In this article we will discuss about the meaning and types of colouration in animals.
Meaning of Colouration in Animals:
All living organisms are adapted to survive in the particular environments they inhabit. For living suitably in a particular environment, living organisms exhibit numerous adaptations. Most of the adaptive features encountered in nature are protective in a variety of ways. Animal colouration is generally regarded as a sort of protective adaptation, where many animals resemble the backgrounds against which they live.
While discussing the colouration in animals, emphasis is given to the ways whereby the colours are product and then we will pass on to the significance of such colouration in the biological world with suitable examples. Instances are numerous in the animal kingdom, but to restrict the discussion only typical cases are cited.
Colour Production in Animals:
Colouration in animals is produced in animals both by physical as well as chemical means. Colour is usually produced by a sort of interference with the beams of white light.’ The physical colouration is caused by surface sculpturing of the body. The beam of light falling upon this surface refracts and breaks up into its component rays. The chemical colouration is primarily due to the presence of pigments.
Presence of pigment is very common in animals. Pigment may be present in the interior of the body as well as in the outer surface. Chemical colouration may be produced by the absorptive power of certain pigment which by oxidation may cause various colours. Haemoglobin and haemocyanin are the pigments of the blood.
Cells containing the pigment are usually differentiated into two broad categories—first cells containing the pigment give an unchangeable colour to the animals and the second category is the chromatophores or changeable pigmented spots which produce flushes of colour over the skin. Various types of chromatophores are seen in animals.
They are melanophores containing black or brown pigment; erythrophores containing red pigments; xanthophores containing yellow pigment and lipophores containing a combination of red, yellow and orange pigments. The ability to change colour is called as metachrosis and is controlled by two agencies—nervous and endocrine systems.
The chromatophores are influenced by the skin and eyes through the pituitary gland and thus produce changes in the hue of the body. The activities of the chromatophores are greatly enhanced if a reflecting layer of guanin is present beneath them.
Types of Colouration in Animals:
Colouration in animals is usually classified under the following categories depending upon its-biological significance.
i. Sympathetic or Cryptic or Concealing Colouration:
The colour of the animals harmonises with its surroundings to such an extent that it completely blends into the background. By this way the animals lose their conspicuousness. This tactic may help them to escape the eyes of the enemies (Protective colouration) or to wait in order to catch hold of the prey (Aggressive colouration).
Hares, weasels, ptarmigan of far North moult into white coat of hair when the surrounding is covered with snow. Many marine invertebrates and the larvae of some fishes are transparent. Bark-inhabiting insects usually fit with the colour of the background on which they live.
The colour of the same animal may vary. Species having a wide range of distribution, vary their body colour to harmonise the different colours of the surroundings. Such colour is designated as the local colour. Several grasshoppers belonging to the family Acrididae exhibit such local colouration. The body colour of the desert adapted gazelle, varies from white (in sandy plains) to dark gray (in the volcanic districts).
Another phenomenon is called as the seasonal colour, where the body colour of the same species varies in different seasons of the year. A number of birds and mammals exhibit such change of colour. The Artie fox, some hare, the weasel change their body colour from summer to winter to harmonise the changing surroundings.
As a consequence of adaptation all animals harmonise the backgrounds they live. The desert forms are usually dull or gray, such as the gazelle, the camel, the lion, etc. Most of the forest insects are green in colour to harmonise the green colour of the plants. Arctic animals are mostly white to harmonise the snowy background. In many sea birds the dorsal side of the body is blue or gray and the ventral side is white.
When viewed from above, these birds harmonize the background of the sea and to the sky when viewed from below. Generally, the upper surface of the body is darker than the lower. This phenomenon ofcountershading is effective in camouflage because it minimises the contrast between the shaded and non-shaded parts in light.
ii. Warning or Revealing Colouration:
The colour of the body of some animals is conspicuously red and yellow and they also advertise their unpalatability. Possession of stout sting and conspicuous yellow and black colours of the body in bees and wasps are the typical examples. Other instances are the coral snakes, tiger salamander, Gila monster. These creatures are almost immune from attack, because of the conspicuous exhibition of a dangerous nature.
Prominent markings on the dorsal side of the hood of Cobra is another instance. The large eye-like spots on the hind wings of a moth, Smerinthus are displayed when they are annoyed, but are ordinarily kept concealed under the front pair.
Recognition marks or signal. These markings are used by some species to help in identifying from other species and to warn them in time of danger. These are of great use particularly in gregarious animals when mutual help in time of danger is necessary.
The conspicuous whitely coloured under surface of the tail of Virginia deer acts as signal at times of danger. The white rump patch on Antilocapra, the cottony-white tail of Lepus sylvaticus are all flashed into prominence when they are frightened. The reorganisation marks are usually used by individuals of the same species and are borne by many insects.
iii. Alluring Colouration:
Certain animals exhibit some colouration which may act as lure for others. Certain carnivorous spiders resemble the orchid blossom very closely in form and colour which is an alluring adaptation. Similar characteristic is present in some African lizards which are protectively coloured excepting a brilliantly coloured patch at the corners of the mouth which Usually acts as a lure.
iv. Mimetic Colouration:
The colour of the body of the animals may resemble very closely between the animal and any other objects. This colouration may be protective in concealing their conspicuousness or may be warning as well.
v. Confusing Colouration:
Many butterflies and moths by exhibiting recognition marks make their position very conspicuous but at the end of flight, they cover the bright area very quickly. This sudden change from extreme conspicuousness to extreme inconspicuousness is very confusing to the predators.
vi. Sexual Colouration:
Colouration in different sexes has significant value in sexual selection. The males of many birds and some other forms are more conspicuously coloured than their females. The actual cause and significance of such colour difference seem to be confusing, but it is a fact that such colour differences exist.
The biological significance of colouration in animal kingdom is quite interesting and has always played an important role in determining the chances of survival in this hostile world. Whatever the cause of animal colouration may be, the adaptive perfection for whatever purpose may they be used, is the result of Natural Selection.