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Thursday, Nov. 28, 2002

NATURAL SELECTIONS

Superfly just f- f- f- fades away, showing that insects age too


Neil Young referred to it with, "It's better to burn out than to fade away" while Pete Townshend echoed the sentiment with the line, "I hope I die before I get old."

Both men (who are now getting on a bit) knew that as humans age our organ function deteriorates, our skin loses elastin and sags, our bones decalcify and become brittle and the neurons in our brains gradually die out. In short, we fade away.

But do all animals age before they die? And if they do, why has natural selection shaped us this way?

Many mammals show similar signs of aging to humans. But many fish and reptiles show no such signs. Some insects (mayflies, for example) live for a few hours only, burning out with a flash of sperm and eggs, but others (such as periodic cicadas) live for years.

Biologists know what happens when animals are kept in a lab (they often show signs of aging), but that doesn't mean they know for sure what happens in the wild.

"Insects in the laboratory are a bit like humans in industrialized societies, in that they tend to live much longer than their ancestors did under 'natural' conditions," said Russell Bonduriansky, a zoologist at the University of Toronto, Ontario, in an e-mail interview.

Studying animals in the field is far harder, and disease, predators and accidents regularly kill wild creatures in their prime of life.

Knowing whether they get less vigorous as they get older as a result of natural aging or of environmental effects is difficult to establish.

Now research conducted by Bonduriansky and colleague Chad Brassil on a tiny fly that lives on the shed antlers of moose and deer has shown for the first time that aging does occur in wild insects. The antler fly, only a couple of millimeters long, lives for only four weeks, but at least has the compensation that it copulates several times for hours on end.

Bonduriansky and Brassil followed hundreds of the flies throughout their lives and found that as each day passed the flies were more and more likely to die. Their reproductive ability also declined. In short, the flies don't die before they get old: They age and decline just like us.

Adult antler flies emerge from the soil where they have been pupating and find themselves a fallen antler to settle on.

Fortunately for the biologists, the flies tend to stay put once they are on an antler, allowing the same flies to be studied each day of their short lives. Bonduriansky and Brassil marked 609 male flies with tiny numbers and measured how their reproductive prowess changed with time.

"We found that the costs of senescence [aging] are very large: The average male lost 20 percent of his potential reproductive success from senescence," said Bonduriansky.

"These costs resulted mainly from the decline of reproductive capabilities with age: Old males were either less capable of defending their territories against younger males, or they were less attractive to females."

As might be expected from an animal with such a short life, antler flies have little time for peaceful resolution of conflicts. They are highly aggressive, attacking even other species of fly far larger than themselves, and, says Bonduriansky, attacking the tip of his pen.

When it comes to sex, the antler fly should arguably be renamed superfly, copulating as it does for more than two hours at a time, courtesy of a prodigious organ.

"The male's penis is longer than the rest of his body," said Bonduriansky. "It's unusual length may help the male to cling more firmly to the female during mating, making it more difficult for rival males to dislodge him."

After copulation the female expels much of the sperm and eats it. The male stays on her back, guarding her from the attentions of other males while she lays eggs into tiny holes in the surface of the antler. The fly larvae live in this bone matrix, climbing to the surface when they are ready to pupate. From the antler surface they drop into the soil and undergo transformation from larvae to adult insect.

Bonduriansky and Brassil, whose study results are published in the journal Nature this week, found that although the number of times the flies mate decreases with age, the sex sessions still last for an average of 2.3 hours. Their explanation for the decline in reproductive rate will chime with many of the millions of men who have made the producers of Viagra rich : exhaustion. The old flies just don't have the energy to do it as much as when they were young.

Rowan Hooper welcomes comments at rowhoop@gol.com


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