Baumans, V., Bennett, K., Donnelly, M. et al. 2013. The problem with the dark bottom-row cage for non-human primates: A discussion by the Laboratory Animal Refinement & Enrichment Forum. Animal Technology and Welfare 12, 111-114.
Since there is evidence that the bottom cages can impact research, this should be a good reason to eliminate the two-tier system with its species-inappropriate bottom-tier cages.