journal club

Go to the Penguin: Regulating Heat

Swarms in nature can teach us how to get organized behavior in a swarm of robots, but with only a few simple rules. We’ve looked previously at ants informing traffic control, moving objects around obstacles, and self assembling into rafts. Now, we’re going to take a look at two other species that lean in on their social networks: bees and penguins.

When bees congregate in their little swarms, things can heat up fast. Within a hive, bees are known to fan the entrance to keep it cool. But within a more unstructured swarm, in which bees huddle together away from a hive, a few rules are needed to keep the temperature manageable.

In their study of heat regulation in these bee huddles, Ocko & Mahadevan (2014) ignored more complex behaviors they observed. For example, young bees produce less heat and tend to find the warmer center of a swarm, but older ones flank the edges and are able to produce more heat. Instead, they focused on a few rules that might apply to all bees, young, old, or in between. They noted that bees tend to gravitate towards higher temperatures, but also followed a temperature-dependent huddle preference: cool bees huddle closer, but warmer bees aim for a loose, less densely packed huddle.

Bees do not communicate across the huddle, but there is a temperature gradient across it. Each insect, however, is only able to detect its local temperature. Thus, individual bees take their cue from the temperature produced by its nearest neighbors, and huddle more or less densely accordingly. That does much of the temperature regulation right there.

But there’s more to temperature regulation than what’s going on in the huddle. If the weather is nice, why huddle up much, if at all? If it’s cold, however, huddles are a pretty good idea. Thus, they found that boundary bees are positioned to read the temperature gradient. Is the huddle marked by a strong temperature difference, an island of warmth in otherwise cool air? Then they will tightly pack into the swarm. If there’s less of a gradient, there is less interest in packing the boundaries tight.

These, then, were the rules the researchers chose for their model. And predictions matched real-life hive dynamics fairly well. With such research, we can better understand not only the heat distribution of a swarm, but also gain insight into the balance of cooperation and competition in the natural world.

Of course, bees are not the only animals that huddle for warmth. The Antarctic’s Emperor Penguin is famous for braving winters down to -45C, all while incubating their presumably vulnerable eggs. Put hundreds of fairly large, 50-100 lb. birds together, and you’re at risk of a traffic jam – among humans, crowds of this size and density can result in chaos if panic strikes.

Not so in penguins. The huddles can boost temperatures up to freezing, and sometimes as high as 37C (almost 100F). To make sure everyone gets their fair share of the heat, they coordinate tiny little steps resulting in a wave through the huddle, like a stadium wave at a sports game. In doing so each penguin moves through the huddle, and is able to warm up with the rest.

The tiny little steps are like a coordinated dance. While researchers observing the huddle weren’t able to identify if one leader or a set of penguins started each wave, once it began, each penguin took a step of 5-10cm every half a minute or so, in the same direction. In this way, they huddle up, warm up, and prevent traffic jams.

Researchers may well be interested in modeling and understanding how this is done so successfully, as a topic that combines macro-scale biology with thermal and wave physics.

References

Ocko, S. A., & Mahadevan, L. (2014). Collective thermoregulation in bee clusters. Journal of the Royal Society Interface, 11(91). https://doi.org/10.1098/rsif.2013.1033

Zitterbart, D. P., Wienecke, B., Butler, J. P., & Fabry, B. (2011). Coordinated movements prevent jamming in an emperor penguin huddle. PLoS ONE, 6(6), 5–7. https://doi.org/10.1371/journal.pone.0020260

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