Science

Clever carpentry helps bees fix wild honeycombs

This false color image shows how much the cells within various section of honeycomb are skewed, with their orientations varying by 10% (yellow), 20% (green), and 50% (pink). Worker bees join them by crafting irregular cells, including lots of pairs of a five-sided cell next to a seven-sided cell.

MICHAEL SMITH; NILS NAPP; KIRSTIN PETERSEN

With its unmistakable hexagonal pattern, the honeycomb is a marvel of animal engineering. Achieving a geometric regularity rare in the living world, bees construct the comb by making row after row of identical six-sided cells, themselves arranged in an interlocking pattern with hexagonal symmetry—so that if you turn it by 60°, the pattern looks exactly the same as it did before. The design maximizes the storage area for honey while minimizing the amount of wax needed. And bees have evolved to make regular comb even in oddly shaped living quarters, such as the inside of tree trunks.

Now, thanks to some help from computer imaging, scientists have a better idea of how they do it. “The gals have got some tricks up their sleeves,” says team leader Michael Smith, a behavioral ecologist at Auburn University.

One of the challenges of making honeycombs, where food is stored and bees develop, comes from their collective and unsupervised construction. Bees start to build combs in different spots in the hive simultaneously, working their way downward. The insects eventually have to join these sections, which often do not line up evenly. So how do the bees connect the pieces into a single, sturdy comb?

To find out, Smith and his colleagues analyzed photographs of 23 wax combs, using a computer to measure the size, shape, and orientation of more than 19,000 individual wax cells in them. The combs in the study differed from the sheets of comb that bees make in commercial hives, which contain embossed templates to encourage consistent construction. Lacking those aids, the bees in the study produced combs like those in nature, with various sections built separately and at different angles.

As those sections grow toward one another, they meet at irregular gaps that standard-issue hexagons won’t fit. Instead, the bees filled the gaps by building cells of irregular sizes and shapes. The cells ranged from four sides to as many as nine, a lot more geometric diversity than bees typically employ, the team reports this week in Proceedings of the National Academy of Sciences.

Scientists already knew that bees make these unusual cells, but measuring them is tedious. The automated image analysis allowed the team to look for and quantify patterns. And, it turns out, the bees do not merely fill the gaps with random assortments of differently shaped cells. Instead, certain combinations of odd cells occur frequently, such as pairs consisting of a five-sided cell next to a seven-sided one.

Chains of such pairs also show up in another of nature’s striking hexagonal patterns, the arrangement of atoms in graphene, a type of crystal made of a single layer of carbon atoms. In a sheet of graphene, the five-seven pairs appear at the boundaries between “grains,” regions in which the orientations of the hexagonal patterns differ.

In graphene, the chains of five-seven pairs reduce the pent-up energy in the material and, coincidentally, make the crystal stiffer and stronger. Smith and colleagues are now trying to find out whether the bees employ the same pattern to increase the strength of the comb or because, geometrically, it’s the easiest thing to do.

The insects must also sometimes join a section of comb built to house a smaller type of female larvae (those that will mature into worker bees) with another section built to house a larger type of male larvae (those that grow up to be reproductive drones). To do that, the six-legged engineers take a different tack. They join the two types of sections with rows of hexagons that gradually increase in size, the team found.

The study “shows how flexible the whole honey bee system is,” says Christian Pirk, a behavioral ecologist at the University of Pretoria, who was not involved with the research. What’s not known is the degree to which bees are solving the architectural challenges with foresight and an overall plan or with sets of simple rules that they apply at each step of the way, he says.

Pirk thinks these construction techniques involve unconscious decision-making. A minimal amount of information—not a master blueprint—is used to build complex structures, akin to coloring by numbers. “The picture will still emerge even if you do not have a clue what you are drawing.” It might also result in part from trial and error, he notes, as bees will quickly tear down wax cells if they don’t fit.

More insights might come from observing the bees building wax combs, says Francesco Nazzi, an entomologist at the University of Udine, who was not involved with the study. That’s difficult to do, as the insects work in the dark, but Smith say it’s a priority for his lab. Experiments in which researchers fiddle with the wax cells to disrupt construction might reveal how much behavioral flexibility and perhaps planning might be involved.

Orawan Duangphakdee, a bee biologist at King Mongkut’s University of Technology Thonburi, who was not involved with the work, says she’s already impressed by the evidence—and by the bees. “How wonderful the bee’s cognitive ability is.”

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