Honey bee swarms are produced in the spring when several thousand worker bees leave their hive with their mother queen to establish a new colony. The swarming bees cluster near the parental hive for a few days while several hundred of the oldest bees in the swarm, the scout bees, find prospective nest sites.
Each scout then returns to the swarm and performs a distinctive waggle dance which tells its hive mates the direction and distance of the best nest site it has found – rather like posting a ‘flat to let’ advert on a post office bulletin board. But how does the swarm, which can number up to 20,000 individual bees, decide which of these available nest sites to choose?
The international team of researchers, including Dr Thomas Schlegel and Professor Nigel Franks of Bristol’s School of Biological Sciences, found that the scout bees use a special ‘stop’ signal to inhibit the waggle dances of other bees causing them to cease dancing, and that each scout targets those scouts reporting sites other than her own (like taking a rival advertisement down from the bulletin board).
This vibrational stop signal, which sounds like a sharp electronic beep, is typically delivered by the sender butting her head against a rival dancer. Dancers usually do not show an immediate response to the signal but an accumulation of such signals increases the probability that a bee will eventually cease dancing. Previous research has shown that bees use this stop signal to prevent their colony visiting food sources that are dangerous – this study is the first to observe its use during the process of choosing a swarm’s future home.
In an experiment conducted at Shoals Marine Laboratory on Appledore Island, six miles off the New Hampshire/Maine coast, lead researcher Professor Tom Seeley (Cornell), Professor P. Kirk Visscher (Riverside) and Dr Schlegel presented the scout bees with two identical nest boxes and labelled the scouts with pink or yellow paint marks depending on which nest they visited. The pink scouts directed their stop signals mainly toward the yellow scouts, while the yellows mostly addressed theirs to the pink. This created ‘cross inhibition’ between the two populations of scout bees.
Video by kind permission of Professor Tom Seeley, Cornell University
This video shows a swarm bee (in the centre) advertising a suitable nest site with a vigorous waggle dance. She previously visited one of the two nest boxes provided by the researchers where she received a yellow paint mark on the back of her thorax. A pink-marked bee, who visited the other nest box, comes along and butts the dancing bee with her head while releasing a brief, but sharp vibrational signal: the stop signal.
Such cross inhibition is key to the collective decision making process as it curtails the production of waggle dances for – and thus the recruitment of bees to – a competing site. When a scout bee stops producing waggle dances for a site she soon stops making visits to that site so the cross inhibition created with the stop signal also inhibits the number of bees visiting a competing site. Cross inhibition thus increases the reliability of swarm decision making by solving the problem of deadlock over equal sites.
The computer-modelling of the bees’ collective decision-making process by Dr Patrick Hogan and Dr James Marshall (Sheffield) also allowed the researchers to draw a parallel between the stop signals in bee swarms and the inhibitory connections in models of decision making in primate brains.
The researchers said: “Honey bee swarms and complex brains show many parallels in how they make decisions. In both, separate populations of units (bees or neurons) integrate noisy evidence for alternatives and when one population exceeds a threshold the alternative it represents is chosen.
“This research shows that a key feature of a human brain – cross inhibition between evidence-accumulating populations of subunits – also exists in a swarm as it chooses its nesting site.
“For neural models of decision making, cross inhibition between integrating populations is crucial for effective decision making, and has been shown to allow optimal decisions under some circumstances. This research indicates that cross inhibition is also present in honey bee swarms, and is very important for their success when making decisions.
“It seems very likely indeed that the ability to implement a highly reliable strategy of decision making is what underlies the astonishing convergence in the functional organization of these two distinct forms of decision-making system: a brain built of neurons and a swarm built of bees.”
Paper
‘Stop signals provide cross inhibition in collective decision making by honey bee swarms’ by Thomas D. Seeley, P. Kirk Visscher, Thomas Schlegel, Patrick M. Hogan, Nigel R. Franks, James A. R. Marshall in Science Express
The study was funded by the Cornell Agriculture Experiment Station, the University of California-Riverside and the Biotechnology and Biological Sciences Research Council (UK).