Quote from tomdavis:
Technically, they're not sedated (e.g., with valium), but they're often given anti-anxiety or anti-depression medications. It's a minor but important distinction. Sedating the tiger would make it completely inactive and uninteresting to the viewing public.
Tigers in the wild have a territory that can be as large as 5 to 30 square miles. They have a deep instinct to acquire, patrol and defend that territory. Even in a large nice-looking zoo exhibit, tigers often feel confined and get anxious. Normal cat behaviors become accentuated as their need to roam is suppressed. For example, "excessive grooming" is a common problem. Normal cats groom themselves to stay clean. An anxious or depressed cat will groom itself excessively to the point that it licks all the hair off large spots on its body (making it unpresentable to the public). Sometimes large cats will pace incessantly in a small section of what appear to be a large exhibit. The animal feels penned in and engages in neurotic behavior such a constant pacing or pawing and digging. Or sometimes they become depressed and just sit around and sulk. These problems are often treated with anti-anxiety or anti-depressant medications.
Big cats are not happy when they're confined, even in what looks like a nice environment to you and me.