Environmental enrichment for laboratory mice: preferences and consequences

Abstract

Current laboratory housing systems have mainly been developed on the basis of ergonomic and economic factors. These systems provide adequate, basic physiological requirements of animals, but only marginally fulfil other needs, such as the performance of natural behaviour or social interactions. Many signs indicate that the current housing conditions affect the well-being of animals negatively. Environmental enrichment may solve some of the problems caused by the restrictive laboratory environments, because the more structured environment allows the animals to perform species-specific behaviour. When introducing enrichment it is important to evaluate the effects of the enrichment programme used. In the first study described in this thesis, the effects of relatively simple environmental enrichment provided in standard laboratory cages on the behaviour of mice in three behavioural tests were investigated. Effects of enrichment on the behaviour of the mice were found, although the effects differed per strain. Choice tests have been used to assess laboratory animals' preferences for different environments or for aspects of the environment. The preferences of the mice for six different types of nesting material and six nest boxes made of different materials were evaluated. All mice showed a preference for cages with nesting material or, in the second series, a nest box. In order to gain some insight in the strength of preference for these types of enrichment, the most preferred nesting material and the most preferred nest box (from the previous test series) were tested against each other. All mice showed a strong preference for the nesting material. This indicates that the presence of nesting material, which can be manipulated, fulfils a need of mice. In a following study the effect of the long-term provision of enrichment was studied. Groups of mice were housed under either standard or enriched conditions (provided with the preferred nesting material). Several behavioural and physiological parameters were monitored to determine the impact of environmental enrichment. The long-term provision of the nesting material showed no major effects on the physiology or behaviour of the mice. These studies led to the conclusion that providing a cage with nesting material can be a relatively simple method to contribute to the well-being of laboratory mice, without jeopardising the outcome of experiments. Thus there seems to be no good reason to deprive laboratory mice from this form of enrichment. In order to further investigate the effects of enrichment on the behaviour of laboratory mice the mice were tested in an open field test. The animals from the enriched housing conditions seem to habituate faster to the test situation, reflected in a (rapid) decline in exploratory behaviour, whereas control animals continued to explore the new environment longer. Furthermore, the behavioural patterns of the mice were registered during 24 h using LABORAS, a newly developed system for the automated registration of behaviour. Environmental enrichment appeared to have strain-specific effects on the time budget of mice, but not on the circadian rhythm of behaviour. In conclusion, the experiments described in this thesis have indicated that environmental enrichment may have profound effects on the behaviour of mice. The degree of complexity of the enrichment however, plays a role in the level of the behavioural and physiological effects. Animals from enriched housing conditions seem to be physiologically and psychologically more stable and could therefore be considered as more refined animal models. This means that eventually their use might lead to a reduction in the number of animals necessary for obtaining valid experimental results.