Primate Gamble Pays Off

30 March 2008


In a study which might help to explain why humans sometimes make rash decisions that don't pay off, scientists have uncovered the evolutionary origin of why some animals are partial to a flutter.

A BonoboHumans usually make low-risk safe-bets when it comes to what they might win, but what about other animals, are they equally risk-averse? To find out Harvard researcher Sarah Heilbronner and her colleagues engaged two of our closest primate relatives - chimps and bonobos - in a gambling game.

Five chimps and five bonobos were shown two upturned bowls of different colours. Beneath one, "the safe option", were always four grapes; but the other bowl, "the risky option" concealed either just one or as many as seven fruit pieces. The bowls were presented to the animals in a series of trials to see whether they were prone to gamble, choosing the bowl that might contain twice as much fruit or virtually none, or whether they preferred the lower-adrenaline predictable option.

The results were striking; the chimpanzees consistently elected to gamble, whilst the bonobos usually plumped for the safe option. The two species are highly genetically related and split apart from each other during evolution just 1 million years ago, so why this strong behavioural difference? The team think that  the environment in which the animals have evolved to live has moulded the decision making rules applied by their brains. Bonobos tend to depend heavily on widely-available and highly reliable ground-based herbaceous vegetation as their main food source. But chimps eat a less terrestrially-dominated fruit diet and also hunt monkeys, both of which tend to be less predictable, riskier but sometimes more rewarding food sources. Consequently, say the researchers, because the chimps don't know where their next meal is coming from they have evolved to favour a gamble.

So what about us? Well it looks like our decision making may well be clouded by our evolutionary origins too; as the team point out, since humans didn't evolve in the context of the modern world, many of our preferences are based on the best strategies to fill the fridge, rather than on sound economic principles!


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