How to tell a joke to an ape

Even before they can speak, most babies will playfully tease adults they know well. An eight-month-old might offer a toy but then withdraw it at the last minute, for example, or attempt to do something 'naughty' while smiling and watching the adult's face for a reaction. 

This tells us that babies of this age understand what adults expect in an interaction — and that they get pleasure from defying those expectations. This sort of behaviour is thought to be the pre-lingual precursor to more sophisticated joking. 

Now, a study in Proceedings of the Royal Society B reports very similar behaviours in four species of great ape. This work suggests that humour evolved in a common ancestor of us and these relatives, at least 13 million years ago. 

Isabelle Laumer at UCLA and colleagues analysed videos of a group of 17 chimpanzees living in a zoo in Leipzig, as well as videos of groups of nine bonobos, four orangutans, and four gorillas, all living at San Diego Zoo. Each of these ape groups included at least one juvenile, aged between three and five. 

The team looked at all of the social interactions captured in these videos. Some had all the characteristics of regular play, which is well-documented among great apes. The team also identified 18 distinct 'playful teasing' behaviours. These included poking, hiding, offering but then withdrawing an object or body part, pulling on hair, stealing (apparently just for the sake of it), quickly leaning in towards another's face and staring, and swinging close by another ape. 

In contrast to what's typical for regular play, playful teasing events had a few distinct characteristic elements: they were typically one-sided — the target of the teasing often didn't reciprocate; they often didn't involve clear play 'signals', such as the 'play face' expressions used by great apes; and they overwhelmingly occurred between individuals with a very different body size or social status. 

In fact, the teasing events almost always involved a juvenile and an adult, and were far more likely to be initiated by the younger ape. In one video clip, for example, a young chimpanzee runs over to an adult female who's calmly hanging out with another adult and slaps her on the back, then hurtles away and watches for a reaction. When she doesn't respond, he does this again — and again. (You can watch videos showing some of these behaviours here.)

The team also observed that juveniles and adults differed a little in how they went about their playful teasing. For example, the two young chimpanzees in the Leipzig group were just as likely to hit or poke the adults, but adults were far more likely to poke rather than strike one of the youngsters. This suggests that in these situations, the adult, with far superior strength, is aiming to be playful rather than aggressive.

The team also notes that teasing mostly happened when both individuals were relaxed, mirroring findings for human babies — and again implying that the behaviours weren't aggressive. As is also the case with human babies, the ape teasers routinely also watched for a reaction and, if there wasn't one, they often repeated or stepped up their antics. 

Though all four ape species exhibited playful teasing, the results did suggest a few differences between the species. For example, orangutan teasing was far more likely to feature hair-pulling (though as the team notes, they do have longer hair than the other apes). Another finding was that while the gorilla youngster often directed his teasing towards his mother, the young orangutan and the young bonobo never did. 

However, as the team also stresses, the sample sizes in this study were small, and the varying numbers of individuals in the different species groups also makes it hard to draw clear conclusions about potential species differences. Also, of course, all the groups were living in captivity. Whether or not wild apes show the same behaviours is yet to be explored (though there are some anecdotal reports that they do). 

Overall, though, this work does provide evidence that all four species engage in behaviours that resemble playful teasing behaviours observed in young humans — and that seem to be different from regular play, or aggression. 

While the teasing of human babies is clearly not as cognitively complex as adult humour, it's thought to spring from the same basic social and conceptual building blocks. The finding that it also seems to be a feature of life for our four closest relatives suggests that our last common ancestor had the cognitive precursors for joking and humour, too. 

Read the paper in full.