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Last Updated: Thursday, 3 July, 2003, 09:58 GMT 10:58 UK
Animal navigates by moonlight
By Helen Briggs
BBC News Online science reporter

It is well known that birds such as homing pigeons use the Sun to find their way home.

Now, biologists say they have found the first case of an animal that navigates by the Moon.

Marie Dacke, Lund University
The beetle, Scarabaeus zambesianus
It is only a humble beetle but the ability to use moonlight as a compass may be widespread in the animal kingdom.

Many birds use the Sun, Moon or stars as a marker in the sky.

But the African dung beetle seems to have even more remarkable skills. It uses the pattern created when moonlight strikes tiny particles in the atmosphere (polarisation) to orient itself and travel in a straight line.

This enables it to make a hasty retreat from competitors rolling a ball of the fresh elephant dung on which it forages for food.

Filter trick

When nights are cloudy, its progress across the ground is more random and it tends to go around in circles.

Experiments have confirmed that the beetle uses the polarisation pattern of moonlight rather than the Moon itself to navigate.

I wouldn't be surprised if birds were able to use polarised moonlight as well as the stars, Sun and the Earth's magnetic field
Ian Dawson, RSPB
When the researchers, from Sweden and South Africa, placed a polarising filter over the beetle and rotated the light, it changed direction.

Special receptors have been found in the eye that detect polarised moonlight, which is a million times dimmer than sunlight.

"It is more than likely that this is more widespread in the animal kingdom," says lead researcher Marie Dacke of Lund University in Sweden, "for example with bees, wasps and other beetles."

Homing pigeons

Previous research has suggested that other nocturnal insects orientate themselves by polarised moonlight, says Dr Frank Krell of the Department of Entomology at London's Natural History Museum.

"Tests of other species of different groups of nocturnal insects are necessary to find out whether this mechanism is unique with dung beetles or widespread in insects or even arthropods," he says.

Birds, too, may navigate by moonlight. Studies of homing pigeons and migrant birds show they have an internal body clock that is calibrated by the movement of celestial bodies across the sky.

"I wouldn't be surprised if birds were able to use polarised moonlight as well as the stars, Sun and the Earth's magnetic field," says Ian Dawson of the Royal Society for the Protection of Birds in the UK.

"The more that is discovered, the more remarkable it becomes."

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