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Dung Beetles, Dancing to the Milky Way – New Yorker (blog)
 

Dung Beetles, Dancing to the Milky Way – New Yorker (blog)

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    Dung Beetles, Dancing to the Milky Way – New Yorker (blog) Dung Beetles, Dancing to the Milky Way – New Yorker (blog) Document Transcript

    • Dung Beetles, Dancing to the Milky Way – New Yorker(blog)“People find them a bit revolting,” Eric Warrant, a biologist at Lund and one of the paper’sauthors, said over the phone. “But they’re fascinating, and they’re the cutest animals you canimagine. When you’re holding one in your hand, they’re quite sweet.”There are some six thousand known species of dung beetle in the world, all of which thrive onfeces: cow, bison, tiger, kangaroo, chimp, what have you—the smellier and more exotic, thebetter. A dung heap is a frenzy of shoving and shovelling. “Never did adventurers hurrying fromthe four corners of the earth display such eagerness,” the French entomologist Jean-HenriFabre once wrote. “They are there in the hundreds, large and small, of every sort, shape andsize, hastening to carve themselves a slice of the common cake.” Some grab what they canand cram it underground on the spot. Others, the ball-rollers, embark on a journey that requiresthe heavens to navigate.The beetle’s head is arsenal and toolshed: horns, ploughshare, spade, sword. With it, a malemeticulously sculpts a large dung ball for himself, then rolls it away from the heap—awkwardly,backward, steadying the ball with his rear legs while pushing against the ground with hisforelegs. He might as well be fleeing with a sack of gold. The dung ball, once buried, will serveas a larder and a nursery; a female will lay a single egg in it, and the larvae will grow toadulthood as it eats its way out. In building a dung ball, the male hopes to lure a mate (“My ballis bigger than his!”), but just as often he attracts pirates—bigger dung beetles that would rathergrab another guy’s dung ball, and his girl, than work for one of their own.“I’ve seen fights go on for half an hour, two males bashing each other with their forelegs,”Warrant said. “All the while, the female is on the side, waiting to get on with the rolling.”A male aims to escape with his prize in as straight a line as possible (circling aimlessly invitesrobbery), and he is remarkably faithful to his vector. Daytime species use the sun as a compass.Sunlight is highly polarized; it shines through the atmosphere in a particular pattern, and dungbeetles, like many insects (but not humans), have specialized photoreceptors in their eyes thatdetect it. When a dung beetle hits a bump or rolls off course, he climbs up onto his ball andspins in a circle, to read the polarization pattern in the sky and regain his bearings. “It’s like ifyou’re trying to use a map and the map gets blown out of your hands, you have to pick it upand reorient yourself,” Warrant said.In 2003, Dacke, Warrant, and others discovered that nocturnal dung beetles can navigate bythe 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 prettystraight.”Other animals, including seals, some birds, and us, can navigate by individual stars, but dungbeetles probably can’t; their eyes aren’t sensitive or well-resolved enough to detect points of 1/4
    • light. More likely, the researchers thought, the beetles were cuing to the Milky Way. North of theequator, one sees only the tail end of the Milky Way; near cities, the sky-glow cast by outdoorlighting obliterates it altogether. But in the Southern Hemisphere it is spectacular, and it is thedominant feature of the night sky; one can readily make out the galactic center. “You’re staringright into the guts of the galaxy,” Warrant said. “You can even see interstellar dust clouds. Youcan see the clouds of Magellan”—the Large Magellanic Cloud and Small MagellanicCloud—“which are two other galaxies entirely.”Marcus Byrne, a zoologist at University of Witwatersrand, and another co-author on the paper,said: “The Milky Way is a great big signal of light across the middle of the sky.” Byrne wasspeaking from the group’s field site on the edge of the Kalahari, some three hundred miles fromJohannesburg; he and Dacke are there for two weeks, studying dung beetles around the clock.(“It’s one of those crazy pack-it-all-in-and-fall-over-at-the-end-of it situations,” he said.) In theevenings, after long days of watching beetles orient to the sun and moon, the researcherswould eat and drink and watch the Milky Way emerge. “We’d look up and say, ‘Howbeautiful!’” Byrne said. “It’s corny, but it’s a highway in the sky, a great big pathway: the MilkyWay. We figured, if we can see it, they can see it.”To test they idea, they built a circular, wooden table several feet in diameter, with a moataround the edge to catch beetles when they fell off. A high wall around the perimeter, lined withblack cloth, blocked the view of trees and other potential landmarks. One by one, a beetle andhis dung ball would be placed in the middle of the arena and timed to see how long it took himto reach the edge. This was all done in the dark. “They were completely unobserved,” Byrnesaid. “It was pretty weird. We’d release them, then you’d hear their footsteps pattering acrossthe 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 severalminutes, if it went in torturous circles. The beetles were quickest when they had an open view ofthe starry sky. When the scientists put tiny black, cardboard hats on the beetles, to block theiroverhead view, the insects meandered hopelessly. “It took them a long, long time,” Warrantsaid. (When the beetles wore clear plastic hats, they rolled straight.) Then the researchersmoved the arena to a planetarium, where they could control the contents of the sky. Sureenough, when only the eighteen brightest stars were turned on, the beetles couldn’t navigate ina straight line. But when all the stars were turned off, and only the fuzzy stripe of the Milky Wayremained, the beetles were quick and direct. 2/4
    • Dung beetles are ideal experimental subjects, Byrne said: “They are so tenacious in what theyare trying to do. They cannot be distracted, they don’t get frightened, they don’t change theirminds, they don’t get stage fright. They are so, so, so determined. If you set up yourexperiment correctly to get a yes or no answer, you will get an answer.” There are plenty moremysteries to explore, like how exactly the orienteering dance works, and which part of the braindoes the computing. “You pick away at a question,” Byrne said. “It’s like unraveling a tapestry.You take it thread by thread, to try to understand the whole system.”The cosmos is nothing if not egalitarian; we are all equally small. It seems fair that Earth’ssanitation workers should benefit from the Milky Way, as the rest of us do. And dung beetleslikely aren’t alone; crickets, moths, nocturnal bees, and other insects probably share their abilityto navigate by the Milky Way and by polarized moonlight. “I’d be surprised if they were the onlyinsect,” Warrant said.One wonders, then, what will happen as the night sky disappears. Thanks to sky glow, ten percent 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 ofmoonlight, which could have a detrimental effect on dung beetles and other insects, at leastaround urban areas.“Dung beetles play an incredibly important role in revitalizing our soil,” Warrant said. “It’s agardener’s dream, to have all this manure pushed into the dirt.” He couldn’t predict what thelong-term biological consequences of sky glow might be, “apart from the fact that it probably willhave some impact.” But he noted that in Australia, in the first half of the century, millions ofhectares of land were ruined by the dung of imported cows. (Native dung beetles prefer the dryfare dropped by marsupials and wouldn’t touch the sloppy, foreign stuff.) Soil quality improvedonly after the country imported dung beetles en masse from South Africa. “You could see whatkind of impact they must have in South Africa,” Warrant said, “and what it would be like if they 3/4
    • weren’t there.” We suppose that we are superior to dung beetles, but are we really? At least dung beetles recycle. We scavenge, hoard, consume…what? Crap, mostly. It piles up around us; increasingly we live on a ball of it. Even light we waste; designed to illuminate, it now obscures. As our celestial guides recede, we risk losing our bearings and will have ever less to consider but ourselves. Photographs by Davis Meltzer/National Geographic and Hoberman Collection/UIG via Getty. Source Article from http://www.newyorker.com/online/blogs/newsdesk/2013/01/dung-beetles-dancing-to-the-milky-w ay.html Waddywood.com Dung Beetles, Dancing to the Milky Way – New Yorker (blog) 4/4Powered by TCPDF (www.tcpdf.org)