There is actually some debate over this topic in the scientific community. Some biologists maintain that big cats such as lions and tigers cannot purr, because they have adapted special physical traits in order to be able to roar which preclude purring ability. Others believe that the big cats can and do purr, although they may not sound exactly like household cats. Study of big cats in a variety of environments appears to bear out the second theory, that they do in fact purr for many of the same reasons that smaller cats do.
The exact mechanism of purring is not completely understood. It is believed that it is accomplished with the use of the hyoid bone, a small flexible bone located in the neck. When the cat pushes air through its voicebox, it rattles the hyoid bone, creating the distinctive sound. Purring may also be caused by false vocal cords, located slightly behind the cat's actual vocal cords. The debate over how cats purr has greatly complicated the issue of whether or not big cats can do so.
Biologists who believe that big cats cannot purr generally support the hyoid bone theory. They argue that big cats have developed a slightly different hyoid bone, which is less flexible, and thus able to serve as the mechanism behind the roar. The sacrifice in flexibility, however, means that the hyoid could not be used to purr. However, other biologists believe that big cats actually can purr, although they can only do so when exhaling, rather than continuously, like a house cat or smaller cat breeds. This may be accomplished through limited vibration of the hyoid, or through the false vocal cords.
The purring noise made by big cats undoubtedly sounds different than the sound of smaller cats. It resembles a cough or a growl, which may have led to confusion about whether or not big cats actually purr. Both lions and tigers purr when they are with a group, and use the noise to communicate with kittens as well. Like a smaller cat, the purr of large cats may resonate at a frequency which promotes healing, explaining why cats make this sound when they are injured or distressed.
Many textbooks state that only small cats, members of the Felix genus, can purr. However, it appears that cats in the genus Panthera, such as lions, tigers, leopards, and jaguars, also do so. It also believed these big cats may not be able to purr while meowing, growling, or eating, unlike smaller cats.