LONDON - Parents of young children might agree wholeheartedly that reproduction ages you - but what about the rest of the animal kingdom? Two researchers in the UK decided to examine the observation that early reproduction leads to earlier death, using the fruit fly, Drosophila melanogaster, as their experimental animal.

Their study is published in Science , dated December 24, 1999, in a paper titled "A delayed wave of death from reproduction in Drosophila." Carla Sgro and Linda Partridge, of the Department of Biology at University College London, wanted to test if there is a delayed increase in death rates when flies reproduce at a younger age.

"We wanted to understand the mechanisms by which normal aging occurs - not the disease states that occur with aging, but just the normal aging process that makes organisms more likely to die and less fertile as they get older," Partridge, who is Natural Environment Research Council Research Professor at University College London, told BioWorld International.

Various factors are known to slow the rate of aging, she said. One is a reduced reproductive rate. In general, if an animal (or plant) reproduces less, it lives longer. The second is reduced consumption of food: animals (including primates) that eat less live longer. Third comes genetic factors: Some mutants have been observed, mainly in well studied "laboratory species" such as yeast, mice and worms, to live longer for genetic reasons. Finally, there are antioxidant drugs which appear to slow aging-related damage by mopping up oxygen free radicals.

The problem is, no one knows how any of these factors operates in lengthening lifespan, nor whether they all operate independently or are part of the same mechanism. For example, said Partridge, "It could be that they are all acting in series with one another: you eat less, so you reproduce less, and this switches off some genes and so you live longer."

In their Science paper, Sgro and Partridge examined the question of whether reproduction actually causes aging. "We wanted to find out whether, if an animal reproduces at a high rate when it is young, that affects its death rate when it is old," said Partridge.

The two researchers decided to address the question using D. melanogaster. This fruit fly lives for about 60 or 70 days, and is sterile by day 40. Sgro and Partridge had at their disposal two lines of D. melanogaster. One, the "young" line, had been bred to produce lots of eggs early in its lifespan - between 0 and 21 days. The other - the "old" line - produced fewer eggs over the early part of its life. The young line flies tended to die younger, while the ones that produced fewer eggs at a young age tended to live longer. This difference in death rate was apparent from day 35 onward.

To find out whether the difference in reproductive rate really causes the difference in death rate, Sgro and Partridge made all the flies sterile. They did this in two ways: by exposing the fly pupae to x-irradiation, and by making the lines genetically sterile using a mutant with a defect that blocks egg production.

They found, Partridge explained, that the difference in death rate entirely disappeared. "This means," she added, "that reproduction is the sole cause of the difference in death rate. Furthermore, if you go back to your fertile females and look at the difference in death rate at different ages, between the young lines and the old lines, you find a delayed wave of surplus mortality in the ones that reproduced a lot when they were young."

Commenting on the paper in the same issue of Science, David N. Reznick and Cameron Chalambor, of the Department of Biology at the University of California in Riverside, Calif., noted that an "unanticipated dividend" of the Sgro and Partridge study is an explanation for "one of the most baffling results to emerge from aging research." In their article, titled "Sex and Death," they wrote: "Evolutionary theory predicts that mortality rates should continue to accelerate with age. But, instead, mortality rates plateau or decelerate in organisms as diverse as yeast, nematodes, insects and humans. This leveling off can be caused by nongenetic differences in frailty or susceptibility to mortality among individuals." Sgro and Partridge suggested that the reduction in mortality at greater ages might reflect the waning of the wave of mortality induced by reproduction.

The challenge now, Partridge said, is to understand how the delayed increase in death rate works. "We want to find out why it is that the effect on death rate takes a while to build up, and what is reproduction actually doing to cause that," she said.

It is possible, Partridge speculated, that reproduction is causing damage that later manifests itself as rapid aging, or that it may be taking resources away from repair and maintenance of the organism, preventing it from looking after itself properly.

And what light, if any, does this study shed on the processes of human aging? "The mechanisms are obviously going to be different," Partridge said, jesting. "All a fly does is pile out eggs but most human females would agree that although pregnancy can be unpleasant, it is not as unpleasant as what follows. My personal observations suggest that if children age their parents they do it by screaming at night and infecting them with colds and flu."