Does size matter? Evaluating the space requirements for laboratory mice.

Mice are the most commonly used laboratory animal – comprising approximately 70% of all animals used for research. Despite their widespread use, surprisingly little is known about how the behavioural biology of mice relates to the social and physical conditions of the laboratory environment. A team of researchers, from the Division of Animal Welfare at the University of Bern, Switzerland, therefore studied systematically how space allowance and group size affects measures of animal welfare in both males and females of two strains of mice. Their findings suggest that within the range of conditions commonly found in laboratory mouse housing, space allowance has little impact on measures of welfare, except for group size, as larger groups may be a risk factor for escalating aggression in males of some strains. The study was funded by the European Research Council (ERC Advanced Grant, REFINE).

The majority of all animals that are used for research are used in procedures that confer little or no pain or distress. For example, in Switzerland, approximately 70% of all research is rated as severity level 0 and 1 (see link for definitions). Therefore, for most of these animals, pain or distress, and therefore fundamental aspects of animal well-being, is not necessarily determined by the kinds of experimental procedures that are performed on them, but by the manner in which we house and care for them (husbandry).

Distribution of animals (%) per severity category across time in Switzerland.

To regulate the housing of mice, many countries provide legal guidelines which specify minimum requirements, and in particular, the minimum space allowance of a single animal in relation to its body weight.

Reviews of the existing literature, however, have revealed that available scientific evidence informing legal guidelines is scarce. First, there are few studies which systematically investigated the space needs of rodents. Secondly, in the limited literature, the results of different studies are ambiguous – with more recent studies suggesting that we can as much as halve the minimal space allowance with no decrement to well-being. Thirdly, nearly all of the existing studies confound group size (i.e., the number of mice per cage) and floor area (i.e., cage size). Stocking density (i.e., group size in a given floor area, e.g., 3 mice/300 cm2) and space allocation (i.e., floor area per mouse, e.g., 100 cm2/mouse) are intimately related. Specifically, if floor area is kept constant and group size is increased, stocking density is increased while space allocation is decreased.

Conversely, if group size is kept constant (e.g., three mice) and floor area is increased, stocking density is decreased while space allocation is increased.

Therefore current legislation which specifies minimal standards for space are not based on evidence but rather common practice. Interestingly, in Switzerland, a similar common practice policy is observed where space allowance for mice that are kept as pets is markedly higher compared to mice used for research.

However, legislation and policy should be based on evidence and not common practice. Therefore, we investigated how space allowance affects measures of animal welfare in mice by systematically varying group size and cage type in both males and females of two strains of mice (C57BL/6ByJ and BALB/cByJ; n = 216 cages, a total of 1152 mice).

This study design allowed us to disentangle the effects of total floor area, group size, stocking density, and individual space allocation on a broad range of measures of welfare, including growth (food and water intake, body mass); stress physiology (glucocorticoid metabolites in faecal boli); emotionality (open field behaviour); brain function (recurrent perseveration in a two-choice guessing task); and home-cage behaviour (activity, stereotypic behaviour).

While increasing group size was associated with a decrease in food and water intake, and more specifically with increased attrition due to escalated aggression in males of one strain of mouse (BALB), no other consistent effects of any aspect of space allowance were found with respect to the measures studied here. Our results indicate that within the range of conditions commonly found in laboratory mouse housing, space allowance has little impact on measures of welfare, except for group size which may be a risk factor for escalating aggression in males of some strains.

These findings highlight the difficulty in making general recommendations for mice – although these results are in line with reviews of previous studies. However, we must echo caution in the interpretation of these findings. First, the present study, as well as previous studies, has focused on relatively crude measures of animal welfare. Further studies, extending and replicating these effects, may be needed using methods to assess the animals’ emotional states depending on specific housing standards. Second, albeit covering most of the range typically found in mouse studies, the range of space allowance, and in particular group size, studied here covers a relatively narrow range of conditions compared to the situation in the wild. For example, home-ranges of wild or feral mice can range from 1m2 under food-rich, densely populated conditions to as much as 80’000m2. Thus, the lack of more pronounced effects may indicate that variation within this narrow range may not matter much to mice. This, however, does not necessarily mean that current space regulations are appropriate, as our results only permit conclusions about relative levels of welfare between the studied treatments, but not about absolute levels of welfare.

Nevertheless, there are good reasons to assume that space allowance per se may not be the most important aspect of housing conditions for the welfare of laboratory mice. First, similar studies investigating effects of space allowance in mice, as well as other rodents have found similar results. Second, other studies varying both space allowance and environmental enrichment generally found that the provision of critical resources (e.g., shelter, nesting material) had stronger effects on measures of welfare than the provision of extra space. Therefore, further studies are needed which investigate the effects of floor area and group size in combination with other relevant variables, such as structural enrichments that promote active engagement with, and control over, the environment. This may be key in mediating social behaviour in ways that prevent escalating aggression, leading to high levels of stress, injury, and ultimately attrition. Before we accept last resorts such as housing males at higher densities or singly to attenuate or prevent escalating aggression, we should first examine more systematically whether it is possible to create housing conditions for mice that account for their behavioural biology without compromising their welfare, which may also compromise the validity of the research conducted with them.

Jeremy D. Bailoo

This research has been published in the journal Scientific Reports and has been awarded the 2017 Prix Jean-Pierre Miéville for outstanding contribution to the field of Animal Welfare Science.

Evaluation of the effects of space allowance on measures of animal welfare in laboratory mice. Bailoo, J. D., Murphy, E., Varholick, J. A., Novak, J., Palme, R., Würbel, H. (2017). Scientific Reports. DOI: 10.1038/s41598-017-18493-6

4 thoughts on “Does size matter? Evaluating the space requirements for laboratory mice.

    1. Legal minimum requirements vary by jurisdiction, but may include climbing opportunities, suitable material to gnaw on, firm floor with suitable litter, social housing (at least pairs) etc.

      It is important to note that these are minimum requirements, which means, at the discretion of the researcher, standards for housing and care can go above and beyond these. This is not a simple issue, as adding something as simple as a shelter, may increase aggression in some strains of male mice. Therefore, most supplementation to standard housing and care strategies, should be pilot tested on the relevant population of animals, before being recommended.

  1. It is certainly true that in comparison with the huge distances that can be travelled by mice in the natural environment, the addition of a few extra square cms of empty space to a very small cage isn’t likely to make that much difference to a mouse’s welfare. Perhaps it’s a bit like saying to someone who needs $100 would you like me to give you 5 cents, 10 cents or 20 cents? However, the provision of additional space – which may not in itself improve welfare – does then allow the opportunity to improve the complexity of the environment and the degree of choice on offer to the animals, which we know is HUGELY important to an animal’s wellbeing.

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