You have full access to this open access article. Among African great apes, play is virtually absent between adult lowland gorillas Gorilla gorilla gorilla. We recorded three playful interactions between the silverback and an adult lactating female. Given the diverse causal and functional nature of play, different factors may join in promoting this behaviour.
In our group, contrary to what has been shown by previous studies in wild and captive gorillas, adult females spent more time in spatial proximity with the silverback than with other females.
Hence, the probability of social interaction including play between silverback and adult females was enhanced. Moreover, the motivation of the lactating female to play might be an effect of oxytocin, a hormone that reaches high concentration levels during lactation and that promotes social affiliation. The fact that play invitations were always performed by the female might support this hypothesis.
Certainly, we cannot exclude the possibility that the play event is a group idiosyncrasy or an artefact of captivity, even though the subjects never showed abnormal behaviour. Structurally, play sessions showed a suitable degree of pattern variability and switching frequency from one pattern to another.
The proportion of offensive patterns was higher in the female during play and in the male during aggression, which conforms to the role reversal play phenomenon. In conclusion, this report confirms that the absence of evidence is not the evidence of absence. In human and non-human primates, social play is widespread among immature individuals and is considered the main social interaction characterizing the juvenile developmental phase Fairbanks ; Cordoni and Palagi This finding supports the diverse causal and functional nature of adult social play behaviour in primates.
