How do bees work together to cool their hives?

For the Asian giant honeybee, keeping cool is a group effort.

Courtesy of Kastberger et al.
Identification of warm and cool zones at the surface of a giant honeybee nest.

When it comes to taking care of a nest, honeybees are in sync. And a hive of hundreds of thousands of bees can make a cool parent.

Asian giant honeybee larvae are sensitive to temperature, so the hive must regulate the temperature of the nest while the brood is incubating. But with nests open to the elements, how do the bees do it?

The obvious solution would be to fan the nest by flapping their wings, but each individual bee would expend a lot of energy in that effort. Luckily, the bees seem to know that teamwork is more efficient.

Like one muscular organ, the bees can move their bodies in a synchronized dance of sorts to suck cool air into the larvae-filled nest or push warm air out, according to a paper published Wednesday in the journal PLOS ONE.

Gerald Kastberger, who has been studying the giant honeybees, Apis dorsata, for decades, compares the hive's thermoregulation to human breathing. 

Just as our nostrils flare and those muscles engage as we inhale, a carpet of bees covering the nest lifts away from the honeycomb in one muscular motion. This action lowers the air pressure inside the nest allowing fresh, cool air to pour in. 

The hive "inhales" when the worker bees press their little legs against the honeycomb and "exhales" when they relax their bodies, pushing the warm, stale air out of the nest.

Although the mechanism is muscular like mammalian lungs, the hive isn't "breathing" to suck in oxygen. Instead it's acting as more of a ventilating air conditioner to keep the nest at the specific temperature of 95 degrees Fahrenheit.

"It's a must for the rearing of the larvae," Dr. Kastberger of the University of Graz, Austria, tells The Christian Science Monitor.

Keeping the nest at just the right temperature is particularly challenging thanks to where the Asian giant honeybees set up camp.

If you're picturing beehives in white boxes, you're thinking of the European honeybee, Apis mellifera, which is half the size of the Asian giant honeybee. That smaller honey-maker likes to build its multi-comb nests in protected nooks and crannies. But the Asian giant honeybee nests are single-comb hives that hang from tree branches, cliffs, or other places out in the open.

To protect their nest from the elements and some predators, the bees form a sort of curtain over the comb. That curtain ends up being the "breathing" blanket of bees.

The first hints of this mechanism came in the form of "cooler nest spots" spotted when Kastberger was observing the nest using a heat-sensitive infrared camera. Cooler sections would appear across the surface of the curtain of bees, particularly during the hottest part of the day. And these splotches could be as much as 10 degrees colder than the rest of the blanket of bees.

Kastberger and his colleagues took a closer look with an infrared camera and a vibrometer to detect the bees' motions and how readily they cooled the nest and came up with the model described in the new paper.

"It’s an intriguing idea, but with no direct evidence," Benjamin Oldroyd from the University of Sydney told The Atlantic.

The researchers couldn't look at what each individual bee was doing inside the nest, just the overall motion, explains Kastberger, because inserting a camera would have disturbed the bees and they might not have behaved as normal.

Still, Chelsea Cook, a postdoctoral researcher at Arizona State University who was not part of the research, called the study "refreshing."

"Because they have these external aggregations – they're not nesting in cavities – you would assume that it would be relatively straightforward" how the giant honeybees thermoregulate their hive, she tells the Monitor, particularly because the wind could help ventilate the nest. Instead, the hive engages in surprisingly complex behavior.

This displays "really dynamic behavior by individuals" on behalf of the collective, Dr. Cook says. "I think a lot of these individual behaviors in these honeybees indicates that in their mode of existing, collective is definitely more important than individual."

And, she says, humans might be able to learn a little bit about our own societies by studying such complex, cooperative animal societies.

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