How do laboratory mice live?

To describe how a laboratory mouse lives, we will first define what a mouse cage is, then examine the macro and micro-environment of these animals used for scientific purposes.

What is a laboratory mouse cage? Dimensions and racks

Laboratory mice live in cages typically made of polycarbonate, which are placed on multi-level racks (see Figure 1). The cage lids can be simple grids in conventional housing, or they may have fine filters that help maintain the mice in clean conditions, thanks to the Individual Ventilated Cage (IVC) system, which allows for control of air intake and exhaust in the cages (Hickman et al., 2017).

A standard mouse cage has a floor area of approximately 500cm², with a minimum height of 12cm covering over 50% of the floor space before the addition of enrichments (European parliament, 2010). The number of mice that can be housed in a cage depends on their weight, with 60cm² required per mouse weighing 20g, and 100cm² per mouse for those weighing over 30g (European parliament, 2010). Standards have evolved between the European directives of 1986 and 2010; the minimum floor area was 180cm² in the 1986 directive and increased to a minimum of 330cm² in the 2010 directive.

The process of cage changing involves moving mice from a dirty cage to a clean one. The frequency of these changes varies widely depending on housing conditions (particularly the use of ventilated racks or not) and can range from twice a week to once every 2 weeks or more (Hickman et al., 2017; Rosenbaum et al., 2009). This frequency should be determined based on two main factors:

• As cages become dirty (mice are very clean animals that ‘tidy up’ their cage by separating the nesting area from the soiling area), humidity and ammonia levels will increase, impacting the health and well-being of the mice (Washington & Payton, 2016).
• Cage changing is a stressful time for mice, as they must re-establish their scent marking, nesting, and hierarchy within the cage (NC3Rs, 2021).

Consequently, cages need to be changed regularly to prevent excessive dirtiness, yet not too frequently to avoid increasing the frequency of stressors for the mice.

At TransCure bioServices (TCS), to maintain Specific Opportunistic Pathogen Free (SOPF) status, mice are housed in groups of maximum 5 individuals in IVC racks. Cage changing takes place every two weeks.

The macro-environment of laboratory mice: housing rooms

Housing rooms can contain multiple racks, each capable of holding up to a hundred cages. Excessive density in a housing room can lead to changes in environmental parameters and increase pheromone exchanges (a stressed or pain-experiencing mouse may transmit its distress to neighboring or co-housed mice) (Sterley et al., 2018).

According to the directives of 1986 and 2010 (European Community Council, 1986; European parliament, 2010), environmental parameters (temperature, humidity, light cycle, ventilation, and noise) must be controlled. Humidity should be maintained at 55% +/- 10% and temperature between 20 and 24°C (European Community Council, 1986), with both parameters requiring daily monitoring. Ventilation and noise should be regularly monitored, with a focus on limiting noise in housing rooms as it can significantly impact the health and well-being of animals. The 1986 directive specifies that a continuous background sound of moderate intensity may help mask stressful sounds (European Community Council, 1986; NC3Rs, 2021).

In the majority of animal facilities, lighting is entirely artificial. Establishing a photoperiod corresponding to the natural cycle of the animals is essential and mandatory. In some cases, it is possible to reverse the light/dark cycle to manipulate mice, nocturnal animals, during their wakeful phase (see Figure 2).

At TCS, mice are kept in a 12h/12h normal cycle. Consequently, minimizing disruption to the mice during their sleep phase is particularly crucial. Noise is constantly monitored and given special attention. A monthly assessment is conducted, and measures are implemented to minimize noise, particularly from personnel working in the animal facility. Everyone must knock on doors before entering to prevent mice from being startled by sudden noise, and background noise is used to attenuate the impact of stressful sounds.

The micro-environment of laboratory mice: the contents of a cage

The basic elements that should be contained within a mouse cage are food, water (available ad libitum unless there is a specific experimental protocol), and bedding. Mouse food consists of dense pellets contained in feeders. Water is provided through water bottles, water pouches, or via automatic watering devices. There are numerous types of bedding that can be used in mouse cages, varying in absorbency and softness. Depending on the type and amount of bedding used, it is sometimes considered as enrichment, allowing mice to build nests and rest in a clean and dry environment (as stated in the 2010 directive).

These standard housing conditions are poor and limit the expression of behaviors typical to mice. Both directives state that animals should be allowed to express the maximum of behaviors typical to their species. These directives do not mandate the addition of enrichments but rather encourage it. When given the opportunity, mice in captivity engage in nesting, digging, gnawing, hiding, climbing, etc. They also exhibit highly developed social behaviors and can engage in elaborate physical and cognitive activities. Enrichments provide animals with the opportunity to manifest some or all of these behaviors.

To build a nest, mice need structuring and soft materials that allow them to construct a three-dimensional structure (see Figure 3), which can completely hide them and increase the temperature within the nest (mice have a thermoneutral zone around 29°C) (Gordon, 2012; Kasza et al., 2023). These small animals are so motivated to engage in this behavior that it is used to measure their anxiety; when a mouse does not build a nest or does so inadequately, it is a sign of distress (Deacon, 2006; Nollet, 2021). Huts or houses made in wood pulp or polycarbonate can be added to assist mice in nest-building.

Digging is a behavior that is challenging to replicate satisfactorily in captivity. It is primarily the thickness of the bedding that will determine the extent to which this behavior can be performed. Guidelines from the Animal Research Review Panel advise a minimum bedding depth of 2cm (Fawcett, 2012).

Gnawing is not only necessary for the well-being of mice, but it is also essential for their health, as their teeth continuously grow and must be used to prevent deformities that can hinder their ability to eat (Brown & Donnelly, 2004). Pellets are dense and enable mice to sufficiently utilize their teeth to avoid health issues. However, additional items such as pieces of wood can be provided to separate feeding behavior from gnawing behavior.

Mice need to hide from others within their group based on hierarchy, and from potential predators like humans. There are various enrichment options available for this purpose, ranging from tunnels to complex structures (Olsson & Dahlborn, 2002). Hideaway enrichments can be made of cardboard, paper pulp, or polycarbonate (which allows for reusability). The shapes vary, although rodents in general particularly enjoy tunnels.

To climb, mice typically have access to the structure of the cage itself. If the lid is a grid, they can easily hang onto it; if not, the feeder provides at least one element to climb on. Other enrichments, such as ropes or ladders, can be added.

Rodents are highly motivated to use a wheel when it is provided to them. Thus, a mouse with unlimited access to a wheel can run several kilometers per day (Zhu et al., 2020; Zhu et al., 2021)! Several types of wheels are available, but this enrichment takes up a lot of space and cannot always be used in the confined space of a cage. Other cognitive-motor enrichments, such as hammocks, can be used instead.

Other types of enrichment can have multiple benefits. For example, the use of sunflower seeds allows mice to perform several behaviors inherent to them, such as digging in the bedding to find food, using fine motor skills to open the seeds, holding a seed between their paws, and carrying it in their mouth.

At TCS, mice have access to a particularly soft bedding added to structural nesting enrichment allowing them to dig and create a high-quality nest. A little house and a tunnel are used so that the mice can hide or sleep. The health of mice teeth is promoted by providing each mouse with an aspen brick and mice receive sunflower seeds as treats (see Figure 4).

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