by Alyssa Foulkes
Central Animal Facility, Building #12, University of Guelph, Guelph, Ontario N1G 2W1, CANADA
In the United States, rodents used in research are commonly kept in minimum sized, barren cages. In Canada, trends toward environmental enrichment have been implemented in many research facilities. However, biomedical researchers do occasionally require animals to be housed in small, wire bottom cages for the purpose of urine and fecal collection. Investigators often argue that the animals do not find such living quarters to be stressful, and that they would not make use of additional space. The present study examines if this assumption is really correct.
The subjects of this project were 24 male Wistar Han rats who were all 31 days old at the beginning of the study. The animals were kept either in small wire bottom cages that had a floor area of 432 cm2 or in large test cages that had a floor area of 1088 cm2. All cages were 18 cm high and of the same design (Figure 1). The cages were either barren or enriched with a PVC tube of 6 cm length and 3 cm diameter plus a Nylabone(tm) (Figure 2). The animals had ad libitum access to pelleted food and water.
Six rats each were assigned to the following four housing conditions:
* small barren cage (Group 1)
* small enriched cage (Group 2)
* large barren cage (Group 3)
* large enriched cage (Group 4)
The amount of time spent moving around, resting on the cage floor, contacting the PVC tube [resting in or on it] and contacting the Nylabone(tm) [gnawing, pushing around] was recorded for each individual rat four times a week from 10-11 am over a period of six weeks. The amount of food consumed was recorded weekly for each animal. The animals' body weights were taken once a week. Their stress levels were estimated also once a week by scoring the porphyrin staining around the nose and back of the neck of each of the 24 rats. Porphyrin is a red pigment that is secreted by the Harderian glands when a rat is stressed. The weights of the rats, their feed, and their porphyrin scores were recorded weekly on Mondays. The study ended on Monday of week seven. No behavioural observations were recorded in week seven.
Results and Discussion
Regardless of the housing condition, the rats of all four groups spent about the same percentage of observation-time moving around (~30%) and the same amount of time resting (~54%). This implies that neither the provision of more space nor the provision of enrichment encouraged the animals to engage in more exercise.
Those rats who lived in enriched cages spent approximately 3% of the test session time contacting the Nylabone(tm) versus more than 40% of the time contacting the PVC tube which, apparently, was much more attractive and/or useful for the animals. Both, in the small and in the large cages, the animals spent more time resting in/on the PVC tube
(~30%) than on the bare floor (~23%). This suggests that rats feel more comfortable and perhaps more secure with a PVC tube offering an elevated vantage point plus seclusion than with the bare floor. The porphyrin scores substantiate this assumption: The scores were significantly higher in rats housed in small or large cages that were barren than in rats housed in small or large cages that were furnished with a PVC tube. The lowest porphyrin scores were found in rats living in the large PVC tube-furnished cages (Figure 3). This housing environment was the least stressful one of the four test situations.
The animals' stress status was reflected in their food consumption with rats living in barren cages eating much more food and gaining significantly more weight than rats living in the enriched cages (Figure 4). Rats kept in the typical, barren standard cages very often become obese. The present findings indicate that this problem is related to stressful living conditions, similar to obesity in modern people living in stressful environments.
Laboratory rats do not benefit from more cage space, unless the space is provisioned with proper enrichment such as a PVC tube. Being confined in a small standard cage is a much more stressful experience for rats than being confined in a larger cage furnished with a PVC tube. Access to a PVC tube reduces a rat's stress level in a small cage, but the stress is still much higher than in a large cage with a PVC tube.
Stress is a sign of poor welfare. Housing laboratory rats in larger cages that are furnished with PVC tubes rather than in small standard cages would, therefore, contribute to the animals' welfare. It would also improve scientific research methodology by reducing the variable stress resulting from inappropriate living quarters.
Thank you Animal Welfare Institute for providing the 2003 Refinement Award to fund this study. Thank you to the 24 rats, of which 10 have already been adopted and placed into good homes. Thank you to Dr. Atkinson, Dr. Duncan, Dr. Widowski, and Margaret Quinton for all your guidance and assistance. Finally, thank you to the staff at Animal Care Services for your dedication to the animals in your charge.