We don't fully understand the complexities of aggression. It's partly the product of genetics and the evolutionary process of natural selection. It's also influenced by the environment. Some forms of aggression are fairly simple to understand -- they involve either animals preying upon other creatures for food or the prey attempting to defend itself from potential predators. In these cases, aggression is a matter of survival for predator and prey alike.

Then there are examples of intra-specific aggression. These are acts that a species commits against other members of the same species. They range from intimidating displays to organized raiding parties searching for victims. In most cases, these acts of aggression determine which animal has access to certain resources, including food, territory and mates.

So which species is the most violent? It depends upon how you frame the question. If you mean which species has the most ferocious attack, you may look at such creatures as the Tasmanian devil or the piranha. The most aggressive animals could include the hippopotamus, which kills more people in Africa than any other animal apart from disease-carrying insects [source: Hammond].

There are other animals with violent reputations. Polar bears and grizzly bears are known for their aggressive behavior and powerful attacks. Crocodiles are dangerous predators capable of killing a human with a quick strike and a violent death roll. Sharks have a bad rap for being dangerous, and although shark attacks are rare, they're certainly ferocious. Killer whales can seem to be both violent and capricious at the same time, occasionally tossing prey like sea lions into the air, battering them before finishing them off. Cats play with prey to tire it out before delivering a killing bite and to teach their young how to hunt.

But if we go by sheer killing power and ingenuity, none of these animals have anything on us. Humans have spent countless resources and devoted centuries to discovering ways to inflict violence upon others. Humans are also capable of considering their own actions. Konrad Lorenz wrote in "On Aggression" that human violence is largely an irrational practice committed by beings who can reason. He suggested that we've inherited our violent nature from our ancestors, and that since we're emotional as well as capable of reasoning, we sometimes commit unreasonable actions.

Whatever the reason, no other species has devoted as much to finding ways to end life. From ancient weapons of war, to methods of execution such as the guillotine, all the way up to nuclear weapons, humans are masters of violence. We can launch into violence in a rage or, perhaps more frighteningly, we can engage in it with a cold and calculating approach.

Learn more about aggression and violence by following the links on the next page.