Out on the African savanna, a fresh and moist pile of fine-grained antelope dung is a nutritious treasure aggressively fought over by a melee of critters. The spoils go to those with the craftiest strategies to snatch and stash a piece of the pie.
To gain an edge in this battle for the poop, the African dung beetle Scarabaeus zambesianus orients itself by the polarized light pattern cast by the moon to make a straight, nighttime escape with its morsel, according to Marie Dacke, a biologist at the University of Lund in Sweden.
"There are so many beetles at the dung pile going after limited food, so they want to escape from the competition," she said.
Once the beetles are a safe distance from the pile, they burrow into the ground and feast on their nugget for days. The burrows also serve as a pathway for air and moisture to get into the ground, giving dung beetles a critical role in the savanna ecosystem.
Many creatures are known to use the polarization pattern of sunlight to navigate, but S. zambesianus is the first animal known to use the million-times dimmer polarization of moonlight, Dacke and colleagues report in the July 3 issue of Nature.
Bruce Gill, an entomologist and dung beetle expert at the Canadian Food Inspection Agency in Ottawa, was at first surprised by this beetle's behavior, but upon reflection he said it makes sense because light from the moon was simply reflected sunlight, and many insects that are active during the day are known to navigate by polarized light from the sun.
"I bet it's going to be a lot more widespread than this particular beetle," he said. "I bet a lot of different insects are using this in their navigation and flying at night."
Polarized light patterns cast directly by the sun and indirectly from the sun's reflection off the moon are invisible to the human eye, but can be seen by a host of others in the animal kingdom.
Regular light vibrates in all directions, but some of this light interacts with particles in the atmosphere and becomes polarized, which causes it to vibrate along a single, distinguishable plane.
Humans wear polarized sunglasses, for example while fishing, to filter out the glare from polarized light that is reflected off the water's surface. This makes the water more transparent and thus humans can more easily see fish swimming in the water.
Scientists have known for a few decades that the eyes of certain insects such as honeybees are sensitive to polarized sunlight, said Gill. The research by Dacke and colleagues extends this known sensitivity to nocturnal insects.
"The eye has to be so much more sensitive, [moonlight] is one millionth…the strength of sunlight," said Gill.
S. zambesianus begins to forage for a fresh pile of dung around sunset. When it finds a dropping, it gathers up a bit into a ball with its front legs and head and rolls it away in a straight line.
At sunset, the beetle is able to orientate itself using the polarization pattern formed around the setting sun, but when the sun is 18 degrees beyond the horizon, known as astronomical twilight, the pattern is lost.
"But the moon will create a new polarized path in the sky," said Dacke. "It lasts for as long as the moon is in the sky, so as long as we have a moon, we have a polarized pattern around the moon."
To find out if the beetles are able to use the polarized light of the moon to navigate, Dacke and her colleagues observed the beetles under the night sky. On nights when there was a moon, the beetles continued to forage and roll their dung balls in a straight line. On moonless or cloudy nights the beetles could not maintain a straight path.
To determine if this continued ability to forage after astronomical twilight is a result of the polarization of moonlight or of the moon itself, the researchers placed a polarizing light filter over a ball-rolling beetle feeding inside a ten-foot (three-meter) diameter arena.
When the researchers changed the pattern of polarized moonlight by 90 degrees, the beetles changed course by 90 degrees. The results indicate that indeed the beetles are using the polarized light of the moon to navigate, which extends their foraging time.
Frank Krell, an entomologist at the Natural History Museum in London, said the work by Dacke and colleagues is "a great, elegant study that demonstrates that the polarization of the moonlight is actually guiding the movements of this dung beetle."
Brett Ratcliffe, a dung beetle expert at the University of Nebraska in Lincoln, said the research provides insight to the foraging behavior of the many species of dung beetles that are active during the night.
"This probably works best in savanna or grassland-adapted species where moonlight is not blocked by tree overstory," he said. "For forest species, further investigation is needed to ascertain it is being utilized there also. And we probably can extrapolate to certain other insects that navigate at night."