Cleverness may carry survival costs

Being smart is not always a good thing in the evolutionary race, suggests a new study by Swiss researchers.

If intelligence were always a positive attribute, it would always be selected for by natural selection. But it is not - people and animals have their dolts as well as their Einsteins.

To evolutionary biologists, that diversity means that theoretically, there must be some cost to being smart. Now for the first time, researchers have shown that in fruit flies at least, it doesn't always pay to be clever.

When Frederic Mery and colleagues at the University of Fribourg, pitted fast-learning fruit fly larvae against their more dimwitted cousins in scarce food conditions - the slower fruit flies came out on top.

"This shows that just having a better ability to learn involves a cost, even when you aren't using it," Mery told New Scientist.

Sabotaged flavour

The team first bred a group of fast-learning flies. They allowed fruit flies to lay eggs on gels flavoured with either orange or pineapple juice. But one or the other was also spiked with bitter quinine. The next time round the flies were given the juice only - but some remembered which had previously been laced with quinine and laid their eggs on the other flavour.

The scientists collected those eggs to breed the next generation of flies, gave those flies the same opportunity to learn, and bred the next generation from those that made the "right" choice.

After 20 generations, most flies from the selected line could learn the task in one go. They were not just better at tasting different juices or more averse to quinine, as ordinary flies could eventually learn to avoid the sabotaged flavour too, but it took them three to five sessions.

When the ordinary flies did learn, they also forgot faster than the selected flies. "That shows we selected a gene that improves both memory and the speed of learning, or two genes that are tightly linked," says Mery.

Neuronal connections

However, when the larvae of the more astute flies were made to compete with ordinary larvae for scarce food, fewer of them survived.

"They are slower at feeding," says Mery. He speculates that the flies may have to invest more energy in making or re-arranging connections between neurons in their brains, leaving them with less energy to forage when calories are limited.

He cautions that the work measures the ability of fruit flies to learn only a particular task, though the group is testing whether their smart flies are also better at learning other things. "But this should open up the evolutionary question," he says. In principle, it should be possible to look for the costs of intelligence even in primates.

The group is now collaborating with molecular geneticists to try and tease out which genetic changes have made their fruitflies so smart.

Journal reference: Proceedings of the Royal Society of London B (DOI 10.1098/rspb.2003.2548)

Debora MacKenzie

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