Giant sperm that ensure paternity

Scientists have discovered the ultimate weapon in the battle of the sexes: giant sperm which guarantee that a male cannot be cuckolded.

A species of fly that lives off fruit has been found to produce inch-long sperm about five times the length of its body. In human terms it is equivalent to a man making a sperm some 10yds (9m) long.

The phenomenal feat of the male fruit fly is believed to have evolved in order to prevent females having their eggs fertilised by other males.

Biologists have long recognised that differences between the sexes have largely come about because of opposing reproductive strategies. Males produce large numbers of small sperm while females have much larger but far less numerous eggs. This has resulted in males not knowing for sure whether their sperm has fertilised an egg - or whether another male's sperm has slipped in. This innate doubt of the male is responsible for a host of behaviours - such as promiscuity - which appear in many animals, scientists say.

The sheer size of the fruit-fly sperm guarantees that the female's egg is fertilised because no other sperm can get near, according to researchers at the French National Centre for Scientific Research, at Gif-sur- Yvette, in an article in the current Proceedings of the National Academy of Sciences.

Dr Daniel Lachaise, one of the research team, said: 'The sperm becomes a nuptial gift that cannot be refused . . . It is a mechanism to ensure that the male is not cuckolded.'

The drawback is that the male fruit fly cannot produce many sperm in a lifetime because of the effort involved. In comparison, the human male ejaculates about 120 million sperm, only one of which usually stands a chance of success.