Dung Beetles Navigate by the Moon, Study Says

John Roach
for National Geographic News
July 2, 2003

Out on the African savanna, a fresh and moist pile of fine-grained antelope dung is a nutritious treasure aggressively fought over by a melee of critters. The spoils go to those with the craftiest strategies to snatch and stash a piece of the pie.

To gain an edge in this battle for the poop, the African dung beetle Scarabaeus zambesianus orients itself by the polarized light pattern cast by the moon to make a straight, nighttime escape with its morsel, according to Marie Dacke, a biologist at the University of Lund in Sweden.

"There are so many beetles at the dung pile going after limited food, so they want to escape from the competition," she said.

Once the beetles are a safe distance from the pile, they burrow into the ground and feast on their nugget for days. The burrows also serve as a pathway for air and moisture to get into the ground, giving dung beetles a critical role in the savanna ecosystem.

Many creatures are known to use the polarization pattern of sunlight to navigate, but S. zambesianus is the first animal known to use the million-times dimmer polarization of moonlight, Dacke and colleagues report in the July 3 issue of Nature.

Bruce Gill, an entomologist and dung beetle expert at the Canadian Food Inspection Agency in Ottawa, was at first surprised by this beetle's behavior, but upon reflection he said it makes sense because light from the moon was simply reflected sunlight, and many insects that are active during the day are known to navigate by polarized light from the sun.

"I bet it's going to be a lot more widespread than this particular beetle," he said. "I bet a lot of different insects are using this in their navigation and flying at night."

Polarization

Polarized light patterns cast directly by the sun and indirectly from the sun's reflection off the moon are invisible to the human eye, but can be seen by a host of others in the animal kingdom.

Regular light vibrates in all directions, but some of this light interacts with particles in the atmosphere and becomes polarized, which causes it to vibrate along a single, distinguishable plane.

Humans wear polarized sunglasses, for example while fishing, to filter out the glare from polarized light that is reflected off the water's surface. This makes the water more transparent and thus humans can more easily see fish swimming in the water.

Continued on Next Page >>


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