Alone but not always lonely: Social cues alleviate isolation induced behavioural stress in wild zebrafish

Abstract

Social isolation has been routinely used for experimental studies on many animal model systems. Additionally, considering the recent increase in the prevalence of isolation in the human population, it has become even more imperative to understand social isolation and measures that might alleviate its effects. Using wild-caught zebrafish, our study aims to elucidate the effect of social cues on reducing behavioural stress caused due to acute social isolation in a socially aggregating species. Fish were kept in groups or individually, with either visual and olfactory cues, only visual cues or no social cues, and tested for behavioural stress every alternate day in a commonly used open tank test. Experimental observations measured a range of behavioural indicators of stress such as geotaxis (ratio of time spent in top half of the tank to time spent in the bottom half of the tank), thigmotaxis (average duration of visit to wall of tanks), activity (homogeneity and extent) and freezing (average duration of freezing bout) among individuals that were kept in differing levels of social isolation. We found that the presence of both visual and olfactory cues from conspecifics significantly decreased stress in individuals as compared to only visual cues being present. Fish that were completely isolated showed significant increase in stress levels, as indicated by all observed indicators of behavioural stress. Over trials, except for completely isolated individuals, fish showed an eventual decrease in stress levels, indicating acclimatisation. Males and females showed differences in behavioural stress, with females exhibiting a lower baseline stress level, as well as less response to isolation. Our findings not only demonstrate the importance of social cues in managing stress when social isolation is unavoidable, but also add to the knowledge about behavioural stress response measures in zebrafish, which is an important vertebrate model used to extrapolate information about stress and anxiety in humans as well.