Short Communication Honeybees Can Be Trained to Respond to Very Small Changes in Geomagnetic Field Intensity

It has been demonstrated repeatedly (Kirschvink & Kirschvink, 1989; Walker et al. 1989; Walker & Bitterman, 1985, 1989) that free-flying honeybees can be trained to respond to local anomalies in the geomagnetic field, but none of the training methods previously available has been sensitive enough for threshold studies. Here we report some determinations of intensity thresholds with a new and more powerful technique that permits objective measurement of response. The training apparatus, set into a laboratory window facing east, consisted of a hinged Plexiglas panel in which two Plexiglas tubes, each 2-5 cm long and 2-5 cm in inside diameter, were mounted 14 cm apart, with their axes aligned horizontally. Around each tube were two coplanar, concentric coils that produced a sharply focused magnetic anomaly extending horizontally from the entrance to the tube. Inside each tube, at the intersection of the axis and plane of the coils, was a food well. The tube design and the pattern of the anomaly are shown in Fig. 1. A screen of fine plastic mesh separated the observer from the animal. The coils around each tube had equal dipole moments (area x current) with antiparallel directions and different diameters. The field from the inner coil dominated and produced the anomaly close to the tube, while the moments of the two coils cancelled each other out further away from the tube. Each coil was doubly wrapped, with the two sets of windings so connected that they could be energized independently to induce parallel or antiparallel fields that summed or cancelled, with the very small amount of heat produced by passage of current the same in both cases. For further details of the coil design, see Kirschvink & Kirschvink (1989). Individual honeybees were pretrained to fly back and forth between their hives and the training window. A single animal was selected at random from a group at a feeding station providing 10-15 % sucrose solution, carried in a matchbox to the window, and set down at a drop of 50 % sucrose solution near the entrance of the tunnel in one of the tubes (the left for some animals, the right for others). The coils of the tube were activated to produce the local anomaly that would be the positive stimulus (S+) at the outset of the subsequent discriminative training; the