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The criminal culture of bumblebee nectar thieves

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TO MOST people, bumblebees are charming, slightly absurd creatures that blunder through garden and meadow with neither the steely determination of the honeybee nor the malevolent intention of the wasp. If you are a plant, though, things look rather different--from the point of view of some flowering plants, many bumblebees are nothing more than thieves. They rob them of their nectar and give nothing in return.

Nectar robbery, in which a bumblebee carves a hole in the side of a flower as a bank robber might cut his way into a vault, was discovered by Charles Darwin. This technique lets bees get at the nectar of flowers whose shapes have evolved to encourage their pollination by insects with long tongues, which can reach down narrow tubes.

Some bumblebees do have such tongues. Short-tongued bees are, however, unwilling to deny themselves the bounty of nectar inside these flowers. Hence the hole-cutting. By breaking in this way, though, a bumblebee nullifies the 100-million-year-old pact between flowering plants and insects: that the plant feeds the insect in exchange for the insect pollinating the plant.

The question about nectar robbery that has intrigued biologists from Darwin onwards is whether the behaviour is innate or learned. Darwin, though he originated the idea that many behaviour patterns are products of evolution by natural selection, suspected it is learned. Insects, in other words, can copy what other insects get up to. Only now, though, has somebody proved this is true.

The observations were made by David Goulson (then at the University of Stirling, now at the University of Sussex), and his colleagues. To test his ideas he had to go from Britain to Switzerland, for only there could he find a flower of the correct shape to conduct the study.

His crucial observation was when the flowers of an alpine plant called the yellow rattle are robbed, the entry holes -- because of the structure of the flower -- tend to be unambiguously on either the right-hand side or the left-hand side. Moreover, preliminary observation suggested the holes in flowers in a single meadow are often all made on the same side. This led him to speculate bumblebees in a particular area do indeed learn the art of nectar robbery from one another and then copy the technique with such fidelity they always attack a flower from the same side.

His team monitored 13 alpine meadows during the summers of 2009 and 2011. They painstakingly recorded the sites of robbery holes in yellow-rattle flowers and studied the behaviour of 168 bumblebees. They tried to follow each bee until it had visited 20 flowers, though they lost sight of some insects before they had reached this score. If they could, they then captured the insect so as not to follow it again on another occasion.

Dr. Goulson found, as he reports in Behavioral Ecology and Sociobiology, that two short-tongued bumblebee species which live in the area, Bombus lucorum and Bombus wurflenii, demonstrated handedness when they robbed flowers. Moreover, if one species was behaving in (say) a left-handed manner in a particular meadow, the other was likely to do the same. This suggests one species can learn from another -- a trick previously thought to be confined to vertebrates.

Handedness in any given meadow, Goulson found, increased as the season progressed. But each summer appeared to start as a blank slate. The handedness that developed in a meadow in 2009 did not predict its handedness in 2011.

The most reasonable explanation, Goulson argues, is each year a few bumblebees which have learned the trick of nectar robbery in the previous season come out of hibernation and start robbing flowers again. By chance, they make more holes on one side of the flowers than the other and as the habit is picked up by other, newly hatched bees, a preference for left or right spreads by a process of positive feedback. The bees have, in other words, created a simple culture. It is a criminal culture, admittedly. But no one ever said nature was pretty.

Republished from the Winnipeg Free Press print edition April 27, 2013 J11

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