the purpose of blushing from the human past, based on the assumption that blushing is unique to humans. This reveals blatant ignorance. Blushing has been observed amongst several bare-skinned, at least bare-faced, birds and mammals, most notably humans, vultures, and turkeys. Consequently its purpose has nothing to do with human prehistory but is a remnant of something else and its origin must be older than the human species.
Blushing occurs when the small blood vessels of the face, the capillaries, are dilated. Then the face becomes suffused with red. This is an emotional reaction caused by an arousal of the sympathetic nervous system. The opposite can happen as well: the colour is drained from the face, giving a very pale or white appearance.
Let me suggest the theory that this happens with all or almost all mammals and birds, it can just not be seen if they have fur or feathers. So what happens? The hairs or feathers in certain places of the body move - rise or shrink. This is an important signal system. The reason why we see it as blushing, a colour effect, is just that we have lost our fur. If this is right, the change of colour is secondary here, not a phenomenon that has developed for the sake of the colour.
Apart from amongst birds and mammals, the ability to shift colour is very common. The chameleon is the classical example, but the real master is the octopus, which can create complex patterns that move over the body. This is done by special cells, chromatophores, containing different pigments. Each chromatophore is capable of contraction and expansion, whereby pigment spreading can be controlled. Chromatophores are used by many reptiles, fish, amphibians, crustaceans and cephalopods, although the exact design of the chromatophores differs between species.
The most primitive blood was probably sea water, able to contain a sufficient amount of oxygen to satisfy the need of the animal. When animals gradually became more complex and active, the capacity of the blood for oxygen transportation had to increase. New methods had to be developed. They were based on colouring matters. All of them consist of a metal bound to a protein. It can be copper, it can be vanadium, or it can be iron.
All vertebrates, including humans, use some sort of haemoglobin, which is based on iron and gives the blood its red colour. ["Vertebrata" is a large division of animals that have a backbone, or spinal column, of bone or cartilage, and early in life have a notochord. The Vertebrata include mammals, reptiles, birds, fishes and amphibians (frogs, etc). All other animals belong to Invertebrata.]
Porphyrins, a group of pigments essential in nature, combine with iron, making heme; which, together with globin, produces haemoglobin. A genetically based enzyme deficiency might disrupt this process, causing a disease called porphyria.
Derived from the blood and the breakdown of haemoglobin, bilirubin, orange-red in colour, is a colouring matter of bile, contributing to the colour of faeces. Another bile pigment is urobilin, darker brown, also occurring in faeces and urine.
Not many animal pigments are or have been used by humans, but there are some. The best-known are:
Sepia - is a secretion of the common cuttlefish, Sepia officinalis, a pigment containing melanin. You can read more about sepia in Sepia, Grisaille & Verdaille: Monochrome Art... and how about Japan?.
Purple - is now made synthetically, but the original source was a gastropod, Murex brandaris, having a shell with many spines and provided with a gland secreting a purple dye. This was extremely expensive, so the use of purple carried enormous prestige. It was used during antiquity to symbolise high status. Other molluscs have been used to get other bluish or purple-like shades. Purple and its related shades are treated separately, so I will not deal with them any further here.
Recent research indicates the presence of anti-tumorous compounds is purple-producing molluscs. Genuine purple or purple-like pigments, or substances involved in their formation, might prove to be a powerful medicine for the future.
Carmine - originally made from Kermes vermilio, later also from Dactylopius coccus, cochineal, an insect. Kermes vermilio was used already in old Egypt. Cochineal, however, stems from the Americas, and was used by the Maya and the Aztecs.
Ivory black - is a result of charred ivory. It is heated to drive out the hydrogen and oxygen and is subject to incomplete combustion. The resulting ivory char is black.
Copyright © 2010, 2021 Meleonymica/Mictorrani. All Rights Reserved.
(The lead image shows a kingfisher, with its Tyndall blue colour. Photo by Lukas Bieri/Pixabay, CC0/Public Domain.)
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