 Heading Home with Dung
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July 2, 2003 — One of nature's curiosities, the African dung beetle, gets its remarkable sense of direction by using the polarisation of moonlight, the first time that this ability has ever been spotted among animals.
Scarabaeus zambesianus is a picturesque scavenger that feasts on the tasty droppings of elephants.
It moulds the dung into a ball, using its front legs and head, and then rolls it away home, taking a straight line from the dung pile and its swarm of competitors. But how does it do this fast and efficient exit trick?
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Swedish and South African biologists say the answer lies in a batch of the beetle's retinal cells which are sensitive to polarised light.
Light from the sun scatters into shimmering, polarised patterns, unseen to the human eye, when it strikes tiny particles in the atmosphere. In 2001, scientists discovered that moonlight, even though it is a million times dimmer than sunlight, does the same.
S. zambesianus starts to forage on the wing for fresh dung at around sunset, using the polarisation patterns formed by around the setting Sun to figure out a straight departure bearing should it come across any food.
But after twilight, the solar cue is no longer available. The only light is lunar.
The researchers monitored the movements of the beetles at night and found that the bugs went off in straight lines away from the dungpile, rolling their prize, when there was moonlight but followed a squiggly path when there was no moon.
The team took their work further by placing a 42-centimetre (16.8-inch) polarising filter over the beetles, shading them from the moonlight, as they rolled for home with their prize.
The filter changed the polarity of the moonlight by 90 degrees. Depending on which way the filter was positioned, the beetles promptly made a right-angled turn in that direction.
That means the beetles use polarised moonlight to maintain a steady course, but not to tell left from right.
"Although, to our knowledge, this is the first description of an animal using the moon's polarisation pattern as a nocturnal compass, this ability may turn out to be widespread in the animal kingdom," the biologists said.
The study, led by Marie Dacke of Sweden's University of Lund, is published on Thursday in the British weekly journal Nature.
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