Bumblebees can fly in ‘economy mode’

We’re learning a lot about bees. In the past year alone, Cosmos has reported on their ability to learn maths and surf to safety, and even their drinking habits.

Now researchers from The University of California, Davis, think they have answered that enduring question: how can bumblebees carry such large amounts of nectar? We’re talking close to their own bodyweight at times.

It appears they have an “economy mode” – though it’s rather counterintuitive.

“They get more economical in flying the more heavily loaded they are, which doesn’t make any sense in terms of energetics,” says Stacey Combes, an associate professor in the Department of Neurobiology, Physiology and Behaviour.

Coombes and biologist Susan Gagliardi decided to look more closely, conducting experiments in which they attached small pieces of solder wire to bumblebees to adjust their weight then measured the energy they expended flying in a specially designed chamber.

They found they have two different ways to cope with increasing loads.

They always increase stroke amplitude (how far the wings flap) when they are more heavily loaded, but this isn’t enough to support the extra weight on its own. To make up the difference, they can increase wingbeat frequency, which generates more lift and increases energetic cost.

But they also have an alternative, subtly different flying mode that allows them to carry heavier loads while expending less energy than when they increase flapping frequency.

It’s not yet clear exactly how this economy mode works, Combes says, but it may involve a change in how the wing rotates to reverse direction between strokes.

“Our results suggest that some bees that were heavily loaded or had already performed a previous flight trial were using alternative mechanisms to enhance force production, rather than increasing wing velocity further by elevating flapping frequency,” they write in a paper published in the journal Science Advances.

So, if economy mode uses less energy, why wouldn’t bees fly like this all the time? 

One possible explanation is that a higher wingbeat frequency has advantages such as maintaining stability in air or avoiding obstacles.

It could also, quite simply, come down to a bee’s personal choice.

“When I started in this field there was a tendency to see them as little machines, we thought they’ll flap their wings one way when carrying zero loads, another way when they’re carrying 50% load and every bee will do it the same way every time,” says Combes.

“This has given us an appreciation that it’s a behaviour, they choose what to do. Even the same bee on a different day will pick a new way to flap its wings.”