A study of fruit flies suggests that reproducing early in life shortens life expectancy.

Scientists at University College, London, compared what happened to female fruit flies bred to reproduce at an old or young age.

They found that young egg-laying females had significantly higher death rates after 30 days than the older flies.

The difference in death rates peaked at day 40, after which it tended to decline. But later reproducing flies had a generally longer life span.

The scientists found that the death rates were linked to reproduction when they X-rayed the flies from the young group to stop them laying eggs.

The group then started living as long as those that laid eggs later in their life span.

Delayed effect

Researchers have long believed that reproduction increases death rates.

But the new study shows that the cost is felt after a delay, with the genetic changes caused by having children only taking effect when the body starts to age.

"The mechanisms by which reproduction acts with a time delay to increase mortality rates require further investigation," says the report, which is published in the journal Science.

"Reproduction may cause damage directly, and the effects may accumulate with time. Reproduction may also divert nutrients from repair and defence, resulting in more rapid accumulation of damage."