Competition among dung beetles for food is fierce, so as soon as one has succeeded in forming a ball of dung it flees. The preferred path is a straight line, because that is the quickest route away from aggressors. But Marie Dacke of the University of Lund in Sweden and her colleagues noticed that the beetles move in a straight line only on moonlit nights. On cloudy evenings, in contrast, the animals' movements are much more erratic. To test whether the light of the moon was guiding the insects, the scientists placed a polarizing filter over ball-rolling beetles, which altered the light's polarization by 90 degrees. Under the filter, the creatures made right-angled turns, suggesting they orient themselves according to the polarization of the moon's light, which is one millionth as bright as the sun.
Although the moon can keep a dung beetle on the straight and narrow, because it follows a symmetrical pattern it can't help the animal tell left from right. Many animals, including honeybees and some ant species, use the sun's polarization to find their way, but this is the first description of an animal using the moon's polarization pattern as a nighttime guide. It may not be the only one, however: the authors note that "this ability may turn out to be widespread in the animal kingdom."
