Sumatran tiger, Toronto Zoo. She's not growling; this is the "flehmen" response, which can be seen in all cat species when they are examining a pheromone or other interesting scent. The curled lip allows air (and smells) to be drawn over a special organ called the Jacobson's organ for analysis.
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They're worn down from age. The two adult Sumatrans that the Toronto Zoo have are quite old for tigers! Which is mostly good, but I guess it sucks when your important body parts start wearing out a little.