How do single housing and group housing arrangements affect the well-being of male mice? How can rodents benefit from a special type of pair housing arrangement known as separated pair housing? These were the questions explored as part of a 12-week study which saw researchers from Charité work alongside colleagues from Freie Universität Berlin (FU) and the University of Vienna. Results from this research suggest that the conditions associated with these three types of housing influence not only the animals’ behavior, but also hormone levels and body weight.
Mice are social animals. In the wild, most mice live in large family groups. Some males, however, choose to live a solitary lifestyle. Both types of living arrangements are entirely normal. What never happens in the wild is male animals living exclusively with other males. For laboratory animals, however, this arrangement represents the norm and is used to prevent uncontrolled breeding. There are exceptions. Aggressive behavior in a male – not a rare occurrence – will result in said male being place in single housing. Similarly, certain experiments will require mice to live apart from others for a period of time, for instance to control food intake or prevent injuries by other animals.
Seeing, hearing, and smelling a cage mate
As single housing may cause stress, researchers from Charité, Freie Universität Berlin and the University of Vienna have studied a type of paired housing with a twist: A cage divided into two compartments by a perforated transparent divider, which enables two males to remain separated yet able to see, hear and smell their respective cage mate.
“We thought pair housing was better for the animals,” explains Dr. Kristina Ullman, who was Charité’s Animal Welfare Officer until September 2020. “Our study, however, was unable to confirm this hypothesis unequivocally.”
In fact, the Charité 3R-funded study showed that animals in pair housing experienced similarly low levels of stress and anxiety as their mates in single and group housing. Conversely, this also means that levels of well-being are generally quite high in male mice kept at Charité.
Study finds no difference in levels of stress caused by different types of housing
“The concern that our conventional housing arrangements could cause the animals more stress than pair housing was not confirmed,” says veterinary surgeon Dr. Ullman. “In that sense, we see our study both as a vindication and as confirmation that mice are very good at adapting to different housing conditions, which is also extremely reassuring.” To estimate and compare levels of quality of life in differently housed rodents, the researchers observed the animals for 12 weeks. They measured a range of parameters, including anxiety-related behavior, social interactions, stress hormones and general well-being.
Nesting and burrowing behavior, for instance, is an important indicator of mouse well-being. If well-being decreases, nest-building and burrowing for materials such as food pellets will virtually cease. This was not observed in any of the groups studied. Anxiety-related behavior, for which the measured parameter was ‘voluntary climbing onto a ladder’, remained within the normal range for all participating mice. Similarly, no animals in any of the groups studied exhibited signs of stress – except on day 1 of the acclimatization period, as indicated by the levels of stress hormone found in mouse hair feces.
Housing system affects hormones, body weight and behavior
Despite the relatively homogeneous results with regard to mouse well-being, the researchers found a number of differences between the housing systems. Levels of sex hormones suggested that animals in group housing are likely to be under constant pressure to (re-)establish a social hierarchy. Body weight results were also higher in group-housed than in single- or pair-housed males. Animals in pair housing were also found to prefer building their nests in the middle of the cage, near the cage divider. The researchers interpreted this as a preference to nest near their cage mate. The nests built by animals in pair housing also showed a higher degree of complexity than those built by animals in single housing. According to Dr. Ullman, both of these findings suggest that pair housing has a beneficial effect on the animals. Differences between the study groups also underlined the importance of considering housing systems during the design, analysis, and reporting stages of animal research.
Use of pair-housing in high-stress situations remains to be explored
“Housing conditions certainly have an impact on the animals. However, benefits are not sufficiently large to warrant the exclusive use of pair housing,” emphasizes Kristina Ullmann. The pair housing cages acquired as part of this study continue to be used at Charité.
It would be useful to test whether pair housing might be a better option for animals in high-stress situations, such as after surgery. Our observations have shown that our mice are very adaptable and show high levels of well-being. Furthermore, while we are already complying with all legally mandated requirements, it may be possible to achieve even higher levels of well-being for the animals.
Text: Beatrice Hamberger
Publication: Hohlbaum K, Frahm S, Rex A, Palme R, Thöne-Reineke C, Ullmann K. Social enrichment by separated pair housing of male C57BL/6JRj mice. Sci Rep. 2020 Jul 7;10(1):11165. doi: 10.1038/s41598-020-67902-w. PMID: 32636413; PMCID: PMC7341880.
Dr. med. vet. Kristina Ullmann
Director Animal Welfare
NUVISAN ICB GmbH
Coordination Communication and Public RelationsCharité – Universitätsmedizin Berlin
Postal address: Charitéplatz 1 10117 Berlin
Campus / internal address:Luisenstraße 13a | 10117 Berlin
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