The Homing Instinct: Meaning and Mystery in Animal Migration. Bernd Heinrich

calculating where they are at all times, probably in measuring distance by keeping a kind of count of their steps, and also keeping track of the angles of their direction from their home relative to the sun’s location. These were not mere speculations, but a hypothesis tested in painstaking experiments that entailed altering the ants’ perception of the sun (holding filters over them that varied the direction of polarized light that they, like bees, use in orientation) and altering their stride length (altering their leg length by gluing on extensions) to find out what information they valued and how they used it. Presumably bees could also have a similar “map sense,” and Randolf Menzel, a neurobiologist in Berlin, was trying to find out how it might work.

      Menzel runs the large and active Institute of Neurobiology at the Free University of Berlin, and one of his projects was the burning question of how honeybees seem to find out where they are in order to be able to go where they want to be. Honeybees are suitable animals with which to study this problem because, like ants, you can count on their motivation to return home after they are loaded with food.

      We can’t look into a bee’s brain and determine what it knows and what it wants. However, clever experiments based on the bee’s natural history permit inferences. We can determine, for instance, where a bee perceives herself to be relative to her hive. If a bee regularly visits a feeding place, she knows where she is, because she always flies off in a straight line from it back to her hive. If we then remove either the hive or the feeding spot, she circles in the area where her target had been. We know what she is looking for, because when we provide the hive and/or the feeding station within the area where she circles, she quickly finds it. But suppose we capture our bee at the usual feeding station after she tanks up on honey or syrup, put her into a dark box, and then carry her “blind” to a place she has never been. As mentioned, most bees will then make a beeline in the same direction they had normally flown to return to the hive. They will fly as far as before but find no hive there. Yet, they usually eventually do make it back home. How do they find their way? What do they do until they reach home? Until Menzel’s experiments, it had not been possible to track them in flight when they were out of sight out in the field. Menzel had a tool — radar but with a unique twist — whereby he could trace bees’ actual flight paths over a kilometer away by radar and record them on a computer. And he invited me to come see the work in progress.

       Bee flight paths. A. A bee’s first trip from a flower patch or bee box back to its bee tree (hive) begins with an orientation flight. B. Later trips are more direct. C. After a bee has been transferred while “blindfolded” to a new spot, she acts as though she perceives herself to be still at the same place as before.

      The problem of tracking small objects such as insects from a long distance by radar had always been that radar would “see” too much. You could not isolate and then plot a single specific bee out of all the extraneous noise of echoes bouncing off all objects. The new insect-tracking radar technique started in 1999, when Joe Riley, a British researcher, applied a radar system able to track very small objects over long distances by attaching to the insect a small device that, after receiving the energy of an electromagnetic sound pulse, would respond with a frequency other than that of the transmitted ultrasound. The receiver is then tuned to amplify only that frequency. In this way, it became possible to track the flight paths of individual preselected bees equipped with the appropriate transponders because the echoes from all other objects were filtered out.

      The Menzel group’s electronics technician, Uwe Greggers, adopted the Riley system in 1999 and 2001 and got interesting results, but then ran into software problems. Nevertheless, given the promise from the data they did get, the scientists contacted a radar specialist at Emden (north Germany) who agreed to develop the system. The Menzel group then needed to find the right site in which to use it. They needed to locate the experiments at a large flat area devoid of trees in order to be able to record the complete flight paths without interference such as the bees’ trying to avoid objects or being attracted to them. The closest suitable area was an expanse of marshy meadow about a two-hour drive from Berlin. The large, idyllic farmstead near the village of Klein Lübben and land associated with it had accommodations for seven or more helpers, making this site amenable.

      One Menzel group experiment in the works when I visited involved training individual bees to expect food at two widely separated feeding stations, but only one station at a time was open to them. I had no idea what to expect, and on my day with the team I was eager not only to watch the bees but also to see the experiment in action.

      It was early in the morning when Menzel picked up Greggers and me for our trip to the experiment site in the Brandenburg countryside. We loaded a large, heavy printer that would be used to handle the large-scale printouts of flight paths, and then we were off down the Autobahn. Two hours later we arrived at Klein Lübben, a quiet village of farmsteads that at least in outward appearance has changed little since medieval times. The fields were several kilometers square, flat, and moist — perfect also for frogs, and hence storks which nest there in baskets attached to the tops of red-tiled house roofs. Swarms of starlings swirled through the air, and a pair of white swans paddled serenely down a canal along a dirt road, followed by a line of five still-downy gray cygnets.

      At one end of the study field stood a steadily turning radar apparatus with a large round antenna for sending out the signal. A smaller dish antenna mounted directly above it would receive the transformed signal bouncing off the transponder on an airborne bee in the field. On the field sat two blue triangular tents and three yellow ones. They were experimental landmarks for bees that could be made available to them, to find out if they used them, and manipulated for experiments by changing their locations. In the distance sat a beehive, and I noticed a man running from it. He was wildly slapping himself, in an obviously defensive mode. He had been assigned to provide food for the bees close to the hive and then was to gradually move the feeder into the field so that a population of bees from that hive would be available for us to study when we arrived at midmorning. He had come too close to the hive, and at that moment it was he who was getting dispersed over the field, not the bees. Also, as we soon found out, there were no bees coming to the two feeder stations, as they were supposed to have been by now; the student had apparently overslept or been otherwise distracted from his assigned job of luring bees.

      The experiment we wanted to do was in doubt. This was serious. Two hundred thousand Euros had already been spent on this study, and the boss was intolerant of negligence. Luckily, bees from hives used previously for another experiment were still coming into the field to search for feeders. He could let some of those bees find the feeders and then train them to come back to specific locations.

      For our experiment we needed to establish two feeding stations, A and B, separated by about three hundred meters. Certain bees were already keyed into the routine. When I walked across the field, one bee started following me. It looked most extraordinary: it had a lot of blue and green color, not just the usual plain brown honeybee attire. As soon as Menzel’s helper and I set up our feeder, this specific bee landed on it and immediately started to suck up the rich sugar solution. Now I could examine her more closely: the green was a plastic tag with the number 29 on it that had been glued to her thorax. The blue was a slash of paint that had been daubed onto her abdomen.

      Within a few minutes an assembly of several differently color-coded bees was lined up around the edge of the syrup dish. All were sucking up syrup. Some had green on the thorax, some had blue, and still others had yellow tags on their thoraxes, with additional daubs of white, blue, or yellow paint on their abdomens. Uwe Greggers and the unfortunate helper immediately started logging a list of the bees that had shown up in a notebook.

      Each bee tanked up quickly, flew off directly toward her hive at the other end of the field, and then came right back to take a next load. Newly recruited (unmarked) individuals were also coming every minute to our site A. At the second feeder (site B) there was a similar flurry of activity, except it involved different individuals.

      Menzel then instructed us to move our food station A one hundred meters closer to the second one, B. Bee numbers 29 and 30 green, both with blue tails, number 2 yellow with white tail, and number