Cho, E., Walsh, C. A., D’Angelo-Gavrish, N. M. et al. 2023. Effects of housing density on reproductive performance, intracage ammonia, and welfare of mice continuously housed as breeders in standard mouse and rat caging. JAALAS 62(2), 116–122.

Maintaining compliance with cage density recommendations in The Guide for the Care and Use of Laboratory Animals precludes continuous trio breeding in standard-sized mouse cages. This study evaluated and compared several parameters of reproductive performance, intracage ammonia concentration, and fecal corticosterone levels in 2 strains of mice, C57BL/6J (B6) and B6.129S(Cg)-Stat1tm1Dlv /J (STAT1 –/– ), housed as continuous breeding pairs or trios in standard-sized mouse cages, and continuous breeding trios in standard-sized rat cages. Reproductive performance data indicated that STAT1 –/– trios raised in rat cages weaned significantly more pups per litter than did STAT1 –/– trios raised in mouse cages, and B6 mice had higher pup survival rates at weaning than did STAT1 –/– mice in mouse cages housing continuous breeding trios. In addition, the Production Index was significantly higher for B6 breeding trios in rat cages than for B6 trios in mouse cages. Intracage ammonia concentration increased with cage density, with significantly higher ammonia concentrations in mouse cages housing trios compared with rat cages housing trios. However, fecal corticosterone levels did not differ significantly regardless of genotype, breeding configuration, or cage size, and daily health checks revealed no clinical abnormalities under any of the conditions evaluated. These results suggest that, although continuous trio breeding in standard-sized mouse cages does not seem to compromise mouse welfare, it offers no advantage in reproductive performance compared with pair breeding, and in some cases, it might be disadvantageous in this regard. Further, high intracage ammonia in mouse cages containing breeding trios might necessitate more frequent cage changes.