Beetles by Moonlight

Although their dining habits tend toward the scatological, dung beetles also have a lofty sensibility, relying on the moonlit skies to provide them with a sense of direction. Behavioral zoologist Marie Dacke at the University of Lund in Sweden suspected the beetles were using the moon for navigational cues when she noticed they walk in crooked paths on overcast nights but move straight ahead when the moon is out. All natural light has a polarized component—some portion where all the rays are traveling in one plane. This might be what the beetles are following, Dacke thought, but moonlight is so much dimmer than sunlight that researchers had thought its polarized light would be impossible for animals to detect. To test her hypothesis, Dacke and her colleagues covered the beetles with filters that changed the polarity of the moon’s rays by 90 degrees. Sure enough, each beetle made a corresponding right-angled ...