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Dung beetle, by Eric Warrant.

Dung Beetles Watch the Galaxy (That’s How They Roll)

From all across the galaxy, the light of billions of stars finds its way to Earth, passes through our atmosphere, and enters the eyes of a small South African beetle rolling a ball of dung. The beetle’s eyes are not sensitive enough to pick out individual stars but it can see the Milky Way as a fuzzy stripe, streaking across the night sky. With two of its four eyes, it gazes into the guts of our galaxy, and uses starlight to find its way home.

Dung beetles eat the droppings of other animals. They congregate upon piles of fresh dung, gather it into tasty balls and roll it home. Competition at a dung pile is intense and the path home can be long, dangerous and tiring. So, it pays the beetle to keep the straightest possible course.

Like many insects, dung beetles that roll in the day can detect the way that sunlight changes as it enters the Earth. Light behaves like a wave that normally vibrates in every possible direction. When it hits particles in the atmosphere, it becomes polarised, so it only vibrates in one direction depending on where the sun is in the sky. Many insects can see these patterns of polarised light, which are invisible to us, and use them to orient themselves.

Keeping a straight line is even harder at night, and even humans veer off in a curve if we can’t see anything. “For beetles with poor-resolution compound eyes, it pays to grab a hold of any visual cues in the night sky to help them steer straight,” says Ken Cheng from Macquarie University. Some use the moon, which also produces patterns of polarised light, albeit a million times dimmer than those from the sun. In 2003, Eric Warrant from Lund University found that some dung beetles, which come out at night can use these patterns to keep their paths straight.

But one beetle, known as Scarabaeus satyrus, was doing something else. Warrant and his colleague Maria Dacke noticed that even on clear, moonless nights, this species could keep a reasonably straight course. “We thought this was odd,” he says. Perhaps the beetles were navigating by starlight? Humans, birds and seals can certainly use the constellations to find their way around, but we all share similar eyes. Could an insect’s eyes use the same cues?

Dacke and Warrant found out by capturing dung beetles and placing them in a circular open-topped arena, surrounded by black walls to obscure the sight of any landmarks. The beetles were raised into the middle of the arena with their faecal cargo. They rolled off and once they hit the edge, they fell through a small gap and made an audible thump. Since each beetle rolls at a constant speed, Dacke and Warrant could measure how far they had travelled by marking the time between their entrance into the arena and the sound of their exit.

Under a full moon, they took 21 seconds. Under a moonless, starry night, their paths wavered more and they took 40 seconds. On a cloudy night, or if Dacke and Warrant covered their heads with a black cap (this isn’t the first time they’ve dressed up dung beetles for science), the beetles took around 120 seconds to reach the edge.

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Dung beetle rolling a dung ball with a cap taped to its head. (This one is transparent, so the beetle can still see through it.) Credit: Eric Warrant

Dung beetles roll their balls backwards, walking face-down and pushing with their hind legs. That seems like the wrong position for stargazing, but they have two pairs of eyes, one pointing up and one pointing down. It’s the upwards pair that sees the stars.

But to work out what they are actually seeing, Dacke and Warrant couldn’t just cover the beetle’s eyes. They had to somehow manipulate the night sky itself. So they took their beetles and arenas to the planetarium at the University of the Witwatersrand in Johannesburg.

If they projected the bright stripe of the Milky Way onto the building’s 18-metre domed ceiling, the beetles could find their way out of the arena reasonably quickly. It didn’t matter whether the team added another 4,000 stars—it was the Milky Way that mattered. If they left out this galactic stripe, or only added the 18 brightest stars, the beetles took much longer to find their way out.

“I really like the simplicity of their experiments. They’re delicious and simple like sushi,” says Lars Chittka, an insect expert at Queen Mary University of London. “It shows that even in the post-genomic world, the best science is still often driven by ideas rather than expensive equipment.”

This isn’t the first time that scientists have brought animals to planetariums to test their navigation. They’ve used these artificial domes to show that indigo buntings, flying south for the winter, navigate by flying away from the North Star. The dung beetles, however, clearly aren’t focusing on a single bright star. Instead, they’re using the collective light of billions of them.

“Have the beetles specifically evolved the ability to navigate by using the Milky Way, or are they just opportunistically using any landmark that stays in place for the duration of their path?” Chittka wonders. “The Milky Way might not have a special salience for the beetles, but just be one of a class of cues that can be used to maintain a constant direction, such as a tree, a mountain, the sun, or the moon.”

Reference: Dacke, Baird, Byrne, Scholtz & Warrant. 2013. Dung Beetles Use the Milky Way for Orientation. Current Biology

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