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Pollen or nectar today?

Ask the wind!

Oct 29, 2015
Iris and Bumblebee

The laws of physics forbid bumblebees to fly, engineers used to say. But fortunately, the bumblebees did not know this and took off happily – even with heavy loads and in stormy weather. Harvard biologist Andrew Mountcastle even calls them “aerial tankers”, for they can carry roughly double their body weight in nectar and nearly half their own weight in pollen.

After studying the load-lifting capacity of bumblebees, Andrew Mountcastle et al. wanted to understand how a bumblebee’s flight is affected by what it is carrying.
And this is what they found out: In windy weather, the flight stability of bumblebees is higher when they carry pollen rather than nectar. The reason is in the laws of physics: Pollen is stored in little “baskets” in the bee’s back legs. This extra weight at the back of the body stabilizes them when the wind knocks them about and allows them to fly faster than their nectar carrying colleagues. For a bumblebee trying to carry food back to the hive on a windy day, this speed difference might be an important factor.
 
On a calm day, however, bees with heavy legs are at a disadvantage. Their stability is gained at the expense of flight maneuverability. They change direction more sluggishly and as a result, they are less agile in tracking a moving flower. On calmer days, bees carrying nectar are much more nimble than their heavy-legged colleagues and again, this boils down to physics: the nectar loads are swallowed and stored in a special pouch inside their abdomen, thus adding mass to near the bee’s center of gravity.
Heavy legs or a fully loaded belly? The study authors suggest that wind conditions may influence the answer, and determine what resources bumblebees choose to gather. These findings may have important consequences for pollinations dynamics and colony fitness.
 
How they did it
Using small steel ball bearings, the researchers simulated pollen and nectar loads. They temporarily glued one ball to each of a bumblebee’s pollen baskets to represent pollen, or glued both to its back to represent a nectar load. The ball bearings added about 15 percent to the bee’s total mass.
The researchers had previously trained the bumblebees to fly toward an artificial flower while in a wind tunnel. Their freight flights were then filmed with high-speed cameras under three scenarios: unsteady airflow, a flower waving back and forth, and both challenges at the same time. All of these scenarios forced the bees to keep making small adjustments to their flight paths.

The study, titled “Nectar vs. pollen loading affects the tradeoff between flight stability and maneuverability in bumblebees” by Andrew M. Mountcastle et al. was published by PNAS, Proceedings of the National Academy of Sciences of the United States of America.

It was also picked up widely by online media like Discovery Magazine and NPR.
To watch a video of the flight of the loaded bumblebee click here.

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