Moonlighting: Beetles navigate by lunar polarity

By watching dung beetles roll their balls of dung at night, an international team of researchers has turned up evidence that the insect aligns its path by detecting the polarization of moonlight.

ROLL ON. A South African dung beetle positioned to roll away its treasure. Inset: Diagram shows a beetle’s path when scientists cover it in midcourse (at black spot) with a filter that shifts moonlight’s polarization. The beetle compensates for the shift, but later when outside of the filter’s influence, it returns to its original path. Dacke

When researchers set up polarizing filters to shift the moonbeams, the African beetle Scarabaeus zambesianus changed direction to compensate, says Marie Dacke of the University of Lund in Sweden. “This is the first proof that any animal can use polarized moonlight for orientation,” she says.

Some insects and other arthropods detect polarity patterns in sunlight and rely on those patterns for finding their way. Researchers are still debating whether some migrating birds and fish orient themselves in this way, too, says Eric J. Warrant, also of Lund and a coauthor of the report in the July 3 Nature.

Dung beetles offer a great opportunity to study orientation, Warrant says, because they roll dung in straight lines. Dung, especially the half-meter-wide deposits from elephants, attracts dozens of beetles. In the species that Dacke and Warrant monitored, beetles roll small balls, 4 centimeters or so across, for their own nourishment, but males in search of mates roll 10-cm specials. A female flies in to cling to the ball for a roller-coaster ride. After the pair mates, the ball serves as baby food.

With food and family at stake, one male often tries to wrestle away another’s dung ball. “The dung heap is a very competitive place,” says Warrant. The best exit strategy, he notes, is a straight line out of the fray, and that’s where the moonlight comes in.

The Swedish researchers, collaborating with colleagues in South Africa, found that beetles active during the day depend on sunlight polarization patterns. However, Dacke noticed that on moonlit nights, one twilight-active species worked particularly late.

It’s only been 2 years since researchers first documented polarization in moonlight, so biologists are just beginning to explore its effects on animals. On moonlit nights, Dacke set out a pile of pig dung, and within minutes, dung beetles came flying. The beetle researchers positioned a sheet of polarizing filter over a beetle, rotating the polarization of moonlight by 90. Tracking 22 beetles, Dacke found that the beetles under the filter deviated from their previous course by turns of nearly 90.

Dung-beetle specialist Franois Génier of the Canadian Museum of Nature in Ottawa says that because so many insects use polarized daylight, moonlight navigation “is the next logical step.” He speculates that this species has taken to moonlight because at night “competition among large rollers is less fierce.”


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Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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