For a species that could also do arithmetic, it’s no surprise that honey bees have also figured out an efficient cooling solution in the summer, when it gets hot in their nests.

Scientists know that when it gets hot inside the nest, usually located in tree hollows with narrow openings, a group of bees crawl to the entrance and use their wings as fans to draw hot air out and allow cooler air to move in, but how do bees self-organize into these living ventilating units?

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Department of Organismic and Evolutionary Biology (OEB) have developed a framework that explains how bees use environmental signals to collectively cluster and continuously ventilate the hive.

“We have demonstrated that bees don’t need a sophisticated recruitment or communications scheme to keep their nests cool,” said Jacob Peters, a postdoctoral fellow in SEAS and OEB, and first author of the paper, published in the Journal of the Royal Society Interface. “Instead the fanning response of individual bees to temperature variations, and the physics of fluid flow leads to their collective spatial organization, which happens to lead to an efficient cooling solution.”

For the study, which was conducted in the summer of 2017, the research team monitored a group of man-made beehives, and used a combination of measurements and computational models to quantify and explain how fanning bees create an emergent large-scale flow pattern to ventilate their nests.

The team measured temperature, air flow into and out of the nest, and the position and density of bees fanning at the nest entrance. They observed that rather than spreading out across the entirety of the nest entrance, the bees clustered at the hottest areas and kept those areas, which had the highest air outflow, separate from the cooler areas with the highest air inflow. Importantly, they also found that different bees had different temperature thresholds above which they would begin fanning, so that collectively they were better at responding to temperature variations.

To determine how bees organize to cool their nests, researchers measured temperature, air flow into and out of the nest, and the position and density of bees fanning at the nest entrance. (Photo Credit: Jacob Peters/Harvard SEAS)

The behaviors observed all linked to the environmental physics of the nest, according to the researchers. Fanning outward allows the bees to sense the upstream nest temperature; different thresholds of temperature allows for more continuous ventilation and more stable hive temperatures; and, because of the physics of friction and flow, clustering to separate inflow from outflow allows more cool air to enter the nest because of the physics of friction and flow.

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“Our study demonstrates how harnessing the dynamics of the physical environment allows for large-scale organization of a physiological process,” said former postdoctoral fellow at SEAS Orit Peleg, who also co-authored the paper.

“Although this is a physics-focused story, biological variation with roots in genetics and evolution likely plays a big role in order for this system to work,” said Peters. “Our theory suggests that not only does individual variability in temperature threshold lead to a more stable hive temperature but also this diversity is critical to the stability of the patterning of fanning behavior which is required for efficient ventilation.”

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