Chimp on trial

Chimp society is complex and violent, and it is reasonably common for one chimp to kill another. But is this murder? At Cheltenham Science Festival, a temporary court was set up to debate this topic, before the audience played the role of jury and voted whether the chimp in question was guilty or innocent. The case for the prosecution was led by Dr Kate Cross (University of St Andrews), and Dr Lewis Dean (University of St Andrews) led the defence. They were joined by expert witnesses Dr Katie Slocombe (University of York), Dr Benedict Douglas (University of Durham) and Dr Amanda Seed (University of St Andrews) to guide us through the case.

The event opened with the case being presented: chimp Jack was out patrolling his territory with his group one evening when he came across chimp Steve, who was from a rival troop. After a violent struggle, Jack killed Steve. Was this murder – or just natural animal behaviour?

Murder is defined as "the killing with intent of a reasonable being, not in self-defence".  In the chimp context, Katie explained that boundary patrols are very tense affairs, which often end in attack if a rival group is encountered. Lone males found in these cases are almost always attacked, although some may escape without much damage.

Chimps are also known to kill within their group - most commonly a new alpha male will kill the offspring of the previous alpha so their mothers become ready to mate again. It has also been known for females to kill other females' young, particularly if they are new to the group. In the case in question, we know that Jack killed Steve, and that it wasn't, in the traditional sense at least, self-defence. So the big question is whether a chimp is a 'reasonable being' and whether there are any other reasons Jack could be exempt from prosecution.

To commit a crime, you need to understand that what you are doing is wrong. So the first question is, can we claim that chimps understand killing, or even death? Dr Slocombe explained that chimps do seem to have an understanding of death - although their reactions to dead troop members aren't often seen as most ill or injured chimps leave the group before they die. However in a few cases, chimps have been seen to beat dead bodies, possibly trying to get them to respond, while other times they will stay with the body, grooming it and trying to keep flies away. Mothers have even been known to carry the bodies of their dead infants around for several days. But do these behaviours really show understanding death, or just instinctive reactions?

We know that chimps can show compassion to other chimps, but only (as far as we know) to members of their own group. If a friend is attacked, others will spend time hugging, comforting and grooming them after the attack is over. There was even a case of a chimp who was second in command defending the alpha male after he fell out of tree and broke leg, so he didn't lose his position. So it seems they do have some understanding of the concept of hurting, or killing others. But do they know that killing another chimp is wrong? Do chimps have a concept of morality?

Dr Slocombe explained that in some cases, they do seem to. They have a strong concept of possession - a more dominant chimp won't take something by force if it is in the possession of a lower ranking chimp, for example. They also have a sense of fairness - studies have found that if one chimp steals food from another, the second chimp will punish the first, even if there is no direct benefit to him. However, we need to be careful when implying that because chimps have some ideas of right and wrong this can map directly onto our own human morality. For one thing, our ideas of right and wrong vary hugely by culture. These findings do seem to suggest that there might be something in the idea of a chimp morality, but it isn't clear if this is enough to say that they can commit murder.

In human courts, children can't be found guilty of murder until they reach the age of ten, as it is believed that until this point they can't understand right and wrong. So if we are to extend the court system to chimps, this implies we believe their abilities are greater than those of a 9 year old human. But is that the case? The answer, it seems, isn't clear cut. The answer depends on what ability it is you are testing them on. Dr Slocombe told us that when it comes to chimp comprehension of human speech, they are able to reach about the same level as a 2.5 year old human, but this is asking them to learn the language of a different species, which would be a difficult task for anyone! We don't yet know enough about their communication within the group to say how sophisticated that is.

It is tempting to try and think of chimp intelligence in the same way as our own, but this is misleading, and it makes it a difficult and complicated question to answer. In other tasks, chimps come out with a much higher human age than in language. Dr Seed explained that they have the ability to solve puzzles, thinking flexibly and changing their actions as the rules of the puzzle change. In this task, some chimps excel, comparing to 3-6 year old children. In other tasks they do even better, perhaps reaching the standard of an 8-10 year old. But in the eyes of the law, this would still put them as too young to really be responsible for their actions. And not all chimps reach these heights - some are unable to pass the tasks at all.

It is also interesting to wonder about how we came to the idea that 10 is the magic age for human responsibility. One of the things that younger children struggle with is the ability to plan or reason about the future, and predict the outcomes of their actions. If you can't do this, then you can't be expected to know that your actions could kill someone. But chimps are rather good at this. They do seem to be able to plan for the future - in the wild, they will carry their favourite tools with them when they go fishing for termites, and there is even a chimp in a zoo who hides rocks from his keepers at night so he can throw them at visitors (his favourite hobby) in the mornings! But is this really planning, or just habit? Lab experiments have shown that chimps can plan for the future to some extent - choosing a straw over a tastier grape, for example, when they know that a couple of hours later they will be given fruit soup that requires a straw to drink it. Make the challenge harder, however, by increasing the delay from a couple of hours to overnight, and most chimps fail.

So does this evidence suggest that chimps can commit murder? I, for one, was not convinced. Dr Douglas discussed the idea of animals having rights, similar to human rights, and I can see the logic of extending some of these to our nearest relatives. There have even been cases come before the courts which argued that chimps in captivity are effectively slaves and should be freed. But even if we were to extend rights like personal freedom to these animals, I think the idea that they could commit murder is still a step too far. Killing is a natural part of chimp life, and while in some ways their lives are socially complex and similar to ours, there are huge differences too. We are the only animal that has devised rules to help us overcome our natural instincts, and it doesn't seem reasonable to impose these rules on other species. It appeared that the audience at this event agreed, with all but two voting that Jack was innocent of murder.

While this event was entirely fictional, it was a fantastic way to raise some fascinating questions; about what makes us special as humans, how we treat our nearest relatives in the animal kingdom, and how much we still have to learn about their intelligence and their society.

- Ginny Smith is a freelance science communicator.