Do male mice prefer to live on their own?

Mice are the most commonly used laboratory animal species. They provide researchers with a model organism of mammalian physiology which recapitulates many of the normal and pathological processes seen in human biology. One of the most important scientific advances in the last few decades has been the ability to manipulate the genetics of the mouse to create novel mutations which can be used to answer important scientific questions. Genetically altered mice now represent a large proportion of the animals being used in research and the management of mouse colonies is an area where huge benefits in animal welfare could be made but equally, if not optimal, can create significant welfare costs and cumulative suffering. The housing environment of these animals is an area where the greatest impact over the duration of an animal's lifetime can be made. Whilst animals often do not show overt signs of distress associated with their housing environment, in male mice this is not the case. Fighting among males as they become sexually mature can lead to serious injury and even death. Even where animals do not show evidence of physical injury, there is the potential for chronic social stress within these group housed animals. Amongst the scientific community there is widespread concern about fighting in male mice and how best to manage this. Current beliefs are that mice are highly social and therefore require social housing in groups to achieve the highest standards of welfare. However, the evidence for this is quite varied and surprisingly limited. In fact, some studies in wild mice suggest that males may prefer not to live with other males. We propose that whilst mice may benefit from social interaction, this may not be offset by the social pressures which arise from group housing in a restricted (laboratory cage) environment. It is also possible that the social stress of group housing has higher welfare costs than the consequences of loss of physical contact with other mice.

A major limitation associated with previous research in this area has been a lack of objective methods to measure the impact of long term, low level social stress and cumulative suffering. In fact, it is only very recently that methods have been developed which can reliably measure behaviours which directly reflect the animals affective state and hence can provide a measure of welfare outcomes. Our research group is one of those which has pioneered methods to measure affective state in rodents. We are one of the leading groups working in the field of rodent models for depression research and we now want to apply this expertise to address our key question, 'Do male mice prefer to live on their own?' We have developed a programme of research which will look at different housing conditions and social structures. We want to provide objective evidence of the best approaches to male mouse housing and husbandry to minimise cumulative suffering and improve scientific outcomes through reduced stress and its impact on the variability of the data and therefore the numbers of animals needed for a particular experiment. Whilst our primary aim is to understand the impact of group versus individual housing, we also have designed experiments to test whether careful management of the cage environment could enable mice to achieve a more natural social structure even within the much smaller cage environment.

We anticipate that these studies will provide clear and objective measures of the welfare implications of different housing conditions. This knowledge can be used by animal technicians, researchers, vets, ethical review bodies, the Home Office and policy makers to make informed decisions about husbandry procedures based on knowledge of the welfare implications of the different options and the balance between providing social contact (sensory and/or physical) but avoiding social stress, fighting, injury and death.

In 2017, the UK used a total of 5.53 million animals in scientific procedures or in non-regulated scientific research, the majority of these were mice. A widely reported welfare issue is aggression and fighting amongst males, which are routinely housed in groups on the basis that current guidelines consider that mice, as a social species, require direct contact with conspecifics. The importance of this issue has been recognised in numerous publications and was the topic of a recent NC3Rs crowdsourcing data project. The experience of animals exposed to aggression is a major welfare concern with some animals suffering significant injury, being killed or requiring euthanasia as a consequence of fighting within groups. There is also the issue of chronic social stress which does not result in injury but is likely to contribute to the cumulative suffering of laboratory mice and their lifetime experience. This project draws on our expertise in studying affective state and chronic stress in rodents to specifically investigate optimal housing conditions for male mice. Our work plans provide an objective assessment of the welfare implications of different housing conditions (group vs individual, optimal enrichment) with the aim that the outcomes of this work will inform future guidance on how to best manage male mice in the laboratory environment. We will use our affective bias tasks to provide an objective and easily interpreted measure of the animal's affective state which we will complement with additional behavioural, physiological and biochemical measures of chronic stress. We will specifically test the hypothesis that sexually mature male mice prefer to live in their own territory but with access to adequate sensory information relating to conspecifics. Based on our own recent pilot research, we will also investigate the impact of temperature and whether this important, but little-studied environmental variable, has confounded previous research in this area.

In 2017, the UK used a total of 5.53 million animals in research. The majority of these were mice (1.1million regulated procedures, 1.45 million non-regulated; 670,000 genotyped July-Dec 2017; 1.7 million creation or breeding GA). Aggression in male mice is a major welfare issue therefore any outcomes which can improve their husbandry will have major welfare benefits. In terms of the 3Rs, our research will achieve:
1. REFINEMENT - reducing chronic social stress, aggression and fighting will improve the lifetime experience of the animals and reduce cumulative suffering.
2. REDUCTION - Aggression and stress are known to impact on data variability. Although currently the impacts are poorly understood, it is likely to increase the numbers of animals required to achieve statistical power.

Specific impacts:
1. CUMULATIVE SUFFERING AND THE WELFARE BENEFITS OF THIS RESEARCH
The experience of animals exposed to aggression is a serious welfare and scientific issue with animals suffering significant injury, being killed or requiring euthanasia as a consequence of fighting. Whilst animals experiencing sustained attacks have obvious welfare consequences, there is also the potential for subordinate animals to experience chronic stress when living in social groups. In addition to the experimental procedures used, experience of social stress and aggression will add to the life time experience of the animal and cumulative suffering. Methods to reduce or avoid aggression are needed and our work will deliver the knowledge needed to achieve this.

2. NEW GUIDANCE FOR MALE MOUSE HUSBANDRY BASED ON OBJECTIVE ASSESSMENT OF AFFECTIVE STATE AND CHRONIC STRESS
The prevalence of aggression in male mice indicates that current practises are not ideal. It also suggests that the research which has informed current recommendations for housing male mice may not have provided a complete picture. We will address this knowledge gap by undertaking a comprehensive and objective assessment of the consequence of group versus individual versus isolation housing in male mice. Although we hypothesise that individual housing for male mice within the constraints of laboratory conditions may reduce cumulative stress, it is also important to consider if group housing with appropriate enrichment can reduce aggression. We will be able to use the arising data to provide guidance for the scientific community and hope that we will be able to work with groups such as the NC3Rs to develop these and their widespread dissemination.

3. SCIENTIFIC KNOWLEDGE
Despite the fact that mice have been used in laboratory research since the 16th century we are only recently seeing research with a focus on the welfare of the laboratory mouse. We propose that the lack of objective methods to assess welfare means that previous studies may be confounded by other factors such as territorial behaviour, the effect of environmental conditions such as temperature and an over-generalisation from studies using social isolation. Our proposed research will provide new data on the social behaviour of mice within the laboratory environment. The scientific knowledge gained will inform future practise and enable researchers to optimise their methods to achieve the best scientific outcomes and welfare for their animals. By bringing together expertise in affective disorders and the neurobiology of emotional behaviour in rodents (Robinson) with animal welfare (Mendl), this programme bridges these two research fields. Our combined knowledge and the resulting dissemination of the research outcomes will impact across disciplines. Both the PI and Co-I are also involved with different funding bodies, advisory groups and organisation involved with animal research including Wellcome Trust, NC3Rs, BBSRC, UKRI Future Leaders Fellowships, AWERB Hubs which will help with the wider dissemination of the research and development of future guidance and applications of the findings.