A review of housing guidelines and published data pertaining to temperature and humidity reveals the gaps in our knowledge that make it difficult to specify precise environmental conditions. Most common laboratory animals are adaptive homeotherms and as such make anatomic, metabolic, and physiologic adjustments in response to their environment to maintain well-being. Environmental adaptation in both wild and laboratory animals suggests that consistency in environment may not be "normal" or perhaps even desirable. To demonstrate effects caused by temperature or relative humidity, it may be necessary to have a complex/ unique set of conditions present unless extreme and clearly unacceptable conditions are utilized. For example, testicular degeneration/infertility in mice has been shown to occur when temperatures exceed 83°F within the secondary enclosure.
It would appear that the rationale for specifying any temperature or relative humidity conditions within the laboratory, other than avoiding extreme conditions that clearly could cause harm to the animals, would be to control research variation caused by unpredictable adaptation to housing conditions. The adaptive processes may, in fact, serve the animals quite well, but those are the processes that may interfere with research results. The question then remains as to how much variation due to adaptive processes is acceptable and, hence, what limitations must be placed on temperature and relative humidity. It also begs the question of why such changes would not be sorted out by the use of appropriate controls. Clearly, a number of variables would affect these adaptive processes, including the type of housing (e.g., pen, run, open cage, microisolation cage, isolator), the type of ventilation system used within the primary and secondary enclosures, the specifics of cage/room coupling of ventilation, as well as stratification of temperature and relative humidity within the room itself.
Interaction of other environmental factors with the thermal regulatory behavior of rodents is also an important consideration. Substantial existing data demonstrate that singly housed mice prefer ambient temperatures between 28°C and 30°C whereas group-housed animals prefer temperatures between 24°C and 27°C. The ability of group-housed animals to share metabolically generated heat by huddling together explains the differences in selected ambient temperatures between group- and singly housed animals. The resultant effective ambient temperatures in both group- and singly housed mice are compatible with the estimates of thermal neutral zones for mice of 28°C to 30°C. This range is at variance with the human comfort zone of 22°C (± 2°C), which appears to correspond more closely to temperatures recommended within guidance documents.
98 SCIENCE-BASED GUIDELINES FOR LABORATORY ANIMAL CARE
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