Warning: If you love giant pandas, reading this article may inhibit your desire to cuddle them, at least in winter. Although the world cannot get enough of videos of giant pandas on slides or playing in bubble baths, one panda behavior hasn't achieved the same viral status – smearing themselves in horse droppings. You might regret knowing they do this, but at least now we know why.
Giant pandas have been observed to not just roll around in horse manure, but to actively smear it on themselves. Professor Fuwen Wei of the Chinese Academy of Sciences wondered why. Was it to mask their scent from predators? Some sort of camouflage? Just pandas' innate playfulness taking them somewhere stupid, again?
In Proceedings of the National Academy of Sciences, Wei and co-authors note that mammals are seldom attracted to feces. While some insects feed on vertebrates' waste or lay their eggs there, mammals usually show aversion, perhaps to limit parasite infections. Indeed, the effect is so powerful that small mammals lose body mass when exposed to the feces of predators. Pandas' status as exceptions makes their behavior intriguing.
For 10 years, Wei's team observed wild pandas in Qinling. They describe four responses to encountering horse droppings – carefully sniffing, rubbing the manure with their cheek, rolling in the stuff, and smearing it all over their bodies. Cameras recorded 38 events in just a one year period.
Crucially, the behavior, which the authors named horse manure rolling (HMR), turned out to be highly weather dependent. HMR only happened from November to April (including, but much longer than, pandas' brief mating season). Almost all events were in temperatures below 15º C (59º F), and most happened near freezing.
Wei also observed pandas like their horse dung fresh. He identified scents in fresh horse manure that decline quickly, sprayed them on hay, and watched panda reactions. Pandas proved attracted to the related chemicals beta-caryophyllene (BCP) and caryophyllene oxide (BCPO).
Putting these facts together the authors wondered if these chemicals make pandas feel warmer. Testing on pandas can be challenging, so the team decided to see if other animals show a similar response. They gave BCP/BCPO to mice and tested their willingness to move between a warm plate and a cold one. Mice given BCP/BCPO proved much more likely than controls to venture into the cold and also huddled for warmth less.
Seeking a molecular explanation for the effects, Wei and co-authors found BCP and BCPO interact with the TRPM8 thermoregulation channel, a sort of all-body equivalent to the sensation of eating chilies.
Quite why pandas, equipped with some of the thickest fur coats around, have discovered the warming effects of horse feces and other animals haven't is a continuing mystery. Perhaps they're smarter than they seem.
There might be a market for BCP/BCPO to help people endure freezing temperatures, but personally we'd prefer a coat of (artificially produced) fur as deep and fluffy as a panda's to smearing some extract of horse dung.