Body signals used during social play in captive immature western lowland gorillas.

The play face is a well-established play signal in nonhuman primates that functions to invite play and convey a playful intent. However, recent evidence indicates that some species display repertoires of play signals that may have more specific meanings related to particular aspects of play. Furthermore, previous studies have inconsistently categorized gorilla behaviors as play signals versus actual play. Here we aim to identify behaviors displayed by two immature captive western lowland gorillas (Gorilla gorilla gorilla) at the Buffalo Zoo that meet three necessary criteria to be considered play signals. Specifically, we (1) investigate whether 21 candidate signals are significantly different from actual play behaviors, (2) and from similar signals used in non-play contexts, and (3) determine whether they predict the occurrence of social play. The results indicate that at least 18 of the 21 candidate signals have strong support for classification as play signals. These findings represent first steps in determining the function of multiple play signals in gorillas.