“This is a complicated navigational feat—it’s quite impressive for an animal that size,” said study co-author Eric Warrant, a biologist at the University of Lund in Sweden.
Moving in a straight line is crucial to dung beetles, which live in a rough-and-tumble world where competition for excrement is fierce.
Once the beetles sniff out a steaming pile, males painstakingly craft the dung into balls and roll them as far away from the chaotic mound as possible, often toting a female that they have also picked up. The pair bury the dung, which later becomes food for their babies.
But it’s not always that easy. Lurking about the dung pile are lots of dung beetles just waiting to snatch a freshly made ball. (Related: “Dung Beetles’ Favorite Poop Revealed.”)
That’s why ball-bearing beetles have to make a fast beeline away from the pile.
“If they roll back into the dung pile, it’s curtains,” Warrant said. If thieves near the pile steal their ball, the beetle has to start all over again, which is a big investment of energy.
Scientists already knew that dung beetles can move in straight lines away from dung piles by detecting a symmetrical pattern of polarized light that appears around the sun. We can’t see this pattern, but insects can thanks to special photoreceptors in their eyes.
But less well-known was how beetles use visual cues at night, such as the moon and its much weaker polarized light pattern. So Warrant and colleagues went to a game farm in South Africa to observe the nocturnal African dung beetle Scarabaeus satyrus. (Read another Weird & Wild post on why dung beetles dance.)
Attracting the beetles proved straightforward: The scientists collected buckets of dung, put them out, and waited for the beetles to fly in.
But their initial observations were puzzling. S. satyrus could still roll a ball in a straight line even on moonless nights, “which caused us a great deal of grief—we didn’t know how to explain this at all,” Warrant said.
Then, “it occurred to us that maybe they were using the stars—and it turned out they were.”
To test the star theory, the team set up a small, enclosed table on the game reserve, placed beetles in them, and observed how the insects reacted to different sky conditions. The team confirmed that even on clear, moonless nights, the beetles could still navigate their balls in a straight line.
To show that the beetles were focusing on the Milky Way, the team moved the table into the Johannesburg Planetarium, and found that the beetles could orient equally well under a full starlit sky as when only the Milky Way was present. (See Milky Way pictures.)
Lastly, to confirm the Milky Way results, the team put little cardboard hats on the study beetles’ heads, blocking their view of the sky. Those beetles just rolled around and around aimlessly, according to the study, published recently in the journal Current Biology.
Dung beetle researcher Sean D. Whipple, of the Entomology Department at the University of Nebraska-Lincoln, said by email that the “awesome results …. provide strong evidence for orientation by starlight in dung beetles.”
He added that this discovery reveals another potential negative impact of light pollution, a global phenomenon that blocks out stars.
“If artificial light—from cities, houses, roadways, etc.—drowns out the visibility of the night sky, it could have the potential to impact effective orientation and navigation of dung beetles in the same way as an overcast sky,” Whipple said.
Keep On Rollin’
Study co-author Warrant added that other dung beetles likely navigate via the Milky Way, although the galaxy is most prominent in the night sky in the Southern Hemisphere.
What’s more, it’s “probably a widespread skill that insects have—migrating moths might also be able to do it.”
As for the beetles themselves, they were “very easy to work with,” he added.
“You can do anything you want to them, and they just keep on rolling.”