An exploration of the postural, location- and social contact- related sub-characteristics of inactive but awake behaviour as a depression-like indicator in mice

Inactive behaviour is essential to life. However, specific forms of inactivity may be indicative of compromised welfare in certain captive conditions. Inactive but awake behaviour (IBA - spontaneous, motionless awake behaviour without interacting with the surroundings) has been documented in some species and may be associated with poor welfare and negatively valenced affective states. In our previous work in laboratory mice, we have identified environmental risk factors (non-enriched housing) and curative factors (antidepressant drug Venlafaxine) for IBA and we hypothesise that greater levels of IBA may represent a depression-like state in this species. Here we aimed to identify which specific sub-characteristics of IBA would show construct validity as a depression-like state by exploring the posture (i.e. lying, curled lying or sitting), social contact position (i.e. in physical contact with a cage mate or not) and location of mice while performing the behaviour during two experiments (respectively investigating the aetiology and the curative factors of IBA). In both experiments we expected that more IBA would be displayed in standard (non-enriched) laboratory cages, compared with large highly-enriched cages and that a move from a highly-enriched to a non-enriched cage would increase IBA, while the opposite treatment would result in a decrease. In our second experiment (curative factors investigation), we predicted that less IBA would be displayed by mice that voluntarily ingested an antidepressant (Venlafaxine) versus a placebo. Because we could not control the number of instances of each IBA sub-characteristic we measured and we had no a priori predictions about which IBA sub-characteristics would match our general IBA treatment predictions, we compared the effect size and the direction of the effect between our treatment groups to explore which of the sub-characteristics matched our general IBA predictions. Overall, we found little variation in the location IBA was performed, with the majority being seen in the nest. Across treatment comparisons in both experiments, overall, the largest effect sizes were measured for IBA performed when in contact with the cage mate and performed when lying and both characteristics generally matched the direction of our treatment-related predictions. We suggest that future work should perform more detailed analyses of the specific characteristics of IBA by identifying behavioural sequences and the co-occurrence of the sub-characteristics to obtain a more complete picture of IBA as a depression-like indicator.