Do pandas get high

Why Do Pandas Eat Bamboo?

It’s often raised as a criticism against our cuddly friend – “Why would a huge bear end up relying so heavily on bamboo as its main food source?” And they’re not kidding either, the giant panda gets over 99% of its food in the wild from the bamboo plant, only occasionally diverting to eat small rodents, fish or eggs when the opportunity presents itself.

Even in captivity where the pandas have access to other foods such as rice, apples or other vegetables they still choose to consume bamboo for up to 75% of their diet.

Part of the giant panda’s charm is undoubtedly its uniqueness in the animal kingdom and a part of that unique character is its complete dependency on one of the densest and most demanding foods imaginable – bamboo.

First of it’s important to put into context just how much bamboo pandas actually consume. You might think you really enjoy your food, but trust us when we say, everyone’s appetite looks tame compared to the pandas love of bamboo.

I’m sorry, how much?

In the wild pandas will consume between 10 and 15 KILOS of bamboo A DAY. Within a fortnight the average panda will have eaten well over it’s own bodyweight in bamboo.

Pandas have to munch this obscene quantity for two reasons:

1. Bamboo isn’t all that nutritious. Although the leaves are much easier to eat, the tough and chewy stalk is a lot of hard work for not all that many calories.
2. Giant pandas have a carnivore’s digestive system with only a simple stomach and short small intestine, unlike some herbivores that rock multi-chambered stomachs (Cows, our black and white brethren, we’re looking at you). This means that pandas do not digest dense plant matter very efficiently.

But why would they do that?

I hear you screaming already – this just doesn’t make any sense, why are pandas eating this stuff if there’s so little gain for them? The simple answer is that bamboo is everywhere in their natural habitats, it is an incredibly plentiful food resource that regrows quickly and that few other animals use.

Some scientists have theorised after looking at pandas DNA that the adaptation occurred approximately 4 millions years ago. At a time when many of the earth’s animals were going extinct, it’s suggested that the panda’s ancestors began to rely on bamboo as one of the few remaining food sources. Even as the animal population rebounded, the panda’s ancestors kept to their new niche diet and the rest is history!

I’d get so bored, how do they keep it up?

The giant panda has adapted in many weird and wonderful ways to it’s diet and all of these changes are things that distinguish this animal as a truly unique specimen.

Firstly pandas have extremely strong jaws, thanks to a denser skull than other bears that supports bigger muscles surrounding the mouth. Panda’s teeth are large and flat, ideal for grinding up and breaking down dense bamboo. This jaw and teeth combo means food can be broken down mechanically before digestion, alleviating the stress placed on the panda’s carnivorous digestive tract.

Secondly pandas have a unique elongated wrist bone that acts as a sort of thumb, allowing them to grip bamboo stalks with added dexterity. This means it’s easier for the panda to strip a bamboo stalk of its leaves quickly  and efficiently! Pandas will always favour eating the leaves where possible and will only result to the denser, less nutrient rich stalk when necessary.

Finally some scientists have suggested that bamboo might produce a reward response in the giant panda’s brain. Similar to the happy feeling humans experience when we eat sugary and fatty foods, pandas also experience a dopamine bump when they eat bamboo. So there you have it, the best news ever, not only do pandas look adorable when they’re sat munching away, they’re also having the time of their lives.

But it has it’s drawbacks…

Even though eating bamboo as your main food source does make you unique, it comes with it’s fair share of drawbacks.

You may be aware that unlike other bears the giant panda doesn’t hibernate and the main reason is that bamboo simply doesn’t provide them with enough calories to fuel the big sleep.

A bamboo diet also means that pandas are not able to producer as high a quality of milk as other bears and so typically spend much longer rearing their young. Pandas only spend 2 to 3 months pregnant compares to most bears 6 months and panda babies weigh just a third of other bear cubs, in a 90 to 130 grams, compares with 300 to 400 grams for other bear species.

Finally and most crucially, the panda’s reliance on bamboo means there are only very specific places in the wild they can survive and thrive and not only that but each panda needs a large area to feed from. These factors contribute massively to the threat of extinction facing our beloved bear. As human settlements encroach further upon the panda’s natural territory, the amount of land that can sustain pandas (and numerous other animals within a complex ecosystem, is in decline.

Bamboo is what makes pandas so dopey

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.

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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.

This panda prefers the vegetarian option.Image: AP Photo / Xinhua, Yang Sanjun

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.

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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.

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…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.

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I’m cool and I know it.Image: AP Photo/Evan Vucci

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.”

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