Commensalism

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In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral (there is no harm or benefit). There are three other types of association: mutualism (where both organisms benefit), competition (where both organisms are harmed), and parasitism (one organism benefits and the other one is harmed).

Commensalism derives from the English word commensal, meaning "sharing of food" in human social interaction, which in turn derives from the Latin cum mensa, meaning "sharing a table". Originally, the term was used to describe the use of waste food by second animals, like the carcass eaters that follow hunting animals, but wait until they have finished their meal.

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Examples of commensal relationships

Commensalism is harder to demonstrate than parasitism and mutualism, for it is easier to show a single instance whereby the host is affected, than it is to prove or disprove that possibility. Often, a detailed investigation will show that the host indeed has become affected by the relationship.

Cattle egrets and livestock

An example of commensalism: cattle egrets foraging in fields among cattle or other livestock. As cattle, horses, and other livestock graze on the field, they cause movements that stir up various insects. As the insects are stirred up, the cattle egrets following the livestock catch and feed upon them. The egrets benefit from this relationship because the livestock have helped them find their meals, while the livestock are typically unaffected by it.

Tigers and golden jackals

In India, lone golden jackals expelled from their pack have been known to form commensal relationships with tigers. These solitary jackals, known as kol-bahl, will attach themselves to a particular tiger, trailing it at a safe distance in order to feed on the big cat's kills. A kol-bahl will even alert a tiger to a kill with a loud pheal (thereby straying into mutualism). Tigers have been known to tolerate these jackals: one report describes how a jackal confidently walked in and out between three tigers walking together a few feet away from each other.[1] Tigers will however kill jackals on occasion: the now extinct tigers of the Amu Darya region were known to eat jackals frequently.[2]

Other examples

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