Leaving home: A study of laboratory mouse pup independence

Juvenile wild house mice leave their mothers at 8 weeks (+). In contrast, laboratory strains of mice (lab mice) are typically ‘weaned’ at postnatal day (PND) 21. Lab mice might mature faster than their wild forebears; but if they do not, standard laboratory weaning likely involves maternal deprivation. We therefore investigated when lab mice voluntarily leave their mothers. C57BL/6J families were housed in home cages (HC) each attached via a tunnel to an identical ‘dispersal cage’ (DC); tunnel-widths allowed pups but not dams to pass. For generality, we used two common cage-types: Ancare ‘shoe-boxes’ (28 cm × 19 cm × 13 cm) and Thoren ‘duplexes’ (30 cm × 14 cm × 14 cm). Measures of pup independence were recorded PND 16–35. Pups first visited the DC at PND 21.5 ± 0.35; utilised its food/nesting material at PND 22.6 ± 1.5; and DC-use increased up to PND 35. However, maternal care continued and even at PND 35, pups spent at least 50% of their time with mothers. This differed between cage-types: ‘shoe-box’ mice matured faster (P < 0.01), becoming indifferent between the HC and DC by PND 35; while same aged ‘duplex’ mice preferred the HC, and received more maternal care (perhaps because they weighed less). Thus, similar to wild house mice, lab mouse independence occurs weeks after PND 21; independence is also plastic, affected by the local environment. ‘Weaning’ lab mouse pups at PND 21 therefore deprives them of maternal care, to an extent varying between different widely used cage-types. The impact this has on stress, abnormal behaviour, brain development, and inter-lab variation now needs investigating.