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COLLECTIVE MECHANICAL ADAPTATION IN HONEYBEE SWARMS

with L. Mahadevan


Honeybees often form swarms that take on an inverted cone shape where the bees hold on to each other, and form a large structure that can be hundreds of times the size of a single organism. We study the mechanisms by which a multitude of bees work together, without an overseer, to create a stable structure that accounts for gravity. 


Nature Physics, doi s41567-018-0262-1 (2018)

 

Honeybee (Apis mellifera) swarms form clusters made solely of bees attached to each other, forming pendant structures on tree branches. These clusters can be hundreds of times the size of a single organism. How these structures are stably maintained under the influence of static gravity and dynamic stimuli (e.g. wind) is unknown:  Movie: honeybee cluster in the wind.

 

A MECHANICALLY ADAPTIVE HONEYBEE CLUSTER

Experimental setup

We created pendant conical clusters attached to a board that was shaken with varying amplitude, frequency and total duration.

Time-lapse of horizontal shaking experiment

Our observations show that horizontally shaken clusters spread out to form wider, flatter cones, i.e. the cluster adapts to the dynamic loading conditions, but in a reversible manner - when the loading is removed, the cluster recovers its original shape, slowly.

 

A MODEL OF MECHANICAL RESPONSE AT DIFFERENT ASPECT RATIOS

We use particle-based simulations of a passive assemblage to suggest a behavioral hypothesis that individual bees respond to local variations in strain. This behavioral response improves the collective stability of the cluster as a whole at the expense of increasing the average mechanical burden experienced by the individual.

Passive simulations to extract local strains

For the same forcing, the maximum amplitude of the local strains increases as the cluster becomes more elongated. This passive signature of a horizontally-shaken assemblage suggests a simple behavioral hypothesis: bees can sense the local variations in the normal strain above a critical threshold, and move slowly up gradients collectively?

Active simulations

Simulations using this rule explain our observations of adaptation to horizontal shaking. Over time, the cluster spreads out to form a flattened cone, with the local behavioral rule that integrates relative displacements that arise due to long-range passive coupling in the mechanical assemblage wherein bees actively move up the local gradient in normal strain.

 

This behavioral response improves the collective stability of the cluster as a whole via a reversible shape change, at the expense of increasing the average mechanical burden experienced by the individual. Introducing elastic interactions between the bees naturally allows for long-range signaling via physical cues, complementing the more traditional view of collective behavior via stigmergy wherein organisms respond to local chemical cues with little or no long range effects. Given the tensorial non-local nature of elastic interactions in assemblages of social insects, perhaps this is just the tip of an iceberg that hides the many ways in which organisms take advantage of physical interactions and simple behavioral rules to adapt to changing mechanical environments.

To learn more about this work, see: 

O.Peleg*, J.M. Peters*, M.K. Salcedo, L. Mahadevan

Collective mechanical adaptation in honeybee swarms

Submitted, preprint available at bioRxiv https://doi.org/10.1101/188953 (2017)

*Contributed equally to this work.