system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't the dorsal fins of captive killer whales stay up?

And why do the fins of female killer whales and dolphins stay up? It seems to only happen to male killer whales (with the huge fin).
gpt
There are a few theories.

1. In their natural habitat the pressure is so great that the collagen in their fins solidifies. In captivity they are in shallower water so the collagen can't solidify.

2. It's also hypothesised that the lack of movement, and more importantly, turning prevents the fin getting enough pressure. It has also been linked to dietary changes and changes in blood pressure.

All in all, it appears to be due to the collagen in the fins not being able to solidify, though the exact causes of that are unclear.