And the answer: snow leopards.
In addition to cheetahs and pumas, snow leopards can mew, growl, and purr— but they can’t roar. For cats of all sizes, roaring and purring are mutually exclusive. Lions, tigers, and jaguars can all roar, but are unable to purr.
The ability to purr versus roar is, surprisingly, quite anatomical. Purring is made possible by tightly connected, delicate bones that run from a cat’s tongue to the base of its skull. By vibrating its larynx, or voice box, these small bones resonate throughout the cat’s head and neck— thus, the purr. Evolutionarily, scientists still aren’t sure just how this function came to exist in the first place, but one such idea suggests that perhaps the purring of a mother cat could have protected her kittens by masking the sounds of their meows. Interestingly, the purr is a constant action done by the cat both when exhaling and inhaling (surprising that no one’s tried to teach a cat to play the didgeridoo yet…).
Meanwhile, the ability to roar requires a whole other set of unique attributes. Alongside the delicate array of bones that run from the skull to the tongue, larger cats also have a length of tough cartilage. This cartilage prevents purring, but provides the larynx with enough flexibility to emit a loud, intimidating roar. These roars can be so powerful that they can be heard up to five miles away!
Although many big cats can not purr, they can do something nearly equivalent: chuff. As it sounds, this noise is done by blowing air quickly out through the nostrils in a sort of breathy snort. This non-aggressive signal is often shared between a mother cat and her cubs, as it is always used to communicate and strengthen social bonds.
Learn more about vocalizations both big and small here.