The New York Times, February 27, 2014
Forcing male flies into monogamy has a startling effect: After a few dozen generations, the flies become worse at learning.
This discovery, published on Wednesday in the Proceedings of the Royal Society, isn’t a biological excuse for men who have strayed from their significant other. Instead, it’s a tantalizing clue about why intelligence evolved.
The new study was carried out by Brian Hollis and Tadeusz J. Kawecki, biologists at the University of Lausanne in Switzerland. They investigated a fly species called Drosophila melanogaster that normally has a very un-monogamous way of life.
To find a mate, the male flies seek out females on rotting pieces of fruit. They often engage in battles to chase their rivals away, and then pick a female to court.
“The males will do this wing song, where they use one wing or the other to generate a song,” said Dr. Hollis. This wing song may last from 10 minutes to an hour.
Virgin females usually accept the overtures. But if a female has just mated, she will reject a new male’s advances. “If a male comes at her from behind and she’s not interested, she’ll kick at him with her rear legs,” said Dr. Hollis. If a couple of days have passed since her last mating, however, the female may choose to mate again.
Seven years ago, while he was a graduate student at Florida State University, Dr. Hollis set out to study how the competition among males shapes their evolution. He began breeding two groups of flies — one polygamous, the other monogamous.
In 2011, he took his flies to the University of Lausanne, where he met Dr. Kawecki, an expert on learning. The two scientists wondered if the different mating habits of Dr. Hollis’s flies had altered their brains.
To find out, the researchers gave the flies a learning test. They began by teaching the flies to be scared of a particular smell. They would put a smelly piece of paraffin into the tubes where the flies lived, and after 30 seconds, the scientists gave the tubes a violent shake. After many such experiences, the flies learned to associate the smell with the shaking.
An hour later, the scientists tested how well they had learned. The flies were put in a tunnel that ended with a T intersection. From one side they smelled the dangerous odor, and from the other they smelled a harmless one. Dr. Hollis and Dr. Kawecki then observed which way the flies walked.
The results were stark. The monogamous flies were much more likely to wander toward the dangerous smell than the polygamous ones. In other words, they had done a much worse job of learning.
“I think this is a compelling and interesting study,” said Emilie Snell-Rood of the University of Minnesota, who was not involved in the research. The experiment, she said, suggests that the struggle to find a mate favors the evolution of better learning.
The evolution of learning remains a puzzle for scientists. A smart animal can learn how to find more food or how to avoid predators. But if learning were such an unalloyed good, then one might expect all animals to be as smart as we are.
They are not because there is a cost to learning. Dr. Kawecki and his colleagues have found that flies that have been bred to be good learners are more likely to die when competing for scarce food with regular flies. Even when they’re not threatened with starvation, their life span is 15 percent shorter than average.
It’s still not clear why that is so. Changes to the nervous system that come with learning may cause long-term damage of some sort, or learning may simply use up energy that could be directed to other uses.
Because of the cost, evolution may increase learning only when its benefits outweigh its drawbacks — such as when it affects mating. Dr. Hollis and Dr. Kawecki suspect that fast-learning males may be able to swiftly recognize receptive females, and thus mate with more of them before they die. Forcing the flies into monogamy, on the other hand, gets rid of learning’s benefits, leaving only the cost behind.
To test this idea, Dr. Hollis and Dr. Kawecki compared the mating prowess of the evolved flies. They put a group of male flies in a vial with one receptive female and five unreceptive ones and tallied how many mated in an hour. The scientists found that the polygamous males quickly zeroed in on the receptive female. The monogamous males, on the other hand, wasted time courting unreceptive females and being rejected.
“They’re just not figuring it out,” said Dr. Hollis.
While no one has yet carried out an experiment like this on other species, Dr. Hollis suspects that the relationship between sex and the evolution of learning might apply beyond flies — perhaps even to our own species.
“I think it really can inform us quite a lot about what’s going on in nature, and why we have the brains we have,” said Dr. Hollis.
Copyright 2014 The New York Times Company. Reprinted with permission.