Reviews Animal play and animal welfare Suzanne D. E. Held a, * , Marek  Spinka b, 1 a Centre for Behavioural Biology, Department of Clinical Veterinary Science, University of Bristol b Department of Ethology, Institute of Animal Science in Prague article info Article history: Received 5 August 2010 Initial acceptance 5 October 2010 Final acceptance 5 January 2011 Available online 2 March 2011 MS. number: 10-00529 Keywords: animal play animal welfare captive animal Play has long been identified as a potential welfare indicator because it often disappears when animals are under fitness challenge and because it is thought to be accompanied by a pleasurable emotional experience. But animal play is a vexing behavioural phenomenon, characteristically flexible and variable within and between species, with its proximate mechanisms and ultimate functions still not fully understood. Its relationship to animal welfare is therefore complex and merits a focused theoretical investigation. We review evidence on four aspects of the playewelfare relationship: first, that play indicates the absence of fitness threats; second, that play acts as a reward and flags up the presence of opioid-mediated pleasurable emotional experiences; third, that play brings immediate psychological benefits and long-term fitness and health benefits, and thus improves current and future welfare; and finally, that play is socially contagious and therefore capable of spreading good welfare in groups. On this basis, we argue that play does indeed hold promise as a welfare indicator and also as a tool to improve it; but we also point to difficulties in its study and interpretation, and identify some unresolved questions. As a welfare indicator, play may signal both the absence of bad welfare and the presence of good welfare, thus covering a wide range of the welfare spectrum. However, play can also increase in stressful situa- tions, in response to reduced parental care, or as a rebound after a period of deprivation and therefore does not consistently reflect favourable environmental conditions. A better fundamental understanding is needed of the varied ultimate functions and proximate mechanisms of play, and the species-specific play patterns of captive animals, in order to be able to explain exactly what an animal’s play behaviour tells us about its welfare state, and whether and how play might be applied as a tool to improve welfare. Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. To suggest a relationship between the play behaviour of animals and their welfare is hardly novel. Play has long been identified as a potential indicator of the current welfare state of an animal (e.g. Fagen 1981; Lawrence 1987). It is easy to recognize, present in a wide array of mammals and can be measured noninvasively (Fraser & Duncan 1998;  Spinka et al. 2001; Barnard 2004; for other taxa see Fagen 1981; Burghardt 2005); it is often expressed in the absence of fitness threats and can drop out of the behavioural repertoire when conditions become more challenging (Lawrence 1987; Fraser & Duncan 1998;  Spinka et al. 2001; Dawkins 2006); it is also commonly linked to the experience of positive emotions in animals (Fraser & Duncan 1998;  Spinka et al. 2001; Barnard 2004; Burgdorf & Panksepp 2006) as it has long been in humans (e.g. Clark & Miller 1998). In human children, for example, play inhibi- tion is one of the core symptoms of depression, both in standardized play tests and in free play situations (Lous et al. 2002). The most recent review of positive emotions in the context of animal welfare assessment therefore singled out play behaviour as one of the three most promising indicators of positive emotions in captive (to include ‘domestic’) animals (Boissy et al. 2007). Play behaviour thus appears to have the potential to flag up challenging conditions, in which an animal’s welfare may be compromised, as well as favourable situations, which induce positive emotions and good welfare. Furthermore, playing can have immediate, delayed and/or long-term benefits and thus cause welfare improvements, not just reflect them. Yet a systematic theoretical or experimental investigation of the relationship between animal play and animal welfare is lacking. The main purpose of this review therefore is to provide a theoretical analysis of how an animal’s play behaviour relates to its welfare. Within this, we put the case that play can not only result from good welfare but also cause it. Defining ‘play’ is difficult because it covers many behavioural categories, varies considerably between and within species, and its single or multiple functional significance is still being debated (e.g. Bekoff & Byers 1998; Power 2000;  Spinka et al. 2001; Burghardt 2005). In the absence of a widely agreed functional or structural * Correspondence: S. D. E. Held, Department of Clinical Veterinary Science, University of Bristol, Langford BS40 5DU, U.K. E-mail address: suzanne.held@bris.ac.uk (S. D. E. Held). 1 M.  Spinka is at the Research Institute of Animal Production, Pratelství 815, Prague-Uh rín eves 104 00, Czech Republic. Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/anbehav 0003-3472/$38.00 Ó 2011 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2011.01.007 Animal Behaviour 81 (2011) 891e899