Usually, when two animals fight to the death spontaneously without human interference, it’s because one is the predator and the other the prey. The former has all his own way, and once it comes to an actual fight the latter tends to be - well, lunch. A lion fighting a gazelle or whatever it is those crazy lions munch on these days never, or at least statistically insignificantly often, loses if it does get its claws and teeth into the gazelle.
What are the exceptions to this? I know that sperm whales fight giant squids and that those fights can go both ways, although I believe the whale is at a statistical advantage. While reading the Wikipedia article on swordfish I learned that they are sometimes attacked by shortfin mako sharks, and sometimes win.
So, what we’re talking about there is:
[ul][li]Two animals[/li][li]of different species[/li][li]who fight[/li][li]fairly frequently[/li][li]in their natural habitats[/li][li]without human intervention[/li][li]with the intention of seriously harming or killing each other (no asserting dominance rituals and similar)[/li]where the outcome varies from occasion to occasion[/ul]Any examples?