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.
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
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?
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.