Honeybee swarms act like superorganisms to stay together in high winds

bee swarm

A bee swarm is like a superorganism

Tim Graham/Getty

A swarm of honeybees acts like a superorganism that responds to physical stress by changing shape – even though doing so comes at a physical cost to some individuals.

Colonies of European honeybees reproduce by releasing a queen from the nest accompanied by an entourage of colony workers. This swarm often attaches itself to the underside of a tree branch – taking on the shape of an inverted cone with the queen safely at the centre – while scout bees search for a good place to build a new nest.

Researchers already know that the cone can withstand temperature changes and rain by changing its shape and appearance. During high winds the cone changes too, typically becoming flatter and hugging closer to the underside of the branch.

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To understand how individual bees work together to generate a swarm-wide response, Orit Peleg at Harvard University and his colleagues attached a bee cluster to the underside of a board hanging in their laboratory and shook the board horizontally to mimic the physical stress of high winds.

Bee quake

As the board shook, the bee cluster began to sway from side to side, like a tree swaying in the wind, and the bee cluster started to flatten. The inverted cone was a few tens of centimetres tall before shaking but lost almost half of its height over the course of 30 minutes, becoming less prone to swaying in the process.

By videoing the swarm and tracking the movement of individual bees, the physicists discovered that bees near the tip of the cluster slowly climbed up towards the base and spread out when it shakes, explaining why the structure flattens.

But this doesn’t explain why the bees know how to respond to shaking by climbing up. It’s unlikely that the bees simply climb against gravity, says Peleg’s team, because it would be difficult to detect gravitational forces while the swarm is violently shaking from side to side.

The researchers produced a model to simulate the physical processes individual bees experience when the swarm shakes. They showed that bees endure local physical strain as they try to cling together, and it’s the bees at the base of the cone — closer to the surface the cone is attached to — that experience the greatest stress.

It’s possible, they say, that it’s this physical strain that acts as a cue. When individual bees experience strain they respond by climbing upwards – even though this means they are moving towards the high-stress area. By climbing, individual bees have to shoulder a greater workload, but the bees seem willing to do this for the greater good of the swarm.

Journal reference: Nature Physics, DOI: 10.1038/s41567-018-0262-1

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