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.

Imagine 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?

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?

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.

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.

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