Has a lot to do with the size and genus of the cat. Because of the supporting structure of the hyoid bone, many big cats, such as lions and tigers, can purr only when breathing out. The hyoid of smaller cats is solid bone. These smaller cats can purr when breathing both in and out. However, Bobcats, Lynx, Cougars and Cheetahs can, apparently, purr... sort of. The big cats can, though, roar, which, thankfully, Tabby cannot...