Millions of us have pet cats. Have you ever wondered why domestic felines purr, but cannot roar like big cats, and why big cats cannot purr? Only four species of big cats (lions, tigers, jaguars and leopards) have a special bone in their throat called a hyoid that allows them to roar but avoids purring. This means that domestic cats can never roar: and it doesn’t matter how much they try, or how big and scary they are.