Dung Beetles Navigate by the Milky Way
Starry night. Using starlight, dung beetles travel relatively straight (lines inside top circle). But when the night sky is hidden, the beetles had much more difficulty keeping a straight path to the periphery (bottom circle).
Credit: (left) Emily Baird; (right) Adapted From M. Dacke et al., Current Biology, 23 (2013)
A day in the life of a male dung beetle goes something like this: Fly to a heap of dung, sculpt a clump of it into a large ball, then roll the ball away from the pile as fast as possible. However, it turns out that the beetles, who work at night, need some sort of compass to prevent them from rolling around in circles. New research in Current Biologysuggests that the insects use starlight to guide their way. Birds, seals, and humans also use starlight to navigate, but this is the first time it's been shown in an insect.
The whole point of rolling dung is to impress the female beetle with provisions—i.e., excrement—for her future progeny and entice her to mate. She then lays an egg in the ball and buries it in a network of tunnels more than a meter deep, where it serves as food for the developing larvae inside.
But rolling dung balls in a straight line is also key to the male dung beetle's reproductive success. Rival males have been known to overtake a slower moving insect and claim the hard-earned treasure as their own. Competition is fiercest near the dung heap, so making a quick and efficient getaway is crucial for mating success.
The discovery that dung beetles use starlight "was an accident more than anything," explains study author EricWarrant, professor of zoology at the Lund University in Sweden. His research group was studying how the beetles used the polarized light patterns of the moon to stay on their paths, when one moonless night they made a surprising observation—the beetles maintained straight trajectories. "Even without the moon—just with the stars—they were still able to navigate," Warrant says. "We were just flabbergasted."
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To learn more, Warrant and his colleagues set up a circular arena filled with sand and measured the time it took the beetles to roll a dung ball from the center to the edge, while filming their trajectory from above. The experiment was conducted both outdoors under the night sky, and inside a planetarium where researchers could manipulate the starlight and hone in on the specific cues that the dung beetles were using.
What they found was that the dung beetles rolled in straight paths and made it to the periphery quickly using natural light from the moon or a moonless starry sky. They did equally well when an image of the night sky or the Milky Way alone was projected in the planetarium. However, under overcast conditions, when their eyes were covered with a piece of cardboard taped to their back, or when they were presented with only a handful of bright stars in the planetarium, they had significantly more difficulty following a linear path.
The researchers concluded that in the wild, the beetles weren't using individual stars, but the bright stripe of starlight from the Milky Way as a sort of compass. The Milky Way is much easier to see in the Southern Hemisphere (South Africa) where the researchers did their experiments, Warrant says. "It's probably much more visible for an insect eye—especially a sensitive insect eye like these beetles have."
Dung beetles "see about 1000 times better than honey bees, which have the best daytime vision," explains James Gould, an evolutionary biologist and an expert in insect communication and navigation from Princeton University. To a dung beetle, the Milky Way probably looks like a dim band of variable brightness stretching across the night sky. "All they have to do is to maintain a heading relative to that landmark and they'll be OK," he says.
Other insect species may rely on starlight too. "There are nocturnal bees and nocturnal moths and nobody has ever asked if they could do this," Gould says. "It's the dung beetles that have provided the reason to go and look."
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