The importance of learning young: the use of nesting material in laboratory rats.

Unlike mice, adult laboratory rats do not spontaneously build nests when nesting material is offered. As a result, nesting material is often regarded as unsuitable environmental enrichment for laboratory rats. Wild rats and pet rats, however, have been observed to build complex nests from nesting material at hand. It was hypothesized that nest building in rats is an acquired behaviour, rather than genetically predisposed. To test this hypothesis, the progeny of three Wistar rats provided with nesting material (Kleenex tissues) during pregnancy and three standard-housed rats were divided in 34 same-sex couples with access to nesting material: (1) from the age of 8 weeks (n=7); (2) from weaning (n=8); (3) from birth (n=17). The latter were subdivided into two groups after weaning, one provided with Kleenex tissues (n=9), the other with Enviro-dri (n=8). At the age of 12 weeks, all couples were provided with both types of nesting material for one week. Amount, shape, and soiling of the nesting material were scored on weekdays, and behaviour of the rats was scored twice weekly. Results indicated that the older the rats were when first provided with nesting material, the greater the amount of nesting material eaten and soiled, and the nests built were less elaborate. Overall, Enviro-dri was eaten less than Kleenex tissues. It is concluded that rats need to learn the proper use of nesting material. When provided from birth, nesting material is a suitable type of environmental enrichment for laboratory rats.