In Tanzania, we watched on many occasions when small flocks of red-billed birds descended upon the backs of giraffes, cape buffalo, elephants and hippopotamuses, prancing around like they owned them. These birds, called oxpeckers, were hunting for insects, ticks and other invertebrates that were trying to make a living on the backs of these huge mammals as well.

It seemed like a perfect example of mutualism, a form of symbiosis (“the living together in more or less intimate (and long-term) association or close union of two dissimilar organisms, as in parasitism or commensalism,” Merriam-Webster Dictionary), but most references I checked called it commensalism. We learned both terms in high school biology, but now I was confused. What was the difference? What other forms of symbiosis are t

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