A honeybee colony can contain tens of thousands of workers, yet only a small number of them leave the hive to search for food at any one time. These searchers, known as scouts, fly across the surrounding countryside looking for flowers rich in nectar and pollen. When a scout finds a good source, she faces a simple problem. The flowers may be several kilometres away, and the other bees in the hive have no way of seeing them. Using a series of body movements, a returning scout can tell her nestmates both the direction and the distance of the food. This behaviour, usually called the dance language, is one of the most studied forms of animal communication in the world.
The meaning of these movements was first explained in the 1940s by the Austrian scientist Karl von Frisch. For many years, beekeepers had noticed that bees moved in unusual patterns on the surface of the honeycomb, but nobody understood why. Von Frisch trained bees to visit dishes of sugar water placed at different distances and in different directions from the hive. He then marked the visiting bees with small dots of paint and watched them inside a glass-walled hive. By changing the position of the dishes, he showed that the dances changed in a regular way. His work was later recognised with a Nobel Prize in 1973.
The simplest form of communication is the round dance. A bee performing this dance runs in a small circle, first in one direction and then in the other, repeating the pattern many times. Round dances are generally used when food is close to the hive, usually within about 50 metres. They tell other workers that food is nearby, but they give little information about direction. For example, a bee that sees a round dance simply leaves the hive and searches the local area.
When food is further away, the scout performs the more complex waggle dance. In this dance, the bee moves forward in a straight line while shaking her abdomen rapidly from side to side. This straight section is known as the waggle run. At the end of the run, she turns and loops back to the starting point, alternating between a turn to the right and a turn to the left, so that the whole pattern looks like the number eight. The length of the waggle run tells the other bees how far away the food is. In general, the longer the run lasts, the greater the distance. In one common European variety, a waggle run of about one second indicates a journey of roughly one kilometre. Some researchers now believe that the round dance is simply a very short version of the waggle dance.
The direction of the food is shown by the angle of the waggle run. Inside the dark hive, bees dance on vertical sheets of honeycomb, so they cannot point directly at the flowers. Instead, they use the sun as a reference. If the waggle run points straight up the comb, the food lies in the same direction as the sun. If the run points straight down, the food is in the opposite direction. A run angled 40 degrees to the right of vertical means that the bees should fly 40 degrees to the right of the sun. In this way, the dancer turns outside information into a pattern her nestmates can follow in darkness.
The system has to deal with two practical difficulties. First, the sun moves across the sky by about 15 degrees every hour, so a direction that is correct in the morning will be wrong by the afternoon. Bees solve this by adjusting the angle of their dances over time, even when they have not left the hive. Second, bees cannot measure distance in metres. Experiments suggest that they judge it by how much the visual scene moves past their eyes during the flight. Bees that were trained to fly through a narrow tunnel lined with patterns, for example, danced as if they had travelled much further than they really had.
Dances also carry information that is not about location. A scout returning from a very rich source dances for longer and with more energy than one returning from a poor source, and this attracts more followers. In addition, the dancer carries the smell of the flowers on her body and shares small samples of nectar with the bees around her. These smells help recruits to recognise the correct flowers when they arrive at the right area. Studies have also shown that a new recruit usually follows a dance for several circuits before leaving the hive.
Honeybees use other signals as well. The stop signal, a short vibration often given while pressing the head against a dancer, can reduce dancing for a site where bees have been attacked by predators. The tremble dance, in contrast, is performed when foragers wait too long for younger bees to unload their nectar, and it encourages more workers to help with storing food. Today, scientists continue to study bee communication using video tracking and even small robotic bees that can perform simple dances. Because bees pollinate many important crops, a better understanding of how colonies find food may help farmers and beekeepers to protect them.