Georgia State University re­search­ers Sar­ah F. Bros­nan and Mi­chael J. Be­ran spent two years working with three chimps to study why animals don't barter.

Only the human world features trade, although chimpanzee culture has been known to include exchanges of services - such as grooming or help in a fight - for food.

In the experiment, pairs of chimps were placed in sep­a­rate en­clo­sures and given to­kens that could be turned in to a a human invigilator for treats.

Symbols were used on the 'currency', which the chimps had been taught from birth, and in half the cases, the chimps had to ex­change the to­kens with each oth­er be­fore the cur­ren­cy could be con­vert­ed to food.

A few of the tokens were also deliberately wrong, so that the chimps would understand the reasons for trade with the correct currency.

While bartering with humans generally went well, it all unravelled in a most uncivilised fashion when the chimps were left to trade with each other.

For a start, they did­n't al­ways re­turn to­kens despite being given their treat by a fellow chimp, leading to a lot of screeching.

And after a while without human observers they completely stopped trading, because it was easier to steal the goods from their peers.

The scientists say the results suggest that bartering appeared quite late in human evolutionary history.