Dung beetles use polarised moonlight - light which has different properties in different directions - to walk backwards in a straight line.

Dr Marie Dacke from the University of Lund, in Sweden and colleagues from South Africa report their investigation of the use of polarised moonlight by African dung beetles in this week's issue of the journal Nature.

The African dung beetle (Scarabaeus zambesianus) flies around foraging for fresh dung at sunset. When it finds a pile, the beetle lands and uses its front legs and head to make a small ball of dung. It then immediately sets out to hide the booty from competitors, making a beeline away from the main pile. The beetle pushes the ball along with its back legs while walking with its front legs, face down to the ground.

The researchers had suspected the beetles were using polarised moonlight when they noticed that on overcast nights beetles made very erratic paths, compared to the straight lines they made when the moon was shining.

"Many creatures use the Sun's light-polarisation pattern to orientate themselves, but S. zambesianus is the first animal known to use the million-times dimmer polarisation of moonlight for this purpose," Dacke and team write.

Sunlight and moonlight scatter when they strike tiny particles in the filter of the Earth's atmosphere and this is what causes the light to become polarised. Light from the sky is most strongly polarised at sunset and sunrise and it is the polarised pattern formed around the setting Sun which the backwards-walking African dung beetles use to keep their bearings during the day.

However once the Sun moves well beyond the horizon, dung beetles can no longer use its polarised light for direction any more. But the researchers observed that the beetles were still able to walk in straight lines on moonlit nights - and they wobbled all over the place when it was a cloudy night. So they set out to test if they were using the polarised light of the moon to navigate.

To make sure it was the polarisation of the moonlight rather than just the moonlight itself that was helping the beetles navigate, the group used a polarising filter over a beetle while it was rolling a ball of dung. The light filter changed the polarity of the light by 90 degrees, and the beetles responded by making a right-angled turn in either direction.

The researchers made doubly sure it was the polarisation (and not something to do with the filter that was changing the beetle's direction) by changing the filter so it filtered light with the same polarisation of the sky. Under these filters the beetles kept walking in a straight line.

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