House-hunting is full of decisions, for us and honey bees. One early decision we both face is where to live. P. Kirk Visscher at the University of California, Riverside, often in collaboration with Thomas Seeley at Cornell University, NY, has long been studying how honey bees make these decisions.
Swarms of honey bees split off from their mother colony and go house-hunting, looking for a secure cavity in a tree or elsewhere that will make a good home for the new colony. In this process, they communicate to each other what they have found by dancing: a scout bee returning from a good site moves over and over in a figure-eight pattern that indicates the direction and the distance to the site, and other scouts read these dances and inspect the site themselves.
Usually, the swarm's scouts find more than one site, in which case the swarm faces a decision that must be made quickly since the swarm is exposed and the season for honey collection is passing. The decision, however, must also be good decision, the future welfare of the colony depending on a good home site.
Visscher, Seeley and colleagues report Dec. 8 in Science Express that they have found another, overlooked, signal that plays a role in this process -- a signal that is similar to those that occur between neurons in the brains of monkeys making decisions. Called the "stop signal," it is a very short buzz delivered by the sender scout while butting her head against the dancer. Its effect is to shorten and ultimately end the dance.
"It appears that the stop signals in bee swarms serve the same purpose as the inhibitory connections in the brains of monkeys deciding how to move their eyes in response to visual input," said Visscher, a professor of entomology. "In one case we have bees and in the other we have neurons that suppress the activity levels of units -- dancing bees or nerve centers -- that are representing different alternatives. Bee behavior can shed some light on general issues of decision making. Bees are a lot bigger than neurons for sure, and may be easier to study!"
To study the stop signal, Seeley, Visscher, and Thomas Schlegel at Bristol University, United Kingdom, set up swarms, one at a time, on an island off the Maine coast that was devoid of natural nesting cavities. They also set out two identical nest boxes. They labeled scout bees visiting the two boxes with paint marks of two colors. They then video-recorded the scouts producing waggle dances and tracked dances produced by the marked scouts with a microphone and videotape to ascertain when they received stop signals, and from which bees.
What the international team observed was that the stop signals were primarily delivered to dancers reporting a particular site by scouts that had been marked at the other site.
"The message the sender scout is conveying to the dancer appears to be that the dancer should curb her enthusiasm, because there is another nest site worthy of consideration," Visscher said. "Such an inhibitory signal is not necessarily hostile. It's simply saying, 'Wait a minute, here's something else to consider, so let's not be hasty in recruiting every bee to a site that may not be the best one for the swarm. All the bees have a common interest in choosing the best available site."
Visscher explained that the kind of cross inhibition seen in stop-signaling by house hunting bees mirrors cross inhibition found in nervous systems. In the research paper, theoretical models by team members Patrick Hogan and James Marshall at Sheffield University, United Kingdom, demonstrate that such cross inhibition helps to insure that a decision will not become deadlocked between equal-quality alternatives.
"This is critical, because the swarm must choose a single nest site, even if two sites of equal quality are available," Visscher said. "This cross inhibition curtails the production of waggle dances for, and thus the recruitment of bees to, a competing site."
Honey bee swarms are produced when, to establish a new colony, many thousand worker bees leave a hive that has become crowded, bringing along their mother queen. The swarming bees cluster near the parental hive for a few days while several hundred scout bees, the oldest in the swarm, locate and advertize prospective nest sites and choose the best ones.
To advertize a nest site, a dancing bee runs figure eight patterns and waggles back and forth while she moves across the middle portion. The angle of her body during this waggling run represents to the other bees the angle to fly. The duration of the waggling portion informs the other bees of how far away the nest site is. It can be thought of as a miniature reenactment of the flight to the goal; the longer the flight, the longer the waggle run, and the angle of flight relative to the sun direction equals the angle of the dance from relative to straight upwards from the swarm.
To be selected as a future home, a nest site must attract a certain number of scout bees. Further, there is competition between sites for the attention of a limited number of scouts. Once a site attracts a "quorum" number of scouts, the bees detect it, and begin to change their signals on the swarm. They then produce a piping signal by vibrating their wing muscles while pressing down on another bee. This signal leads the swarm bees, most of which simply hang quietly in the swarm during the decision-making process, to warm up in preparation for takeoff.
The piping signal is also associated with a change in the stop signal behavior. After piping begins, the stop signals are no longer delivered reciprocally; instead dancers begin to receive stop signals from scouts that had visited their own nest site, as well as the alternative nest site.
"Apparently at this point, the message of the stop signal changes, and can be thought of as, 'Stop dancing, it is time to get ready for the swarm to fly,'" Visscher explained. "It is important for the scouts to be with the swarm when it takes off, because they are responsible for guiding the flight to the nest site."
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