It was a strange sight: In the winter of 2007, scientists in China spotted a wild giant panda romping about in horse manure, diligently smearing itself with excrement until its fur became a poo-muddled mess. It wasn’t the last time the researchers would spot this strange behavior.
But figuring out why pandas do this would take the team 12 years and a scientific trek through the fields of animal behavior, chemical ecology and neurophysiology. But now, researchers think they have an answer.
Pandas may roll in poop, oddly enough, to feel warm. Researchers identified a chemical present in horse droppings that confers cold resistance to laboratory mice and could inhibit a cold-sensing protein present in giant pandas (Ailuropoda melanoleuca), they report December 7 in Proceedings of the National Academy of Sciences.
“I’m a panda expert, and this is one of the strangest panda papers I’ve ever 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, but these researchers deserve a lot of credit.”
Students of Fuwen Wei, an ecologist at the Chinese Academy of Sciences in Beijing, first glimpsed the bizarre behavior deep in the Qinling mountains of central China. The region is crisscrossed by ancient trade routes well-trod by horses, so the researchers say horse manure may have been common.
Rolling around in poop isn’t unheard of among animals — consider the dog. But many mammals actively avoid the fecal matter of other individuals and species, as poop can harbor pathogens and parasites, says Cécile Sarabian, a cognitive ecologist at Kyoto University in Japan who wasn’t involved in the study.
“Behavior is a story of compromises,” she says. “In this case, the benefits of getting in contact with fresh horse manure may override the [potential] risks.”
To understand what those benefits might be, the researchers first had to catch more manure maneuvers. They set up a series of motion-sensitive cameras along a road in the Foping National Nature Reserve. The cameras captured 38 panda-poo interactions from June 2016 to June 2017, suggesting that the initial observation wasn’t just a freak incident. The camera setup also recorded the time and air temperature for each behavior, revealing a clear pattern: Giant pandas rolled in poop only in colder weather. The majority of observations were captured when temperatures were between –5° Celsius and 5° C.
Pandas were picky about poo, too. It had to be fresh; manure more than a few days old 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 pandas’ preference for fresh manure, the researchers figured they might be attracted to BCP/BCPO. At the Beijing Zoo, the team presented six captive giant pandas with piles of hay suffused with the chemicals, or with other substances. The pandas spent significantly more time investigating hay covered in BCP/BCPO. One panda, named Ginny, spent six minutes covering herself with the treated hay.
Armed with these clues, the researchers next tested whether the chemical somehow affects temperature sensation. Pandas aren’t exactly amenable to laboratory experiments, so the researchers applied BCP/BCPO to the tiny paws of lab mice and subjected the mice to a battery of cold tolerance tests. Compared with saline-treated mice that shivered in the cold, mice treated with BCP/BCPO seemed unfazed.
Sophisticated molecular biology experiments revealed more clues. BCP/BCPO interacts with the pandas’ 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 also gets activated by menthol, the chemical behind peppermint’s cooling sensation.
In cells exposed to the chemical in the lab, BCP/BCPO had the opposite effect, inhibiting TRMP8. That effectively makes the protein less able to detect cold, the researchers say.
From camera trap to petri dish, the evidence suggests wild pandas have stumbled onto an environmental resource that acts as a sort of analgesic against the cold, perhaps helping them acclimate to winter, the researchers conclude.
“It’s a really remarkable study,” says Elena Gracheva, a neurobiologist at Yale University who wasn’t involved in the study. “It shows the value in exploring behaviors out in the wild and looking for their molecular mechanisms.”
But she says it will take more direct evidence to truly demonstrate that pandas roll in poo to resist the cold. Extrapolating the effects of BCP/BCPO on mice to pandas “isn’t a totally fair comparison,” she says. There may be other reasons for donning a coat of dung. Pandas are known to cover themselves in natural scents, which may ward off parasites or act as a territorial signal.
Also unclear is whether horse poop actually prevents pandas from feeling cold temporarily, or if it just makes feeling cold less unpleasant, Gracheva says.
If BCP/BCPO really does confer cold resistance, McShea, the Smithsonian biologist, wonders if other animals might also roll around in the occasional pile of dung. “Everyone likes some relief from the cold every once and awhile.”