Marcus Byrne: The dance of the dung beetle
«Daytime species use the sun as a compass. Sunlight is highly polarized; it shines through the atmosphere in a particular pattern, and dung beetles, like many insects (but not humans), have specialized photoreceptors in their eyes that detect it. When a dung beetle hits a bump or rolls off course, he climbs up onto his ball and spins in a circle, to read the polarization pattern in the sky and regain his bearings. “It’s like if you’re trying to use a map and the map gets blown out of your hands, you have to pick it up and reorient yourself,” Warrant said.
In 2003, Dacke, Warrant, and others discovered that nocturnal dung beetles can navigate by the polarized light of the moon—the first animal shown to do so, although many probably can, Warrant said. “But we noticed that on many nights the moon didn’t come up until much later,” he said. “Yet our beetles kept on rolling in straight lines—not quite as straight, but pretty straight.” (…)
To test their idea, they built a circular, wooden table several feet in diameter, with a moat around the edge to catch beetles when they fell off. A high wall around the perimeter, lined with black cloth, blocked the view of trees and other potential landmarks. One by one, a beetle and his dung ball would be placed in the middle of the arena and timed to see how long it took him to reach the edge. This was all done in the dark. “They were completely unobserved,” Byrne said. “It was pretty weird. We’d release them, then you’d hear their footsteps pattering across the woodwork, then they’d fall into the trough with a thump.”
The trip could take as little as twenty seconds, if a beetle went straight, or as long as several minutes, if it went in torturous circles. The beetles were quickest when they had an open view of the starry sky. When the scientists put tiny black, cardboard hats on the beetles, to block their overhead view, the insects meandered hopelessly. “It took them a long, long time,” Warrant said. (When the beetles wore clear plastic hats, they rolled straight.) Then the researchers moved the arena to a planetarium, where they could control the contents of the sky. Sure enough, when only the eighteen brightest stars were turned on, the beetles couldn’t navigate in a straight line. But when all the stars were turned off, and only the fuzzy stripe of the Milky Way remained, the beetles were quick and direct. (…)
Dung beetles are ideal experimental subjects, Byrne said: “They are so tenacious in what they are trying to do. They cannot be distracted, they don’t get frightened, they don’t change their minds, they don’t get stage fright. They are so, so, so determined. If you set up your experiment correctly to get a yes or no answer, you will get an answer.” There are plenty more mysteries to explore, like how exactly the orienteering dance works. (…)
One wonders, then, what will happen as the night sky disappears. Thanks to sky glow, ten per cent of the world, and forty per cent of Americans, no longer view a night sky that is fully dark. This troubles ecologists as well as astronomers. A paper published in 2011 by Christpher Kyba, a physicist at Free University, in Berlin, found that light pollution washes out the polarization of moonlight, which could have a detrimental effect on dung beetles and other insects, at least around urban areas.»
– “Dung Beetles, Dancing to the Milky Way“, The New Yorker, January 27, 2013.
«Despite having tiny brains, dung beetles are surprisingly decent navigators, able to follow straight paths as they roll poo balls they’ve collected away from a dung source. But it seems the insects’ abilities are more remarkable than previously believed. Like ancient seafarers, dung beetles can navigate using the starry sky and the glow from the Milky Way, new research shows.
“This is the first time where we see animals using the Milky Way for orientation,” said lead researcher Marie Dacke, a biologist at Lund University in Sweden. “It’s also the first time we see that insects can use the stars.”
After locating a fresh pile of feces, dung beetles will often collect and roll away a large piece of spherical dung. Last year, Dacke and her colleagues discovered the beetles climb on their dung balls and dance around in circles before taking off. This dance is not one of joy, however; the insects are checking out the sky to get their bearings.
“The dorsal (upper) parts of the dung beetles’ eyes are specialized to be able to analyze the direction of light polarization — the direction that light vibrates in,” Dacke told LiveScience. So when a beetle looks up, it’s taking in the sun, the moon and the pattern of ambient polarized light. These celestial cues help the beetle avoid accidentally circling back to the poo pile, where other beetles may try to steal its food, Dacke said. [Photos of Dung Beetles Dancing on Poop Balls]
In addition to these cues, Dacke and her team wondered if dung beetles can use stars for navigation, just as birds, seals and humans do. After all, they reasoned, dung beetles can somehow keep straight on clear, moonless nights.
To find out, the researchers timed how long dung beetles of the species Scarabaeus satyrus took to cross a circular arena with high walls blocking views of treetops and other landmarks. They tested the insects in South Africa under a moonlit sky, a moonless sky and an overcast sky. In some trials, the beetles were fitted with cardboard caps, which kept their eyes to the ground. Overall, the beetles had a difficult time traveling straight and took significantly longer to cross the arena if caps or clouds obstructed their view of the sky.
From the experiments, “we thought that they could be using the stars [for orientation], but dung beetles have such small eyes that they don’t have the resolution, or sensitivity, to see individual stars,” Dacke said.
So the researchers moved their setup into a planetarium to tease out the information the beetles were extracting from the starry sky. They repeated the experiment under several different conditions, such as showing only the brightest stars, showing only the diffuse band of the Milky Way and showing the complete starry sky. The beetles took about the same amount of time to cross the arena when only the Milky Way was visible as when they could see a full star-filled sky. And they were slower to cross under all other conditions.
Previous experiments showed another dung beetle, S. zambesianus, is unable to roll along straight tracks on moonless nights when Earth’s galaxy, the Milky Way, lies below the horizon, Dacke noted. Taken together, these results suggest dung beetles navigate using the gradient of light provided by the Milky Way. However, this technique would only work for beetles living in regions where the Milky Way is distinct. “What they are doing in the Northern Hemisphere [of Earth], I don’t know,” she said.
The researchers are now trying to determine the relative importance of the different sky cues dung beetles use. “If they have the moon, polarized light and the Milky Way, will they use all cues equally?” Dacke said.
The research is published online today (Jan. 24) in the journal Current Biology.»