There are four big cats that can roar: lion, tiger, leopard and jaguar - all of which belong to the genus Panthera.

In these species, the epihyal bone, part of the voice box, is replaced by a ligament. This can be stretched, creating a larger sound-producing passage and thus a wider range of pitch. The more the ligament extends, the lower the sound generated when air passes across the vocal cords. In addition, the cords are large, unbroken and fleshy, which produces deeper sounds.

In contrast, in the 'small' cats, the bones of the voice box form a fixed structure, with divided vocal cords that vibrate with both in and out breaths. While this design enables these cats to purr continuously (unlike their big cousins), it limits the range of other sounds and prevents them from being able to roar. Even large species, such as pumas and cheetahs, whose voice boxes are proportionately big, possess small-cat anatomy.

Interestingly, the snow leopard - also a member of Panthera - cannot roar. Its vocal cords lack an all-important layer of fatty, elasticated tissue, which, in other big cats, gives the vocalisations an uneven rumble that emerges as a roar. Some scientists therefore argue that the snow leopard deserves its own genus: uncia.