Sunday, February 16, 2003		Nature

WHY does any animal suddenly start looking for a safe place to lay eggs or give birth to young ones in a particular season only? Why does it begin searching for food when it has shown no interest in it for some hours? In other words, what is it that makes animals behave in a particular way, often giving up one activity and suddenly taking up something quite different? For instance, an animal which ignores food when it is well-fed, will eagerly eat the same food when it is hungry. What has made it change its responses?

Watching a hungry animal seeking food gives the impression that it is being forced to look for it, ignoring everything else. It looks as if it is being 'driven'. This gives rise to the idea of 'drive', of some inner urge that orders them to behave in a particular way. When looking for food animals are under the influence of the hunger drive, when looking for a mate, a mating drive propels them and so on. The drives are present in an animal but remain latent until they are aroused. The word 'drive' is a convenient shorthand, but when it is necessary to explain just what a drive is, we are forced to examine the physiology of animals to discover why animals behave one way one minute and another way the next.

An active animal uses its blood sugar to provide energy for its muscular movements, etc. This blood sugar comes from stores in its liver and directly from the food in its gut. If it has gone unfed for a time, the level of blood sugar will have dropped, and the reduced level will affect areas in the brain. These are stimulated to produce the activity, which we see as a search for food or 'drive'. If the animal is successful in its search, it takes more food into its gut, the blood sugar level rises once again and the search is suspended. Incidentally, when the blood sugar is low, nerve impulses pass from the brain to the stomach which contracts in a way that makes itself felt as hunger pangs.

Hormones - the chemical messengers produced by ductless glands - play a large part in determining the behaviour of vertebrates; their role in insects and other invertebrates is also beginning to become clear.

The cycle of changes, which goes on in a female mammal as she comes into season brings about changes in her behaviour. She becomes sexually receptive and accepts the advances of a male, which, at other times, she rejects. This period is known as oestrus.

The increase in bird songs in spring is so noticeable that it has been recognised to herald the change from winter to summer for hundreds of years. It stresses the fact that birds, like many other animals, do not breed just at any time of the year. They begin when the temperature is increasing, plants have started flowering and insects come out of their winter hiding places. In other words, they breed when food, which will be needed for the young ones, is beginning to be abundant. Breeding time seems to be marked by an increase in the length of days, which becomes evident in early spring and continues steadily until mid-summer. It is possible to bring birds into breeding conditions earlier than usual by providing them with increasing periods of artificial light each day during the early part of the year. The greater length of day gives birds a longer time in which to find food for their young. Thus spring is the appropriate time for mating.

Other animals have cycles of different lengths. Many animals on the seashore, for instance, follow the tidal cycle. They will continue to show the rhythm in a tank in the laboratory where there is no rise and fall of the tide to stimulate them into activity. Yet others follow a lunar cycle. The Palolo worms of the Pacific swarm in October and November during the third quarter of the moon. The worms break off the ends of their bodies, which are by then stuffed with reproductive products. The ends wriggle their way to the surface to discharge the eggs and sperms in clouds, turning the sea milky.