A Pollination Moment
from Pollinator Paradise Farmer's Market E-mail reminders

6/19/2004

Orienting to the Zenith

Club Solstice Cartoon

Sunday is Father's Day. As it happens, it is also the summer solstice, the day that the sun will be overhead at noon on the Tropic of Cancer. By definition, the Tropic of Cancer is the furthest north that the sun gets when it is in the zenith, i.e., directly overhead, at noon.

You may be aware that honeybees use the sun as a compass for determining the direction of a food source. When they find a good source of food in flowers, they come back to the hive and dance their waggle dance to tell other honeybee workers the direction and distance to travel to find the food source.

The information about direction is contained in the orientation of the dance on the vertical honeycomb.  If the bee orients its dance straight upward on the hive, this means: "Fly toward the sun to find the food source".  Other bees will observe the dance, then head out of the hive flying over the land in the direction of the sun in the sky.  If the food source lies 20 degrees to the right of the sun's direction in the sky, then a forager bee will guide the others by orienting its dance 20 degrees to the right of straight up.  Because the sun moves slowly across the sky during the course of the day, but the food source stays in the same place, the orientation of the bees' dance changes during the day, reflecting the changing angle between the sun's position and the location of the food.

Are you following my drift? This waggle dance is an amazing phenomenon, but what happens at noon on days when the sun is directly overhead? In this situation, the sun's position in the sky would lie in no particular direction from the hive.  How do the bees orient their dances?

What looks to us like the sun's zenith - the point when it passes directly overhead - is usually not exactly the true zenith.  Honeybees register with great discrimination all but the slimmest of deviations of the sun's position from absolute vertical.  The famous behaviorist, Martin Lindauer, discovered this by means of a classic experiment. He traveled to Paradeniya, Ceylon (now Sri Lanka) where the sun passes through the zenith at 12:06 pm on 9 April.

As described by Karl von Frisch in "The Dance Language and Orientation of Bees" (Harvard University Press, 1967), Lindauer's research ran thus:  "As long as 3 weeks beforehand he himself was no longer able to make out the southerly direction at noon from the position of the sun - with the sun still 8o from the zenith. The bees' eyes proved to be superior. Even at noon they still pointed with well-oriented dances toward an artificial feeding station after the sun had already approached within 3 degrees of the zenith. "

"But on 2 April with the sun 2.5 degrees from the zenith, they first gave evidence of their 'embarrassment' - at all events in an unexpected manner, which almost prevented a clear decision: from 11:45 a.m. to 12:20 p.m. they took a midday siesta and stayed at home."

"Lindauer found a way out. During the morning he gave the bees but little food, with numerous interruptions. Then toward noon he served them a full meal, where-upon they became so eager that they kept on harvesting. Their eagerness to dance was less than usual, but nevertheless during those days altogether 87 tail-wagging dances were observed; these were disoriented."

I like the siesta idea.  If you can't figure out which way you are going, take a nap. When you wake up, check the sun for direction.