Sticky bees: How honeybee colonies stay safe outside their hives.

Original article: Collective mechanical adaptation of honeybee swarms

A honeybee colony can only exist when many individual bees cooperate. When a hive becomes too crowded, about 10,000 of the workers and a queen leave the hive to form their own colony. While the scout bees are searching for a new nest site, the rest of the bees are exposed to all of the dangers of the outside world, such as predators and storms, and have to stick together for protection. They form a “cluster”, which hangs on a nearby tree branch (as in Figure 1a) until a new suitable nest site is found. Sometimes, beekeepers hang these clusters from their faces as a “bee beard”.

In this study, Orit Peleg and colleagues investigated how these bee clusters stick together against the forces of gravity and the wind by shaking them and tracking how the shape of the cluster changed. Their experimental setup consisted of a board attached to a motor that shook it horizontally at frequencies between 0.5 and 5 Hz and accelerations up to 0.1 times gravitational acceleration. Peleg and colleagues put a queen bee in a cage attached to the board, leading the rest of colony to cluster around her, as shown in Figure 1b. Once the cluster formed, the researchers turned on the shaking and filmed how the bees behaved.

Figure 1. a) A bee cluster in the wild. The worker bees are protecting the queen until they find a new hive. b) A bee cluster in the lab, with the queen attached to the top board in a cage. Figure adapted from original article.

As the bee cluster was shaken horizontally, its tip swung from side to side at about 1 Hz, or one cycle per second. Peleg and colleagues tracked the bees moving from the tip of the cluster to the base as the cluster flattened over about 30 minutes. A flatter cluster does not swing nearly as much as an elongated one.  Once the shaking was turned off, the cluster elongated again after 30 minutes to two hours, longer than it took the cluster to flatten. This is shown in Figure 2.

Figure 2. A honeybee cluster adapting to shaking filmed from the side and the bottom over time. While the shaking is on, the cluster spreads out along the base, and becomes shorter. When shaking is turned off, it returns to its original form. Figure adapted from original article.

Peleg and colleagues observed that individual bees responded to the variations in strain near them. At the base of the cluster, the strain was high, since the base bore the load of the entire swinging cluster. The bees at the base stretched their limbs to hold the rest of the cluster as it swung back and forth. The strain at the tip of the cluster was lower, since the bees there did not have to stretch as much to hold on. As more bees reached the base of the cluster, it flattened, making it swing less and decreasing the local strain on all the bees. The cluster was much flatter after 30 minutes of shaking, as in Figure 2. The bees at the base then didn’t have to stretch as much to hold on, and the cluster was safe from being torn apart.  Even though an individual bee moved towards a greater strain, which may have been less comfortable for it, this collective bee behavior ultimately decreased the strain on the entire colony.

The researchers hypothesized that when a bee experienced a “critical strain”, a high value that might endanger the cluster, it moved to where the strain was higher — up towards the base —changing the cluster’s shape. To show that moving in the direction of increasing strain is a possible explanation for how the cluster flattens, Peleg and colleagues simulated honeybee clusters of different shapes under horizontal shaking (Figure 3). Each bee was modeled as a spherical particle experiencing gravity and attraction to neighboring bees. The simulated bees could not overlap with each other.

For their first simulation, the researchers simulated an entire cluster in 3D with stationary bees subject to horizontal shaking. They wanted to investigate the relationship between cluster shape and the strain.  In this simulation, longer bee clusters experienced a higher strain when they were shaken, as shown by the color gradient in Figure 3a, with yellow corresponding to a higher strain than blue.

A second set of simulations allowed bees to break their connections with their neighbors and move in the direction of increasing strain if their neighboring strain was above a critical value. As expected, bees moved towards the base when shaking was simulated, and the cluster flattened out forming a shape similar to what was observed with real bees in Figure 2, shown in Figure 3b.

Figure 3. a) Three clusters simulated with horizontal shaking, from flat to elongated. Longer clusters have much stronger strains at the base. Blue colors correspond to lower strains while yellow corresponds to high strain. b) When shaking is turned on, simulated bees move towards higher strain (in the direction of the arrows) and flatten the cluster. Red and yellow colors correspond to higher strains. Figure adapted from original article.

This behavior lets bees keep the queen safe and the colony together on the tree when the cluster swings side-to-side in the wind. The cooperation of bees following simple rules lets the colony survive until it finds a new home.

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