Bumblebees found to enter "economy mode" when carrying heavier loads
Regularly tasked with carrying loads of nectar that weigh almost as much as themselves, bumblebees must use their energy wisely when flying back and forth to the hive. To better understand the capabilities of these pint-sized power lifters, scientists at the University of California, Davis have used high-speed cameras to observe them in flight, finding that they can enter a type of “economy mode” when carrying particularly hefty cargo.
“They can carry 60, 70 or 80 percent of their body weight flying, which would be a huge load for us just walking around,” says study author Susan Gagliardi. “We were curious to see how they do it and how much it costs them to carry food and supplies back to the hive.”
To do this, Gagliar and her fellow researchers carried out experiments designed to measure the energy expenditure of bumblebees in flight. This meant placing them inside emptied-out snow globes, attaching adjustable wires to their bodies in order to simulate different payloads, and measuring the carbon dioxide the insects produced to calculate their energy expenditure.
“They are mostly burning sugar so you can tell directly how much sugar they are using as they are flying,” Gagliardi said.
Through the use of the high-speed cameras, the team found that bees adjust their flying style under heavier loads in a couple of ways. Unsurprisingly, they respond by increasing both the frequency of the flapping and the stroke amplitude, or how far the wings flap, in order to create more lift and deal with the extra weight.
But what the scientists didn’t expect to see was a slightly different style of flight that enables them to travel using less energy, despite the heavier loads. It is not clear at this stage exactly how the bees achieve this “economy mode” form of flight, though the researchers suspect it could involve a different way of reversing their wings in between strokes.
“They get more economical in flying the more heavily loaded they are, which doesn’t make any sense in terms of energetics,” says co-author Stacey Combes.
What they do know is that the bees are selective about when they enter this energy-saving mode. When the insects are well-rested or carrying lighter loads, they tend to increase the frequency of their flapping, possibly for performance benefits like extra stability. When burdened with a heavier load, however, they enter this enigmatic economy mode, seemingly able to to carry the extra weight with only a small increase, or even decrease, in wing-flapping frequency.
“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 load, another way when they’re carrying 50 percent load and every bee will do it the same way every time,” says Combes. “This has given us an appreciation that it’s a behavior, they choose what to do. Even the same bee on a different day will pick a new way to flap its wings.”
The research was published in the journal Science Advances, while you can see footage from the experiments in the video below.
Source: University of California, Davis