During reproductive swarming, honeybee clusters of more than
10,000 individuals that hang from structures in the environment
(e.g. tree branches) are exposed to diurnal variations in ambient
temperature for up to a week. Swarm clusters collectively modulate
their morphology in response to these variations (i.e. expanding/
contracting in response to heating/cooling) to maintain their internal
temperature within a tolerable range and to avoid exhausting their
honey stores prematurely. To understand the spatiotemporal aspects
of thermoregulatory morphing, we measured the change in size,
shape and internal temperature profiles of swarm clusters in response
to dynamic temperature ramp perturbations. Swarm clusters showed
a two-fold variation in their volume/density when heated from 15°C to
30°C. However, they did not reach an equilibrium size or shape when
held at 30°C for 5 h, long after the core temperature of the cluster had
stabilized. Furthermore, the changes in cluster shape and size were
hysteretic, contracting in response to cooling faster than expanding in
response to heating. Although the base contact diameter of the
cluster increased continuously when the swarm was heated, the
change in length of the swarm (base to tip) over time was non-
monotonic. Consequently, the aspect ratio of the swarm fluctuated
continuously even when held at a constant temperature. Taken
together, our results quantify the hysteretic and anisotropic
morphological responses of swarm clusters to ambient temperature
variations while suggesting that both mechanical constraints and heat
transfer govern their thermoregulatory morphodynamics.