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The term ‘commensalism’ derives from the Latin commensalis and means ‘at the same table together’. Most definitions state that one species benefits from the association and the other is unharmed (0+). Including the concept of ‘harm’ within any definition is seldom a good idea because harm is difficult to measure and varies with the circumstances. Similarly, a ‘benefit’ may not be immediately apparent, and some associations commonly cited as commensal might involve a degree of benefit to both parties (++) albeit they may not benefit to the same extent. A commensal association may be ‘facultative’, in which both species can live independently of one another or ‘obligatory’, in which one of the associates must live in association with its partner. For example, in many warmer parts of the world, the cattle egret (Bubulcus ibis) perches on the back of cattle and big game from which it swoops down periodically to capture lizards and insects that are disturbed as its ride moves through the undergrowth. The egret is perfectly capable of living apart from cattle, but it benefits from its mobile vantage point‐cum‐beater. The egrets probably do not remove many ectoparasites from the cattle and they get their Arabic name Abu Qerdan ‘father of ticks’ from the abundance of ticks associated with their nesting colonies. The cattle, therefore, appear to gain little from the relationship although the egret acts as an early warning system of the approach of predators. African Cape Buffalo (Syncerus caffer) have a good sense of smell but a notoriously poor eyesight: they are therefore vulnerable to predators approaching from downwind. The red‐billed oxpecker (Buphagus erythrorhynchus) is sometimes said to have a similar commensal relationship with cattle, but this is almost certainly false. Unlike cattle egrets, the red‐billed oxpecker has an obligatory relationship with cattle and big game and far from removing ticks it feeds primarily on scabs and wound tissue pecked from their host. Their feeding delays wound healing and thereby makes the affected animal vulnerable to infections and infestations with blowfly larvae.

The amoeba, Entamoeba coli (not to be confused with the gastro‐intestinal bacterium Escherichia coli, which is also abbreviated to E. coli) is a common commensal that lives in our large intestine. Unlike its pathogenic cousin, Entamoeba histolytica, E. coli feeds on bacteria and gut contents and does not invade the gut mucosa or consume red blood cells. Therefore, E. coli is of little interest per se, although a study in Mexico suggested an association between moderate‐heavy infestations and childhood obesity (Zavala et al. 2016). The most important feature of E. coli is that its morphological similarity to E. histolytica means that one must be careful to distinguish between the two species in microscope surveys of faecal samples.