Housing and husbandry: Mouse

Laboratory mice are derived from the wild mouse Mus musculus. Although they are partially adapted to captive life, laboratory mice perform similar behaviours and have similar behavioural needs to their wild counterparts. Different strains of laboratory mouse can show considerable differences in the expression and intensity of their behaviours [1,2]. 

Mice are most active during dawn and dusk and may retreat to shelters and nests during the daytime. During their active hours, mice spend their time eating in short and frequent bouts, grooming, gnawing, exploring, climbing, building nests and digging [3]. Laboratory mice may also perform abnormal behaviours known as sterotypies. These repetitive and apparently functionless behaviours can indicate that the welfare needs of the animal are not being met, but this may be strain dependent [4].

Mice are prey species and this should inform how humans interact with them during general husbandry and procedures. Mice respond best to gentle habituation and being picked up using refined methods (i.e. not by the tail). Mice accustomed to being picked up using refined methods (e.g. a tunnel or cupped hands) are less stressed and more cooperative than mice that are picked up the tail, even following restraint and procedures (see the mouse handling research papers table for examples from the literature). Recent advances in technology have led to the development of automated systems that can continually monitor animals with minimal human disturbance. These tools refine studies by minimising the stress associated with being handled and moved from the home cage.

Mice communicate using ultrasonic vocalisations (USVs) [5], scent [6,7], body language and physical contact. Disrupting their ability to communicate can impact social behaviour. For example, cage cleaning removes scent cues and can disrupt the social hierarchy, leading to increased levels of aggression [8, 9]. 

Free-living mice

The free-living mice from which laboratory mice are derived typically live in groups with a single dominant male, several females and their offspring and subordinate males [1]. Free-living animals can manage their social interactions and demonstrate social flexibility in ways that can be impaired or impossible when they are captively housed.

Social housing for laboratory mice

Social housing can facilitate the behavioural needs of mice within the research environment and can provide benefits including social enrichment and social buffering [1,2,3]. Housing mice in family groups to imitate their wild counterparts is not always feasible within a research setting. Instead, housing mice in same sex groups is a common approach. 

Within the research environment, mice are exposed to husbandry conditions that can disrupt social stability (e.g. removal of cage mates for experimental use and the removal of familiar odours through frequent cage cleaning). Cage size can also limit the ability of subordinate animals to escape from aggressors. In general, it is advised to keep adult mice in stable groups and avoid constantly removing and introducing new individuals as this could destabilise the established social hierarchy [4,5]. 

It is advised that when mice are weaned they are housed with familiar littermates. However, when this is not feasible, for example when weaning transgenic strains and only one mouse in a litter has the desired genotype, the ‘companion mouse’ strategy can be used. This strategy involves keeping a ‘companion’ littermate that does not carry the gene of interest in order to avoid the need for single housing. 

Single housed male mice

Male mice can be particularly territorial and aggression between group housed males can present a serious welfare concern within the laboratory environment. Steps should be taken to minimise aggression between group housed male mice and this may include single housing of males. 

There is some evidence to suggest that male mice may prefer living alone and housing male mice individually is preferential to disharmonious group housing where aggression is constant, severe and cannot be resolved. However, balancing the welfare needs of male mice in respect to social or single housing is context dependent and should be considered on a case by case basis [6]. If mice are single housed it is essential that they are provided with additional nesting material and/or another heat source to allow them to maintain their body temperature.

Pup survival

The social environment is important for pup survival and wellbeing. The presence of two litters in a single cage with a large age difference between them has been identified as a contributing factor to high pup mortality [7]. Therefore, care should be taken to avoid housing two or more litters of different ages together, potentially by removing and temporarily single housing the pregnant females prior to parturition or opting to set up breeding pairs instead of breeding trios.

The minimum space allocations for laboratory mice in the UK are given in tables 2-2-1 to 2-2-4 of the Code of Practice for the Housing and Care of Animals Bred, Supplied  or Used for Scientific Purposes. The minimum legal requirements will vary depending on location. The aim should always be to exceed the minimum and provide sufficient space and environmental complexity to meet the welfare needs of animals.

Mice should be able compartmentalise their space. For example, water and food should be provided in one end of the cage leaving the other end free to be used for urination and defecation [1].

The space within the cage should be maximised to enhance environmental complexity and increase the usable space, for example, by making use of vertical space. Climbing is an important locomotor activity in mice, which can be facilitated by incorporating a grid section in the cage lid. Grid space is limited to the food hopper in Individually Ventilated Cages (IVCs), however additional climbing devices, such as mouse lofts and mezzanines can be provided.

Mice prefer solid floors with suitable bedding, such as wood shavings [2]. Use of grid or wire mesh floors should be avoided as this can lead to injuries. If grids or wire mesh floors are used due to scientific purposes, a solid or bedded area should be provided for the animals to rest on unless specific experimental conditions prevent.

Environmental enrichment provides sensory and motor stimulation for captive animals and allows them a greater choice of activity and some control over their environment [1,2]. Bedding and nesting material, refuges, and gnawing sticks are the most commonly used enrichment resources for mice and should be provided in every mouse cage unless there is sound scientific justification not to.

Giving mice the opportunity to perform species-typical behaviours, such as nest building or foraging, can significantly improve their lives in the laboratory. Gnawing is also an important behaviour for mouse welfare; as their teeth grow continually through their lives, an inability to gnaw can lead to malocclusion (overgrowth of incisors) and consequently malnourishment. Barren enclosures are associated with signs of impaired welfare, such as anxiety and abnormal repetitive behaviour [1]. During breeding, lack of enrichment has been shown to result in higher pup mortality, reduced pup weight, and delayed development [3].

Mice prefer nesting material over every other form of enrichment [4], including prefabricated refuges, manipulanda and social contact [5]. Nest building is well preserved from wild‐type progenitor mice and is undertaken equally by female and male mice. Assessing the quality of the nest is a great cage-side tool to identify health and well-being issues (such as thermal stress, aggressiveness, sickness, and pain) and nest scoring protocols are available in the literature [6-9]. Even mice at the preclinical stage of a disease (i.e. not yet showing any clear clinical symptoms) may fail to build adequate nests, if they manage to build a nest at all [6,7].

Common nesting materials for laboratory mice include facial tissue, shredded paper strips, compressed cotton squares (nestlets), wood wool, and wood shavings [10,11]. Mice seem to prefer paper-derived materials (e.g. shredded paper) over wood-derived materials (e.g. cotton squares), however, preference for a particular nesting material might depend on the structure rather than the nature (paper or wood) which determines how easily they can manipulated to create a nest [12]. Providing appropriate nesting material can allow even “nest-building challenged” strains (e.g. C3H mice) to build nests comparable to stains with strong building skills (e.g. BALB/c) [13]. Additionally, care should be taken when choosing nesting material for some groups of mice, such as breeding mice or post-surgery mice, as some types of nesting material might have a negative effect on animal welfare [11]. For example, pups and adult mice could get entangled and injured in cotton wool [14,15], and mice with head implants risk getting tangled while trying to build a nest using facial tissue [16].

Mice show preferences for different bedding materials, shelters and gnawing resources, which can be sex or strain specific [17-19]. Therefore, provision of environmental enrichment shouldn’t be a random process whereby different objects are included in a mouse cage based merely upon their availability, instead care should be taken to assess the true beneficial effect on the animals’ overall well-being.

In addition to the citations given within the text, the following guidance on environmental conditions is based on the UK Home Office Code of Practice for the Housing and Care of Animals Bred, Supplied  or Used for Scientific Purposes.

Lights and sounds

Mouse rooms typically have a 12-hour light-dark cycle. It is recommended that light levels within the enclosure are low, particularly for albino animals as they are more susceptible to light-induced retinal damage [1]. Racks should ideally have shaded tops in order to keep light intensity to a minimum and reduce the risk of retinal degeneration.

Mice are sensitive to ultrasound, which they use for communication. Sources of ultrasound in laboratories include dripping taps, trolley wheels and computer monitors, which may affect mouse behaviour and breeding cycles [2,3]. As some sources of ultrasound cannot be eliminated (e.g. laboratory equipment) playing background music may help mask stressful sounds [4]. It is advisable to minimise sudden irregular noises as they create more disturbance in breeding rodents than continuous or predictable sounds and can increase the risk of mis-mothering or cannibalism.

It is possible to make recordings and take measurements of the audible and ultrasonic noise within an animal facility [5,6]. Knowledge about the acoustic environment of animal facilities can be used to minimise disruption to animal welfare and the impacts on scientific outcomes.

Temperature

According to UK Home Office guidelines, the room temperature for mice should be 20–24°C (68–75°F) and kept as stable as possible. However, the thermoneutral zone of mice is between 26°C and 34°C (79–93°F) and therefore they may experience cold stress in the laboratory unless nesting material is provided for them to regulate their body temperature and the microclimate within the cage [7,8]. Sufficient nesting material should be provided that enables them to create good nests, which can reach temperatures of 30–32°C (86–90°F) [8,9]. This is particularly crucial for new born pups which lack the ability to thermoregulate and therefore their survival depends on the warmth of their mother and siblings, which is retained by the nesting material [10].

Ventilation and humidity

The air in the mouse room should be renewed at frequent intervals. Typically, a ventilation rate of five to 20 air changes per hour is adequate for a fully stocked room of mice. The relative humidity should be kept at 45 to 65%.
For IVCs, the number of air changes per hour typically ranges between 25 and 120. As air is blown into the cage at relatively high speed, the intra-cage ventilation rate could lead to chronic stress as well as heat loss due to the draught [8]. Therefore, mice in IVCs should be monitored for any signs of anxiety or discomfort, and the air change rate should be adjusted accordingly. Signs that the mice are reacting to the high air flow include change of location of the nest or building protective barriers against the draught using bedding material [9]. If such changes in behaviour are detected care should be taken to decrease the intra-cage draughts and provide mice a comfortable environment to live in.