'No sex' rule for longer life

The research reveals that hormones are the key to mating insects having a shorter life expectancy.

The findings may also give clues as to why the same principle appears to hold true for other creatures - including humans.

Mealworm beetles, more commonly bought as food for pet reptiles and birds, were used in the research by Dr Jens Rolff and Dr Michael Siva-Jothy at the University of Sheffield.

A hormone released after mating adversely affected an enzyme vital to keep the immune system functioning.

The beetles were therefore more susceptible to infection, and less likely to live as long.

Although it has long been known that mating in insects can shorten life expectancy, the research by the two doctors is the first time a reason why it happens has been uncovered.

Dr Siva-Jothy said: "Those organisms that mate the most, and are therefore more successful in evolutionary terms, reduce their own life expectancy in the process."

When insects' immune systems are depressed they become susceptible to parasites, so the findings could have implications on pest control, which may be more effective when the insects are mating.

The research could also help explain how sexually-transmitted diseases evolve - by exploiting the depressed immune system when humans have sex.

Such diseases also lead to the possibility of a reduction of life expectancy in humans.

The scientists' findings are to be published by the National Academy of Sciences in America.