A hemp rope or a cardboard pipe as supplemental environmental enrichment in male C57bl/6 mice: does it affect agonistic and social behavior, stress and anxiety?

Environmental enrichment is increasingly being used to enhance welfare among laboratory animals, as environmental surroundings such as housing conditions have a great impact on the animals. However, when introducing environmental enrichment it is important to investigate whether a negative effect on behavior and physiological parameters and thus on the welfare of the animals occur. This study is a randomized trial investigating the effect of supplementing standard housing with a hemp rope or a cardboard pipe, as part of environmental enrichment, on agonistic and social behavior, stress and anxiety level in male C57bl/6 mice. Moreover, it was investigated whether a possible increase in agonistic behavior was correlated with high levels of testosterone. The social interaction test measured agonistic and social behavior, number of wounded companion animals was used as a surrogate measure of agonistic behavior, fecal corticosterone levels measured stress, the elevated plus maze measured anxiety and testosterone was measured in serum. Three hundred thirty six male C57bl/6 mice were randomized to (1) standard environment, (2) supplement of a hemp rope or (3) supplement of a cardboard pipe. We found no agonistic behavior in any of the three groups, but a significantly higher number of wounded companion animals among the hemp rope and the cardboard pipe groups. Both intervention groups showed more social behavior and produced more corticosterone. No difference was detected in anxiety and testosterone levels between the three groups. Future studies are needed to investigate the effect on welfare in more depth.