Dung Beetles Navigate By The Light Of The Milky Way

Birds, seals and humans are known to use the stars for navigation. Now, according to new research, there's a new animal to add to that list: dung beetles.

A group of scientists from Lund University in Sweden and the University of Pretoria and University of the Witwatersrand in South Africa were intrigued by the fact that dung beetles can still roll their little balls of material along on straight lines when the night is moonless.

"This led us to suspect that the beetles exploit the starry sky for orientation -- a feat that had, to our knowledge, never before been demonstrated in an insect,” Lund University researcher Marie Dacke said in a statement Thursday.

In a paper published on Thursday in the journal Current Biology, Dacke and her colleagues describe how they figured out how Scarabaeus satyrus, an African dung beetle, follows the light of the Milky Way as it merrily rolls its ball of dung along.

To test whether or not the beetles were using the stars to orient themselves, the researchers put the insects inside an outside arena of flattened sand, surrounded by a wall. Some beetles were fitted with little cardboard hats that blocked their view of the sky. The hat-wearing beetles took a significantly longer path to the edge of the arena than the beetles not wearing hats.

In the next step, some beetles were fitted with transparent plastic caps to see if the longer path was an effect of hat-wearing alone. But the beetles with see-through hats did not take much longer to roll their balls of dung along than the hatless beetles.

“Thus, the straight-line orientation of S. satyrus is significantly impaired if they are prevented from seeing the starlit sky,” the authors wrote.

The scientists also wanted to rule out the possibility that the beetles were using overhead landmarks, like treetops, to guide their dung-rolling. To that end, they made a second arena with an elevated wooden floor, enclosed by a wall of black cloth that obscured all but the night sky above. They then timed how long it took for beetles to exit the arena, as observed by an “audible 'thump'” when the beetles fell off the platform into a collecting trough below.

Under a full moon, beetles exited the arena in an average of around 21 seconds. On moonless nights with visible stars, the beetles took a little bit longer to navigate out of the arena, with an average exit time of 40 seconds. But when the beetles were again prevented from seeing the night sky with cardboard hats, they foundered, taking an average of 124 seconds to exit. Likewise, when the sky was overcast and neither moonlight nor stars were visible, the beetles rolled around the arena for an average of 117 seconds before they reached the edge.

So, clearly the beetles were using the stars to navigate, “but the vast majority of these stars should be too dim for the tiny compound eyes of the beetle to discriminate,” the researchers wrote.

To see what kind of information the beetles could be gleaning from the stars, despite their limited eyesight, the next phase of experiments took place inside a planetarium in Johannesburg. They used the arena setup again under five different conditions: a completely starry sky with 4,000 stars and the Milky Way visible; the Milky Way only; an arrangement of dim stars; an arrangement of the 18 brightest stars in the sky; or total darkness.

Under conditions where the full starry sky or the Milky Way alone was projected, they found the beetles took about the same amount of time to exit the arena.

“This indicates that on a moonless night, S. satyrus orientates using the bright band of light produced by the Milky Way,” the scientists wrote.

The authors noted that previous experiments with another kind of dung beetle, S. zambesianus, found that this insect was not able to roll along straight tracks on moonless nights in October, when the Milky Way is close to the horizon after sunset. This seems to confirm that dung beetles rely on the Milky Way to guide them.

Though technically all stars visible to the naked eye are part of the Milky Way, we use the term to refer to the broad band of light arcing across the sky. The Milky Way galaxy appears as a smeary streak to creatures on Earth, because we're looking at it from the inside of its disk. It's possible that this strategy of navigating by the light of this band might be more widespread among insects and, indeed, the entire animal kingdom.

“Since the moon is potentially visible for only half of all nights, the stars, and more importantly the Milky Way, can provide a more dependable celestial cue for orientation,” the scientists said.

SOURCE: Dacke et al. “Dung Beetles Use the Milky Way for Orientation.” Current Biology published online 24 January 2013.