Ric O'Barry, former trainer of the dolphins used for the 'Flipper' TV show (now animal rights activist) tells a story in the documentary 'The Cove' how one of the dolphins swam up to him one day, looked him in the eye, took a last breath and then sank to the bottom of the tank and died [1]. Given that dolphins have to make a conscious choice for every breath they take gives some indication for a conscious decision for suicide.

[1] https://web.archive.org/web/20110926051048/https://blog.sfga...

Reportedly also Peter, the dolphin in the Margaret Lovatt research in the 60's https://www.theguardian.com/environment/2014/jun/08/the-dolp...

Eh. . . . yeah? I'm not sure that's a very strong criticism. We're much more similar to other mammals than we are different from them, so our understanding of human behavior provides a very strong basis for understanding other animals' behaviors.

There's a whole book about that sort of thing, by a fairly well-respected scientist in the field: https://en.wikipedia.org/wiki/The_Expression_of_the_Emotions...