Clustered environmental enrichments induce more aggression and stereotypic behaviour than do dispersed enrichments in female mice

Adding environmental enrichments to a previously resource-poor cage or enclosure can sometimes cause elevated aggression in socially housed animals, due to competition over the provided resources. Here, using female C57BL/6J mice, we investigated whether the way that environmental enrichments are distributed affects the risk of negative interactions between individuals and whether familiar siblings are less likely to compete than unfamiliar, unrelated animals. Twenty adult females were each subjected to four treatments in a 2 × 2 factorial design with familiarity to partner (familiar versus unfamiliar) and resource distribution (environmental enrichments clustered versus dispersed) as the main factors. The resources used were: running wheels, nesting material of two types, and wooden chewing sticks coated with peanut butter. The behaviour of each female was observed 18 times for 5 min per treatment during the week they were housed in the treatment. In addition, stress levels were measured after each treatment by corticosterone metabolites in the faeces. The results showed increased aggression (P = 0.035) and stereotypic behaviour (P = 0.007), and a trend towards higher rates of displacement of one mouse by another from resources (P = 0.057), in the clustered environment compared to the dispersed environment. However, no effects of conspecific familiarity were found. Furthermore, the elevated aggression and stereotypic behaviour co-varied in the clustered treatment in a way not seen in our distributed condition, suggesting common underlying causal factors. However, we could not detect any treatment effects in the stress level measured. Our results suggest that physically arranging valuable resources like enrichments in a dispersed way, so that they are easy to share and hard to monopolize, is better for welfare than clustering.