The naked truth: Breeding performance in nude mice with and without nesting material

In laboratories, mice are housed at ambient temperatures between 20 and 24 °C, which is below their lower critical temperature of 30 °C, but comfortable for human workers. Thus, mice are under chronic thermal stress, which can compromise many aspects of physiology from metabolism to pup growth. These effects may be exacerbated in nude mice. We hypothesized that provision of nesting material would allow nude mice to behaviourally thermoregulate, reducing heat loss to the environment, and that this reduction would improve feed conversion as well as breeding performance. Mice of one inbred strain (CAnN.Cg-Foxn1nu/Crl) and one outbred stock (Crl:NU-Foxn1nu) were housed in shoebox cages at approximately 21 °C either with or without 8 g nesting material in breeding trios (two heterozygous haired females: one homozygous nude male; 59 cages) for 6 months. Nest quality, feed consumption, and reproductive outcomes were recorded. At weekly cage cleaning, fresh nesting material was provided to cages from the nesting treatment. Mice with nesting material built better nests than controls with just bedding material (P < 0.001). Nest quality decreased in nesting cages when litters were present (P < 0.001) but no differences were seen in controls (P > 0.05). Nesting material significantly increased the number of pups born (P = 0.04) and weaned per cage (P = 0.018). The amount of feed needed to produce 1 g of weaned pup was reduced when mice were provided nesting material (P = 0.002), but the total feed consumed in each treatment was not significantly different (P > 0.05). The breeding index (pups weaned/female/week) was significantly higher when nesting material was provided (P = 0.006). The energy conserved by nesting material was reallocated from heat generation to improved breeding performance. Thus nests lessen the thermal stress of cool laboratory temperatures on nude mice. Together these data show that good welfare can be good business and good science.