Dung Beetles Navigate by the Moon, Study Says

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Scientists have known for a few decades that the eyes of certain insects such as honeybees are sensitive to polarized sunlight, said Gill. The research by Dacke and colleagues extends this known sensitivity to nocturnal insects.

"The eye has to be so much more sensitive, [moonlight] is one millionth…the strength of sunlight," said Gill.

Moonlight Navigation

S. zambesianus begins to forage for a fresh pile of dung around sunset. When it finds a dropping, it gathers up a bit into a ball with its front legs and head and rolls it away in a straight line.

At sunset, the beetle is able to orientate itself using the polarization pattern formed around the setting sun, but when the sun is 18 degrees beyond the horizon, known as astronomical twilight, the pattern is lost.

"But the moon will create a new polarized path in the sky," said Dacke. "It lasts for as long as the moon is in the sky, so as long as we have a moon, we have a polarized pattern around the moon."

To find out if the beetles are able to use the polarized light of the moon to navigate, Dacke and her colleagues observed the beetles under the night sky. On nights when there was a moon, the beetles continued to forage and roll their dung balls in a straight line. On moonless or cloudy nights the beetles could not maintain a straight path.

To determine if this continued ability to forage after astronomical twilight is a result of the polarization of moonlight or of the moon itself, the researchers placed a polarizing light filter over a ball-rolling beetle feeding inside a ten-foot (three-meter) diameter arena.

When the researchers changed the pattern of polarized moonlight by 90 degrees, the beetles changed course by 90 degrees. The results indicate that indeed the beetles are using the polarized light of the moon to navigate, which extends their foraging time.

Frank Krell, an entomologist at the Natural History Museum in London, said the work by Dacke and colleagues is "a great, elegant study that demonstrates that the polarization of the moonlight is actually guiding the movements of this dung beetle."

Brett Ratcliffe, a dung beetle expert at the University of Nebraska in Lincoln, said the research provides insight to the foraging behavior of the many species of dung beetles that are active during the night.

"This probably works best in savanna or grassland-adapted species where moonlight is not blocked by tree overstory," he said. "For forest species, further investigation is needed to ascertain it is being utilized there also. And we probably can extrapolate to certain other insects that navigate at night."

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