Hughes, H. C., Campbell, S., Kenney, C. 1989. The effects of cage size and pair housing on exercise in beagle dogs. Laboratory Animal Science 39, 302-305.

Dogs in the single cage size spent slightly less time moving, but they moved at a greater speed than when housed in double sized cages [probably because of the extreme confinement condition leading to frustration and possibly abnormal behavior patterns, and because of the absence of any structures in the cage]. When a pair of dogs were housed in a barren cage 25% smaller than recommended, the distance traveled and the amount of time spent moving increased significantly [probably because they were no longer able to keep adequate social distances and hence were quasi-forced to keep moving all the time in a barren cage without any structures or visual barrier]. From this data, it is apparent that human contact is the single most consistent and important factor in encouraging dogs to be active [in an otherwise unstimulating environment].