The cost of reverie. Protein production in the part of the fruit fly brain responsible for long-term memory (shown in green) may be the trade-off that keeps flies from evolving more smarts in the wild.

What Price Recollection?

We may take our long-term memory for granted, but it doesn't come cheap. According to a new study in fruit flies, such memories can shave off hours of life when the going gets rough.

Scientists can breed flies to have better memories, but flies in the wild haven't evolved the skill themselves. This has stumped researchers, who assume that remembering the details of a dangerous situation would help flies live longer. So some have wondered whether making such memories comes at a significant biological cost.

To test the idea, evolutionary biologists Frederic Mery and Tadeusz Kawecki, of the University of Fribourg, Switzerland, gave fruit flies a bad experience to remember. They wafted a stinky chemical at flies in a tube and then vigorously shook the tube, so the flies would learn to associate the smell with danger. Some flies were given a 20 minute rest between shakes--a pause that allowed them to manufacture proteins required for the development of one type of long-term memory, while others got no break, permitting them only to develop a less permanent form of long-term memory.

The next day, the researchers quizzed the flies by seeing if the creatures would avoid going down a maze path toward the chemical. Both groups were equally wary, while control flies didn't care. When the team stressed the flies by withholding food and water for 2 days, the flies that had only formed the less permanent form of long-term memory went on to live about 20% longer than the flies with longer-lasting long-term recall. The results indicate that the latter type of memory comes at a price because it requires additional protein manufacturing. This could explain why fruit fly memory is limited in the wild, says Mery, whose team reports its results today in Science.

"This is a really elegant piece of work," says Victoria Braithwaite, an evolutionary biologist at the University of Edinburgh, U.K. By separating the different types of memory in flies, she says, "they've devised an ideal model system" for studying learning and its biological pricetag.

Related sites
Mery's site
Braithwaite's site