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Abstract
[...] it has been suggested that cooperative behaviour may represent a costly display that signals an individual's prowess as a rival or its potential quality as a mate25, though as yet there is little evidence that this is the case26. [...] to other species, reciprocal exchanges of costly services or valuable resources between non-kin are widespread and often involve considerable time delays between assistance given and received and extensive opportunities for cheating16,84.Alikely reason for this contrast between humans and other primates is that reliable exchanges of valuable resources require some form of language (and associated psychological capacities) to establish the intentions and expectations of both parties regarding the nature and timing of exchanges84,85 as well as the social norms that discourage cheating83,86.