Lots of animals learn, but smarter isn’t better

Learning is remarkably widespread in the animal kingdom. Even the microscopic vinegar worm, Caenorhadits elegans, can learn, despite having just 302 neurons. It feeds on bacteria. But if it eats a disease-causing strain, it can become sick. The worms are not born with an innate aversion to the dangerous bacteria. They need time to learn to tell the difference and avoid becoming sick.

Learning turns out to have dangerous side effects that make its evolution even more puzzling. Kawecki and his colleagues have produced striking evidence for these side effects by studying flies as they evolve into better learners in the lab. To produce smarter flies, the researchers present the insects with a choice of orange or pineapple jelly to eat. Both smell delicious to the insect. But the flies that land on the orange jelly discover that it is spiked with bitter-tasting quinine. The flies have three hours to learn that the nice odor of oranges is followed by a nasty taste.

To test the flies, the scientists then present them with two plates of jelly, one orange and one pineapple. This time, neither has quinine. The flies settle on both plates of jelly, feed, and the females lay their eggs. “The flies that remember they had a bad experience with orange should continue to avoid orange and go to the pineapple,” Kawecki said.

Kawecki and his colleagues collect the eggs from the quinine-free pineapple jelly and use them to produce the next generation of flies. The scientists repeat the procedure on the new flies, except that the pineapple jelly is spiked with quinine instead of orange.

It takes just 15 generations under these conditions for the flies to become genetically programmed to learn better. But the flies pay a price for fast learning. Kawecki and his colleagues pitted smart fly larvae against a different strain of flies, mixing the insects and giving them a meagre supply of yeast to see who would survive. The scientists then ran the same experiment, but with the ordinary relatives of the smart flies competing against the new strain. About half the smart flies survived; 80 percent of the ordinary flies did.

Reversing the experiment showed that being smart does not ensure survival. “We took some population of flies and kept them over 30 generations on really poor food so they adapted so they could develop better on it,” Kawecki said. “And then we asked what happened to the learning ability. It went down.” In a paper to be published in the journal Evolution, Kawecki and his colleagues report that their fast-learning flies live on average 15 per cent shorter lives than flies that had not experienced selection on the quinine-spiked jelly. Flies that have undergone selection for long life were up to 40 per cent worse at learning than ordinary flies.

Kawecki says it is worth investigating whether humans also pay hidden costs for extreme learning. “We could speculate that some diseases are a byproduct of intelligence,” he said.