Sunday, September 14, 2003		Nature

Courting troubles
Nutan Shukla

Oysters don’t have physical contact for mating

THE majority of animals reproduce sexually, a method that requires two separate sexes to come together and it is possible only during the breeding season when animals display extraordinary behaviour. Even the most bizarre courtship rituals are strictly functional and entirely necessary. The act of reproduction brings together genes from two distinct hereditary lines and ensures that the offspring of a species will differ to some extent from its parents.

Thus, unless the two sexes interact in some way they will be incapable of reproducing. Imagine the problem: two animals that have never seen, let alone encountered, each other have to come together in close, harmonious physical contact. How do they recognise each other as being of the same species, and at the same time of the opposite sex, and how do they know that having encountered each other, both are going to be ready and ripe for mating? To solve this problem, each species has evolved an extraordinary mode of behaviour known as courtship, the essence of which is to bring the males and females together.

For some species, courtship is simple or almost non-existent. For example, members belonging to the same species of oysters or the sea anemones never come into close physical contact with each other to mate. Nonetheless, when they release their sperms and eggs into the water, it is not at a random place. Instead, the eggs and sperms are released more or less simultaneously into established breeding grounds and fertilisation can therefore take place readily. Like the above mentioned creatures there are many other lower forms of life that react purely to certain environmental conditions of light and heat combined with some chemical signals from the mate. They come together and shed their eggs and sperm, and immediately go their separate ways. In such cases there are no displays or dances and no elaborate rituals to observe. But higher up the evolutionary scale, social life becomes more complex and difficulties arise. The first concerns the location of the mate.

The animals that live in groups face no trouble in finding a partner, but those that lead a solitary life or form pairs only during the breeding season, like insects, do face the problem of finding a mate. For them there is a need to transmit or advertise some kind of signal, in the form of smell, sound or sight, to announce their presence and willingness to mate.

Many insects rely on their sense of smell for the purpose, but in birds it is the sound that usually plays the most important role. In migratory birds it is the males that arrive at the new destination slightly before the females, chart out individual territories and then start to sing their loud songs to announce their whereabouts to potential mates. Birds are not the only singing species though they are certainly the most melodious. The singing or chirping of insects, croaking of frogs and bellowing of mammals also fill the air in the breeding season.

In the non-breeding season, many animals usually live in flocks that consist of an anonymous group of animals that do not recognize each other individually. But with the advent of the breeding season they become territorial and their behaviour changes drastically. Males especially form their own territories and defend them against intruders. This change in behaviour serves many purposes. For instance, it promotes mating, it provides an area where the young can be cared for and can be protected against predators, it prevents overcrowding. Therefore, it also ensures an adequate food supply and at the end it affects the dispersal of the species.