A male killer whale's dorsal fin can grow to as much as 6 feet tall. Despite the fact that the dorsal fin is very straight, it is primarily made of cartilage. A wild orca often travels far, and quickly, in deep water. The water provides pressure to the fin, keeping the tissues inside healthy and straight.
In captivity, a killer whale is usually confined to a small pool, where it spends much time at the surface and often has to swim in a circle. Consequently, the tissue in its dorsal fin doesn't get much of a workout, and starts to atrophy (wastes away due to lack of use), and it falls over. The same thing can happy to the whale's tail flukes, which often flop over at the ends.
According to SeaWorld, dorsal fin collapse "isn't an indicator of the animal's health or well-being," but it's certainly not natural. Dorsal fin collapse is very rare in wild killer whales.
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