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

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СКАЧАТЬ to predators and also precluded the bees from living in huge areas of the globe, those with cold climates.

      Homes in hollow trees allowed the bees to live in areas where they would otherwise be excluded because of cold and/or nest predators. But in such safer homes the combs hung from the roof of a cavity and left no horizontal dancing platform, and additionally the “dance floor” was now in darkness, so bees could not point directly toward the food. Even if they could, they would not be seen. But a breakthrough for indicating horizontal directions on vertical surfaces became possible after some bees started using the hanging flat surfaces of combs as their dancing platform while indicating the sun’s location as the up or “toward” direction in their dance. Additionally, tactile rather than visual orientation became predominant for recruits in reading the code within the nest.

      It is amazing enough for an animal to be able to navigate to a location it has never been to before. But some ants do something even more amazing. In North Africa, desert ants live in underground homes where they are protected from the heat. But they must venture out onto the searing surface periodically to forage by scavenging on heat-killed prey. The ants are fast runners that have evolved a very high tolerance for heat. Still, at times it is a matter of life and death even for them to make it back to their cool underground home; they cannot afford to wander on the sand surface for an extended time without access to their shelter to cool down and replenish body fluids. This is where their homing ability comes in; they may have zigzagged in all directions to find a heat-killed insect, but after finding one they must make a straight “ant line” directly back home. This begs the question, Since they are often on a featureless plain and have not kept a steady course, how do they know in what direction to head home?

      If one captures bees in one pasture and releases them in another, they usually depart in the direction they would have flown from the original field. That is, they act as one would expect if they do not realize that they have been moved to a new location. Rüdiger Wehner and his colleagues at the University of Zurich came to the same conclusion about desert ant homing in their lifelong experimental studies. The ants use the sun as a compass, but a compass is not enough; the ants, when released from a point they had not themselves traveled to, like the bees caught in one pasture and released in another, apparently got lost.

      For homing you must know where you are on “the map” before you head off in the correct direction. The desert ants can return home, but only if they walk to where they find themselves. Wehner concluded that the ants’ homing mechanism involves somehow 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 СКАЧАТЬ