Rivalry helps fruit flies maintain brainpower

The demands of outmaneuvering other guys when courting may help male fruit flies stay mentally sharp.

After more than 100 generations in the lab without male competition, male fruit flies didn’t do so well in a standard test of learning, reports evolutionary biologist Brian Hollis of the University of Lausanne in Switzerland.

The male Drosophila melanogaster still courted with normal enthusiasm and success when alone with a female. But in groups, the long-sheltered males lagged when competing with another strain of males in siring young. A series of other tests suggests that the competition-free populations faltered not because of physical weakness but because they had lost some of their smarts for coping with complex mating crowds. Male competition and other forms of sexual selection may be unappreciated evolutionary forces for maintaining a species’ smarts, Hollis and Tadeusz Kawecki of Lausanne propose February 26 in the Proceedings of the Royal Society B.

The results don’t mean that “monogamy makes men dumb,” Hollis says. The fly experiments create a “caricature” of monogamy to study an evolutionary question. Real-world demands of reproducing, for humans as for many other animals, involve much more than being alone in a lab vial with one member of the opposite sex.

Pairing fruit flies with just one partner is how the researchers created their artificial world without male competition, starting in 2007. After 100 generations of such solitude males were still courting lone females as successfully as normal flies.

Fruit flies usually mate in crowds drawn to food. Virgin females may accept male advances after merely 10 or 20 minutes of courtship moves such as face-to-face stares and intense wing vibrations. A female who has already mated, however, is much harder for a male to persuade. “She’ll kick at him, or vibrate her wings in a characteristic way,” Hollis says. “There are lots of rejection behaviors. A male can spend hours.”

Other research has shown that males learn signs of rejection from their early attempts at courtship. When the Lausanne researchers mixed an inexperienced male from the noncompetition group with one virgin female and five mated ones likely to reject him, he did seem to learn a bit. After 20 minutes, these males were more likely to focus attention on the receptive females. But males from the normal polygamous population became even more likely to focus.

The researchers also tested learning ability with odor. They gave fly vials an earthquake shake during exposure to one odor but not when a different one wafted by. The noncompetition fruit flies didn’t score as well as the regular flies in learning to flee the earthquake smell.

Females of the noncompetition population, however, matched the odor-learning performance of regular females. Maybe the challenges of egg-laying or some other aspect of female reproduction had preserved their mental edge, the researchers speculate.

The results from the male flies run counter to research on vertebrates. Among bats and nonhuman primates, scientists found species investing more resources in brainpower when they faced less intense pressures of courting and competing.

“Primates are highly intelligent and very social; flies are not,” says evolutionary anthropologist Michael A. Schillaci of the University of Toronto Scarborough. He’s not sure the flies shed much light on primates, but they could help scientists understand mammals that don’t display a lot of social complexity.