Why scarab beetles dance on a ball of dung

The ancient Egyptians would have nodded sagely: scarab beetles perform a dance to the sun atop a ball of dung. They're not worshipping a sun god, though: the beetles dance to orient themselves and ? crucially ? to roll their dung ball in a straight line.

Dung beetles were sacred in ancient Egypt, their dung-rolling linked with the nocturnal activity of Khepri, the god of the rising sun. Khepri was supposed to roll the sun through the underworld at night, pushing it over the horizon in the morning. Now Emily Baird of Lund University in Sweden and colleagues have shown that a diurnal dung beetle in South Africa (Scarabaeus nigroaeneus) uses celestial cues to ensure it keeps going in a straight line away from the dung pile.

Beetles collect dung from a pile and form it into manageable balls. Making a ball costs time and energy, and competition for dung can be intense, so it's best for a beetle not to hang around when it's got a precious new ball ready to roll.

"As a fresh dung pat can attract many beetles, it is necessary for individual beetles to try to avoid the others that may try to steal their ball," says Baird. "To do this, the beetles roll their ball away from the dung pile in the most efficient manner possible. That is, in a straight line."

Beetle boogie

When beetles have fashioned their ball, they climb to the top and "dance" ? actually rotate on the surface ? before returning to the ground and pushing. Their aim is to find a suitable patch of ground where they can bury the ball and eat it. The beetles shove the ball facing backwards, with their head down and rear legs pushing the dung.

Through an ingenious series of experiments, the biologists found that the beetles perform the dance before moving away from the dung pile, and also when they encounter an obstacle or lose control of the ball. Baird's team sent the beetles down semicircular tunnels to put them off course, made them roll balls down rotating pathways, and used a mirror to change the apparent position of the sun. In both cases, most beetles performed the orientation dance before changing the direction they pushed the ball.

"Similar behaviours are seen in ants, which rotate about their vertical axis to memorise landmarks near the nest, and sandhoppers, [small seashore crustaceans] which rotate back and forth to find a particular magnetic orientation," says Baird.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0030211

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