Many animals can be horrifically violent for no reason, it seems, other that the sheer desire to kill and maybe even to torture. It took many, many years for people to finally realize that dolphins, for instance, aren’t benign, perpetually smiling sea creatures they look like to us. Instead, dolphins are big-brained animals who commit gang rape, brutal killing of dolphin "children", and the mass murder of porpoises. [...] With dolphins, researches have pretty much reached the conclusion that much of the killing they do serves no evolutionary purpose. Dolphins will slaughter hundreds of porpoises at a time. The only imaginable evolutionary reason for this would be if porpoises compete with dolphins for the same scarce resources, like food. But they don’t. Porpoises eat different food than dolphins do. Killing a porpoise doesn’t increase a dolphin’s chances of surviving and reproducing in any way. The only conclusion is that dolphins kill porpoises because they want to.

I don’t know why animal violence happens, but when I read through the research literature I’m stuck by the fact that animals with the most complex brains are also the ones who engage in some of the nastiest behavior. I suspect people and animals probably pay a price for having a complex brain. For one thing, in a complex brain there may be more opprtunities for wiring mistakes that will lead to vicious behavior. Another possibility is that since a more complex brain provides greater flexibility of behaviors, animals with complex brains become free to develop new behaviors that will be good, bad, or in between. Human beings are capable of great love and sacrifice, but they are also capable of profound cruelty. Maybe animals are, too.