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Ant colonies attack queens that don't produce largest broods, study finds

Tuesday, March 16, 2010

Even in the domain of insects, uneasy lies the head that wears a crown

Life in some ant colonies, it turns out, is quite Shakespearean, with worker ants killing off queens, and queens attempting clever reproductive strategies to survive.

Luke Holman, a University of Copenhagen ant researcher who recently published a paper in the Proceedings of the Royal Society B about ants in multi-queen colonies, said that queen ants are wilier than previously thought, protecting themselves even if their strategy hurts the colony's survival.

When an ant colony is forming, several queens will often work together to produce large broods of worker ants. But within about 45 days, those initial worker ants turn on the queens, biting them and spraying them with acid, apparently so that only one will be left to reign.

According to Holman and his co-authors, some queens in these multi-queen colonies will start to produce smaller broods, apparently in an effort to conserve energy to survive the battles with the worker ants. (In colonies with single queens, the researchers found no reduction in offspring production.)

Holman said the strategy did not appear to work, because the worker ants, using chemical cues, could determine which queens were most fertile and thus most likely to keep the colony going. The workers were more likely to attack "selfish" queens that had reduced their fertility.

Not that the ants think in those terms. Holman said in an e-mail that they were just doing what evolution had built into them for survival: "Natural selection has shaped the evolution of behavior." The queens that became less fertile, he said, "might be sensitive to some sort of cue found on other queens and ant brood. When they detect this cue, some sort of physiological pathway reduces their hormone levels and makes them a little less fertile, giving the impression that queens 'decide' not to reproduce."

Similarly, he said, "workers might have an inbuilt tendency to be aggressive towards queens, but this is inhibited by a chemical on the queens, which is in turn correlated with how fertile the queen is. Therefore, workers appear to us to be sniffing out fertile queens and preferentially sparing them, but really it is just that natural selection has favoured workers whose aggression is inhibited by an odour associated with fertile queens."

Lady Macbeth would have understood.

-- Margaret Shapiro

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