A long-term goal of our laboratory animal welfare research group is to identify management and housing factors that affect animal welfare and influence the validity of research findings. Validity is an important component of scientific methodology. One aspect of validity refers to the degree to which the findings from a study are repeatable—in other words, whether the findings would hold true if the same study was conducted again in other laboratories.
Because rats are used often in research, the discovery of husbandry methods to improve their welfare can have a major impact on laboratory animal care. Our research team is currently investigating how laboratory rats are affected by the degree to which they can see out of their cages, as well as the location of their cages in a rodent rack. These factors are almost never mentioned in the reporting of biomedical research involving animals, and yet they may contribute variability to the data. If so, this could increase the number of animals required to detect treatment effects and reduce the comparability of findings from different laboratories.
Rats used in biomedical research are commonly housed in shoebox cages made of polypropylene (opaque white) or polycarbonate (clear) material. Opaque white cages prevent visual contact with the surrounding environment. Since rats are social animals, visual contact between individually housed animals could reduce the negative impact of social isolation. Clear cages allow a view of the room and facilitate visual social contact with neighboring rats. However, they may induce anxiety due to lack of visual cover.
It is possible, therefore, that cages with partially covered walls, rather than completely clear or opaque walls, would be beneficial to rat welfare. Walls with alternating opaque and clear stripes could create a "Venetian blind" effect that allows the animals to observe the surrounding environment, while providing a sense of security and control over exposure to visual stimuli outside of their cages.
Rat cages are usually placed on multi-shelf racks. On different shelves, rats are likely to be exposed to multiple varieties of light intensity, temperature, humidity, sounds and views of the surrounding area. When studying laying hens, birds housed in upper cages are reported to be more fearful than birds housed at middle or lower levels. In laboratory rats, the effect of shelf height is unknown. Furthermore, there may be interactions between shelf height and degree of visual cover provided by the cage walls, especially when rats are housed individually. A better understanding of the effects of shelf height and cage cover could lead to improvements in housing that increase the well-being of laboratory rats used for research.
We are comparing the responses of male Sprague-Dawley rats housed in three different cage types: clear plastic cages, opaque cages (clear plastic cages covered with white cardboard walls), and partially covered cages (clear plastic cages covered with vertical bands of white cardboard). Our research group is also interested in knowing how rats may be affected by shelf height. Therefore, the three cage types are spread among three different shelf heights: top (at or above human head), middle (at human body level) and bottom (at human leg level).
While the rats are housed in these cages, we assess effects of cage cover and shelf height on body weight, chromodacryorrhea secretion, and behavior. Chromodacryorrhea is a reddish porphyrin secretion that stains the skin and fur around the eyes and nostrils during times of stress. We measure chromodacryorrhea secretion weekly, following two routine procedures:
body weight measurement and cage cleaning. The anxiety levels of the rats, and their reactions to the anticipation of being handled by a human, are assessed at four-week intervals.
Preliminary results suggest that body weight, growth rate and secretion of chromodacryorrhea are not affected by the degree of visual cover provided by the cage walls or the height of the cage in the rodent rack. The rats are reacting similarly in anticipation of being handled. They are curious and approach the experimenter when the cage lid is removed. However, after eight weeks of housing, we are finding differences in the behavior of the rats according to cage type and shelf height. Rats housed in partially covered cages show less anxiety-related behavior than rats housed in clear and opaque cages when tested in an elevated plus maze. Additionally, rats housed on the bottom shelf in opaque cages or on the top shelf in clear cages are more active during behavioral testing compared with other rats. It still needs to be determined whether this increased activity is related to exploration or escape attempts, but these results suggest cage types and location may modulate the animals' fear responses.
Our results indicate that the degree of visual contact with the surroundings can affect the outcome of behavioral tests used in biomedical research. The use of cages providing partial visual cover may reduce the anxiety level of rats and thus reduce the time taken by rats to habituate to the cage, humans and routine handling. This simple change in housing could improve the welfare of laboratory rats, and the external validity of research data from behavioral tests performed outside the home cage.
This research project was made possible through funding from an Animal Welfare Institute Refinement Award.