Sunday, January 19, 2003		Nature

Behaviour determines global distribution
Nutan Shukla

EVERY animal needs a special environment suited to the requirements of its physical existence because each species has a different kind of behaviour that has the effect of bringing it to the place where conditions are right for it. The total effect of all these many different behaviour traits is the distribution of animals across the globe.

There are some animals that need such special conditions in the place where they live that they are distributed in a very restricted way. The parasitic wasp, nemeritis canescens, will only live naturally in one host, the moth, ephestia kuhniella. Its behaviour is such that it responds only to the smell of that host, even when raised artificially in the laboratory on a different host species.

At the other extreme, the honeybee is widely distributed and survives well in many environments from the sub-arctic regions through the temperate zones to the tropics. The behaviour of these animals helps them to find suitable areas to live in, and thus propagates the species as a whole.

This last point is important. It is no good having too many of the same species in one suitable environment if there is only a limited amount of food available for them. There must be some behaviour mechanisms causing the animals to spread out more and seek similar places to live in.

Darwin first pointed out that food resources are the ‘ultimate’ factor determining the upper limit of animal population density. Each habitat can support a certain number of animals. This number can vary with the season (the supply of food fluctuates). It can also vary over the years as the climate changes.

The number of mouths to be fed is balanced as nearly as possible by the availability of food. Apart from behaviour other things, such as the mortality rate, and certain physiological changes, can assist this balance.

Very few complete patterns of behaviour in invertebrates have been analysed properly. Those that have been show the complex nature of the stimuli to which the animal reacts.

Lepidiochitona cinera, a species of chiton, lives in the upper part of the tidal zone on the seashore among rocks. When the tide is out, chitons can be found beneath stones, but when the tide comes in and the stones are covered by water, chitons move to the upper surfaces of their stones.

They are a group of primitive marine molluscs, also called as coat-of-mail shells from the eight hinged plates that make up their outer coat. Chitons are flattened marine animals that graze on algae on rocky shores by using the radula (in most molluscs, chitinous strip, constantly renewed, with rows of teeth transversely across its surface, located on the floor of the buccal cavity and used for rasping food) to rasp the thin film of algae off the surface. Attaching themselves to rocks like limpets (various gastropod snails with conical shells), they can also crawl slowly using their long, single ‘foot’, and can roll up into a ball for protection.

The movements of these small moluscs are governed by responses to gravity and to light.

When the stones are uncovered the chitons react positively to gravity and move downward.

Their responses to any light falling on them at this time is a change in speed; bright sunlight makes them move downwards more quickly than duller light. Once they reach the underside of their stones the light is cut off and the chitons stop.

As the tide comes in their response to gravity changes and when covered with water the chitons move upwards. The sun’s light is more constant underwater and the chitons can move about more freely without having to stop in dark places. In this way, simple reactions to stimuli interact to give a much more complicated behaviour pattern. The end result is that the animal’s distribution changes with tides.