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Planet Earth

Beetles by Moonlight

By Elizabeth SvobodaSeptember 9, 2003 5:00 AM

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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 turn.

Dacke theorizes that the dung beetle’s sophisticated nighttime method of guidance helps it gather food more efficiently. As night falls, each beetle finds a dung pile, rolls it up into a ball, and quickly takes off with the prize to keep it away from rivals. Traveling straight ahead, reckoning by the moon’s polarized rays, often provides the fastest route to safety. “It’s the best way for them to escape the fierce competition of the dung pile,” Dacke says. This is the first recorded instance of an animal navigating by polarized moonlight, but she suspects that other insects, such as night-flying bees, may have similar capabilities.

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