Honeybee Apis mellifera swarms form large congested treehanging clusters made solely of bees attached to each other 1
.How these structures are maintained under the influence of
dynamic mechanical forcing is unknown. To address this, we
created pendant clusters and subject them to dynamic loads
of varying orientation, amplitude, frequency and duration.
We find that horizontally shaken clusters adapt by spreading out to form wider, flatter cones that recover their original
shape when unloaded. Measuring the response of a cluster to
an impulsive pendular excitation shows that flattened cones
deform less and relax faster than the elongated ones (that is,
they are more stable). Particle-based simulations of a passive
assemblage suggest a behavioural hypothesis: individual bees
respond to local variations in strain by moving up the strain
gradient, which is qualitatively consistent with our observations of individual bee movement during dynamic loading. The
simulations also suggest that vertical shaking will not lead
to significant differential strains and thus no shape adaptation, which we confirmed experimentally. Together, our findings highlight how a super-organismal structure responds
to dynamic loading by actively changing its morphology to
improve the collective stability of the cluster at the expense
of increasing the average mechanical burden of an individual.