Enrichment of the home cage may provide laboratory animals with an environment in which they can perform more species-specific behavior than in standard laboratory cages. Enrichment also often allows the animals more control over their environment, which may enhance their well-being. Evaluation of the effects of enrichment can be performed in different ways. An overview of the results of a series of experiments that evaluated environmental enrichment for laboratory mice are presented here. The experiments described in this paper have indicated that short- and long-term evaluation studies can be used for interpreting the impact of environmental enrichment on animals.
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The environment of an animal consists of a lot of stimuli and can be divided into the living environment, that is, the social environment (conspecifics and humans) and the nonliving or physical environment, like the cage and its contents. The animal interacts with all these aspects of his environment. Cages for laboratory rodents should satisfy the physiological and ethological needs of, for example, resting, grooming, exploring, hiding, searching for food, and gnawing. The current environments of laboratory animals provide only the basic physiological requirements, such as nutrition and sanitation. The confinement of animals to these standard environments with a lack of stimuli may negatively affect the well-being of animals and may lead to the performance of abnormal behaviors such as stereotypies or passiveness (1-4). When the well-being of the animals is compromised, the validity of the animal models used in biomedical experiments is also compromised, which could lessen the reliability of results and conclusions (5, 23).
Environmental enrichment, which provides a more structured environment, allows the animals to express more behaviors, that are within the range of the animals species-specific behavioral patterns. This will improve the biological functioning of animals, and as a consequence, their well-being may improve (1, 6-9). Enrichment can focus on several aspects of the environment. For example:
Housing animals in pairs or groups is a form of social enrichment, which will be beneficial only if the pairs or groups formed are harmonious and stable. Formation of a group does not mean simply adding individuals together, since each individual plays a role in the group, dictated by its age, sex, and position in the hierarchy (10). Mice and rats, being social animals, are often housed in groups, although this is not a natural way of living for the males. In some strains, especially in mice, aggression may give rise to problems, and the males have to be separated. Because humans provide the daily care, they are part of the social environment of laboratory animals.
Another aspect to consider is foraging as part of the nutritional environment. In nature, animals spend a lot of time searching for food. In the laboratory, food can be easily obtained from the food hopper and is mostly provided ad libitum. For enrichment purposes, food particles can be scattered in the bedding so that the animals spend time searching.
The cage and its contents are part of the physical environment. Small cages may increase the incidence of stereotyped movements and other nonlocomotor abnormal behaviours (2). Enlarging the available space may not be the only way to enhance well-being. A better way of enlarging the available space is to structure the environment with, for example, climbing accessories, shelters/refuges, or exercise devices. This also gives the animals a certain degree of control over their environment, as a lack of control may cause stress. In laboratory cages, the possibilities for animals to control their environment are restricted. Providing a shelter or refuge gives them the opportunity to withdraw from frightening stimuli. Plastic tubes (11) or old drinking bottles (12) are simple solutions for shelters for rodents. Providing rodents with nesting material like tissues, hay, or wood-wool enables them to build a nest which offers shelter (figure 1) and it also enables them to hide from too much light, which may have deleterious effects on the eyes.
When introducing enrichment in the laboratory, consider the costs and the practical use of enrichment items. Objects introduced into cages should be easy to remove for workload reasons and should be easy to clean for hygienic reasons. On the other hand, enrichment should be interesting and stimulating for the animals; they must respond to it. Thus, it is important to evaluate enrichment before introducing it. This can be done in several ways.
The effects of the introduction of enrichment can be monitored in the home cage of the animals (home cage observations), by assessing the behavioral patterns before the enrichment was introduced (baseline behavior) and afterwards. The changes in responses to the enrichment can be quantified. Changes that can be seen include an increase in species-typical behavior and/or a decrease in abnormal behavior.
The effects of environmental enrichment can be evaluated by submitting animals from enriched environments to behavioral test situations, to monitor whether their behavior in such a new environment has been changed (for example, an increase in exploratory behavior or a decrease in freezing behavior). Another type of behavioral test that can be used to evaluate enrichment is a preference test. Preference tests have been used to assess the preferences of laboratory animals for environments or for different aspects of the environment (13-16). Furthermore, the strength of preference has been established in order to measure the importance that an animal attaches to a preferred option (17, 18). Allowing animals to choose between several enrichment items may prevent introduction of enrichment items in which the animals show no interest or that may even harm them (19). Preference tests may also reveal how the animals use the enrichment and may help in determining some species-specific properties of enrichment devices (9, 20).
Enrichment may have consequences not only for the behavior of animals, but also for the physiological state of animals. Therefore, physiological variables (for example, food and water intake, body weight, hormonal levels in plasma or urine, heart rate, and immune status) can be useful parameters to monitor (6, 21, 29). It is also important to assess whether the changes in behavior due to enrichment are maintained over a long period. After introduction of enrichment, the animals may not be interested or might lose interest soon (1, 20, 22).
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In the next part, an overview is given of the results of a series of experiments in which environmental enrichment for laboratory mice was evaluated (23). In a first experiment, the effect of enrichment objects in the home cages of groups of male mice (C57BL/6J and BALB/c) on their behavior was studied. Simple enrichment objects were put in standard laboratory cages such as: a nest box, a metal grid, a plastic tube, and nesting material. The enrichment had strain-related effects on the behavior of the mice in their home cage but also in behavioral tests (24).
In the next series of experiments, male and female mice of two strains (C57BL and BALB/c) were subjected to choice tests (during 48 hours) in order to study their preference for nesting material or nest boxes as a source of enrichment. Both types of enrichment were used by the mice in the first experiment and they are easily applicable: they can be provided in standard cages and can be sterilized if necessary. Furthermore, nest boxes can be used for a longer period, and nesting material allows mice to build nests.
The preference test system used has been described in detail by Blom et al. (13). In short, a preference test system was used consisting of either two or four test cages, connected by non-transparent tubes to a central cage. The movements of the mice between the test cages were detected automatically by means of photo-electrical devices in the passage tubes. The signals were sent to a computer that calculated dwelling times per cage. The behavior of the mice was also recorded on videotape.
In the first preference test series, six different types of nesting material were evaluated. No significant differences in preference were found between the strains nor between the sexes. All mice showed a clear preference for cages with paper towels or tissues as nesting material. The results also suggest that the nature (paper or wood) of the nesting material is less important than its structure, which determines the nestability of the material. The fact that the animals are able to manipulate the materials, thus controlling several aspects of their environment, might be an important factor determining motivation for the preference for this type of enrichment (25).
In a second preference test series, six nest boxes made of different materials were offered as enrichment. The choices of the mice were less uniform as compared to the nesting material preferences. But in general, the mice showed a preference for cages with a nest box made of grid metal or of perforated metal; these materials allow olfactory cues to pass (26).
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. In a next experiment, a choice was offered between nesting material combined with a grid floor, previously found to be avoided (27) and a nest box combined with bedding material. Since no major differences in preferences were found between the sexes in the previous test series, only female mice were tested in this experiment. Even under these circumstances, all mice chose the cage with the nesting material, despite the presence of the grid floor (figure 2). This seems to indicate that the presence of nesting material that can be manipulated is an essential condition for mice in laboratory cages (28).
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Figure 2. Results of the preference test with nesting material provided on a grid floor and a nest box on bedding material. Mean relative dwelling times and (SEM) per cage for day (=12 h), night (=12 h) and total (=48 h) period, for female mice of two strains (N=24). Preferences for the cage with the nesting material on the grid floor were significant for all mice during all three time periods (all P<0.001) |
Preference tests only measure short-term choices of individual animals. Therefore, in a following study, the consequences of providing such enrichments over a longer period to groups of mice were studied. Groups of three male or three female mice of the same two strains (C57BL/6J and BALB/c) were housed under either standard (only bedding) or enriched (bedding and the previously preferred nesting material: paper tissues) conditions for 11 weeks. During this period, several behavioral parameters (two behavioral tests and a handling score) and physiological parameters (food and water intake, body weight, and corticosterone levels) were monitored to determine the impact of the enrichment. All mice used the nesting material to build nests throughout the study.
The main result was that mice from enriched conditions weighed more than mice housed under standard conditions, although the latter consumed more food. This might be due to the fact that nesting material provides good insulation or that the enriched animals are less stressed. The long-term provision of the nesting material showed no other effects on the physiology or behavior of the mice (29).
These studies led to the conclusion that environmental enrichment may have profound effects on the behavior of mice in their home cage and in behavioral tests. The degree of complexity of the enrichment, however, plays a role in the level of the behavioral and physiological effects. Combining behavioral and physiological parameters provides data that can be used for interpreting the impact of environmental enrichment on the welfare status of an animal. Providing a cage with a preferred nesting material can be a relatively simple method to contribute to the well-being of laboratory mice, without jeopardizing the outcome of experiments. Thus, there seems to be no good reason to deprive laboratory mice of this form of enrichment.
To: Contents | Abstract | Introduction | Overview of Experiments | References
To: Contents | Abstract | Introduction | Overview of Experiments | References
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