- Written by Nadia O'Carroll
In amphibians, fish, reptiles, crustaceans and cephalopods these cells are called chromatophores and are grouped into subclasses depending on colour – xanthrophores (yellow), erythrophores (red), iridophores (iridescent), leucophores (white), melanophores (black) and cynophores (blue). In mammals and birds these cells are called chromatocytes and are limited to one cell type – melanocytes.
Many creatures have the ability to change their colour, some automatically and others deliberately, for a variety of reasons including breeding, communication, camouflage protection and environmental adaptation.
Some species of birds and mammals exhibit dramatic colour change due to seasonal adaptation –in cold climates the colour of their fur or feathers changes according to the season, in winter they become white which blends in with the ice and snow background, and in spring they revert to their darker colours. These colour changes are triggered by complex hormonal reactions to temperatures and length of day (photoperiod) which effect biochromes on the skin. As a result the feathers or fur moult and are replaced by the new coloured feathers or fur. Examples are Arctic fox, Arctic hare, Ermine/weasel and Ptarmigan. There are different degrees of whiteness of winter coat – animals in the far Arctic north turn pure white while those in warmer areas may turn bluish or piebald.
Probably the most spectacular colour changers are the cephalopods (squids, cuttlefish and octopus). These intelligent and sophisticated animals use their brains to create almost instantaneous waves of colour change for communication, deceptive or background camouflage, disruptive patterning and deimatic behaviour.
The cephalopod changes colour when its brain activates the chromatophore units under its skin, each of which is composed of a chromatophore cell which contains an elastic sac of pigment granules surrounded by nerve, muscle and sheath cells. The brain sends an impulse to the chromatophore unit nerve cells, which control the chromatophore muscle cells, and these then change the shape, size and form of the elastic pigment sac in a certain way and this alters the cell’s colour. Each chromatophore unit is controlled separately so complex patterns can be produced. Under the chromatophores is a reflective layer of iridophores, which provide metallic greens, blues, golds and silvers, they can also be changed by hormonal activity. The last layer is the leucophores, which produce spots that reflect the predominant wavelength of the environment, so in white light the spots are white and in blue light they are blue. In addition many cephalopods possess photophores that produce light (bioluminescence) which can remain constant or flash. Some species use sacs of resident bioluminescent bacteria to produce the same result.
We humans can only perceive colours within the visible spectrum, other species who see beyond the limitations of our vision may be using colour changes that we cannot see, for instance it is now thought that some fish species communicate through ultraviolet light signals and patterns on their scales.