A bear with a meat-eater's stomach
Why Do Pandas Only Eat Bamboo?
A panda has the gut of a carnivore and eats nothing but a tough grass. It solves the mismatch by eating for twelve hours a day.
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Step 1 of 5
Its gut is built for meat.
Its gene for tasting meat is still in there, and broken. A meat-eater has a short simple gut, because meat is easy to break down. A plant-eater needs a long one with fermenting chambers. A panda has the short one and eats the plant.
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Step 2 of 5
So most of the bamboo goes straight through.
A panda digests only a small fraction of what it swallows. Most of a day's bamboo comes out the other end barely changed, still recognisably leaf and stem.
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Step 3 of 5
The answer is volume and time.
A panda eats for ten to sixteen hours a day and gets through a huge weight of bamboo. Taking a small share of an enormous amount still adds up to enough.
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Step 4 of 5
It grew a thumb to hold the stem.
A wrist bone has stretched into a stubby extra digit that presses against the paw. It is not a real thumb, but it grips a bamboo stalk well enough to strip it.
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Step 5 of 5
And it moves as little as it can.
A panda saves energy everywhere else: it walks slowly, does not hibernate, and its body runs cooler than other bears. Low income means low spending.
A panda still has the gene damage that makes meat taste of nothing to it. Its ancestors ate meat; it can no longer taste why.
The short version
A panda has a meat-eater's short gut, so it survives on bamboo by eating for half the day and spending as little energy as possible.
Try it yourself
Look at how much of a salad leaf is water and fibre. Now imagine that being your only food — you would eat all day too.