Chimpanzees take risks but bonobos play it safe

Blogging on Peer-Reviewed ResearchWould you gamble on a safe bet for the promise of something more? Would you risk losing everything for the possibility of greater rewards? In psychological experiments, humans tend to play it safe when we stand to gain something - we're more likely to choose a certain reward over a larger but riskier one. Now, we're starting to understand how our two closest relatives deal with risk - bonobos, like us, tend to be risk-averse while chimpanzees usually play the odds.

Sarah Heilbronner from Harvard University studied the attitudes of five chimps and five bonobos to risky decisions. All the animals had been born in captivity in the Liepzig Zoo, Germany and were fed well on a regular schedule. Heilbronner presented the apes with one of two upside-down bowls of different shapes and colours. One 'fixed' bowl always contained four grape pieces and the other 'variable' one had a fifty-fifty chance of concealing either one grape piece or seven.

After a few trials with each bowl to get them accustomed to the options, Heilbronner let the animals choose between the two. The chimps were most likely to take a gamble on the variable bowl and they only chose the fixed one 36% of the time. The bonobos on the other hand mostly liked to play it safe and picked the fixed bowl 72% of the time.  On an individual level, all of the five bonobos showed risk-averse behaviour and four of the five chimps demonstrated a penchant for risk-taking.

It's not that one of the two species learns about the choice more slowly than the other; the differences in their behaviour were apparent in the very first trials of the experiment, and only grew larger with experience. Nor was it the case that either chimps or bonobos had better mathematical skills than the other. When they were offered the bowls face-up and could see what was inside, they almost always picked the one with the most grape pieces.

Risky business

i-a4c8f42023901b96f976426fcf8cd21d-Chimpanzee.jpgHeilbronner suggests that the different attitudes to risk shown by the two closely-related apes stems from their habitats. Compared to bonobos, chimps have to cope with greater uncertainty in their food supplies. Both species eat substantial amounts of fruit, a food source that is patchily distributed and available only at certain times in the year. But bonobos are more likely than chimps to supplement their diet with other plants that are more consistently available year-round, and because they have access to larger fruit patches, they face less competition for a meal.

Chimps are also active hunters and prey on monkeys. That's an inherently risky strategy. The rewards - the calorie-rich flesh of a colobus - may be substantial, but animal food can run away and the chimps risk wasting a lot of energy in a failed hunt. The risk-seeking behaviour of the captive chimps may reflect inbuilt preferences that help them cope with risky situations in their natural environment.

However, Heilbronner notes that it will be important to test other groups of chimps and bonobos to see if they show the same preferences. Different populations of chimps are known for having different cultures, and this single experiment can't rule out the possibility that this group's affinity for risk-taking was specific to them alone.

Gains vs. losses

Animals, of course, don't behave in the same way in all situations. Their choices will depend on how hungry they are, how widely available food is and how difficult it is to snag it. Chimps, for example, hunt more often when fruit is plentiful, which suggests they take more risks when they can afford to.

Thanks to the work of psychologists Daniel Kahneman and Amos Tversky, we know that humans react differently to risks depending on whether we stand to gain or lose something. If people are offered a certain chance of getting £100 or a 50% chance of getting £200, they are usually risk-averse and choose the safe bet. When the choice is between definitely losing £100 or a 50% chance of losing £200, they're more likely to take the gamble.

A wide variety of work across different species suggests that most animals behave in a similar way. When decisions affect how much food they can get (a question of gains), they tend to be averse to risk as the bonobos were, or at the very least, indifferent to it. But when decisions lead to delays in receiving food (a question of loss), animals tend to be more risk-prone.

It will be interesting to see how chimps and bonobos react to risky decisions that entail losses. For the moment, the Liepzig chimps provide one of the very few instances of an animal showing a proclivity for risk even when making decisions about future gains.

The strategy could allow them to exploit unreliable food sources, but Heilbronner suggests that, in most environments, this strategy is likely to be counterproductive. Individuals that gamble frequently may lose often enough that it costs them their lives, while those that play it safe may earn smaller rewards but will live to choose another day.

Images by Thomas Lersch and Kabir Bakie.

ReferenceHeilbronner, S.R., Rosati, A.G., Stevens, J.R., Hare, B., Hauser, M.D. (2008). A fruit in the hand or two in the bush? Divergent risk preferences in chimpanzees and bonobos. Biology Letters, -1(-1), -1--1. DOI: 10.1098/rsbl.2008.0081

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Ed - Have you done much reading on game theory? I wonder if there are similar experiments to this that could be done on other primates that might shed some light on evolved risk-assessment strategies. Could this be a way to test some of the claims in evolutionary psychology?

By Matlatzinca (not verified) on 26 Mar 2008 #permalink

I have to wonder if this expression of behavior has ever been examined from the perspective of neotony. It's almost as if bonobos were an expression of what we think of as adult chimpanzees but carrying along a lot of the traits from adolescence such as gracile skeletal structure, instinctual social structure more focused on mothers and females as would a less mature chimp, and now the issue of risk taking where one would expect a younger chimp not under the full sway of its full potential for testosterone might be less inclined to take risks.
It seems to me for humans to judge what is a riskier environment in which Chimps and Bonobos have evolved would depend on our knowing just how the chimps and bonobos saw those environments during their developemental stages and so far human understanding of the paleo-environment has been sketchy at best with lots of extrapolation, interpretation and not a few plain old good guesswork. I don't exactly know where the genetic differences that set bonobos apart from chimps are but I wouldnt' be surprised if they were located along the line we associate with those areas associated with degree of sex hormone production.

Frankly, sample size of 5 is too small.

Consider that somebody tested 5 related British and 5 related French. Obviously everybody would agree that this tells nothing about all British and all French.

Matlatzinca - I've written some stuff on game theory and it's definitely a field I want to know more about.

Doug - interesting theory. Other readers will probably be able to comment more on the issue of neoteny but I do agree that it would be interesting to see Hielbronner's experiments repeated on chimps and bonobos at different stages of development.

Jerzy - the sample size is indeed small and means that we should be cautious about interpreting the data. But it's not a reason to discount them, especially since the attitudes to risk were still consistent at an individual level.