It is always exciting when Hare – an expert in animal cognition – publishes an article comparing chimpanzee and bonobo behavioural responses to a task, since it gives us a chance to consider which of these two of our closest living relatives is most like us.
How do chimpanzees differ from bonobos?
Chimpanzees are political, aggressive, hunt for meat, and engage in primitive war with neighbouring groups. Bonobos have sex for pleasure as well as procreation, and make a variety of almost bird-like vocalisations.
Chimpanzee societies are more “patriarchal” or male dominated, whereas bonobos live in more female-centred or “matriarchal” societies.
Both species are intelligent, are long-lived with long childhoods, develop friendships and laugh when they are tickled.
They are both so much like us - only more hairy - but with bonobos easily recognised by their centre-parted hairstyles and cheek whiskers! Genome sequencing has shown more than 3% of our human genome is more closely related to either chimpanzees or bonobos than they are to each other.
What is a chimpanzee or bonobo tantrum?
Anyone who has worked with chimpanzees or bonobos knows they can throw tantrums and scream VERY loudly, with hearing protection signs located in areas where zoo keepers work near these apes.
They respond just like human toddlers (and sometimes adults!) who are denied a treat or are frightened or upset by something.
If chimpanzees and bonobos want something from a friend or relative or human care-giver, and the other ape is taking too long to respond or denying them their treat, they start to show frustration, impatience or annoyance in ways that are very recognisable to humans.
They may pull a pouty face and whimper or moan, show displacement behaviours (e.g. scratching the head or body) and, as they become more cross or angry, they hit, bang or bash things with their hands or feet.
Rosati and Hare were interested in whether those emotional responses would appear in two studies in which chimpanzees and bonobos had to make an “economic” decision, which involved:
- being patient and waiting to get a bigger reward
- taking a risk with a 50/50 chance of getting a “good” or “bad” food reward rather than the safe option of a reward that was neither liked nor disliked
How do our hairy relatives respond to waiting for a treat or risky choices?
The PLoS ONE study took place at two sanctuaries in the Republic of Congo – Tchimpounga Chimpanzee Sanctuary in Pointe Noire and Lola ya Bonobo Sanctuary in Kinshasa – both of which care for confiscated apes mostly orphaned as a result of the illegal bushmeat trade.
Non-invasive cognitive-behavioural studies such as this one provide us with the opportunity to find out more about how wild-born, semi-free-ranging apes solve problems and “think”, while at the same time allowing these intelligent beings the chance to take part in tasks that they find fun, stimulating and enriching.
The 23 chimpanzees (ranging in age from 7-20 years) and 15 bonobos (ranging in age from 7-10 years) took part in two different tasks: a temporal task and risk “preference” task. Each ape was tested separately, sitting at a table with a sliding top across from the experimenter, but separated by wire mesh.
The participants indicated their choice by pointing at one of two options (one piece of food on the left to be received straight away or three pieces of food on the right to be received after one or two minutes).
In the “Temporal Preference” study, each ape could either choose a small reward (a piece of food) and receive it straightaway, or choose a big reward (three pieces of food) but have to wait for either one or two minutes to receive it.
Neither species showed signs of being frustrated or annoyed when the small reward was received straight away. But much like humans, both species showed negative emotional responses when waiting for a bigger reward, scratching themselves and banging the table and mesh.
Chimpanzees also screamed and whimpered and moaned on about half of the trials, whereas bonobos were quieter and vocalised on only 5% of trials.
In the “Risk Preference” study, each ape had to make either a risky or safe choice. The risky gamble either resulted in receiving a favourite treat (chimpanzees preferred bread and banana; bonobos preferred apple and banana) or a least-favourite treat (chimpanzees didn’t really like papaya and cucumber; bonobos didn’t really like peanuts and lettuce) with an equal chance of getting either.
They couldn’t see what the outcome was until they had made their choice. If the apes didn’t want to take the risky gamble, they could choose the safe option and receive a treat that was neither really liked nor disliked (peanuts for chimpanzees; papaya for bonobos).
To make things more interesting, the size of the reward also differed (one, three or six pieces of food). About 65% of chimpanzees took the risk of a better pay-off, whereas about 60% of bonobos chose the safe option without the preferred outcome at the end.
Chimpanzees only chose the risky option if they were “gambling” for one or three pieces of food, but took the safe “gamble” if the outcome was six pieces of food. The most emotionally-charged situation was when the apes received a “bad” outcome (less preferred food) after making their risky “gamble”, with almost three times as much negative emotion expressed (such as vocalisations, scratching and banging) than for a good or safe option.
Interestingly, in 30% of cases the apes tried to switch their choices (pick the other option) as soon as they saw their risky choice had resulted in a bad outcome. This is a bit like people regretting a choice and trying to change it if they can.
What is the evolutionary significance of differences in economic decision-making?
For chimpanzees and bonobos, food represents a resource that’s at the heart of decision-making in any environment. As with humans, chimpanzees and bonobos have different personalities and may choose different strategies to solve problems or make decisions related to resources, but this study highlights the fact there are differences between species.
The evolutionary significance of such differences is a puzzle, but bonobos evolved in a relatively tiny area of Africa (south of the Congo River) in lush resource-rich forests, separated from other great ape species, with whom they would have had to compete for resources. Chimpanzees have evolved alongside gorillas, their competitors for fruit and other resources.
Bonobos share food readily with other individuals, whereas chimpanzees have to beg and coerce and negotiate between each other for special food items.
Perhaps this is why bonobos are less willing to wait for a special treat than chimpanzees?
As always, we’re left with plenty of questions – but also, perhaps, with the hint of an answer.