What do pandas mostly eat?

Bamboo, for ten to sixteen hours a day, up to 40 kilograms of it - and they are bears, with a bear's short simple gut and no equipment for digesting cellulose. Perhaps a sixth of what they eat is extracted. Everything else about panda life is a consequence of that thin margin.

The genome still reads as a carnivore's. The genes for digesting meat are intact, and one of the umami taste receptor genes has gone dead, which fits an animal that stopped caring whether things tasted of meat about two million years ago. The thumb is not a thumb but an enlarged wrist bone that evolved separately into a pad for gripping stems.

Occasionally they eat meat - carrion, small animals, the odd bird - and there is a long-running argument about the bamboo diet being a specialisation rather than a trap. What is clear is that it leaves no energy for anything else: pandas cannot afford to roam widely, defend large territories or be particularly active, and the reputation for reluctant breeding turns out to be mostly a captivity artefact and a very narrow annual fertile window that zoos took decades to identify.