The phrase “go ape” means to become very angry or get very excited; it’s probably also a lot more accurate than its inventors realised, because we inherited our personality traits from the great ape ancestors that we share with chimpanzees; and that means that chimps have them too. And they’re under evolutionary selective pressure: a favourable trait will mean you live and reproduce more. Drew Altschul, at the University of Edinburgh, has been looking at which ones really matter to chimps…
Drew - So we wanted to understand the evolution of personality, both in humans and in our closest relative, which was chimpanzees. Humans have five main personality dimensions, which are agreeableness, extroversion, conscientiousness, neuroticism, and openness; and chimpanzees share these same five dimensions, plus they have a dominance dimension. We believe that there is an ancestor of humans and chimpanzees that shares these dimensions. And so, by understanding how chimpanzee personality relates to chimpanzee longevity, we can understand one aspect of how natural selection is shaping the development of personality.
Chris - So how might you predict it will? What sorts of personality traits - before you you've gone anywhere near any results in this study - what went through your mind in terms of what will be related to what?
Drew - There's information coming from the human literature as well as the animal literature. So, in humans, high neuroticism and low conscientiousness tend to result in increased mortality. So if you're more neurotic you tend not to live as long; if you're more conscientious, you tend to live longer. Whereas, in animals, it tends to be more of a focus on social traits; so more extroverted individuals to live longer; more dominant individuals tend not to live as long; and, to a lesser extent, the more agreeable individuals live longer as well.
Chris - So that's what you're sort of expecting to see. How did you therefore do the study?
Drew - We used a questionnaire that's been validated across many years. We basically asked people who know the chimpanzees really well - those are keepers and researchers and other staff - to rate them. And by bringing together multiple questionnaires where everyone has filled out the same items we can basically add these items up and use our predefined structure to come up with a bunch of scores one for each personality dimension.
Chris - How many animals did you consider?
Drew - We had 538 in the final analysis.
Chris - Was that males and females?
Drew - Yes, both males and females.
Chris - And when you pull all the data together just in broad terms first of all what were the findings?
Drew - We found that male chimpanzees who were more agreeable tended to live longer, and more open females also tend to live longer. But we didn't find any associations for any of the other personality dimensions.
Chris - That's interesting isn't it; why do you think you saw that effect for the males in terms of getting on well with other males but you didn't see it for the females, because these are social animals are they - they hang around in big groups?
Drew - Yes they are. Agreeableness is one aspect of sociality, and among the males agreeableness is essentially the opposite of aggression and there's evidence to show that more aggressive individuals - particularly males - tend to live less long. And so what's possibly happening is that the more agreeable chimpanzees are better at forming coalitions and making it through tumultuous social circumstances and end up living longer because they end up in basically fewer harmful circumstances.
Chris - But what about the females, because they also work together and live together and look after each other don't they?
Drew - They do. I think that there's probably not as much interaction between females with the males. They spend a lot of time interacting with their children, and that probably disproportionately puts more emphasis on agreeableness for the males rather than the females.
Chris - And where do you think that openness influences the outcome for the females then; why does that make a difference to them but not the males?
Drew - Openness has to do with exploratory behavior. So these are individuals who tend to explore more thoroughly. There is a theory that this is the result of them living in captivity, because there's no downside to basically them exploring more. In the wild you would expect that individuals who explore too far would end up getting caught by a predator or contracting disease by poking around too much. And because there isn't any such dangers in captivity there's no downside to being more open.
Chris - The fact that you've got a range of different personality types, all within one big group of animals, does that mean then that - from an evolutionary point of view - nature is maintaining a range of personalities because they're are different outcomes for different individuals. If an individual is very aggressive, is very dominant but gets lots of mating in early but then gets killed nonetheless they've passed on a lot of genes; whereas an individual that is very agreeable, does get on very well with all the others, is well socially supported gets fewer mating opportunities but lives longer to invest more in their offspring. Is that what you think is emerging here?
Drew - That's definitely one of the possibilities. It's easy to understand how an individual who has many mating opportunities and has many children at a young age passes on their own genes. It's harder to understand how individuals who live longer are exactly profiting from that extra lifespan, but certainly rather more carefully investing in offspring over the course of a longer lifespan. It's definitely one way the individuals can improve their fitness...