The behavioural significance of animal play
Play is frequently reported to be difficult to define but nevertheless easy to recognise. Other workers' attempts at definition are reviewed. These commonly involve initial subjective recognition of play followed by description in terms of characteristics which are frequently shared with the play of other species. Play appears to have certain associated costs and is therefore assumed to benefit the animal in some way in order to justify its existence in evolutionary terms, but the nature of this benefit is unknown. Theories regarding the functions of play are discussed in the light of its properties and potential costs. Chapter 2 describes an experiment to test the common assumption that observers agree on what constitutes play. Comparison of the judgements of the ten naive observers on the behaviour of young rats indicated that the majority agreed, and the activities which they called play formed the basis of the working definition of rat play which is used in the studies described below. A longitudinal study of aspects of the play and other behaviour of littermate groups of rats is described in Chapter 3. The quantitative findings are used to test the validity of certain characteristics for rat play. The experiments described in Chapters 4 and 5 examined aspects of the rat's motivation to play. The nature of recent social experience was found to influence the rat's tendency to play, and a series of choice experiments showed that play was highly reinforcing by comparison with other forms of social experience. The extent to which existing definitions of play can be applied to that of rats is examined in the light of the observational and experimental evidence described in Chapters 2-5 concerning its characteristics. The cost of play for rats is estimated using indirect evidence and tentative suggestions are made as to its functions.