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Dung beetles steer by the Milky Way

Planetarium experiments show that the insects need only starlight to orient themselves

5:04pm, January 24, 2013
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Even a collector of animal waste can keep its eyes on the stars. By tracking the dung beetles skittering across a darkened planetarium, researchers have shown that like seals, birds and people, the feces-eating insects are capable of taking directional cues from the sky.

“This is the first time we have shown that insects can use stars to guide them for orientation,” says neuroethologist Marie Dacke of Lund University in Sweden, “and it’s also the very first proof that animals can use the Milky Way for their orientation.” She and her colleagues report the results January 23 in Current Biology.

Dung-rolling insects are excellent for studying orientation behavior because they collect their prized food source and single-mindedly roll it as directly as possible away from competitors and predators. Putting the beetles in weird get-ups during experiments doesn’t deter them. “They are so attached to their dung balls,” Dacke says, “that under all circumstances they just want to roll the ball in a straight line.”

Earlier work showed that beetles can orient using the sun and moon as beacons or by the patterns of polarization in sunlight and moonlight. Beetles don’t use landmarks like rocks and trees — or, scientists thought, starlight.

That conclusion came from a 2003 publication by Dacke herself, among other researchers. In that work, she and her colleagues had reported that beetles lost their sense of direction if they could see the stars but not the moon. So she was mystified years later, when she observed beetles under starlight in a different experiment and found that they weren’t lost at all.

To figure out why, the researchers performed outdoor experiments blocking the insects’ view of the heavens with caps or using high-walled arenas that allowed them to see nothing but sky. The beetles could orient themselves when they could see the band of light made by the Milky Way but not when they could see terrestrial landmarks.

Testing fully whether beetles orient by starlight would require turning the stars on and off, so Dacke and her colleagues borrowed the Johannesburg planetarium. With the planetarium dome darkened, the nocturnal dung beetle Scarabaeus satyrus fumbled and curlicued around. But showing just the Milky Way let the beetles kick along balls of dung in fairly direct paths.

Knowing that the dung beetles actually can orient using the Milky Way, Dacke realized why her earlier experiment had gone wrong: She had tested the beetles in October in South Africa, when the Milky Way was so low in the sky that the animals couldn’t get a good view.

The beetle doesn’t steer by the Milky Way with the same understanding that a person does, says Paul Graham of the University of Sussex in England. Rather, he says, the blur of stars is a stable feature for orientation. Since the beetles only need to move quickly away from the original dung deposit, any visual cues would work. Nocturnal dung beetles, he says, are a lovely example of an animal’s adapting to its environment — and to its universe.

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