Can animals develop depression? An overview and assessment of ‘depression-like’ states

Abstract

Describing certain animal behaviours as ‘depression-like’ or ‘depressive’ has become common across several fields of research. These typically involve unusually low activity or unresponsiveness and/or reduced interest in pleasure (anhedonia). While the term ‘depression-like’ carefully avoids directly claiming that animals are depressed, this narrative review asks whether stronger conclusions can be legitimate, with animals developing the clinical disorder as seen in humans (cf., DSM-V/ICD-10). Here, we examine evidence from animal models of depression (especially chronically stressed rats) and animals experiencing poor welfare in conventional captive conditions (e.g., laboratory mice and production pigs in barren environments). We find troubling evidence that animals are indeed capable of experiencing clinical depression, but demonstrate that a true diagnosis has yet to be confirmed in any case. We thus highlight the importance of investigating the co-occurrence of depressive criteria and discuss the potential welfare and ethical implications of animal depression.

Do greater levels of in-cage waking inactivity in laboratory mice reflect a spontaneous depression-like symptom? A pharmacological investigation

“…The symptoms and causes of depression are not unique to humans [11][12][13], and some depression-like features have been modelled in non-human animals (hereafter animals) for decades e.g. [14,15], although a full diagnosis involving the co-occurrence of depressive criteria has yet to be confirmed in non-human animals [13]. Validated through their induction by stressors and alleviation by anti-depressants, these include anhedonia e.g.…”

“…First, mice displaying greater IBA show symptomatic similarities with features of human depression. This includes elevated inactivity that may reflect psychomotor retardation [13,24]; reduced preference for sucrose (Trevarthen et al in preparation), i.e. a common proxy for anhedonia in animal model of depression; greater immobility in forced swim tests, i.e.…”

Waking inactivity as a welfare indicator in laboratory mice: investigating postures, facial expressions and depression-like states

“…This evidence is not sufficient to confirm depression in IBA mice, since it is the co-occurrence of five or more specific diagnostic criteria that is crucial [6,26]. However, it does indicate the value of now testing for co-occurring additional signs of depression, such as cognitive deficits, anhedonia, and low mood as inferred from 'pessimism' in judgement bias tasks (see MacLellan et al [6] and Resasco et al [72]). Furthermore, the sleep and body weight data suggest that if the depression hypothesis is supported, mice may experience the 'atypical' subtype (as opposed to melancholic; [26,79]).…”

“…However, it does indicate the value of now testing for co-occurring additional signs of depression, such as cognitive deficits, anhedonia, and low mood as inferred from ‘pessimism’ in judgement bias tasks (see MacLellan et al . [6] and Resasco et al . [72]).…”

“…These are disturbing, clearly abnormal and widely recognized as indicators of poor welfare. But other, less active responses are now attracting attention and concern [5,6]. For example, the hunched postures of research primates isolated in metal indoor cages [7,8], the excessive time spent hiding by wild-caught striped mice brought into the laboratory [9], and the ‘flat necked’ postures of unresponsive, anhedonic horses [10,11] have become recognized as signs of poor wellbeing.…”