Scientists believe they have unraveled the mystery of why giant pandas seem to have an irresistible urge to roll in horse manure.
Researchers at the Chinese Academy of Sciences, working with staff at the Beijing Zoo, believe the manure “bath” helps the pandas tolerate cold conditions.
The study team, writing in the Proceedings of the National Academy of Sciences, have described their 13-year efforts to explain the unusual behaviour.
Their interest was first piqued in 2007, when they witnessed a panda in the wild rolling enthusiastically in fresh horse dung.
Later, using motion-capture technology for a year in a nature reserve inhabited by wild-living pandas, they recorded the behaviour 38 times.
The rolling pandas seemed intent on covering their entire bodies with the fresh manure.
Wenliang Zhou and his colleagues noted in their report that the pandas were drawn to the manure only in the coldest months, in temperatures below 15º Celsius. Indeed, most of the observed rolling behaviour was at even colder temperatures, between –5° C and 5° C.
That suggested the pandas were using the manure to somehow ward off the cold, but how did it work?
They turned their attention to the manure itself.
The study team isolated two strong-smelling chemicals from the dung, beta-caryophyllene and caryophyllene oxide, the odors of which tend to dissipate rapidly. Both of these chemicals were found to be abundant in fresh manure, but scarce in older samples.
Beta-caryophyllene is most familiar as a major constituent of cloves and adds to the spiciness of black pepper, while caryophyllene oxide is found in lemon balm and eucalyptus.
Using zoo pandas, they applied the chemicals to hay and found the six animals treated it like horse manure, rolling enthusiastically in the dried plant material. One panda spent six minutes covering themselves in the hay.
An experiment involving mice followed, with the study team finding the mice were more tolerant of the cold when exposed to the two chemicals.
Zhou and his colleagues believe the chemicals likely provide a sense of warmth on the skin, giving the pandas (and mice) a feeling of warmth, not unlike a heat rub that athletes might apply.
It was likely that the manure chemicals inhibited a cold-sensing protein present in panda skin, the researchers said, effectively serving as an analgesic against cold.
The full study team comprised Zhou, Shilong Yang, Bowen Li, Yonggang Nie, Anna Luo, Guangping Huang, Ren Lai and Fuwen Wei, with the Chinese Academy of Sciences; and Xuefeng Liu, with the Beijing Key Laboratory of Captive Wildlife Technologies at Beijing Zoo.
Why wild giant pandas frequently roll in horse manure
Wenliang Zhou, Shilong Yang, Bowen Li, Yonggang Nie, Anna Luo, Guangping Huang, Xuefeng Liu, Ren Lai, and Fuwen Wei.
Proceedings of the National Academy of Sciences, December 7, 2020; https://doi.org/10.1073/pnas.2004640117