15Investigating depression-like conditions in animals is methodologically challenging, but also revealed that horses scored as withdrawn then remained significantly likely to display the 24 behaviour. We measured sucrose intake, a classic measure of anhedonia never previously 25 applied to horses. Flavoured sugar blocks, novel to these subjects, were mounted in each stall 26 and weighed 3h, 8h, 24h and 30h after provision. We predicted that if affected by depression-27 like states, the most withdrawn horses would consume the least sucrose. This prediction was 28 met (F1,18 = 4.65, two tailed p = 0.04). This pattern could, however, potentially reflect general 29 appetite levels and/or food neophobia. To control for these confounds, hay consumption was 30 measured over 5 days, as were subjects' latencies to eat a meal scented with a novel odour. 31Although low hay consumption and long latencies to eat scented food did predict low sucrose 32 consumption, statistically controlling for these confounds did not eliminate the relationship 33 between being withdrawn and consuming less sucrose (although reducing it to a strong trend): Table 1) that include low, hopeless moods, 49"not caring", social withdrawal, and fatigue (APA, 2013). These are present for many days or Miller et al., 2007, and sub-57 normal levels e.g. Strickland et al., 2002). 58In terms of aetiology, a common trigger is chronic stress, for instance that arising from 59 aversive life events or chronic pain or illness ( e.g. Banks and Kerns, 1996; Blackburn-Munro 60 and Blackburn-Munro, 2001;Tafet and Bernardini, 2003; Munce et al., 2006;Siegrist, 2008; 61 Hammen et al., 2009; APA, 2013). Two types of cognitive change can often be observed before 62 the illness fully develops, and these may act as mediators in some subjects, being hypothesised 63 to help cause the onset and/or maintenance of the disease (Beck, 1967; Gotlib and 64 Krasnoperova, 1998). One is 'learned helplessness', which is proposed to occur "when highly 65 5 desired outcomes are believed improbable or highly aversive outcomes are believed probable, 66 and the individual comes to expect that no response in his repertoire will change their 67 likelihood" (Abramson et al., 1978). The second involves negative biases in attention, memory 68 and/or judgment (Beck, 1967; MacLeod and Byrne, 1996; Gotlib and Krasnoperova, 1998). 69Thus depressed people are prone for example, to judge ambiguous stimuli as being unlikely to 70 be positive ('cognitive pessimism'), and to recall unpleasant memories more readily than 71 pleasant ones. 72The symptoms of depression may not be unique to humans. A Web of Science literature 73 search using the terms "rats" OR "mice" OR "monkeys" AND "depression" yielded over Engel, 2002 p174, Brune et al., 2006 Ferdowsian et al., 2011; Hennessy et al., 2014) 96 and maternally deprived monkeys ( e.g. Harlow and Suomi, 1974;Suomi et al., 1975; Hennessy 97 et al., 2014). Horses, too, have been suggested to sometimes display depression-li...