A Bonobo in the Hand or Two Chimps in the Bush?

Bonobo Week continues! I'm donating whatever proceeds I receive from my blogging shenanigans for the entire month of June to help the bonobos at Lola Ya Bonobo.

This post was chosen as an Editor's Selection for ResearchBlogging.orgResearchBlogging.orgImagine that you're wandering in the desert and you come across two magic lamps. One lamp grants three wishes. It's your standard sort of magic lamp with a genie in it. (No wishing for extra wishes, of course.) The second magic lamp is, well, a moody magic lamp. It's inconsistent. Sometimes it grants one wish, and sometimes it grants seven wishes. But the thing is, you don't know for sure whether, when you rub the lamp and genie pops out, if he's going to grant you just one or the full seven. But let's make things more interesting. You only get to use one of the lamps. As soon as you rub one of the lamps and the genie comes out, the other lamp disappears. And you are in the Desert of Infrequent Lamps. Tomorrow you could chance upon two more lamps, with the same rules. But you might not come across any more lamps for many days. So which lamp will you decide to use?


Figure 1: If you're lucky, the genie will have the voice of Robin Williams and will sing to you.

Decades of studies indicated that, as humans, we tend to avoid risk. When it comes to potential gains, we prefer the safe option over the risky option. But in the Desert of Infrequent Lamps, you might be tempted to take the gamble, especially since you don't know when you'll be lucky enough to stumble upon your next lamp. Resources (in the shape of magical wish-granting lamps) are scarce. After all, you're wandering through the Desert of Infrequent Lamps. You gotta get what you can, when you can. Sucks to be you.

Animals face similar risks on a daily basis, though in the context of things like food acquisition and predator avoidance. So it makes sense that natural selection would, over generations, favor certain cognitive decision-making mechanisms that most effectively addressed those risks. Risk preference patterns in animals are variable though. That variability in risk preference has been observed, at least under experimental conditions, suggests that animals can adjust their strategies given the parameters of the immediate environment. For example, when the riskier option may not be very costly, or when plenty of food is available in the environment, the animal may opt for the riskier choice, and under those circumstances that may actually be the optimal decision.

But is there a relationship between foraging ecology and cognitive decision-making mechanisms? Or are observed inter-species differences in risk preference simply due to experimental task demands? This is what a team of researchers from Harvard and Duke (including our hero this week, Brian Hare) wanted to figure out. They hypothesized that differences in feeding ecology in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) have resulted in differential patterns of risk preferences.

Chimpanzees and bonobos are phylogenetically closely related; they diverged from their common ancestor less than one million years ago. They are morphologically and behaviorally very similar, though striking differences have been found in terms of dominance structure, sexual behavior, and aggression. Most important for this study, the two species live in geographically different areas, with different resources. Several previous studies observed that while chimps and bonobos rely mainly on fruits and terrestrial vegetation, bonobos may rely more heavily on vegetation which is found in greater abundance both in terms of space (i.e. amount) and also in terms of time (i.e. available throughout the year). By relying more heavily on their vegetable crudite, bonobos may avoid some of the risk that chimpanzees must contend with in their "frugivorous foraging." (New goal: use the word "frugivorous" in a journal article.) It is also possible that there are more or larger fruit-bearing tree patches available to bonobos, resulting in less competition for that fruit, when compared with chimpanzees. Another key difference is that chimpanzees hunt monkeys for their meat, which is both costly and risky, though has a high pay-off. Taken together, it appears that bonobos have more reliable food sources, and chimpanzees have less reliable food sources, which may result in increased competition. This leads to the hypothesis that, compared with bonobos, chimpanzees would be more risk prone. And compared with chimpanzees, bonobos would be more risk averse. Put in other terms, bonobos live in the Forest of Many Lamps. Why try for the riskier 1-wish or 7-wish lamps when the 3-wish lamp is a sure thing? Especially since there are plenty more lamps to rub. Chimpanzees, however, live in the Forest of Infrequent Lamps. Why choose the 3-wish lamp and risk missing out on four potential additional wishes, especially considering that you don't know when you might find another magical wish-granting lamp?


Figure 2: Researcher Brian Hare with Malou, a bonobo from Lola. Click to embiggen.

Five chimpanzees (3 males, 2 females) and five bonboos (3 males, 2 females) were tested at the Primate Research Center at the Leipzig Zoo, in Germany. All ten apes were born in captivity, were never food deprived, were socially housed, and had ad libitum access to water, including during testing. All had previously participated in other experiments of cognition and behavior. Both species were fed fruits and vegetables every day, and cooked meat once per week (this pattern was maintained throughout the testing period). Since the animals were born in captivity and had equal easy, regular, predictable access to food, any differences in risk preference behavior when it came to food can reasonably be ascribed to evolutionary differences.

First, the researchers assessed number discrimination: for example, could the apes discriminate four grape halves from seven? All participants had sufficient number discrimination, and therefore could participate in the experiment.

i-024e03824e916a17c2f4a3e9323d2764-riskprefs apparatus.jpg

Figure 3: Experimental Apparatus. Chimps and bonobos chose between fixed and risky rewards, hidden under the bowls.

In the forced-choice task, the individual would be presented with two upside-down bowls that differed both in color and shape. Under one bowl, four grape-halves could always be found. Under the second bowl, either one grape piece or seven grape pieces could always be found, with equal probability. This meant that in half the trials, the second bowl covered up one piece, and in half the trials, the second bowl covered seven pieces. The four pieces bowl represented the fixed reward, while the one-or-seven pieces bowl represented the risky reward.

Before the test began, the individual was familiarized with the reward contingencies associated with either bowl. Sometimes just the four-pieces bowl was presented, and sometimes just the one-or-seven pieces bowl was presented. After completing both the number-discrimination and the familiarization trials, the individual completed three testing sessions, each with twenty forced-choice trials.

i-6bb8d5824c04f3efe943cdc6b7febae8-riskpref results.jpg

Figure 4: Results. Bonobos in slashed bars, chimpanzees in black bars. Values represent the proportion of trials when the fixed option was chosen, with standard error.

The results are pretty straightforward. Chimpanzees were risk-seeking, significantly preferring the one-or-seven risky option, both within each session and combined across the entire experiment. In fact, chimpanzees became slightly more risk-seeking as the sessions progressed. Bonobos, in contrast, were risk-averse, significantly preferring the the reliable four-pieces bowl. Comparing the two species, chimpanzees were significantly more risk-seeking than bonobos. Comparing individual response patterns, four out of the five chimps displayed risk proneness, and all five bonobos displayed risk aversion. There was no effect of sex or age.

The difference could not have been due to numerical cognition, since both species were highly successful at choosing the larger reward in the number discrimination trials. That they could not only discriminate, but consistently chose the larger amount, suggests that the differences were not due to motivation. Chimpanzees chose successfully 95% of the time on these trials; bonobos 94% of the time. Both species were sufficiently motivated to acquire the larger rewards.

The chimps and bonobos tested in this experiment used highly different decision-making strategies when confronted with the same task. Chimpanzees preferred risky choices, while bonobos played it safe. Because there were slight differences in living conditions in the zoo as well as general cultural differences between the two species, the effects of experience can't be completely ruled out. However, the fact that these animals were all born in captivity and had similar and reliable access to food and water suggests that these differences reflect decision-making mechanisms that emerged due to natural selection because of differential environmental circumstances, and was not due to experience.

Additionally, converging evidence for these findings comes from a different study concerning decisions regarding delayed rewards. Chimpanzees were willing to wait significantly longer for larger rewards, compared with bonobos. In this case, the increased delay could represent a significant risk. Chimps may be more willing to wait longer for a larger reward because they are more willing to incur the risk.

At this point, you may be asking why we should care about the risk preference patterns and decision-making mechanisms of bonobos and chimpanzees. And one answer is that humans didn't evolve to engage with modern economics. Indeed,

many of our preferences are probably tailored to providing adaptive foraging and other evolutionarily relevant decisions. An evolutionary approach to economic preferences can therefore offer keen insights into the nature of human and animal decision-making.

Heilbronner, S., Rosati, A., Stevens, J., Hare, B., & Hauser, M. (2008). A fruit in the hand or two in the bush? Divergent risk preferences in chimpanzees and bonobos Biology Letters, 4 (3), 246-249 DOI: 10.1098/rsbl.2008.0081


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The obvious question that the above study leads to... why grape halves? Why did someone go to the trouble of cutting grapes in half, rather than just putting whole grapes under the bowls?

Super interesting--this makes me wonder, as a finance/economics guy, if we could extrapolate some sort of fundamental efficient frontier (the graph of the minimum reward demanded for a given level of risk) on the basis of biology.

humans didn't evolve to engage with modern economics

I would argue that modern economics evolved to explain humans, not vice versa. Of course, as a species, we are completely insane, scatterbrained, egotistical, and hugely susceptible to all manner of cognitive bias, so we constantly violate rational expectations and therefore economics doesn't do all that good a job of expaining our behavior. But behavioral economics can bridge a lot of the disconnect between traditional economic models and our actual behavior.

By Dave Lukas (not verified) on 11 Jun 2010 #permalink

Something's backwards here:

Most people would choose the sure thing, and summon the three-wish genie, especially since they don't know when they'd be lucky enough to stumble upon their next lamp. Resources (in the shape of magical wish-granting lamps) are scarce. After all, you're wandering through the Desert of Infrequent Lamps. Sucks to be you.

this is the opposite of the conclusion that's drawn later in your post.

Super interesting--this makes me wonder, as a finance/economics guy, if we could extrapolate some sort of fundamental efficient frontier (the graph of the minimum reward demanded for a given level of risk) on the basis of biology.

@4: The other important problem to consider when thinking about human economic decision making is our brains simply aren't equipped to really understand the magnitude of large numbers. We hear about deficits of trillions of dollars, for example, and rely on our approximate large number system. We don't really know what $1,000,000,000 exactly means, as distinct from $2,000,000,000, for example. Though we can easily distinguish 1 from 2 objects, for example, thanks to our small number system. I think I feel another blog post coming on.

@7: Part of my [implicit] point is it is far from obvious which is the "natural" solution to scarce resources. Your original explanation made good sense, too, afterall.

Right. Certainly the thought experiment only captures a certain amount of the complexity of the problem.

Interesting, but I admit almost all of these kinds of intelligence(s) tests with apes, Chimps and Bonobos especially, are intriguing to me. They invariably bring to mind that experiment done in a fox farm in Russia some time ago in which, as an attempt to breed domesticated foxes, the scientist selected foxes for breeding based on levels of aggressive behavior. After a few generations the foxes displayed some interesting morphological traits that are suggestive of the kind of retained junenile traits that we typically ascribe to the dogs in contrast to wolves. I wonder if a population of chimps were selected for breeding based on their innate levels of aggression, if they'd begin to express traits more associated with Bonobos; more gracile forms and less aggression/dominance display in adult groups, greater tendency to avoid conflict. I'd wonder what genes controlled their individual levels of adrenalin and other markers for stress/aggression. Maybe the level of aggression is related to the impulsiveness or willingness to take a risk, which could be significant depending on the kind of habitat and availability of food throughout the year. Cheers.

I wonder if the question might also be put as, do chimpanzees experience a greater reward rush than the bonobos when they get a big prize? So the chimps are more hopefull of the bigger prize rather than risk averse per se. As overall there is no difference in outcome just a difference in intensity. A big high for a seven score but only a little dissappointment for the 1 score. Perhaps this is what is meant by risk averse anyway.

By Mark Fyfe (not verified) on 12 Jun 2010 #permalink

If early hominid females typically did the gathering and males did the hunting, would you expect modern human females to be more risk-averse than males? Is there experimental evidence for it? I think it is consistent with common stereotypes.

By Bonnie Packert (not verified) on 20 Jun 2010 #permalink

@13: It's a reasonable hypothesis, I suppose, but pretty hard to test. If I've learned anything in my own research, it's that the results are rarely in accordance with intuitive predictions. For your hypothesis to be true, it would have to be somehow sex-linked, I think. It's rarely that straightforward. I'm skeptical.

Makes one wonder whether we are witnessing a similar speciation event spliting sapiens into risk-adverse, peace-loving social security liberals and risk-friendly corportate warrior winner-take-all conservatives -- it's the same meme. Just saying...

By James Hathaway (not verified) on 22 Jun 2010 #permalink

Re: James Hathaway's comments.

What a load of specious nonsense. If anything, "corporate warriors" will be among the most risk-averse of human primates in any society, right up there with civil servants and employees of "defense" companies. Mostly they are concerned with pensions and the vast majority will require social security to augment whatever meager pensions they qualify for (which includes their 401Ks) though there are the notable exceptions at the top of the food chain who negotiate pensions as soon as they are granted the ceremonial capacity.

Risk takers will eschew corporations like the plague and do their own thing. Bill Gates is a notable example who dropped out of college to start his own small company (though that might have been less risky than it seems given his parents, and in fact it was his mother's business contacts that got his company its big break with IBM). Gates is well-known to be a liberal, by the way. And of course, the real innovator and risk-tasker in all this was Gary Kildall. His story vis-a-vis Gates might teach you something about "economics" but I am guessing you are a nine year old. Perhaps not chronologically, but just saying...