Summary

Social grooming and rough-and-tumble play, along with caressing and hand-shaking, have something important in common, touching. Physical contact with another can be an essential ingredient of social communication – gentle touching can place the other animal at ease, whereas rough contact can do the opposite. Although the underlying neurobiology is still to be fully mapped, it does appear that there is a common set of neurochemical pathways that regulate these touch-induced changes in mood across mammals. Given its potential value in the manipulation of the affective state of social partners, it should not be surprising that touch is an important component of communication. A close analysis of the comparative and neurobiological literature on rough-and-tumble play, or play fighting, suggests that there are two levels of control over this touch-based communication. Firstly, there is the subcortically regulated emotional state of the interactants. Secondly, there is the cortically mediated modulation of the touching behavior that allows animals to use touch in a more strategic manner. How these two levels interact and what social conditions foster the need for additional cortical control over touch remains to be determined.

Introduction

A hostile donkey is rendered peaceful by the human object of its ire vigorously rubbing the base of its tail (Ewer 1967), an anxious monkey is calmed down after being groomed by a social partner (Goosen 1981), and agitated rats relax after social play (Arelis 2006; Darwish et al. 2001). What do all these situations have in common?

INTRODUCTION

Voric runs up to Victor from behind. As he runs past him, he grabs Victor by the tuft of hair on the crown of his head. Victor jerks his body back, bracing against being pulled forward; this brings Voric to a standstill facing Victor. After looking at each other for a moment, Victor turns his head and lunges, with his mouth wide open, and attempts to bite Voric's hand, which is still grasping Victor's hair. As Voric releases his grip and withdraws his hand, he simultaneously lunges forward, grabbing Victor by both shoulders, and tries to bite the side of his neck. Victor ducks and rolls onto his side. Voric follows, falling on top of him. They then grapple, and, when possible, gently deliver bites at each other. After a few seconds, Voric jumps up. He begins to run away, but then slows down, and with a wide-open mouth, looks back over his shoulder at Victor. While slow to rise at first, Victor now bolts upright and chases after Voric, who disappears into the bushes.

This description of play-fighting in two Tonkean macaques (Macaca tonkeana) taken from our research videotapes (Primate Research Centre, Strasbourg, 1996) could well represent almost any mammal. Intriguingly, two researchers who watched young rhesus macaques (Macaca mulatta) play like this drew fundamentally different conclusions about why they played. One claimed that the monkeys were rehearsing skills they would need as adults to be effective in combat (Symons, 1978).

Introduction

In the ongoing commentary on Smith's (1982) target article on play, Moran (1985) wrote a critique entitled ‘Behavioral description and its impact on functional inference.’ In response, Smith (1985) wrote a counter-critique entitled ‘Functional hypotheses and their impact on behavioral description.’ While seemingly at odds, these two points of view are actually complementary. If play fighting did not resemble serious fighting, in at least some crude ways, then it is unlikely that the hypothesis that the former served as practice for the latter (Groos 1898) would ever have been considered. In this regard, description preceded functional inference. However, once formulated, such an hypothesis makes predictions about other features of the behavior, in this case, play fighting, which are not known. Therefore, behavioral description informs functional inference, which in turn, influences further description.

That play fighting functions as a means of refining the skills necessary for combat is an hypothesis which continues to receive widespread support (e.g., Caro 1988; Fagen 1981; Pellis 1981b; Smith 1982; Symons 1978a). Indeed, the hypothesis that play serves as practice is widely endorsed in lay treatments of the topic, as can be ascertained from both television nature documentaries and from popular books. For example, Angier (1995) asserts that ‘through play, animals can rehearse many of the moves they will need as adults’ (p. 133). With regard to play fighting, this hypothesis asserts that this form of play serves to practice the tactics of attack and defense, which would otherwise only occur in dangerous situations (Symons 1978a). The enhancement of fighting skills that such practice affords is often argued to be responsible for shaping the form of play fighting (Smith 1982).