Play is commonly used to assess affective states in both humans and non-human animals. Play appears to be most common when animals are well-fed and not under any direct threats to fitness. Could play and playfulness therefore indicate pre-existing positive emotions, and thence optimal animal welfare? We examine this question by surveying the internal and external conditions that promote or suppress play in a variety of species, starting with humans. We find that negative affective states and poor welfare usually do suppress play (although there are notable exceptions where the opposite occurs). Furthermore, research in children suggests that beyond the frequency or total duration of play, poor welfare may additionally be reflected in qualitative aspects of this heterogeneous behaviour (e.g. display of solitary over social play; and the 'fragmentation' of play bouts) that are often overlooked in animals. There are surprisingly few studies of play in subjects with pre-existing optimal welfare or in unambiguously highly positive affective states, making it currently impossible to determine whether play can distinguish optimal or good welfare from merely neutral welfare. This therefore represents an important and exciting area for future research.

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We are honored to write the opening article of this focus issue of Lab Animal. This focus issue could not occur at a more important time for biomedical research and the use of animals in science in general. The progressive worsening of success rates in human trials (currently 1 in 9 drugs entering human trials will succeed) 1-4 , combined with the explosion of interest in the reproducibility crisis 5-8 and the recognition that most drugs fail in human trials due to insufficient efficacy 1,2,4 , has led to a growing suspicion that the failure of translation from animal work to human outcomes may in some way reflect issues in animal research itself 5-19 -after all, every drug that fails in humans "worked" in an animal model. Indeed pharmaceutical companies continue to disinvest in internal animal R&D, a trend begun in the last decade, passing on the cost and risk to academia and startups 5,20 . Even this approach is not foolproof as pharmaceutical companies often cannot replicate the results of published work from academia 5,8,13 . Accordingly, there is a growing trend to focus on human, not animal, work for basic discovery 17 .Discarding animal research entirely is not the answer. When properly used, animal models have incredible value, not least the ability to follow biomarkers from birth to disease onset in a year or less (in the case of mice), which is impossible in humans 17,18 . There are patterns and principles that can help us identify models and results that are more or less likely to translate, and there are also easily realized, simple changes in the execution of animal work that will inherently improve translation [16][17][18] . This isn't a new concept; looking back over the last 10-15 years we can see many authors have been candid about the merits, strengths, weaknesses, reproducibility, and translatability of various animal models 1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] . Our goal with this article is to unite the common themes in this broader emerging literature and this special issue.Thus, the central point is that we (i.e., refs. 17,18,26-28,39,41) do not represent a voice in the wilderness, but one voice in a chorus and that this emerging literature 1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]42,43 reflects a nascent discipline which can be codified as the study of how knowledge is gained from animal research. We propose the title "Therioepistemology" This focus issue of Lab Animal coincides with a tipping point in biomedical research. For the first time, the scale of the reproducibility and translatability crisis is widely understood beyond the small cadre of researchers who have been studying it and the pharmaceutical and biotech companies who have been living it. Here we argue that an emerging literature, including the papers in this focus issue, has begun to congeal around a set of recurring themes, which themselves represent a paradigm shift. This paradigm shift can be c...

For captive animals, living in barren conditions leads to stereotypic behaviour that is hard to alleviate using environmental enrichment. This resistance to enrichment is often explained via mechanisms that decouple abnormal behaviour from current welfare, such as 'establishment': a hypothetical process whereby repetition increases behaviour's predictability and resistance to change. If such hypotheses are correct, then animals with enrichment-resistant stereotypic behaviour should still find enrichments rewarding. Alternatively, this behaviour could reflect a failure to improve welfare: plausible because age and chronic stress increase neophobia and anhedonia. If this hypothesis is correct, animals with enrichment-resistant stereotypic behaviour should value enrichments less than conspecifics. We tested these hypotheses using C57BL/6 mice, Mus musculus, aged 10e11 and 6e7 months, raised in barren laboratory cages. We observed their behaviour in both these and large enriched cages. Enrichment was more effective on the younger animals. However, contrary to ideas about establishment, the spontaneous predictability of stereotypic behaviour did not increase with age; nor was enrichment less effective on more predictable or timeconsuming forms. We assessed the reward value of enriched cages by allowing access via progressively weighted doors (maximum weight pushed corresponding to peak motivation). In older mice, those individuals whose stereotypic behaviour was least reduced by enrichment were also the least motivated to gain access to enrichment. This suggests that the welfare of middle-aged-animals, as well as their stereotypic behaviour, is harder to improve using environmental enrichment. Ó