Many animals engage in some type of social play-that is, playing with others, including chasing and fighting. Three functions of social play have been proposed: Social play may lead to the forging of long-lasting social bonds, help develop much needed physical skills, such as those relating to fighting, hunting, and mating, and aid in the development of cognitive skills. One cognitively related benefit of social play revolves around the idea of self-assessment. Here, animals use social play as a means to monitor their developmental progress as compared to others. For example, in infant sable antelope (Hippotragus niger), individuals prefer same-age play partners. While this preference could be due to numerous factors, Kaci Thompson has hypothesized that it is primarily a function of infants attempting to choose play partners that provide them with a reasonable comparison from which to gauge their own development.

Since many of the behavior patterns seen during play are also common in other contexts-hunting, mating, dangerous aggressive contests-how do animals know they are playing and not engaged in the real activity? And even more to the point, how do they communicate this information to each other? Bekoff has proposed three possible solutions to this important, but often overlooked, question. One way that animals may distinguish play from related activities is that the order and frequency of behavioral components of play is often quite different from that of the real activity. That is when play behavior is compared with the adult functional behavior that it resembles, behavioral patterns during play are often exaggerated and misplaced. If young animals are able to distinguish these exaggerations and misorderings of behavioral patterns by, for example, observing adults that are not involved in play, a relatively simple explanation exists for how animals know they are playing.

A second, somewhat related, means by which animals may be able to distinguish play from other activities is by the placement of play markers. These are also known as play signals and can serve both to initiate play and to indicate the desire to continue playing and to warn adults that the young are playing and not in danger of injury. In canids (the animal group that includes dogs and wolves) for example, biting and shaking are usually performed during dangerous activities such as fighting and predation. Yet, biting and shaking are also play behaviors of young canids. Bekoff found that play markers such as a bow (lowering the head) would precede biting and rapid side-to-side shaking of the head to indicate that they were not dangerous behaviors. The bow would communicate that this action should be viewed in a new context-that of play. Another play marker might be a particular kind of vocalization-for example, chirping in a rat, whistling in a mongoose, panting in a wolf or a chimpanzee-before or during a play interaction. Or there might be a distinctive smell that indicated that the animals were engaged in play.

Play markers have also been found in primates such as the juvenile lowland gorilla. Juvenile lowland gorillas play with each other often, and play ranges from what Elisabetta Palagi and her colleagues call gentle play to rough play. Palagi's team discovered that when juvenile gorillas-particularly males-were involved with rough play, the play was often preceded by a facial gesture they call the play face. This facial gesture, which is not seen in other contexts, includes slightly lowered eyebrows and an open mouth. In addition to using this facial gesture during rough play, juvenile gorillas also displayed it when a play session was in a place that made escape (leaving) difficult-another context in which it may be important to signal to others that what is about to occur is play.

Yet another way by which young animals may be able to distinguish play from related behaviors is by role reversal, or self handicapping, on the part of any older playmates they may have. In role reversal and self-handicapping, older individuals either allow subordinate younger animals to act as if they are dominant during play or the older animals perform some act (for example, an aggressive act)at a level clearly below that of which they are capable. Either of these provides younger playmates with the opportunity to recognize that they are involved in a play encounter.