Extended early experience in the maternal environment and the welfare of the laboratory mouse
Early maternal loss can reduce animals' abilities to cope with stressors. However, laboratory mice are removed from their dam (weaned) at postnatal day (PND) 21, although wild mice naturally disperse much later. In a standard research facility (Expt.1), offspring were therefore weaned at PND21 or PND35 and assessed for adult anxiety and abnormal behaviour. As predicted, delaying weaning reduced anxiety in elevated plus maze and startle tests; and alopecia in females. However, at a commercial facility (Expt.2), delaying weaning did not benefit offspring. To test if caging differences affected the benefits of delayed weaning, a third experiment observed pup development and behaviour in 'shoebox' cages of Expt.1 or 'duplex' cages of Expt.2, both modified to allow pup dispersal. All pups stayed with the dam beyond PND21; however, pups in Expt.2's cages were developmentally and behaviourally retarded. Beneficial effects of weaning at PND35 are therefore dependent on rates of offspring development.