How much bamboo must a giant panda eat each day?

Up to 40 kilograms, for ten to sixteen hours a day. It has to, because it is running a carnivore's digestive system on a diet of grass. Pandas have the short, simple gut of a bear, no rumen, and none of the specialised anatomy that lets a cow or a deer extract much from cellulose - so perhaps a sixth of what goes in is digested.

They are bears that switched food roughly two million years ago and kept the old equipment. The genome still carries the genes for meat digestion, and one of the genes for the umami taste receptor has become inactive, which fits an animal that stopped noticing whether things tasted of meat. The famous thumb is not a thumb but an enlarged wrist bone, evolved separately as a pad to grip stems.

The strategy works only because bamboo is abundant, indigestible to almost everyone else, and available all year. It leaves no margin: a panda cannot afford to travel far, defend a large territory, or do much of anything energetically. The reputation for being disinclined to breed is largely an artefact of captivity - and of a very narrow annual fertile window, which zoos spent decades failing to identify.