Gibraltar is home to Europe's only wild monkeys: Barbary macaques, about 230 of them

Gibraltar is home to Europe's only wild monkeys: Barbary macaques, about 230 of them. Tourists feed them ice cream, crisps and bread, despite a fine of £4,000. Yes, Gibraltar is a British territory, disputed by Spain.

A recent study records that in response to this diet the monkeys have developed a habit of eating soil, known as geophagy. And they eat a particular substrate: 83% of all recorded cases of geophagy involve "terra rossa", the red clay characteristic of Gibraltar.

Red clay creates a barrier in the stomach that reduces the absorption of harmful compounds and makes up for the lack of fibre in tourist food. Macaques become lactose intolerant after weaning, so ice cream hits them particularly hard.

But the most interesting thing is that in 30% of cases geophagy takes place in groups, with several animals eating earth together from a single outcrop, and in 89% of all cases it happens in the presence of other macaques who are watching. In other words, this self-medication is social in nature: what we have before us is neither instinct nor accident, but an acquired cultural practice that the animals pass on to one another through observation and imitation.

Moreover, one group, living on the western slopes near the Apes' Den, prefers not red clay but earth from potholes in the asphalt, saturated with road tar. Fully 70% of their geophagy involves precisely this substrate, although red clay is available to them too. We may call this a local cultural tradition.

Were it not for the stupid tourists, the monkeys would not have developed their own pharmacopoeia from materials to hand and passed it on to the next generations, and we would have had no occasion to talk about primate culture.