Ant species can be cruel and fierce, but they can be benevolent, too--provided there's something in it for them.

One type of ant enjoys a symbiotic relationship with milkweed plants and tiny aphids. The milkweed gives off a sap that the aphids eat; then they secrete honeydew, a delicious snack for the ants. For their part, the ants run a protection service, fighting off predators that would prey on the other two.