Mechanisms of collective decision-making in bees
Thomas Seeley gave a talk yesterday about how honeybees collectively decide on a new home, and how they use the evidence accumulation/drift-diffusion model to make decisions! When bees are ready to start a new colony, they’ll find somewhere to hang out and swarm. Foragers will then periodically wander off to try to find a good home: they like spacious holes high up on tall trees. When a forager finds a home that it likes, it will report back to the swarm what its found in the form of a waggle dance. Bees will shake their little bottoms as they walk in the direction of the potential home, and the longer they shake the further away their new home is. But the job of the bee is more of that of a politician or proselytizer trying to get other bees to follow their lead to the new nest site that they found. By dancing more and more, a scout bee will impress other watchful bees to go check that site out; generally, the more the scout likes the nest, the more time and energy its willing to invest in its wagglin’.
But politics is rough and tumble. Scout bees don’t only advertise nest sites they like, they’ll actively go find the other scouts advertising other nests sites and headbutt and buzz at them. Needless to say, the more an Opposition bee gets headbutted, the less likely it is to continue advertising its own preferred site.
What this gives us, though, is a feedback loop where better nests cause more waggles and more inhibition of other nest sites which recruits even more scouts to check out the nest to do more waggles and more headbutts. In this way, bees will essentially always find the best nest.
This resembles nothing more than the noisy evidence accumulation that is used to explain human and other animal behavior, but on a large scale; now it is not neurons or brain regions accumulating evidence, but a society. Every time a scout brings back its opinion on the nest quality, it will recruit more bees (evidence accumulation in favor) as well as inhibit other scouts (evidence accumulation against). When enough evidence has been accumulated, the swarm reaches a threshold and off they all go! Interestingly, the threshold is always when 15 bees have reached the potential nest site.
I had a couple of questions for Thomas Seeley which he unfortunately did not know the answer to. I was curious whether the bees that waggle were the same ones that headbutt. That is, are are bees just ferociously in favor of their personal site and will do what they can to promote it? Or are some bees bullies and some bees charismatic politicians? Each has a different set of implications. I also wondered whether the threshold changes with swarm size. Too many bees in a swarm and you can reach the tipping point too soon; too few and it can take too long. Seely didn’t have evidence on that either and seemed to misunderstand the decision-making model, unfortunately (he kept explaing, “physicists say this is how it is done.”).
Griffin, S., Smith, M., & Seeley, T. (2012). Do honeybees use the directional information in round dances to find nearby food sources? Animal Behaviour, 83 (6), 1319-1324 DOI: 10.1016/j.anbehav.2012.03.003
Seeley, T., Visscher, P., Schlegel, T., Hogan, P., Franks, N., & Marshall, J. (2011). Stop Signals Provide Cross Inhibition in Collective Decision-Making by Honeybee Swarms Science, 335 (6064), 108-111 DOI: 10.1126/science.1210361