The Interaction Engine Hypothesis postulates that humans have a unique ability and motivation for social interaction. A crucial juncture in the ontogeny of the interaction engine could be around 2-4 years of age, but observational studies of children in natural contexts are limited. These data appear critical also for comparison with non-human primates. Here, we report on focal observations on 31 children aged 2- and 4-years old in four preschools (10 h per child). Children interact with a wide range of partners, many infrequently, but with one or two close friends. Four-year olds engage in cooperative social interactions more often than 2-year olds and fight less than 2-year olds. Conversations and playing with objects are the most frequent social interaction types in both age groups. Children engage in social interactions with peers frequently (on average 13 distinct social interactions per hour) and briefly (28 s on average) and shorter than those of great apes in comparable studies. Their social interactions feature entry and exit phases about two-thirds of the time, less frequently than great apes. The results support the Interaction Engine Hypothesis, as young children manifest a remarkable motivation and ability for fast-paced interactions with multiple partners. This article is part of the theme issue Revisiting the human interaction engine: comparative approaches to social action coordination.

Past research on interspecies communication has shown that animals can be trained to use Augmentative Interspecies Communication (AIC) devices, such as soundboards, to make simple requests of their caretakers. The recent uptake in AIC devices by hundreds of pet owners around the world offers a novel opportunity to investigate whether AIC is possible with owner-trained family dogs. To answer this question, we carried out two studies to test pet dogs ability to recognise and respond appropriately to food-related, play-related, and outside-related words on their soundboards. One study was conducted by researchers, and the other by citizen scientists who followed the same procedure. Further, we investigated whether these behaviours depended on the identity of the person presenting the word (unfamiliar person or dogs owner) and the mode of its presentation (spoken or produced by a pressed button). We find that dogs produced contextually appropriate behaviours for both play-related and outside-related words regardless of the identity of the person producing them and the mode in which they were produced. Therefore, pet dogs can be successfully taught by their owners to associate words recorded onto soundboard buttons to their outcomes in the real world, and they respond appropriately to these words even when they are presented in the absence of any other cues, such as the owners body language.