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Unlike some of their ursine cousins, panda bears do not hibernate and must brave colder winter temperatures.

Fuwen Wei

Scientists finally think they know why these pandas like to roll in horse poop

The panda bear may one of the world’s cutest animals, but it also has one of the world’s grossest habits: Giant pandas in central China’s Foping National Nature Reserve like to rub horse manure on their necks and faces and roll around in it to cover their entire bodies. Now, researchers say they have an explanation for these dung baths. The horse poop contains compounds that might help the animals deal with colder temperatures.

Droppings are something like an identification card for animals. Creatures sniff the feces of their own kind to pick up clues to sex and mating status, and the scat of other species can tell them when a predator is nearby. But none of this explained why wild pandas (Ailuropoda melanoleuca) at the reserve were attracted to horse poop. Although horses sometimes pass through the reserve, towing supplies to local farmers, the solitary panda does not interact with these hoofed animals in the wild.

To get to the bottom of things, researchers analyzed 38 instances of dung rolling, captured on infrared cameras at the reserve from June 2016 to July 2017. The bears tended to roll in horse poop less than 10 days old. These feces contained natural compounds, called beta-caryophyllene (BCP) and beta-caryophyllene oxide (BCPO), that are scarce in older dung, say the scientists, led by researchers at the Chinese Academy of Sciences’s Institute of Zoology.

The team then added this compound to the hay of pandas at the Beijing Zoo, and found the animals favored the hay treated with these compounds, sometimes rubbing it all over their bodies. What’s more, the pandas tended to roll in horse poop in colder weather, at temperatures between 5°C to 15°C. Could BCP/BCPO help keep the giant pandas warm?

As giant pandas are a national treasure for China, there are strict limitations on conducting research on these protected animals, so the team turned to mice. Covering mice in a diluted BCP-BCPO solution boosted the animals’ cold tolerance, the authors report today in the Proceedings of the National Academy of Sciences. Treated mice more readily walked on colder surfaces and did not huddle together in below freezing temperatures, unlike mice given a saline solution. In cells, the researchers discovered BCP-BCPO blocks receptors that sense cold in pandas.

Although it’s not concrete proof, the authors “provided solid evidence from the molecular level to elegantly explain the unique behavior,” Fan Yang, a biophysicist at the Zhejiang University School of Medicine, wrote in an email. The same thermo-regulating receptors are present in many animals, including elephants, penguins, and humans, he notes. So it is possible that using natural compounds to regulate body temperature “may actually be a general strategy widely adopted in other animals.” For example, a compound in chili peppers, called capsaicin, activates a receptor that makes humans feel warmer, which is why we sometimes sweat while eating spicy food.

Staying warm in the winter can be challenging for pandas, notes Zejun Zhang, an ecologist at China West Normal University, because they don’t hibernate and their low-calorie diet of bamboo makes it hard to store extra fat for insulation. It’s possible, the authors say, that pandas have used horse poop in this way for thousands of years, as ancient trade routes crossed through this mountainous area.

But Malcolm Kennedy, a professor of natural history the University of Glasgow, is not convinced. Pandas will smear on their bodies “anything they find unusual or interesting,” he says, and these bears could just be attracted to the strong smell of horse poop. And although blocking thermosensing receptors would theoretically help animals from feeling cold, their bodies would still need to use more energy to function at lower temperatures. If limiting the animal’s cold sensitivity stopped them from seeking shelter, he says, that could be “potentially suicidal.”

*Update, 8 December, 3:45 p.m.: This story has been updated to include a critique of the study from Malcolm Kennedy.