Beetles stay on course using lunar navigation.

L ike sailors navigating with a sextant, some bees, ants, spiders, and birds use sunlight to orient themselves. But, African dung beetles are really much more sensitive: they can steer by moonlight, which is a mere one-millionth the sun’s brightness.

The beetles, Scarabaes zambesianus, don’t even use the moon directly but rather its polarized light. When moonlight strikes air molecules if Earth’s atmosphere, the rays scatter into planes that point in different directions. It’s this grid of polarized light that results that really guides the beetles.

At sunset the beetles vie for fresh antelope or warthog dung—their main food. When found, each beetle make a dung ball and quickly rolls it away from the others, walking backwards with its head down and rolling the ball with its strong hind legs.

“This also allows the parts of its eyes that are sensitive to polarized light to view the sky,” says biologist Marie Dacke of Sweden’s University of Lund. By using the polarized moonlight, each beetle can keep its ball rolling in a straight line and thus avoid bumping into competitor beetles. On moonless nights the beetles don’t forage, but in experiments simulating such nights “they weave randomly and steal each others’ dung balls from their reserves.” Dacke says.

Once back at their burrows, the beetles feast for days. In some species female lay eggs in the dung, so the hatchd larvae can feed on it.

by: John L. Eliot

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