My kids love building with Legos. So much that I built them a giant Lego table that takes up about a quarter of our living room. On that table, I get to watch their architectural talents revealed, or perhaps improved is a better word, from one day to the next.
Each time I open up one of my honey bee colonies, I think the same thing. There’s almost always new comb that’s been built, allowing me to marvel at the bees’ architectural talents. While most comb is made up of spectacularly efficient worker-sized hexagons (more on that later), there’s always something that isn’t regular hexagons. A transition from worker to drone cells, merging of worker cells with slightly different orientations, or something else.
But are honey bees actually architects, or do individual bees simply stumble along, building one cell and then another, eventually ending up with a comb? How common are irregularly shaped cells? Are irregular cells made individually or in predictable combinations? Is there a common tactic for how transitions are made across a comb? In other words, do honey bees have a repertoire of building behaviors that suggest cognitive, architect-like processes are at play? These are the topics for our forty-sixth Notes from the Lab, where we summarize “Imperfect comb construction reveals the architectural abilities of honeybees,” written by Michael Smith and colleagues and published in the journal Proceedings of the National Academy of the United States of America .
For their study, Smith and colleagues provided twelve colonies of Apis mellifera ligustica with wooden frames that did not contain any foundation or wire supports. This allowed the workers to choose where, how much, and what type of comb to build (Photo 1). Comb was built on 23 of the frames over a period of 19 days, then the frames were removed from the colonies and high-resolution photos were taken of each comb.
Three general areas of interest were identified on each comb: “perfect comb,” “transitions,” and “merging.” “Perfect comb” was defined as areas of repeated worker- or drone-sized cells that were not near frame edges, transitions, or merging areas. “Transitions” were defined as locations where workers transitioned from building worker-sized cells to drone-sized cells. “Merging” areas were defined as locations where workers merged two pieces of comb into one.
Next, automated image analysis was used for every cell in every comb to determine wall lengths, wall-to-wall length, interior cell angles, cell areas, and cell tilt. This allowed the authors to assess how regular the cells were in “perfect comb,” and to assess how irregular the cells were in “transitions” and “merges.” To test whether workers built combinations of irregular cells (“motifs”), the authors compared observed vs. expected incidence of irregular pairs and triplets based on chance. Finally, to test whether “merging” occurred haphazardly or by integrating information across multiple cells, Smith and colleagues developed two competing mathematical models from the perspective of merging two, perfectly regular, hexagonal tilings. One model was purely local and did not attempt to construct any nonideal cells, while the other model used global optimization to find a complete covering of space over multiple cells with possibly deformed cells.
So, what did they find? How perfect was “perfect comb”? Cells in “perfect comb” were highly consistent, which shouldn’t surprise anyone who’s taken a good, long look at comb before. For anyone who’s wondering, after measuring 4,414 worker cells, the authors found that cell area = 25.7 ± 0.9 mm2, wall length = 3.2 ± 0.1 mm, wall-to-wall length = 5.4 ± 0.2 mm, and cell angles = 120.0 ± 2.5°. By measuring 2,586 drone cells, it was found that cell area = cell area 37.5 ± 1.5 mm2, wall length 3.8 ± 0.2 mm, wall-to-wall length 6.6 ± 0.2 mm, and cell angles 120.0 ± 3.2°.
What about transitions? Was cell size and/or shape adjusted during construction? Yes and yes. To transition between worker- and drone-comb, or to merge comb within the same plane, workers either built intermediate-sized cells or cells with irregular shapes as they approached the transition. As seen in Figure 1a-b, 4.4% of cells were ….