Birds do it, bees do it, even turtles in the sea do it

Natural History, Sept, 1997 by Fred C. Dyer

At first glance, using the sun as a compass seems a relatively easy, uncomplicated way to maintain a straight course: just adopt a fixed orientation relative to the sun and hold a steady course. Unfortunately, unless you will only be traveling for a brief time and covering a short distance, you will soon find yourself off track. The trick is to compensate for the apparent movement of the sun relative to a fixed geographic heading. For the last seventeen years, I have been studying one of the acknowledged masters of this trick -- the familiar honeybee.

I work in Michigan. A bee leaving one of my hives at midday in May heads directly toward the sun to reach a feeding place south of her nest. As she repeats the trip throughout the afternoon, the bee has to take into account the gradual, westward shift of the solar azimuth (the sun's compass bearing -- that is, its direction relative to the horizon) along the southern horizon. Complicating her calculations, the rate of movement of the azimuth varies through the day. And as spring turns to summer, the bee will have to contend with additional complications: the daily pattern of solar movement -- known as the ephemeris function -- changes as the path traced by the sun's arc moves north and south with the seasons.

If I were to take my bees to Florida, they would face still further challenges because the arc of the sun in the sky also varies with latitude. No bee can predict where, or during which season, it will spend its short life, and therefore which of the many possible patterns of solar movement it win see. Nor can it even count on staying in the same place for its whole life. African honeybees and the Asian rock bee, for example, migrate more than a hundred miles seasonally. And colonies of bees are successfully trucked from state to state to pollinate crops, further proof that they are up to the challenge.

How do they do it? As with the many other animals that use a solar compass, the challenge for an individual bee is to determine the particular pattern of solar movement where it lives. To do this, the bee uses a compass with both innate and learned components.

Scientists have long known that honeybees use landmarks surrounding the nest to help them memorize the sun's movements in their area. Bees learn this pattern during their first few flights outside the nest. Experienced bees draw on this memory guide them on completely overcast days. After discovering a source of food, forager bees return to the nest and "dance" the directions for their nestmates. In these dances -- first described by ethologist Karl von Frisch in the 1940s -- honeybees integrate information about the sun's position and how far away the food is with information about the time of day (the latter provided by the bees' built-in biological clock). Details of the dance -- especially changes in the orientation of the dancer's body-enable the bees to compensate for changes in the sun's position.

As impressive as their learning capabilities are, however, the bees simply wouldn't have time to build their substantial navigational skills from scratch; once a bee leaves the nest to begin foraging, it lives for only a couple of weeks. Fortunately for the bees, many elements of the sun's movements never change: no matter what the latitude or season, the sun always rises roughly opposite from where it sets; azimuth position changes slowly during the morning hours, moves quickly from the eastern to the western part of the sky at midday, and then slows down again for the rest of the afternoon. The bees, it turns out, are innately aware of these approximate dynamics of solar movement. This intuitive knowledge is enough, as was observed long ago, for them to estimate the sun's position at times of the day when they have never seen it. An Asian honeybee I have studied can estimate its position at night, using landmarks visible by moonlight to interpolate positions of the solar azimuth between sunset and sunrise.

Together with graduate student Jeffrey Dickinson, I have investigated how bees are able to perform such remarkable feats. We reared a colony of bees in an incubator, so we could be sure they didn't see the sun until we were ready to release them. Then we let them out of the hive every afternoon for a few hours before sunset. After a week or so of this regime, we released them in the morning as well, choosing an overcast day so they had nothing to guide them but their own internal estimate of the sun's position. When the bees returned to the nest and danced for their nestmates, we compared their morning "directions" with the ones we had watched them give on previous afternoons.

What we found was that in the morning dances, the bees placed the sun exactly opposite where they had seen it in the afternoon; at noon, they switched their dances 180 degrees, using the afternoon azimuth for the rest of the day. The bees clearly had an innate sense of the sun's movement across the sky, but it was approximate and lacked any gradations. With experience, the bees learned to put together a more accurate picture. Natural selection has thus preprogrammed bees with a rough sketch of solar movement and left it up to them to fill in the details.