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 39 green with red tail (who had all been present at A) then almost immediately started showing up at B, the new location. When crowds of bees had done the same, we removed one station and put the remaining one into the middle, between the two original sites. Next we moved our feeder to site B. Most of the bees, such as 30 green with blue and 39 green with red, who had been at the previous site, s
howed up. That is, we had trained bees who had been at one site to come to the second site, so we knew they now knew two sites and could potentially use either as a reference site to return home.
For the planned experiment it was important that the bees forget the intermediate sites that had been instrumental in getting them to go to the two widely separated sites. So, for the rest of the day, we alternately fed the bees, first at site A, then at site B, and monitored which individuals were showing up at both sites (most of the individuals continued to forage at either one or the other site).
We were now, near the end of the day, finally ready to move from training to trials. The experimental plan was to select one of the bees who knew both sites. This bee would, after feeding at one site and getting ready to leave, be captured in a dark box and thus “blindfolded” and then brought to a third feeding site where she had never been before. Here she would be released after being equipped with a radar-tracking transponder. We presumed that she would do at least one of three things: she might recognize where she was and fly straight home; she might instead fly off in her original (now wrong) direction; or she might immediately know that she was at an unknown location and search until she found one of her two feeding sites and from there take a direct beeline home. Knowing her exact flight path would allow us to distinguish among the alternatives, which would be essential to ultimately decoding her homing mechanism. Setting up this experiment had taken a long time, but I would now, possibly, be treated to an exciting demonstration of bee homing, one I could never have imagined possible.
Menzel picked up his walkie-talkie to call the radar station: “Mike, we’re now going to put a transponder on a bee—are you ready?” Mike had spent some years in the army where he was trained on radar, and he was now working part-time while getting a university degree in computer technology. He replied yes, he was ready. Menzel then took me to feeding station A, where a whole lineup of bees was coming and going.
“Which one do you want?” Menzel asked me. I wanted a bee that I had gotten to know over the course of the day, so I chose 39 green with red-tipped abdomen. We waited for her to arrive and let her feed for a while. As planned, Menzel then held a glass vial over her while she was distracted sucking up syrup. When she was full, she walked up into the vial, and Menzel corked it shut and darkened it by wrapping his hand around it. We then took her to a site distant from both feeders, a place where she had not previously fed and from where we would now release her.
The vial holding “39 green” had a plunger at the bottom with a wide-mesh screen at the top. Menzel gently pushed the plunger in and forced the bee up against the screen, held her there, and picked up a tiny transponder (a wire holding a diode with a sticky pad at one end). With fine tweezers, he deftly removed the protective paper from the sticky pad and glued the transponder onto the top of the bee’s thorax. “Ready?” he radioed Mike.
“OK.”
Menzel removed the plunger and held the vial with the open end up, for the bee to crawl up. She hesitated at the lip of the glass, groomed her antennae, and then lifted off. She showed no strain in flight. (The transponder’s weight is twenty milligrams, and a bee can fly with double her body weight, carrying a hundred-milligram load of nectar in her honey stomach plus two pollen packets on her hind legs.) However, she flew only two or three meters before dropping down into the grass, stopping to preen herself some more. But a couple of minutes later, she finally took off again. Mike, who was now monitoring her flight, radioed us. At intervals we heard: “She is heading south-north-east-north-west-south.” Then, finally, Mike continued: “Now her path is straightening out—now she is heading directly for her hive!”
She had suddenly oriented correctly. This was the crucial point: she had apparently recognized something that had “placed her on the map,” so that she then “knew” in what direction to fly to reach home. Assuming she had taken a path she had never taken before, did her successful homing suggest a “map sense”?
I ran over to the radar tent where Mike showed me the radar screen and the dots where the three-second successive readings traced the bee’s path. A computer screen, where software had converted the time and directions of the bee’s flight path into different-colored images for easy reading, showed that the bee’s original flight direction was toward where the hive would have been had we not moved her from her feeding spot. In other words, she acted as though she didn’t know where she was when we released her. As expected, however, after she reached the area where her hive would have been, she flew loops in several directions. Then, after she had flown ever-farther away from both her real and “would-have-been” hive locations, she suddenly seemed to orient and flew directly toward the hive. Amazingly, it was along a route that had not been her normal foraging route from her two feeding sites. Had she perhaps seen a blue or a yellow tent and, having learned their relationship to each other during previous orientation flights, transposed that information to fix her new location? Only more bees could tell.
Other bee homing experiments with hundreds of bees were ongoing. And in the group’s final publication two years later, the thirteen-author research team headed by Menzel concluded that honeybees incorporate information for flight direction from both their previously learned flights as well as landmarks and from the flight directions learned from hive mates within the hive. But they can redirect their flight vectors to and from the hive and perform novel shortcut flights between the learned and the communicated vectors.
“The” homing instinct, recognized and traded on by every American beeliner to get honey, and used by von Frisch to decipher the bee language, is a source of fascination and mystery still. Von Frisch had likened it to a “magic well” from which the more you take, the more runs back in. The “well” is still doing that, three-quarters of a century after his prophetic pronouncement.
Getting to a Good Place
THE TENT CATERPILLAR MOTH, MALACOSOMA AMERICANUM, is common in North America. It emerges from its light yellow silk cocoon in late summer, and the female is then ready to deposit her batch of over a hundred eggs. She searches for an apple or a cherry tree, and somewhere out on a thin twig of just the right diameter—about a half centimeter—she exudes her eggs along with sticky foam to form her egg mass into a ring that wraps around the twig. The foam dries and hardens, encasing the clutch of eggs and gluing them solidly to the branch where they stay through the coming winter. But the larvae develop inside the eggs during the summer and, while confined in their eggs through the winter, hatch at almost precisely the day, about nine months after the egg-laying, when their tree breaks its buds.
The moth is named for the conspicuous communal homes of silk, called “tents,” that its caterpillars make, and in the spring of 2013 I found a just-made tent on a young black cherry tree next to my Maine cabin. Like nearly everyone else in this part of the country, I was long familiar with these caterpillars but had not deemed them worthy of a closer look. The tents act, I learned, like miniature greenhouses and warm the new caterpillars at a time when nightly frosts are still common. But, despite its advantages, to have any home is to incur costs: it has to be made, and it takes time, energy, and expertise to make, and the wherewithal to travel to and from it. For the time being, I wanted to know where the caterpillars making this home had come from. To my surprise, the ring on the twig with the now-emptied eggs I was looking for was almost a meter from the tent. How had the many hatchling caterpillars “decided” or been able to stay together and then coordinate to make their tent? Squinting against the sun, I could see a glistening trail of fine silk leading from the emptied egg-case ring to their home, so here was at least a hint as to how they crawl together to end up at the same place.
On the second day after I found the tent, May 1, there was still snow on the ground in the woods. There was as yet no sign of fresh green anywhere. But I wrote in my journal, “Black cherry buds ready to pop leaves.” These trees are the first to leaf out, and the caterpillars could not have fed yet
. What would they do? An hour after the sun came up, the tiny caterpillars emerged from their tent and massed on its sunny side. An hour later they started milling about, and then a few started crawling, seemingly aimlessly, several centimeters up and down the trunk and branches of the cherry tree.
As I had anticipated, some of the tiny caterpillars started to crawl back onto the same branch they had come from, possibly following their previously made silk trail. But they went only six centimeters before turning back. Others went down the trunk of the tree. Always some would turn back, and then the others followed one behind the other in a line. Finally, by 7:30 a.m., a contingent of about twenty of them had progressed nine centimeters down the tree trunk, although two were coming back up. Then more started to leave the tent, and eventually all were in one long line, going only down the trunk and then angling up another branch. In half an hour the leaders had traveled seventy-three centimeters and reached a bud. The rest were strung out all the way to the tent, but their two other travel-direction options had been abandoned. All were eventually massed at the same cherry bud, three-quarters of a meter from their tent, and in an hour and a half they had all returned to their tent, one following the other in a long train.
The Homing Instinct Page 4