Bumblebees carry heavy loads in ‘economy’ flight mode

Bumblebees can carry surprisingly heavy loads of nectar, a new paper explains, potentially bearing up to their own body weight in the sweet liquid. Furthermore, the insects use a more energy-efficient flight pattern when heavily encumbered.

Image credits Suzanne Williams.

The humble bumblebees definitely lift, the authors report. In fact, they may be the ‘big lifters’ of the insect world. The team set out to understand how the bumblebees manage to fly with such impressive loads, and uncovered the surprising flexibility and adaptability of their flight mechanics.

The burdens we bear

“[Bumblebees] can carry 60, 70, or 80 percent of their body weight flying, which would be a huge load for us just walking around,” said Susan Gagliardi, a research associate in the College of Biological Sciences at the University of California (UoC) Davis and co-author of the paper.

“We were curious to see how they do it and how much it costs them to carry food and supplies back to the hive.”

For the study, the team emptied a snowglobe (to be used as an experimental chamber) and released bumblebees inside it. Each insect had various lengths of solder wide attached to it in an effort to adjust its weight. High-speed video cameras were used to record their wing beats and movements, while the team charted how much energy each bee needed to expend.

“We have the bees in a little chamber and we measure the carbon dioxide they produce. They are mostly burning sugar so you can tell directly how much sugar they are using as they are flying,” Gagliardi said.

Unlike our aircraft, which generate lift from the smooth flow of air over their fixed, horizontal wings, bees move their wings at a high angle to generate tiny wind vortices. These churning bodies of air curl around the insect’s wings and lift them up. The team explains that while the bee’s approach does generate more lift than the smooth-airflow approach out planes rely on, it’s also more unstable mechanically — the vortices are chaotic and they break down very quickly. Bees are only able to fly because they move their wings rapidly to re-generate the vortices.

We didn’t know, however, the energy-efficiency of this mode of flight. It seems reasonable to assume that the bees would use less energy the lighter their load is, but the team was surprised to find out this isn’t the case: bumblebees are actually more efficient per unit of weight when they’re heavily laden. In other words, they’re more “economical in flying” when they’re heavily loaded — “which doesn’t make any sense in terms of energetics,” says Stacey Combes. Combes is an associate professor in the Department of Neurobiology, Physiology, and Behavior at the UoC and the paper’s lead author.

The team explains that bumblebees have two ways to deal with heavy loads. They can either increase the amplitude of their strokes (i.e. how far the wings flap), which helps but isn’t enough on its own for the heaviest of loads, or increase the frequency of their wingbeats, which helps them stay aloft but costs more energy. However, they also observed an alternative flying mode being used — one the team calls their “economy mode” — in which the bees can carry lots of nectar while using less energy than faster flapping requires.

Exactly how they do this is still unclear, Combes said, although the team believes it may involve the wings rotating when reversing direction between strokes. However, it seems to be something that the bees themselves can choose to do, or not. The team explains that overall, when lightly-loaded or rested, the bumblebees were more likely to increase the frequency of their wingbeats. However, they switch to the ‘economy mode’ only when heavily loaded, which produces more lift without an increase in flapping frequency.

“It turns out to be a behavioral choice they are making in terms of how they support the load,” Combes said.

But why don’t they always fly in this mode? The team is still unsure, but it may be that high wingbeat frequency brings other advantages to the table that are more attractive to the bees in a lighter-load scenario.

“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,” Combes adds.

“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 paper “Kinematic flexibility allows bumblebees to increase energetic efficiency when carrying heavy loads” has been published in the journal Science Advances.

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