Pandas are known for their distinctive black-and-white fur, but when the weather turns cold, pandas in China’s Foping National Nature Reserve like to change it up. The bears find fresh piles of horse manure and get to work rolling, rubbing and smearing it until their fur is stained brown.
Fuwen Wei, a biologist at the Chinese Academy of Sciences, first watched a panda partake in this poopy practice in 2007, Jonathan Lambert reports for Science News. With a team of researchers, Wei began a research project that would last over a decade to answer these questions: How often do pandas cover themselves in horse manure? And why do they do it?
Camera traps set up around the nature reserve captured dozens of instances of the behavior, and temperature measurements recorded alongside the photographs showed that pandas tend to slather up when the temperature is around freezing. Additional experiments with captive pandas, mice and lab-grown cells suggest that pandas might like the way that chemicals in horse poop can take the edge off of cold air. The results were published on December 7 in the Proceedings of the National Academy of Sciences.
“I never would’ve thought of that in a million years,” says Smithsonian Conservation Biology Institute wildlife ecologist Bill McShea, who wasn’t involved in the study, to the New York Times’ Katherine J. Wu. “I would look at that behavior and say, ‘Wow, that’s weird.’ They took it to the next level.”
Between June 2016 and June 2017, camera traps recorded 38 instances of pandas covering themselves in horse manure. And the iconic bears were incredibly thorough, first rolling around on the ground in the fresh feces, then using their paws to rub it into every patch of fur to make sure everything is entirely covered. Over years of tracking, the researchers noticed that the bears tended to do this when the temperature outside was brisk, between 23- and 59-degrees Fahrenheit.
The behavior is “definitely frequent and typical,” Wei tells the New York Times in an email.
The bears also only rolled in fresh manure—at most a week and a half old. Fresh manure is full of chemicals that degrade as it gets older, so the researchers began investigating whether there was a connection between those chemicals and an uncomfortably cold environment.
Pandas handle the cold differently than other bears. They don’t hibernate, since their low-calorie diet of bamboo makes it hard to build up fat stores, ecologist Zejun Zhang of China West Normal University tells Science magazine’s Lucy Hicks.
The researchers focused on a set of chemicals called sesquiterpenes, that are usually found in plants but also present in fresh horse manure. For their first experiment, the team offered hay covered in sesquiterpenes, among other options, to bears at the Beijing Zoo. The bears preferred the sesquiterpene hay—one panda named Ginny spent six minutes luxuriating in the treated hay, per Science News.
Unable to bring pandas into the lab for practical and legal reasons, the researchers then transitioned to working with mice. The team put sesquiterpenes on the fur and paws of one group of mice, and exposed them to the cold, then compared their reactions to a set of mice that didn’t get the same treatment. The sesquiterpene-treated mice weren’t bothered by the cold, while their counterparts shivered and huddled together.
Finally, the researchers investigated the effect of horse manure’s sesquiterpenes on the molecular level. Cells have one switch that can sense when it interacts with something cold. The switch is activated by menthol, the cooling chemical in peppermint. But sesquiterpenes can turn off that switch and dull the sensation of cold.
The study authors conclude that pandas might cover their fur in fresh horse manure to numb themselves to the cold. Pandas in the region might have even been familiar with horses for about 1,000 years because of trade routes that criss-cross the forests.
“I’m a panda expert, and this is one of the strangest panda papers I’ve ever read,” says McShea to Science News’ Lambert. “There’s still a lot of work to be done, but these researchers deserve a lot of credit.”
The research gives one explanation for why pandas would expose themselves to poop, which many animals treat as a dangerous thing. Avoiding feces is an “evolved strategy to avoid parasites and infections,” says Anglia Ruskin University behavioral ecologist Claudia Wascher, who wasn’t involved in the study, to the Guardian’s Natalie Grover. “This reminds me a bit about self-medication in some species: for example, primates are known to self-medicate, so eat specific types of plant when they feel sick.”
There are other possible explanations for the pandas’ behavior, experts say. The bears could avoid the cold by hiding in caves, out of the wind. And the manure may just have the benefit of masking the pandas’ scent. Still, the way the study crosses from animal behavior to molecular biology is unique.
“It’s a really remarkable study,” says Yale University neurobiologist Elena Gracheva, who was not involved in the study, to Science News. “It shows the value in exploring behaviors out in the wild and looking for their molecular mechanisms.”