Rats seem indifferent between their own scent-marked homecages and clean cages

Previous large-scale, long-term studies suggest that rat welfare is little affected by cage-cleaning frequency. Here, we investigate this further using a preference test: arguably a more sensitive welfare indicator than those used previously. Nine pairs of rats were each housed in two interconnected cages of differing cleanliness. One cage was cleaned every 3–4 days, while the other remained uncleaned for 18 days. All rats were handled at each cage-cleaning (regardless of which cage they chose) so that this aspect of the cleaning routine remained consistent. Furthermore, ammonia build-up was negligible. We could thus see whether rats prefer their own scent-marks over clean bedding, without the confounds of differential handling or the potential harm induced by ammonia. Dwelling, resting, feeding, drinking, defecation, and ammonia concentrations were compared between the two cages. None were found to differ significantly between the cages over the course of the experiment. Power tests and confidence intervals support the conclusion that the rats had no meaningful preference for self-scented, familiar areas over clean areas. A weak preference cannot be ruled out, due to the small sample and the space limitations within laboratory cages. Also, the lack of ammonia means that the threshold at which rats would start to avoid soiled areas that do generate this compound remains undetermined. Nevertheless, together with previous studies showing no clear effects of cage-cleaning frequency on rat welfare, these choice tests indicate that it is unlikely that olfactory disruption during cage-cleaning is an important welfare concern for stable non-breeding groups of rats.