Sunday, July 20, 2003		Nature

Sound signals of grasshoppers
Nutan Shukla

A grasshopper's song, the bright colours of a bird's plumage and the smile of a chimpanzee all involve communication. These simple 'languages' are specially adapted to an animal's way of life. However, no animal has as complicated a language as a human being; the vocabulary of a grasshopper, for instance, may contain only 13 different sounds that are created by rubbing its hind legs across the wings folded along its body. On the inside of its legs there is often a row of pegs, each of which strikes the wing in turn when the legs are moved over them like a bow is moved on the strings of a violin. This sets the wing vibrating, at the natural vibration frequency of the wing material.

The sound thus produced does not vary in note, for the sound has the frequency of the natural vibration of the wing. But each time a peg hits the wing, the vibration increases in intensity, for the amplitude of vibration increases. Thus, the song of the grasshopper is amplitude modulated. It is the pattern of these modulations, which renders the song of one species unique. And this pattern can be altered by a change in the rate at which the pegs strike a wing. Different species have different patterns of pegs or move their legs at different speeds from the others.

In grasshoppers, sound signals are very important, especially during courtship. In the breeding season, all males sing a song together, competing with each other to attract females. As a female approaches a particular male, he immediately switches to a courtship song and while mating he sings a song of triumph. Their repertoire has more songs, including the rivalry song, for different occasions. These songs seem monotonous but when their frequency changes they become far more variable.

Unlike in visual signaling (described in the previous article), when sound is used for communication the sender and receiver need not come out in the open to be in each other's view. Instead, they can remain hidden and still communicate. The disadvantage of this method is a shorter reach because the distance sound can travel is often restricted. Naturally, this kind of signaling is used over shorter distances than the visual one.

Scent is another medium through which animals communicate, but only those who have an acute sense of smell use it. There are a number of species of moths whose males are highly sensitive to the smell released by females of their own species. It has also been observed that males are even attracted to an empty box into which a female has been. This is due to the presence of sexually stimulating substances, also known as pheromones, produced in the glands of a female moth's body. At least one of these substances has been isolated and synthesised in the laboratory for the purpose of pest control. Under the name of Gyplure it is used to attract male gypsy moths to insecticides. The feathery antennae of a male insect pick up the feminine scent even if the female is sitting at a mile's distance.

Chemicals are not only used to attract the opposite sex; in fact, they play a very significant role in organising the activities of thousands of individuals living in colonies, like bees, wasps, ants and termites, etc. For instance, ants mark their trail with the chemicals they themselves produce. These pheromones remain attractive for a short time only and new marks are laid only if the food supply is good. If the marks are not reinforced they soon become ineffective so that workers are not drawn along paths that lead to an exhausted food source.

Marking of a territory with scent is a common activity among mammals. Dogs use their urine to mark areas and the urine of a bitch in heat is particularly recognisable, advertising her presence to dogs in the neighbourhood. Many hoofed animals are also in the habit of marking their territory. They smear the ends of twigs or branches with a substance from a gland on their faces while others have glands in their split hooves for the purpose.

Now, in the light of the above-mentioned facts if we compare the language of animals with that of homo sapiens we will find that latter's language has uniqueness, which is absent from all animal activities that comprise modes of communication. It can convey ideas, humour, justice, abstract thoughts etc; it can also make the other person understand not only the present moods and desires of a communicating person, but his future intentions too. On the other hand, methods used by animals for communication, in many cases highly sophisticated, lack many of the above qualities that human language possesses. Still, all animal language, simple though it may be, is highly adapted to the life that an animal leads.