in animal data (Clough 1982; Milligan et al. 1993; Sales et al. 1999; Shoji et al. 1975). Noise in dog kennels can reach levels that can damage human hearing, and that may well have an impact on dog hearing and physiology as well (Hubrecht et al. 1997; Sales et al. 1997). Vibration has been demonstrated to be a stressor in both poultry (Abeyesinghe et al. 2001) and pigs (Perremans et al. 1998); investigators should, therefore, strive to minimize common sources of vibration such as ventilation machinery in adjacent rooms or on cage racks (Clark 1997). In addition to such continuous background disturbances, the effects of isolated environmental insults may also cause distress in some species and models.

For experimental and comfort reasons it is best to maintain animals in their thermoneutral zone (NRC 2006, pages 39-45). The Committee notes the discrepancy between the temperature recommendations for housing rodents in the Guide for the Care and Use of Laboratory Animals (NRC 1996, page 32) and those put forth in the Guidelines for the Humane Transportation of Research Animals (NRC 2006, pages 39-45) due to new evidence used in the later publication. Rabbits are very susceptible to heat stress (Marai et al. 2002). Singly housed mice prefer ambient temperatures of 28-30°C while group-housed mice prefer a slightly colder environment with ambient temperatures of 24-27°C (NRC 2003b, page 97). The provision of nesting materials, refuges, or nest boxes for rodents, or areas within an enclosure with different levels of heating or cooling (i.e., heated areas for dogs) allows the animals to control their microclimate. Moreover, long-term housing in cages with mesh floors where adequate bedding or nesting materials cannot be provided can also result in stress, distress, or more obvious deleterious effects, such as foot ulcerations and arthritis.

Enrichment

Barren environments may not meet the species-specific needs of an animal. In addition to their impact on welfare these conditions can adversely affect the validity of experimental data (Sherwin 2004). Such environments can cause distress as shown by the development of abnormal behaviors (see Chapter 3) or by experiments in which animals, when given the choice to self-medicate with anxiolytics, consume larger proportions of midazolam-water solution than their littermates housed in enriched environments (Sherwin and Olsson 2004). In contrast, supporting evidence has shown that biologically relevant enrichment can help avoid the development of abnormal behaviors (see Chapter 3), although it may not alleviate previously established patterns. Moreover, as Olsson and Dahlborn have shown, some animals exhibit clear preferences and will work to access these enrichments (Olsson and Dahlborn 2002). In mice, environmental



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