The elastic hyoid, combined with the fibroelastic tissue on top of the big cats' undivided vocal folds, acts like a slide trombone, enabling the big cats to roar. Only snow leopards, which lack the specialized vocal folds, are not able to roar. Because of the elastic hyoid, big cats can purr only when breathing out. The hyoid of smaller cats is solid bone. These cats can purr when breathing both in and out, but they can't roar.http://answers.yahoo.com/question/index ... 249AA3t4El
However, 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 purr 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 purring 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 purr 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 purr.
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 purr 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.http://www.wisegeek.com/do-lions-and-tigers-purr.htm