Making the effort

Chimpanzees are one of the small number of animals that are known to use tools for various purposes. But how do they decide to use a tool or not and do they do this every time or just under certain circumstances?

Thibaud Gruber has been working out the Budongo Conservation Field Station in Uganda where his field operatives have been following and monitoring individual chimps for nearly a decade. He presented these animals with a challenge to solve and then looked back at their recent past activity to see what might be behind their motivation to use a tool. Chris Smith spoke to him to find out more...

Thibaud -The experiment is called “honey-trap experiment”. The idea is you have a log and in the log, we’re drilling a hole. Inside this hole, we put some honey. The catch is that the honey is too deep to be reached by your hands. And so, the chimpanzees who encounter the experiment have to find a way to get it.

Chris - So, what sorts of tool do they need to fashion to retrieve the honey?

Thibaud - Well, it actually depends on the chimpanzee community cultural background because I ran this experiment in several communities in what I would say a controlled community. So that was in Kibale forest. The chimps, they used sticks to get the honey. But the other community where I have been working which is actually the focus in this study is the Sonso community of Budongo forest. These chimps, they don’t know how to use sticks. So, what they actually did for – so 10 chimps out of the 50 we tested, they actually use leaf sponges. Basically, they take some leaves from the vegetation, they roll them in their mouth and then they use this to get the honey.

Chris - What was the variation that was occurring in the experiment that you were seeking to test here?

Thibaud——基本上,有两种类型的杂文集tion that we could study. The first one was how much time chimpanzees are going to spend trying to access the honey. Sometimes we would have chimps who would just engage with the experiment for like 2 seconds and then just leave. And other times, we had chimps who would spend 20 minutes. This was one of the first things we wanted to test. The second variation was only a few occurrences of tool use. So that means most chimps actually never managed to get the honey. So, what would actually make you use a tool?

Chris - How did you actually assess this? What was the method of processing these findings?

Thibaud - To do that, it’s very great benefit of working in a long term field site because we have a lot of information about the individuals and it is also a very well organised system to collect data about the behaviour of each individuals. What we have is a database of behaviour. What we did was extract the behaviour of the individuals who had been engaging with the experiments. So basically, if you imagine that our chimps- one chimp called Night would engage with the experiment on the first of January then we could go back in the data, look at what he was doing in December and November before engaging with the experiment because we wanted to know how much she was traveling, what she had been feeding on and try to see this has an influence on engagement with the experiment.

Chris - I wondered if that was the direction you were going to take this and you were going to argue that what was basically driving it is whether or not the animals are really hungry and therefore motivated to want to get to that honey because honey is quite a rewarding thing to find for them if they're hungry.

Thibaud - We don’t know actually if honey is nice treat or something that’s really a good thing to consume just to level out your energy. There has been like some conflicting result. I don’t know if it’s really about being hungry or not. What we found was, they would engage most of the experiment when they really had a bad period. They had been traveling a lot, they did not eat a lot of fruits which is normally their favourite food in the forest. When this happened then they were engaging a lot. So that would really suggest that they would really engage with the experiment out of necessity.

Chris - When you say engage with the experiment, did that mean that they spent longer fiddling with the log because they wanted to get the honey out or was there also a difference in whether or not they resorted to tool use in that setting?

Thibaud - We found two main results. So, for the engagement, what we found was a kind of long term effect. So there was basically a combination of not eating your favourite food much and having to travel a lot to find it, would basically build up some oversetting of engaging with the experiment and that was even more pronounced if the situation had been quite bad for a long time. For tool use, what was interesting is that we didn’t actually find an effect of what they were feeding on although it was kind of suggestive of it. But the really main big effect that we found was how much was travelled in the week preceding the experiment. Somehow the tool use seems to be more driven by an immediate need. Maybe it’s because it’s more costly and that drives you to think – I don’t know – maybe more creatively. It was really connected to the probability of using a tool.