A meandering beetle could have its dung stolen. Credit: source:

An African beetle relies on moonlight's polarisation to roll its dungball in a straight line, researchers have discovered.

Several species, including some insects, spiders and even birds, orientate themselves by the plane of the sun's rays. But this is the first time that an animal has been shown to use moonlight - which is around a million times dimmer - as a compass.

Having successfully claimed a dungball for dinner, Scarabaeus zambesianus beetles try to roll it to safety, explains Marie Dacke of the University of Lund in Sweden, who led the study1. "It's important to move in a straight line or they steal each other's balls," she says.

Dacke and her colleagues discovered that beetles chart a straight course only on moonlit nights. Without the Moon, they meander randomly.

To test whether the beetles use light polarization, rather than the position of the Moon, to maintain a steady bearing, the researchers made sure the insects could not see the Moon. They then placed a polarizing filter over a ball-rolling beetle to turn its light through 90°.

The beetles made an abrupt turn either to the left or the right. This shows that the bugs use the sky's polarized moonlight to follow a straight course, but not to tell left from right, Dacke says.

Dacke anticipates that more nocturnal species will be found to navigate by the light of the Moon. "This ability may turn out to be widespread in the animal kingdom," she says.