One moonless night a little more than a decade ago, Marie Dacke and Eric Warrant, animal vision experts from Lund University in Sweden, made a surprise discovery in South Africa.
The researchers had been watching nocturnal dung beetles, miniature Sisyphuses of the savanna, as they tumbled giant balls of dung. The beetles seemed to be able to roll remarkably straight, even though they had no clear landmarks to reference.
“We thought maybe they were using our cameras, maybe someone had lit a fire somewhere,” Dr. Dacke said. “We were really confused.” Then they realized the beetles were guided by the 100,000 light-years-long streak of the Milky Way.
We humans are famous for this sort of thing. The stars beckoned our species to cross seas and kindled the sciences that later let us putter up toward them in rockets. From culture to culture, the Milky Way served as backdrop and inspiration for stories about rivers, trees, gods, serpents and, of course, exploration.
But we weren’t the only ones looking.
Researchers like Dr. Dacke suspect that a wide swath of the animal world might sometimes navigate by starlight — and might be lost as our city lights drown out ever more of it. Her team’s newest study, published Thursday, found that dung beetles became confused under light-swamped skies. The result adds to a small and scattered body of research, conducted over decades, on what the night sky might mean to the other earthlings who can sense it.
These experiments raise the same few, difficult questions: Can animals see the stars? Can animals use them? And what happens when they lose them?
In the 1780s, the astronomers William and Caroline Herschel scoured the skies for nebulae, finding some that resembled spiral seashells. In the 1920s, Edwin Hubble, at the helm of an enormous telescope, discovered that the Milky Way, Andromeda and Herschel spirals were galactic islands in a vast cosmic sea.
And in the mid-2000s, Nick, a researcher of sorts in Cologne, Germany, swam to his own telescope and plopped into an underwater chair. Then he stuck his head in a tube and, if he saw a star, pressed a paddle. He was rewarded with fish.
Nick, a harbor seal, entered the annals of astronomical history when Guido Dehnhardt, a marine biologist now at the University of Rostock, was studying how marine mammals orient themselves. If seals could discern stars, Dr. Dehnhardt and his colleague Björn Mauck hypothesized, that might help explain how the animals are able to complete long swims across otherwise featureless seas.
To test a seal’s astronomical skills, Dr. Mauck devised what must be two of scientific history’s most wide-eyed, wonder-infused experiments.
First the team constructed their seal-o-scope — a tube with no lens, through which Nick was given a tour of the night sky. He consistently pressed his paddle when bright points like Venus, Sirius and Polaris came into view; he could not see as many faint stars as humans can, the researchers determined, but plenty of possible celestial landmarks were still available to him.
Next Dr. Mauck built something even bigger. This time two seals were invited to participate, Nick and his even cleverer brother, Malte.
When ushered back into the pool at a Cologne zoo, the seals entered a dome measuring 15 feet across, its rim resting on a floating ring. The insides of this bespoke aqua-planetarium were lit up with 6,000 simulated stars. “They were immediately swimming across the planetarium and looking at stars like, ‘Oh, what’s that?’” Dr. Dehnhardt said. “You got the impression that they really recognized what this is.”
First the researchers used a laser pointer to direct the seals to where the rim of the dome met the water nearest to Sirius, the Dog Star. If a seal swam over and touched that precise part of the dome with its snout, it received some fish. Then the pointer was aimed directly at Sirius. Again the seal’s task was to swim toward the star and touch the point on the rim directly beneath it.
Then the researchers dispensed with the laser pointer. No matter which way the star-projector was oriented, both seals could eventually swim in the direction of Sirius. That showed, the researchers argued in a 2008 paper, that seals traversing the open sea had the capacity to use lodestars to guide their travels.
It had taken Nick, the more seasoned astronomer, 11 sessions to reliably touch the wall right under Sirius. But Malte did it the very first time they left him alone in the planetarium. “Malte was an absolutely brilliant seal,” Dr. Dehnhardt said.
Coming of age under Polaris
While seals might steer by the stars, some birds definitely do, as more than a half-century of experiments inspired by Stephen T. Emlen have demonstrated.
In the early 1960s, Dr. Emlen, then a graduate student at the University of Michigan and now an emeritus professor at Cornell, began carrying indigo buntings into a planetarium in the evenings. “I was nocturnal myself,” he said recently.
Indigo buntings migrate at night, flying as far as 2,000 miles. Before they start out, they hop in the direction they plan to go, making their intent apparent to a navigation researcher. Dr. Emlen showed that if indigo buntings were exposed to the stars, the birds hopped in a manner indicating that they knew which way was north.
Human observers know that at night the stars trace circles in the sky as Earth spins on its axis. Those circles shrink for more northern stars — and the northernmost, Polaris, doesn’t seem to circle at all. Once you’ve learned that, you need only to remember a constellation in the vicinity of north — the Big Dipper works well — to get oriented.
Dr. Emlen wondered whether buntings employed a similar trick. If he covered up Polaris in the planetarium, could they still find north? Indeed they could, he discovered. What if he blotted out just the Big Dipper, or just Cassiopeia? Now the birds’ performance varied. And if he erased all the northernmost constellations together? The birds were lost.
Later Dr. Emlen enlisted young indigo buntings that had just left their nests. Over the summer he tweaked the planetarium projector so that the sky rotated around Betelgeuse, the red giant star at Orion’s shoulder, rather than Polaris.
Sure enough, come autumn, the buntings indicated with their hops that they thought Betelgeuse guided the way north. This suggested that the bird’s stargazing skills were learned, not derived from some star map encoded in their genes, Dr. Emlen and his colleagues argued. In the glittering dark, each young bunting had apparently spent some time looking up, studying, as the stars traced circles in the night sky.
Lost on a starless night
Around the time Nick and Malte learned to spot Sirius, Dr. Dacke and Dr. Warrant had their eureka moment in South Africa with dung beetles.
After a dung beetle arrives on a dung pile, it painstakingly cobbles together a snowball of dung larger than itself. Then it climbs up onto the ball and spins around, scanning, as if deciphering the celestial scene.
Through a dung beetle’s compound eyes, stars appear as blobs, not as points of light. But those same eyes are more sensitive than ours to dim objects like the dappled patterns of the Milky Way.
After its spinning scan, a beetle rolls its ball in a straight line away from the dung pile for a few minutes, on a random heading. (This seems to minimize the likelihood that two beetles will meet, which often results in a fight.) Dr. Dacke found that the beetles seem to keep themselves rolling straight by confirming that the intragalactic scene they were heading toward still matched the earlier reference image.
But a funny thing happened as Dr. Dacke’s team kept studying this behavior: They had an increasingly difficult time locating the Milky Way in the sky. “I ended up spending one night cycling off into the forest,” said James Foster, who joined the project as a graduate student and is now a postdoc in zoology at the University of Würzburg in Germany.
That’s a common experience; roughly four in five Americans, two in three Europeans, and one in three people in the world reside somewhere too bright at night for the Milky Way to be visible, a 2016 study showed, and the fraction of our planet that is illuminated at night grows larger every year.
Dr. Dacke and Dr. Foster resolved to test whether washed-out skies might also matter to their study subjects. In their latest experiments, published Thursday in Current Biology, they shined spotlights on the beetles, and brought them to the roof of the Wits University biology building in central Johannesburg.
They found two effects, neither of them good. When a beetle’s sky was dominated by a single glaring light, it could still go straight — but instead of rolling off in a random direction, it made a beeline for the beacon. More confusing were the featureless, light-swamped skies you might expect in suburbs: The beetles just went in circles.
Other species may be similarly affected. The stars have always been more or less constant while landmarks erode and the planet’s magnetic field shifts. But now, rather abruptly, they are fading out. “What we’ve seen on a much smaller scale with the dung beetles could have huge impacts on birds and seals and migratory moths,” Dr. Foster said.
With their studies concluded, science’s pioneering animal astronomers have gone on to different fates. A few of the South African dung beetles now live in the lab in Lund, where the researchers sometimes study them under a fully simulated sky.
Dr. Emlen kept the Betelgeuse-beguiled indigo buntings in his lab through the winter and released them in the spring. Hopefully, he said, they recalibrated to the true north before beginning their first migration.
And Nick the seal is still an active contributor to fields like acoustics, hydrodynamics and optics, although Malte died a few years ago. Nick now lives in a sprawling harbor lab that opens to the Baltic Sea; at night, he and the other seals in the enclosure can see fantastic skies overhead, Dr. Dehnhardt said.
“But whether they use it,” he said, “I don’t know.”
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