Dancing under a Polarized Sky

"Each bee on her return is followed by three or four companions . . . how they do it has not yet been observed" Aristotle, Historia Animalium, IX "In the summer of 1944 a few very simple experiments led to a result that was just as unexpected as it was thrilling" Karl von Frisch on the discovery of the dance language of bees [1]. Soon after the end of World War II in war ravaged Germany, Frisch was observing the dance of bees and "reading" the language he himself had recently deciphered. In a way, he was feeling ecstatic: he could eavesdrop in the bee conversation and interpret their symbolic language. He understood the eight-shaped dance meant, for example, nectar 1.5 km away and at 30 degrees from the current position of the sun. On several occasions he had astonished neighbors by telling them that his bees were feeding from sources on their farms which he had not seen. The human and insect brain had never communicated in such a way before. But at the same time he was baffled. How could the bee know the position of the sun? At that time he was studying the bee dance on a comb placed horizontally. Previous experiments had proven conclusively that bees used the sun as a compass. He could even rotate at will the dance by replacing the sun with a lamp. If the horizontal comb was covered and illuminated by diffuse light, the dances were disoriented. But somehow they became oriented again if the bee could see a small patch of blue sky. As hard as it was to believe at the time, Frisch concluded that the bee could see the polarization pattern of the sky!  Later, other researchers discovered many other animals sensitive to polarized light (eyes), some of which could use it for navigation, as the bee does. But this capacity was discovered in honeybees first because they gave away their secret through their dance language.

The Dance Floor Bees returning to the beehive after finding a good supply of food will communicate to other bees by dancing at a particular region in the comb: the dance floor. The dance floor is generally close to the entrance but sometimes moves, e.g. goes further inside when it is cold or closer to the entrance when there is lots of activity. In Nature honey combs are vertical, so the dance is generally performed on a vertical plane. This is of great significance for the bee dance as the language must provide information of horizontal directions on a vertical plane. However, when the weather is very warm the dance floor may move outside the entrance to a horizontal flight board. It is also horizontal in some primitive bee species and can always be made horizontal by the human experimenter. Dances on oblique dancing floors can also happen, mainly on the obliquely rounded lower edge of a free-hanging comb or on the rounded swarm cluster bees form when looking for a new nesting place. Notice that in nature the vertical dancing floor is inside the hive and thus quite dark while the horizontal one is generally under the open sky. The Bee Dancing Repertoire Round Dance When a foraging bee finds food close to the beehive, it performs its simplest dance, the Round Dance. This dance doesn't provide much information, it is more of an arousal signal. The forager bee runs in a small circle, leaving a single cell inside it. Every one or two circles it suddenly reverses orientation and this goes on from seconds to minutes. The bees recruited follow the dancer on the floor and then fly off by themselves looking for the food. If these bees haven't been feeding at a particular place before, they will look for food in every direction in the proximity of the beehive. However, the dancing bee also gives away odors that can be recognized by bees frequenting the same flowers, who will fly directly to them. Tail-Wagging Dance When the goal is further away, the bees need more sophisticated means of communication. If food is scarce, bees have been known to feed up to about 15 km (~10 miles) from the beehive. In relation to the small size of this animal these distances are outstanding. Although a bee flying to a known source of food uses as references conspicuous landmarks in addition to the sun compass, it can only communicate information about the latter to fellow bees. The Tail-Wagging dance tells the other bees very accurately at what distance and in which direction the food is, so they can look for it by themselves.  Some European honeybees start to perform it when the source of food is more than 100 meters away. Other bee species will do them for closer sources, up to just a few meters away in the case of some Indian bees. For intermediate distances there is a gradual transition between the round dance and the tail-wagging dance. In a typical tail-wagging dance the honeybee (Apis Melliphera) runs straight ahead for a short distance, returns in a semicircle to the starting point, again runs through the straight stretch, describes a semicircle in the opposite direction and so on in regular alternation. The straight part of the run is given particular emphasis by a vigorous wagging of the body (rapid rhythmic sidewise deflections). In addition, during the tail-wagging portion of the dance it emits a buzzing sound. Interestingly, the dance followers can make the dancer pause and give them a taste of the nectar by using a squeaking sound. Distance Information With increasing distance the number of circuits (8's) per unit time decreases and the length and duration of the individual circuits increases. For example, for a goal at 100 meters it makes 10 short circuits in 15 seconds but at 3 km only 3 long circuits in the same time. The duration of the wagging part has the best correlation with distance. The distance is calculated based on the expenditure of energy on the flight towards the source (a head-wind increases it). Each recruited bee averages many dance circuits or even several dances from different bees to calculate the distance. For each bee species a distance-frequency curve can be plotted.  It is remarkably precise, especially if the distance is not close to their foraging range limit. Compass Information If the dance floor is horizontal (the least common case in Nature), the indication of direction is straight-forward: the wagging (straight) portion of the eight-figure dance points towards the food source (and in the same direction as the bee runs through it). But, what does the dancing bee use as compass to accurately point in the right direction? The bee reference is the direction of the sun. This can be demonstrated easily by covering the sky and using a lamp as an artificial sun: the direction of dancing will rotate, always maintaining the same angle with the lamp as the angle with the sun during direct flight towards the food. If the dance floor is vertical the indication of direction requires a higher-level language that can communicate horizontal directions with an indirect, symbolic, representation. In a vertical plane the natural reference is gravity, so the dancer replaces the real reference, the sun, by the "UP" direction. For example, if the bee maintained the sun 70 degrees to her left when flying towards the nectar, the wagging portion of her dance will point 70 degrees in the clockwise direction from the upwards vertical direction. The bee transposes the solar angle into a gravitational angle!  On an oblique comb the gravitational transposition works well up to an angle of about 10 degrees to the horizontal.

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