Everything Exists in Nature.

Polyergus, also known as Amazon Ants, are a genus of so-called "slave-raiding" ants. Its workers are largely incapable of caring for the colony. The species subsists as a social parasite, enslaving ants of the closely related Formica genus in massive colony-to-colony raids to carry out foraging, nursing, and other colony upkeep duties. New queens will enter foreign colonies of other species, kill the existing queen, and force the colony's workers to care for her brood.
-http://en.wikipedia.org/wiki/Polyergus

The human species is free to choose what other species we most wish to emulate.