In one of nature’s more endearing displays, a panda paws at a narrow, basil-colored stick of bamboo. Propping the chute up like a flute, the panda tries to meander the bamboo into its mouth. It leans back just a little too far, awkwardly rolling onto its back as it gnaws on the chute like a toothless child. After the victorious snap of fiber, the panda props itself up toward the zoo crowd around it, flashing a hint of pride as it goes for another bite.
Pandas eat almost 80 pounds of bamboo a day, gorging themselves mostly on the stringy, wooded stalks. But every summer, the panda shifts its diet toward the leaves, signaling the start of a digestive breakdown that, according to a recent study conducted by UW-Madison researchers, births bizarre, mucous-caked fecal pellets and could ultimately lead to reproductive troubles for captive pandas.
“Pretty much once a year… [pandas in captivity] undergo this very painful ordeal where they excrete these things called mucoids,” said Garret Suen, an assistant professor of bacteriology and co-author whose lab studied fecal samples from captive pandas. “It’s very painful for the pandas, enough that they will stop eating for a time.”
According to Suen, the digestive problems are likely tied to bamboo, a cornerstone of a panda’s diet that evolutionarily seems out of sync with the panda’s digestive system. Though largely herbivorous, pandas maintain the carnivorous physiology of other bears.
“They are part of the bear family,” Suen said. “Their jaw morphology is that of a bear. Their gut morphology is that of a bear. There seems to be some movement toward maybe better digesting bamboo, but they’re not very efficient at it.”
When bamboo passes through the panda’s gastrointestinal tract, it might be only partially digested, with the panda’s gut microbes targeting only the easiest access to nutrients. That’s why, according to Suen, pandas’ feces often resembles chewed up bamboo.
“We think what happens is that it’s passing through the panda very quickly, and microbes are going after the really simple to degrade sugars,” Suen said. “But they’re not really going for the high energy sources of plant polysaccharides like cellulose.”
This inefficiency, according to the study, might lead to what it calls a dysbiosis, where jagged splinters of bamboo carve into the mucus lining of the panda’s gut and force a “resetting” or shedding of the intestine’s microbe community. The panda expulses that lining as a mucoid, a mucosal fecal pellet resembling Jello-caked shards of bamboo.
That resetting, according to the study, frequently occurs in the summer, when mucoids become common, and pandas shift their diet from a bamboo stalk to its far more agreeable leaves. During this time, pandas might stop eating outright, becoming lethargic and apathetic as they wait for their gut to finish resetting.
These digestive problems occur during the panda’s reproductive phase, when pandas traditionally go into heat or begin nurturing a pregnancy. According to Suen, this could contribute to panda breeding’s notorious difficulty. Zookeepers often struggle with disinterested pandas during the breeding season, and some captive pandas, like Tian Tian at Edenburgh’s Zoo, have historically had difficulty carrying a pregnancy to term.
While mucoids have been observed in the wild as well, they appear to be more extreme among pandas in captivity, Suen said. According to Suen, this may have something to do with stress and access to diet.
“These animals have large territorial ranges, and suddenly they’re in an enclosure inside of a zoo. That has to be challenging for that type of an animal,” Suen said.
According to Suen, wild pandas’ wide ranges and freedom of movement allow them some greater choice between different kinds of bamboo, as bamboo’s qualities may change over a growing season.
Zoos do monitor panda preferences in bamboo, though. At the Memphis Zoo, whose pandas Ya Ya and Le Le served as subjects for Suen’s lab study, their pandas’ bamboo preferences are closely monitored. According to Courtney Janney, the Curator of Large Mammals at the Memphis Zoo, pandas are fed 12 times a day according to those preferences.
“If they're eating [one species] really well, we continue to ask for that to be cut for them while still making sure to offer at least two or three other options per day,” Janney said in an email. “So we're able to readily identify trends if their tastes change.”
According to Suen, the logic behind a panda’s choice in bamboo isn’t exactly clear.
“It could simply be that one is tastier than the other,” Suen said. “It could also be related to the fact that these animals have to go in and rip it apart, and maybe different types of varieties of bamboo are more difficult to rip apart. Over the growing season, the bamboo itself changes.”
The leading theory is that a panda’s choice relates to nutritional value, Suen added. Memphis Zoo’s pandas’ preferences can be as dictated by species as they are the fields they’re grown in, according to Janney.
Suen’s study was conducted by researchers from both UW-Madison and Mississippi State University, as well as the Memphis Zoological Society, and was funded by the United States Forest Service, the United States Department of Agriculture, the Memphis Zoological Society, the Mississippi Agricultural and Forestry Experiment Station, and the Leo Seal Family Foundation.