Dr Dominy's huge database allowed him to test each of these hypotheses.

ECONOMIST: Young leaves, as well as old ones, are often coloured red

In a study just published in Oikos, Nathaniel Dominy of the University of Chicago and his colleagues pooled existing information on the leaves of hundreds of tree and shrub species from Central America, Africa and South-East Asia.

ECONOMIST: Young leaves, as well as old ones, are often coloured red

"We're finding that when you rub a natural biological surface that has an orientation - that has a grain, if you will - and you run that grain perpendicularly to the long axis of the ridges, we find a dramatic increase in friction, " Dr Dominy told BBC News.

BBC: Artificial finger tests evolutionary origin of prints