Galef Jr., B. G. 1999. Environmental enrichment in laboratory rodents: Animal welfare and the methods of science. Journal of Applied Animal Welfare Science 2, 267-280.

Some years ago in Canada, where I work, the Canadian Council on Animal Care mandated that all rats should be kept not in the 16.8-cm-high cages then standard in Canadian laboratories, but in cages 20 cm in height. That is, the Council insisted that the height of cages in which rats of all ages were to be kept should be increased by about 3.5 cm. The cost of buying new cages for all rats ... must have been tens of millions of dollars. .. I thought it might be worthwhile, before my department bought $100,000 worth of new rat cages .. to ask whether Norway rats were more comfortable in cages of the new height than of the old. .. We reasoned that if rats found a 16.8-cm-high cage in any way less comfortable than a 20-cm-high cage, then, when given the choice between cages of the two heights, they would spend more time in the more comfortable, taller cage than in the less comfortable, shorter one. When given a choice between a barren 20-cm-high cage and a barren 16.8-cm-high cage, rats exhibited no preference. It was inferred that these results failed to provide support for the hypothesis that rats were less comfortable when held in shorter cages than when held in taller ones. .. Increasing cage height may not be a particularly appropriate way to expend finite resources in the attempt to increase the welfare of laboratory rats... I have examined 125 cages containing a single rat and a length of PVC tubing [15 cm long, 7.5 cm diameter]. I found only 5 of the animals in their shelters. .. The results of these informal observations suggest that individual rats actually avoid the PVC tubes.