It was a strange spectacle: in the winter of 2007, Chinese scientists saw a wild giant panda jumping through horse manure, diligently staining itself with excrement until its skin became a confusing mess. It wasn’t the last time investigators saw this strange behavior.
But finding out why pandas do it would take the team 12 years and a scientific journey through the fields of animal behavior, chemical ecology, and neurophysiology. But now, researchers think they have an answer.
Pandas can roll poop, curiously, to feel warm. Researchers have identified a chemical present in horse excrement that confers cold resistance to laboratory mice and could inhibit a cold-sensitive protein present in giant pandas (Ailuropoda melanoleuca), reported on Dec. 7 in Proceedings of the National Academy of Sciences .
“I’m an expert on pandas, and this is one of the weirdest pandas I’ve read,” says Bill McShea, a biologist at the Smithsonian Conservation Biology Institute in Front Royal, Va. "There's still a lot of work to be done. Be done, but these researchers deserve a lot of credit."
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Students of Fuwen Wei, an ecologist at the Chinese Academy of Sciences in Beijing, first glimpsed the strange behavior in the Qinling Mountains of central China. The region is traversed by ancient trade routes well trodden by horses, so researchers say horse manure may have been common.
Rolling with poop is not unheard of among animals: consider the dog. But many mammals actively avoid the feces of other individuals and species, as poop can harbor pathogens and parasites, says Cecile Sarabian, a cognitive ecologist at Kyoto University in Japan who did not participate in the study.
“Behavior is a story of commitment,” she says. "In this case, the benefits of coming in contact with fresh horse manure can nullify the (potential) risks."
To understand what those benefits might be, researchers first had to pick up more manure maneuvers. They set up a series of motion-sensitive cameras along a road in the Foping National Nature Reserve. Cameras captured 38 panda-poo interactions from June 2016 to June 2017, suggesting that the initial observation was not just a strange incident. The camera setup also recorded the time and temperature of the air for each behavior, revealing a clear pattern: giant pandas rolled in poop only in the coldest weather. Most observations were captured when temperatures were between -5 ° C and 5 ° C.
Pandas were also picky about poop. It had to be cool; manure of more than a few days was largely ignored. Chemical analysis revealed that two volatile compounds, often found in plants, were abundant in fresh poop but scarce in older samples: beta-caryophyllene (BCP) and beta-caryophyllene oxide (called BCPO).
Given the pandas ’preference for fresh manure, the researchers thought they might be attracted to BCP / BCPO. At the Beijing Zoo, the team featured six captive giant pandas with piles of hay full of chemicals or other substances. Pandas spent much more time researching hay covered in BCP / BCPO. A panda, named Ginny, spent six minutes covering herself with the treated hay.
Armed with these clues, the researchers then tested whether the chemical somehow affects the temperature sensation. Pandas are not exactly susceptible to laboratory experiments, so researchers applied BCP / BCPO to the small legs of laboratory mice and subjected the mice to a battery of cold tolerance tests. Compared to saline-treated mice shivering from the cold, BCP / BCPO-treated mice appeared impassive.
Sophisticated molecular biology experiments have revealed more clues. BCP / BCPO interacts with the panda version of a well-known cold sensing protein called TRPM8. Found in the skin of many mammals, TRPM8 alerts the rest of the body to cold, but is also activated by menthol, the chemical that is behind the cooling sensation of mint.
In cells exposed to the chemical in the laboratory, BCP / BCPO had the opposite effect, inhibiting TRMP8. That effectively makes the protein less able to detect cold, the researchers say.
From the camera trap to the Petri dish, evidence suggests that wild pandas stumbled upon an environmental resource that acts as a sort of cold analgesic, perhaps helping them acclimatize to winter, the researchers conclude.
“It’s a really remarkable study,” says Elena Gracheva, a Yale University neurobiologist who didn’t participate in the study. "It shows the value of exploring behaviors in the wild and looking for their molecular mechanisms."
But she says more direct evidence will be needed to truly prove that pandas roll poop to withstand the cold. Extrapolating the effects of BCP / BCPO on mice to pandas “is not a completely fair comparison,” she says. There may be other reasons to wear a layer of manure. Pandas are known to be covered in natural odors, which can ward off parasites or act as a territorial signal.
Gracheva says it’s also unclear whether horse poop prevents pandos from feeling cold temporarily or whether it makes the cold less unpleasant.
If BCP / BCPO really confers cold resistance, McShea, the Smithsonian biologist, wonders if other animals could also roll in some dung occasions. "We all like a little cold relief from time to time."