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"A mutualistic relationship is when two organisms of different species "work together," each benefiting from the relationship. One example of a mutualistic relationship is that of the oxpecker (a kind of bird) and the rhinoceros or zebra. Oxpeckers land on rhinos or zebras and eat ticks and other parasites that live on their skin. The oxpeckers get food and the beasts get pest control. Also, when there is danger, the oxpeckers fly upward and scream a warning, which helps the symbiont (a name for the other partner in a relationship)". In other words they both help each other out, one with cleaning that would be the bird eating the bugs off the Zebra or Rhino and the the Zebra and Rhino get to be clean.
http://www.necsi.edu/projects/evolution/co-evolution/mutualistic/co-evolution_mutualistic.html