The Backstory
In May 1949, shortly after the end of World War II, Martin Lindauer, a biologist at the University of Munich, happened upon a swarm of bees hanging in a bush outside the Munich Zoological Institute. Several scout bees were performing a waggle dance on the backs of the other bees, a technique usually used to communicate the location of flower patches to hive mates. But Lindauer noticed something odd: none of the dancing bees was carrying pollen loads, and many of the dancers were black with soot and red with brick dust.
Lindauer lingered by the swarm, kept observing the dancers, and gradually came to suspect that these dirty bees were nest-site scouts that had discovered potential nesting cavities amid the rubble of bombed-out Munich – an unused chimney here, a cavity in some collapsed brick wall there. Over the course of the next six years, he went on to prove that the dirty dancers really were house hunters. He described how these house-hunting bees conduct an extended debate about which candidate nest site is best and eventually come to an agreement about the swarm’s new home.
In the 1970s, I renewed the study of the swarm bees’ collective decision-making in the hope of answering some of the many questions that remained. How do scouts evaluate potential dwelling places? Back at the swarm, how do they convey the quality of a discovery? How is agreement reached without anyone supervising their debate? What ensures that the consensus converges on the best option? How do the scouts, having reached an agreement, inform all the others in the swarm that it is time to move to their new residence?
Over the next 35 years, working in collaboration with many other scientists, I was able to find answers to many of these questions. By presenting swarms with controlled nest-box choices, I was able to determine the real-estate preference of honeybees. By labeling every individual bee in a swarm and then video-recording the scout bee’s dances and the subsequent “debates,” I was able to document how several hundred scout bees in a swarm work together to choose a new home. To determine how the scouts achieve a dissent-free decision, I observed their behavior not only on the swarm cluster but also at the candidate nest sites, a feat that required cornering the honeybee real-estate market. I did this by taking bee swarms and nest boxes to Appledore Island, a treeless, windswept island six miles off the coast of Maine. Here I could control the home sites available to a swarm, and I could monitor the responses of its scout bees to each site. By deliberately simplifying the complex environment in which bees hunt for homes, I was able to see exactly how they perform their amazing feat of democratic decision-making.
House Hunting Begins
In the late spring and early summer, when a honeybee colony becomes overcrowded in its hive, it will cast a swarm. When this happens, about a third of the worker bees stay at home and rear a new queen, thereby perpetuating the mother colony, while the other two-thirds of the workforce – a crowd of some ten thousand – rushes off with the old queen to set up a daughter colony. The migrating bees travel only about 100 feet before coalescing into a beardlike cluster hanging from a tree branch. If an enterprising beekeeper comes quickly with a ladder and shakes the swarm into an empty hive, he or she can take these freebies back home.
If, instead, the swarm is left alone, it will remain bivouacked for a few days. During this time, several hundred of its oldest bees will spring into action as nest-site scouts, explore about 30 square miles of the surrounding landscape for potential nesting cavities in trees and buildings, locate a dozen or more possibilities, and democratically select a favorite for their new dwelling place. Shortly after completing their selection process, the swarm bees fly, en masse, to their new home.
The story of house-hunting honeybees presents us with many intriguing questions. How can a bunch of tiny-brained bees, hanging from a tree branch, make such a complex decision and make it well? And how can a swirling ensemble of ten thousand airborne bees navigate a cross-country flight, a journey whose destination is typically a small knothole in an inconspicuous tree in a remote forest corner?
Life-or-death Choice
Choosing the right tree-cavity home is a life-or-death matter for a honeybee colony, so it is not surprising that it is a choice made by a sizable search committee rather than just a few bees. If a colony occupies a cavity that is too small to hold the 40–60 pounds of honey the colony needs for winter heating fuel, or that doesn’t provide the bees with good protection from cold winds, then the colony will perish over winter.
Honeybees are unique among insects living in cold climates in that they don’t surrender to winter’s cold by going into a chilly dormancy. Instead, they stay warm and active inside their nest. They accomplish this by pressing together to form a tight, well-insulated cluster of bees about the size of a soccer ball and generating heat by shivering with their powerful flight muscles. All winter long, the cluster’s surface temperature is kept above 50°F (warm enough to keep the outermost bees alive) and its core temperature is held at about 95°F (warm enough to rear more bees). The heat output of the four or so pounds of bees in a winter cluster is only about 40 watts, like that of a small incandescent light bulb. But if a colony has tucked itself inside a tree cavity with a knothole entrance just an inch or two across, so that it is thoroughly sheltered from the wind, then this rate of heat production will enable a colony to survive winter just fine.
The fuel for this winter-long heating process is the forty-plus pounds of honey that the bees stockpiled in their nest over the previous summer. It is essential, therefore, that the bees occupy a cavity spacious enough to accommodate combs for holding this much honey, plus additional combs for rearing bees. This means the house-hunting bees must find a nesting cavity with about 10 gallons of living room.
The real-estate preferences of honeybees include more than just a large living space and a small entrance opening. A dream home for honeybees has its front door facing south, so that the home is warmed by the sun, with the door located at the bottom of the cavity, to retain the bees’ warmth, and high off the ground, so that it is easily overlooked by bears, skunks, and other predators.
One thing the bees are not choosy about, however, is the type of wood that forms the walls of their homes. When I was a teenager, I located 45 bee trees in the woods around my home outside of Ithaca, New York, and I found that the bees were occupying many types of trees: red and white oaks, black walnuts and butternuts, bitternut and shagbark hickories, sugar and red maples, quaking and bigtooth aspens, as well as hemlocks, white pines, elms, white ashes, beeches, horse chestnuts, and tulip trees. All these bee trees, except the elms, were sturdy and healthy even though fungal decay had rotted out a large cavity in the heart of each one. I suspect that the soundness of these trees was due in part to the bees living inside them, for these tidy creatures perform remedial work on their living quarters.
The first thing that a swarm of honeybees does upon occupying a tree hollow is scrape off and dump out the loose, dark punkwood that covers the cavity’s walls. They do so to create a solid surface for attaching their beeswax combs. But before the bees build their combs, they varnish the newly cleaned walls with a layer of resins collected from the buds and wounds of various trees, including birch, beech, pine, spruce, and especially aspen. When fresh, these resins are sticky – so much so that a colony’s resin collector bees need assistance from other bees to get unloaded – but over time the resins dry and form a tough, shiny coating. Resins have strong antimicrobial properties (which is why plants synthesize them) and are an important weapon in a bee colony’s defense against fungi, bacteria, and viruses.
Democratic Decision Making
How do the nest-site scouts choose their swarm’s new residence? For these bees, as for any search committee, the first challenge is to identify their options. The house-hunting scout bees search widely for prospective dwelling places and typically discover a dozen or more candidate home sites.
The scouts’ success reflects two things. First, they are a large group, usually several hundred individuals, so they bring considerable bee power to the search for possible places to live. Second, they are a diverse team of explorers, with no two individuals probing the same region of the surrounding countryside. While one scout examines the dusty knotholes that she finds in trees on a hillside, her fellow scouts will set out in various other directions and inspect the cracks in buildings, abandoned woodpecker nests, and any other possibilities they encounter. This variety makes it likely that one of their finds will provide an excellent home.
When a scout locates a potential nesting site, she scrutinizes it for about 30 minutes, making 10–30 journeys inside the cavity, each lasting about a minute. While inside, she walks all around. Three-dimensional reconstructions of the walking paths of individual scouts inside an experimental nest box reveal that when a scout has finished her inspection, she has walked 200 feet or more inside the cavity and has traversed all its inner surfaces. These interior tours give her information about the cavity’s volume and draftiness, and about the size of the entrance opening and whether it is located near floor or ceiling. Her short journeys inside alternate with brief surveys outside, during which the scout bee acquires information about the height of the entrance off the ground, possibly whether the entrance is sunny or shaded, and perhaps even the sturdiness of the tree.
Once a scout has finished her multipoint inspection of the site, she knows whether it is a dream home, a fixer upper, or something in between. If she decides that the place is good enough for further consideration, she flies back to the swarm cluster and announces her discovery by performing a waggle dance, an extraordinary behavior that a bee uses to tell others about valuable resources. Bees living in a beehive perform waggle dances to share information about patches of flowers brimming with sweet nectar or nutritious pollen, but the homeless bees in a swarm perform these dances to indicate the locations of potential homesites. In performing this eye-catching dance, the scout bee walks straight ahead on the swarm’s surface, waggling her body from side to side. Then she stops the “waggle run” and turns left or right to make a semicircular “return run” back to her starting point, whereupon she produces another waggle run followed by another return run and so on. Each waggle dance consists of a series of dance circuits. It is the waggle run in each dance circuit that indicates the direction and distance to the site being advertised. Nest-site scouts that haven’t yet found a possible homesite will trip along behind the dancer through several circuits of her dance, learning from the successful scout the location of her find. These dance followers take the direction and distance information, fly out to the site, and perform their own inspections. The more highly the successful scout bee values her find, the more dance circuits she performs, and the more as-yet-uncommitted scout bees she recruits to her site.
Meanwhile, other scouts that have found other potential nesting sites will be vigorously advertising their proposals as well, so the uncommitted scouts are being actively recruited to various camps. All this makes the surface of the swarm look at first like a riotous dance party, but eventually the scouts choose a winner.
They do so in a most ingenious way. It works much like a political election, for there are multiple candidates (nest sites), competing advertisements for the different candidates (waggle dances), individuals who are committed to one or another candidate (scouts supporting a site), and a pool of undecided voters (scouts not yet committed to a site). At times, supporters for a site can become apathetic and rejoin the pool of undecided voters. The election’s outcome is biased strongly in favor of the best site because this site’s supporters produce the strongest dance advertisements and thus gain supporters the most rapidly, and because the best site’s supporters will revert to neutral-voter status the most slowly. Ultimately, the bees supporting one of the sites build up a large majority and return to the swarm cluster to initiate the swarm’s move.
Flying to a New Home
Once a winning site has been chosen, the scouts run about on the swarm, making vibrational beeps and pipes. Beeps are produced when a scout encounters another scout still performing a waggle dance; she beeps the dancing bee with a brief head-butting maneuver that includes vibrating her flight muscles. This rough treatment informs the dancer that a nest site has been chosen and the time for waggle dancing is over. Pipes are produced in the same way as beeps, but they last much longer (1–2 seconds), and are directed at the quiescent bees in the swarm rather than the dancing scouts. A piping bee grabs her swarm mate with her two front legs and shakes the recipient rather than head butting her. Piping signals tell everyone that it is time to warm up their flight muscles in preparation for the trip to their new home.
After an hour or so of preparations, every bee will have her flight muscle warmed to the flight-ready temperature of 95°F, whereupon the swarm cluster suddenly disintegrates and each bee takes flight, filling the air with the roar of thousands of airborne bees. This flying mob immediately begins moving off in the direction of its chosen home and in another minute or two it will have vanished from sight.
How a swarm of bees steers itself to its new home is an almost mind-boggling puzzle. Somehow, a school bus sized cloud of some ten thousand bees manages to sweep straight from bivouac site to new dwelling place. The path of its flight can stretch for miles, traversing fields and forests, hilltops and valleys, and swamps and lakes. Perhaps most amazing, the airborne colony pilots itself over the countryside to one specific point in the landscape: a single knothole in one particular tree in a certain patch of forest. And as the group closes in on its destination, it gradually lowers its flight speed (from about six miles per hour, tops) so that it stops precisely at the front door of its future residence.
Only the scout bees from the winning site know its location, so it is not surprising that they play the leading role in guiding the swarm to its new home. They point the way by flying in the right direction at high speed through the top of the swarm cloud until they reach its front, then flying slowly back along its border, and then again shooting to the front. The other bees notice the streaking bees and align themselves with their fast-flying guides.
Eventually the swarm comes to a graceful stop at its new home, whereupon the scouts alight at the entrance and release a chemical signal from a gland in the abdomen to pinpoint the swarm’s final destination. Soon the other bees will begin blanketing the tree trunk around the nest entrance. A few minutes later, the queen will join them and slip inside without fanfare, and within another 10 minutes, all the bees will be safe inside their new home.
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