and may affect the experimental outcome. It should therefore be considered an independent variable and appropriately controlled.

Some scientists have raised concerns that environmental enrichment may compromise experimental standardization by introducing variability, adding not only diversity to the animals’ behavioral repertoire but also variation to their responses to experimental treatments (e.g., Bayne 2005; Eskola et al. 1999; Gärtner 1999; Tsai et al. 2003). A systematic study in mice did not find evidence to support this viewpoint (Wolfer et al. 2004), indicating that housing conditions can be enriched without compromising the precision or reproducibility of experimental results. Further research in other species may be needed to confirm this conclusion. However, it has been shown that conditions resulting in higher-stress reactivity increase variation in experimental data (e.g., Macrì et al. 2007). Because adequate environmental enrichment may reduce anxiety and stress reactivity (Chapillon et al. 1999), it may also contribute to higher test sensitivity and reduced animal use (Baumans 1997).

Sheltered or Outdoor Housing

Sheltered or outdoor housing (e.g., barns, corrals, pastures, islands) is a primary housing method for some species and is acceptable in many situations. Animals maintained in outdoor runs, pens, or other large enclosures must have protection from extremes in temperature or other harsh weather conditions and adequate opportunities for retreat (for subordinate animals). These goals can normally be achieved by providing windbreaks, species-appropriate shelters, shaded areas, areas with forced ventilation, heat-radiating structures, and/or means of retreat to conditioned spaces, such as an indoor portion of a run. Shelters should be large enough to accommodate all animals housed in the enclosure, be accessible at all times to all animals, have sufficient ventilation, and be designed to prevent buildup of waste materials and excessive moisture. Houses, dens, boxes, shelves, perches, and other furnishings should be constructed in a manner and made of materials that allow cleaning or replacement in accord with generally accepted husbandry practices.

Floors or ground-level surfaces of outdoor housing facilities may be covered with dirt, absorbent bedding, sand, gravel, grass, or similar material that can be removed or replaced when needed to ensure appropriate sanitation. Excessive buildup of animal waste and stagnant water should be avoided by, for example, using contoured or drained surfaces. Other surfaces should be able to withstand the elements and be easily maintained.

Successful management of outdoor housing relies on stable social groups of compatible animals; sufficient and species-appropriate feeding and resting places; an adequate acclimation period in advance of seasonal

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