New research from scientists at the University of Sheffield has produced an explanation for a perplexing phenomenon; insects that mate have shorter lives than those that do not. The findings may give a clue to why the same principle appears to hold true for other categories of living organisms - including humans.

In a report published this week, Dr Jens Rolff and Dr Michael Siva-Jothy, of the University's Department of Animal and Plant Sciences, explain that a vital reproductive hormone released after mating weakens the immune system of mealworm beetles.

Dr Siva-Jothy explained, "The reproductive hormone, known as juvenile hormone, has an antagonistic effect on the insect's immune system. We found that levels of an enzyme critical for immune function, phenoloxidase, were more than two times lower in recently mated beetles compared with virgin beetles. When we inhibited juvenile hormone release this effect no longer appeared. A similar process is likely to occur in all insect species - which make up 80 per cent of the earth's biodiversity - because all insects have juvenile hormone and the immune-system enzyme.

"We believe that this is the first demonstration of a physiological trade off underpinning a life-history phenomenon; those organisms that mate the most, and are therefore most successful in evolutionary terms, reduce their own life expectancy in the process."

When insects' immune systems are depressed they become susceptible to parasites, so these findings have implications for pest control, which could be more effective during the times in which insects are mating.

The research could also help to explain how sexually transmitted diseases evolve; they may exploit the depressed immune system at the time of copulation.