It has long been a mystery how giant pandas, which have a gut ideal for digesting meat, can survive eating almost exclusively bamboo. Now our research has found that they can cope with this low-quality diet because they have an extremely slow metabolic rate. This may also explain why they are so inactive and have comparatively small organs for their body size.
Hoops and hurdles
There are few animals on the planet as iconic as the giant panda. Its role as the logo for the World Wide Fund for Nature, the perilous nature of its existence in the wild and the fact that it has been exported worldwide as a symbol of Chinese political friendship for decades, continue to sustain its status.
Despite this phenomenal popularity, its political importance and threatened conservation status make it extremely difficult to do research on pandas. There are many, and justified, hurdles to jump to do scientific work on animals in general, but they are even higher when it comes to pandas.
For this reason, there are many fundamental measurements that have been made on other species but that are lacking for pandas. However, we are slowly starting to fill in these gaps.
Gut of a lion…
One bit of biology that many of us are familiar with is that the panda is a carnivore that became a vegetarian. The mammalian order Carnivora includes several families of animals including the canids (wolves, dogs and foxes), felids (cats), mustelids (weasels and so on), pinnipeds (seals, walruses and sea-lions) and ursids (bears). All these groups apart from the bears subsist by killing and eating other animals.
Because meat is easily digested these animals have short digestive tracts. Apart from the polar bear, many bears include various amounts of vegetable material into their diets. The panda has taken this to the extreme: eating almost exclusively bamboo.
Although pandas have many adaptations for eating bamboo (like an extra “thumb” to help hold the shoots) these do not include a long digestive tract. The panda also has the guts of a lion: ideal for digesting meat, but very inefficient for digesting bamboo. So they have to eat lots of it, perhaps as much as 22 to 44 pounds (10 to 20 kilograms) per day. Scientists have long speculated that to survive on such a low-quality food pandas must have a low rate of metabolism.
However, until now nobody had managed to measure exactly how much energy they use. To do this, we used a technique called the doubly-labelled water method, which measures the rate at which animals eliminate stable isotopes from their bodies. We did this for five captive pandas at Beijing zoo and three wild pandas living in the Foping nature reserve.
We found that the pandas’ metabolic rate is exceptionally low. Corrected for their body weight of about 198 pounds (92 kilograms), it is substantially lower than almost all other mammals. In fact, the rate is closer to what would be predicted for a 90kg reptile.
…mobility of a sloth?
How they achieve such low rates of energy use was the focus of the second part of our paper, published in the journal Science. Much of the energy that our bodies use is burned up in relatively few organs, including the brains, kidneys, heart and liver. Using historical autopsy data we found that pandas have small organs for their body size.
Their brains are only 82% of the expected size, their kidneys only 74.5% and their livers a remarkable 62.8% of the expected size for a 200 pound (90 kilogram) mammal. Plus if you ever went to see a panda in a zoo you will know that they are not the most active of animals. Indeed, using GPS loggers we found that in the wild pandas move on average at just 88.3 feet (26.9 meters) per hour.
A key physiological system involved in regulating our metabolism is the thyroid hormone system. We suspected that pandas might have something unusual going on with their thyroid hormones—a hunch that turned out to be correct.
Pandas have very low levels of the main thyroid hormones T4 and T3. We were able to trace these low hormone levels to a unique mutation in the panda genome, which affects a critical gene involved in thyroid hormone synthesis. People who have low thyroid hormone levels often complain that they feel cold. This is potentially because their lowered metabolic rate is insufficient to keep them warm.
So how does the panda manage to keep warm? Despite living in semitropical habitats, it does have a really thick fur coat. This serves to trap what little heat their metabolism produces inside their bodies. A direct consequence of this is that their surface temperature (measured using a thermal imaging camera) is about 50 degrees Fahrenheit (10 degrees Celsius) cooler than the surface of other black and white animals like the zebra. Pandas, it seems, are literally cool.
Incidentally, I also had a cool experience while carrying out this research. One day at the zoo I got distracted while feeding one of the pandas, and he reached through the bars and took a swipe at me to get the bamboo. However, he missed and ended up taking a small chunk out of my rather expensive leather jacket instead. This may seem annoying, but I chose to view it like a badge of honor. If someone asks how I damaged my coat I could in all honesty say that I got it in a “close encounter with a giant panda.”