Sunday, July 6, 2003		Nature

PERHAPS there is no animal on this planet that can exist without any contact with the fellow members of its species. There are few needs that no living being can ignore, and passing one’s genes to the next generation is one of them.

At some time in the lives of the great majority of animals, the sexual products of the male must find those of the female for the purpose of reproduction. In such a situation, it is not surprising to find that creatures of the opposite sex act in such a way that the behaviour of the other animal is altered. The special behaviour thus produced is mating. Even the most inactive animals can’t remain uninfluenced in such circumstances.

For instance, sea-urchins, relatively inactive sea animals, are influenced by others; the release of sexual products into the sea by one member of the species stimulates the others to discharge their eggs or sperm likewise. This is communication by chemicals.

In this way, the act of courtship reveals clearly the kind of information that may be transmitted from one animal to another. For the phenomenon to work efficiently, it is important that the species must first be identified so the signal must preferably be one which is unique to that species; secondly, the sex of the animal has to be signalled. Thirdly, the physiological state of the animal - in this case, its readiness to mate - must be conveyed, and finally, its position given, for if it is to be located by the other animal it needs to indicate where it is. This is a good example of the range of information that can be conveyed by a relatively simple act of an animal.

The Siamese fighting fish changes colour on seeing a rival

Sent by a sender and received by a receiver, the essence of communication is a signal that bears information and it can be in any form that the receiver can sense. There are visual, auditory, touch and chemical signals that the animals use to communicate, but each kind has its own advantages and disadvantages, making one suitable for one way of life and another adapted to some other habitat.

Even the colours have communication value, that is why some animals, insects, fish and birds have bright colours. For instance, black and yellow stripes on a wasp’s body, serve as warning signals marking out distasteful prey to possible predators; birds too learn to avoid cinnabar moth caterpillars, bearing same kind of stripes, after their first unpleasant encounter with them. Some colours help animals in camouflaging, but very many hues are useful for display which make an animal conspicuous to its species mates. One of the many examples is the male of the Siamese fighting fish that turns to brilliant bluish colour on seeing a rival. In birds coloured plumage, tufts and patches of feathers are displayed during the courtship movements.

Some of the most amazing are the feathers and plumes of birds of paradise. Ribbon-tailed bird of paradise trails a pair of two-foot long white feathers, tipped with black, from its tail. But it is not only the splendid colours and their patterns that are used for display purposes in animals, the pattern or sequence of movement of some part of the body may also play important role during courtship. For instance, different species of fiddler-crabs can be distinguished by the way in which their enlarged claw is raised and brought down again - the gestures of a male attempting to lure a female. These signals are so distinctive and species-specific that as many as five species live side by side on the same beach without creating any confusion in the population.

Fireflies, insects that are active only in the night, too use visual signals by flashing light produced at special spots on their abdomens. Males of one common species flash lights in a short series of ‘Morse’ dots, and the female responds by flashing her lights exactly two seconds after the end of the male’s signal. Males respond by heading towards the female. In experiments it has been proved that the male will only move to lights that are flashed at exactly the right time interval after the end of his. This light code ensures that the females, only of his own species, should attract the male.

Facial expressions also play a great part in communication not only in humans, but in animals too. In monkeys and apes ‘smile’ with teeth exposed or covered by lips makes a lot of difference in conveying information.

This shows that visual signals are of varied forms, but they all require sender and receiver to be in view of each other. It means that in most of the cases both of them will have to come out of the cover in open, exposing them to danger from watchful eyes of predators. Advantage of the system is that the signals can be visible over a great distance, but when it is employed, perhaps, its possible disadvantages have been weighed, by natural selection, against the advantages and found wanting.