Plants could turn out to be one of the more chatty organisms. Recent studies have shown they can communicate with a surprising range of cues. Now it turns out they could be sending out electrical signals, too.
As they fly through the air, bees – like all insects – acquire a positive electric charge. Flowers, on the other hand, are grounded and so have a negative charge. Daniel Robert at the University of Bristol, UK, and colleagues set out to investigate whether bumblebees (Bombus terrestris) were able to make use of these signals.
To test the idea, the team created artificial flowers, filling some with sucrose and others with quinine, a substance bees don't feed on. To start with, the bees visited these flowers at random. But when a 30 volt field – typical for a 30-centimetre-tall flower – was applied to the artificial blooms containing sucrose, the team found that the bees could detect the field from a few centimetres away and visited the charged flowers 81 per cent of the time. The bees reverted to random behaviour when the electricity was switched off.
"That was the first hint that had us jumping up and down in the lab," says Robert. The result suggests the bees may use the electric field as an indicator of the presence of food, much like colour and scent do. In the absence of a charge, they forage at random.
Next, his team looked at whether the bees were influenced by the shape of a flower's electric field, which is determined by the flower's shape. By varying the shape of the field around artificial flowers that had the same charge, they showed that bees preferentially visited flowers with fields in concentric rings like a bullseye: these were visited 70 per cent of the time compared to only 30 per cent for flowers with a solid circular field.
Ruthless evolution
The researchers don't know exactly what information is contained in the flowers' electrical signals, but they speculate that flowers could evolve different shaped fields in their competition to attract pollinators. "Flowers are a ruthless expression of evolution," says Robert. "They exploit the bees."
It's likely that a flower's electric charge reinforces the cues provided by its colour and scent, says Robert, in much the same way as TV commercials use a mix of visual and aural cues to convey their message. The team showed, for example, that bees took a shorter time to distinguish two very similar shades of green when an electric cue was applied. "Electricity is part of their sensory world," says Robert.
When a bee visits a flower it transfers some of its positive charge, incrementally changing the flower's field. With repeated visits, the charge may alter significantly, which could tell other bees that the nectar supply has been diminished. "The last thing a flower wants to do is lie to a bee," says Robert. "Electricity is a way to change cues very quickly: 'I look perfect, I smell nice, but my electrics aren't quite right – come back later!'"
Of course, there may be a few cheaters out there that won't budge a millivolt when visited, he says. But both flowers and bees have limited control over their charge. "All that comes for free," says Robert. "It's just atmospheric physics." He hopes to find out whether other pollinators – including bats – also use electrical cues.
Dishonest advertising
Robert Raguso at Cornell University in Ithaca, New York, agrees that the changing electric field may signal that nectar is running low. "Flower colours and scents change slowly, but nectar or pollen can be removed quickly by a pollinator, creating a situation in which the just-visited flower still advertises, dishonestly," he says. The rapid change in electric charge would cut through those out-of-date cues. "Just as the chemical marks left by bee feet can be used by subsequent bees to avoid visiting a depleted flower," he says.
Lars Chittka at Queen Mary, University of London, also thinks it is an interesting finding. He notes that an electrostatic charge can cause pollen to jump short distances from flower to bee, making it easier for the bee to pollinate – another reason bees may favour flowers with a charge.
However, Chittka points out that we cannot yet say with certainty that the bees' ability to detect an electric charge is a true sixth sense. It may be that when a bee hovers over a flower it simply feels the static charge making its hairs bend, in the same way that hairs on our arm bend towards a charged balloon.
If, however, bees do have a true electrical sense, they will join the ranks of certain fish and amphibians. They would be the first animal found to detect electrical fields in the air. "It's previously only been seen in animals in soggy environments," says Chittka.
Journal reference: Science, DOI: 10.1126/science.1230883
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