As far as I know, domestic cats have a flap of skin in their throats that resonates when it vibrates causing the purr. This is lacking in the big cats which is why the sound they make is more a cough than a proper purr. Someone has to have done a web page on it! I do know that cheetahs & siamese cats are the only cats that cant retract their claws. Personally, as far as exotic pets go, I kinda prefer the hyena. I also desperately want a Lappet-Faced vulture... ugly little suckers!