Which big cat cannot roar?

The cheetah. Roaring depends on a specialised larynx and a stretchy ligament in place of a rigid bone in the throat, found in the four Panthera species - lion, tiger, leopard and jaguar. Cheetahs have the rigid version, which is the arrangement that allows purring instead, and they also chirp in a way that sounds startlingly like a bird.

The rule is neat and almost holds: the cats that roar cannot purr continuously, and the cats that purr cannot roar. Snow leopards spoil it by being a Panthera that does neither properly.

The cheetah is unusual in most other ways too. It is the only cat whose claws do not fully retract, using them as running spikes; it is the fastest land animal but overheats within about half a minute; and it has so little genetic diversity, after a population crash perhaps 10,000 years ago, that skin grafted between unrelated individuals is accepted as the animal's own. Cheetahs were tamed and kept for hunting for thousands of years, by Egyptian, Persian and Mughal courts - Akbar reportedly kept a thousand - and in all that time nobody managed to breed them in captivity.