Standard laboratory housing doesn’t allow the mice to spatially segregate their daily activities in this way and may therefore negatively impact their welfare. One aspect of housing that is relatively neglected, is the preference of mice to nest away from their urine and faeces. Substandard housing conditions can lead to abnormal behaviours such as increased aggression, stereotypies and barbering, with some strains being more prone to developing those behaviours. The physical and social environment in which mice are housed has important consequences both for their welfare and the quality of research data derived from them. Housing and husbandry conditions can have a major impact on laboratory mice throughout their lives. Figure 2. C57BL/6 mice build cup-shaped nests. Levels of aggression are also thought to vary by strain, and several investigators have reported that DBA/2 mice are more aggressive than C57BL/6. Similarly, there are considerable differences between strains in preference for enrichment items. Nest building is a good example of such behavioural differences between strains, with BALB/c mice being better nest builders than other inbred strains, like C57Bl/6 and CD-1 (Figure 2). Laboratory mouse strains show considerable differences in the expression and intensity of their behaviours, and these differences are thought to be genetically determined and dependent on their origins. However, recently there has been a lot of interest to systemically study and categorise mouse USVs in order to investigate whether they can be used to assess welfare. So far, and contrary to rats, USVs in mice have not been shown to correlate with negative or positive emotions instead their main purpose seems to be the facilitation or inhibition of social interactions. Mice emit a variety of ultrasonic vocalizations (USVs), in a wide frequency range of 20–100 Hz, which they use as communication signals in different social contexts, such as parental care, during courtship or during territorial and aggressive behaviours. Being curious by nature, they will explore their surroundings and memorize pathways and elements in their habitats, such as food sources, shelter and obstacles. As prey animals, mice will generally avoid open spaces and instead prefer to move along the edge of features in their environment (thigmotaxis). Mouse hearing extends into the ultrasonic frequencies and ranges from one kHz to about 100 kHz whereas the human hearing range is between 20 Hz and 20 kHz. For their daily activities, they rely heavily on their sense of smell and acute hearing. Mice have poor eyesight however, they have good peripheral vision which allows them to detect movement and they use their whiskers and bodies to sense their surroundings.
During their active hours, apart from eating, which they do in short and frequent bouts, they spend their time exploring their territories, running, climbing and digging. As nocturnal animals, mice are mostly active from dawn to dusk and retreat to shelters during the daytime.
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