By "natural", I mean according to the laws of physics, and all higher-order laws or principles which arise from them (basically, everything else). So, yes, narrow, but no, not. What we call "nature" is largely a fiction, a story that we impose on the universe (the "natural world") to help us navigate our place in it and understand it in abstract ways. Humans occupy a strange place, in that we are manifestations of "natural laws" (physics, chemistry, biology, psychology), but at the same time, able to understand and rationalize about our own existence and place in that world in ways that most (all?) other animals are not. I think suicide (vs. self-sacrifice, which is an entirely different thing) is a very human (or human-like) phenomenon, and can only arise from a high-level consciousness which can rationally/emotionally think in terms of "Is living my life worth the cost I am paying (in emotional or physical terms)?". No "less conscious" living thing would ever make that kind of calculation, because they will always follow their instincts to survive as a default (again, differentiating between sacrificing oneself for a goal vs. ending ones' own suffering on purpose). Not saying that it's impossible that "nature" has enabled such a mechanism in a species (though it would violate the "selfish gene" principle), but I don't see it.