Experiment of the month

Natural History, Oct, 2003 by Stephan Reebs

Question: How did the dung beetle cross the road? Answer: By moonlight.

This past March this column noted the first-ever evidence of an animal--the elephant hawkmoth--seeing color by night. Now another insect has joined the ranks of the perceptually advantaged.

When a dung beetle finds a pile of fresh manure, it selects the choicest, least-fibrous bits and crafts them into a large ball that will supply its own dinner for days to come or serve as both shelter and food for its developing young. Rolling the ball away from the raw clung heap as quickly and efficiently as it can (so that the ball won't be stolen and the work wasted), the nocturnal beetle heads off in a straight line--if the Moon is out. On moonless nights, though, it zigzags all over the place.

Orienting itself by the direct light of the Moon would not win the dung beetle any space here; many other animals do that. But when Marie Dacke, a biologist at Lund University in Sweden, and several colleagues shielded the rising Moon from the beetles' direct view, the insects still moved in a straight line. Could the beetles have been navigating by polarized moonlight--extremely feeble illumination that is scattered by minute particles in the atmosphere--rather than by the Moon itself?

To test the idea, the biologists placed a polarizing filter over moving beetles. If the filter was aligned along the sky's dominant axis of light transmission, the beetles kept moving in the same straight line. But if the filter was rotated ninety degrees, the beetles, too, suddenly veered off at right angles: an elegant demonstration of an amazing visual ability. ("Insect orientation to polarized moonlight," Nature 424:33, July 3, 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior 'n the Aquarium and in the Wild (Cornell University Press).