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’.

Waggle danceBut 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.

Bees drift and diffuse

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.”).

References

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

Return of the Neuroecologist

I’ve been gone!  First was preparing for the big neuroscience conference SFN, then was SFN itself, then was a nasty cold, followed by catching up on the other work I missed in all those weeks.  While I write up new posts, here are some tasty links for you to peruse:

– I hadn’t realized that cockroaches were social.  Did you know that cockroaches form mixed-family herds, and that given the choice of hiding out individually in shelters or hanging in a group, they will choose to collect in one large group?  Me either.  When raised on their own, the now-sad little critters will develop isolation syndrome.

– Do the same neural circuits process social and nonsocial information?  Probably not.

– Are animals political, and do they choose their leaders?  The Scorpion and the Frog has a typically great post about animal politics, and Slate talks about bees.

Old bees get a new lease on life (through glutamate!)

Have you ever heard a story about an elderly person who seems surprisingly fine and with it in the outside world, but is then transferred to a nursing home where they quickly slide from their mental peak?  Have you ever stayed at home all day, playing video games (ahem) and feeling a bit sluggish only to go back to mentally stimulating work and feel more alert?  No matter what people say, our work is our life.

Honeybees spend the first two or three weeks of their life as nurses, taking care of the young, tending to the queen, building out and cleaning the hive.  When they get older, they get reassigned to a job outside the hive as a forager.  Now they have to search out nectar and pollen and live in the dangerous outside world.  They are quick to die off as the stress of the outside environment and downright intense physical work causes them to age.  Not only are there physical effects, but mental ones, too: their ability to learn and associate is impaired.

But not all is lost for these bees!  Sometimes disaster falls a hive and more nurses are needed.  When this happens, some forager bees return to become nurses.  Baker et al studied these bees to see how returning to the hive affected them.  Although in some ways the returned bees looked like their foraging compatriots, in terms of learning and memory they were identical to their younger nursing brethren.  They had a new lease on life!

Some of these returned nurses did better than others.  Baker et al looked at what proteins were differentially expressed between these two groups, and the data pointed to proteins that affected physical structure (alpha-tubulins), stress and cell maintenance, and neuronal functioning and signaling.  One of the most abundantly different proteins was the glutamate transporter homologous to EAAT2.  Glutamate is the primary neurotransmitter in the brain, and is the basis for the most common form of long-term learning.  The glutamate transporter will remove glutamate from the extracellular space, so different amounts of glutamate transporter will change the concentration.  This means that cells will be generally more or less excitable and will have different levels of plasticity.

There are clearly a couple of problems with this study which can basically be labeled statistics.  Do the bees learn better because they have returned to nursing?  Or do they return to nursing because they are the better bees?  This is selection bias.  Also, if the bees are learning better, is it because of this change in proteins?  Or were those differences in proteins there before they returned, and something totally different has changed?

What the paper may provide evidence for, though, is the social brain hypothesis.  This hypothesis suggests that the reasons humans got smarter is because we lived in social groups, and the fittest individual was the one that was smartest at dealing with the social group.  Perhaps the bees that return need to be the smartest because they have to return and deal with a social environment, a possibly more intellectually demanding environment.  These bees have more to keep track of, a variety of other bees to placate.  Not only does your job affect you, but so does your social environment.

Well, it’s something to think about at least.

References

Baker et al (2012). Age-related learning deficits can be reversible in honeybees Apis mellifera
Experimental Gerontology DOI: 10.1016/j.exger.2012.05.011

Photo from