Snug as a Bug in a Rug
Research Models
Brianna Gaskill

Snug as a Bug in a Rug

Mice are cold stressed at standard laboratory temperatures1. This is more than just an animal welfare concern--it is also a concern for scientific research. Increased metabolic rates and chronic stress brought on by cold temperatures can alter many aspects of physiology. But, although mice prefer warmer temperatures2-5, simply turning up the thermostat is not a solution, as mouse temperature preference differs with age, gender, and behavior2-3,6. This makes it exceedingly difficult to identify an optimal temperature. Furthermore, increases in ambient temperature, even within the recommended range of 20-26oC, can increase aggressive interactions7.

Nest building is an adaptive behavior used by wild mice to survive in cold conditions8-9 and laboratory mice will similarly build nests according to temperature3-4. Through a series of studies, my co-authors and I aimed to validate the benefits of appropriate nesting material and to establish the causal chain from enrichment, to behavior, to thermal microenvironment, to physiology, to wellbeing and Reduction and Refinement of mouse use.

We hypothesized that nesting material would allow mice to alleviate cold stress by building insulating nests and creating a unique microenvironment within the cage. The aim of our first experiment4 was to determine how much material (0-10g) was preferred by mice at different ambient temperatures. Within standard ambient temperatures, all mice preferred locations with 6-10g of nesting material. Thus, 6g provides some degree of insulation, but 10g or more (especially for females) may be needed to alleviate thermal stress.

We also found some interesting information on primary modes of behavioral thermoregulation. BALB/c mice appear to be primarily nesters. These mice chose to stay in a location and build a nest, while other mice, C57BL/6, preferred to relocate to a warmer temperature. Since providing a thermal gradient within the home cage is not practical, it becomes increasingly important to provide these mice with material they readily and efficiently build with. CD-1 mice on the other hand, shifted between modes depending on the temperature and resources available.

Our second study10 tested if 8g of nesting material altered thermogenesis compared to controls, which received only bedding. Mice that built dome-like nests lost less heat to the environment. Mice that received nesting material also consumed less food, indicating a reduction in overall metabolism and energetic needs. This result was more pronounced in males than females. Only one of our three types of mice tested had reduced non-shivering thermogenesis, based on brown fat UCP1 protein expression. Nests can alleviate thermal discomfort by decreasing the amount of radiated heat and reduce the need for thermogenesis, particularly in males and certain strains.

Last we tested if reduced thermogenesis from insulating nests would free resources and improve reproduction in a large scale breeding facility11. All mice provided nesting material showed improved breeding performance without increased food consumption. One of the three types of mice tested showed an increase in pup survival to weaning. This improvement has the potential to significantly reduce the number of breeding mice needed to achieve a certain level of production for sale or for scientific purposes.

These studies are the first to assess behavioral thermoregulation differences, the tradeoffs between them, and the physiological impact of nest building in three common laboratory mouse strains under typical husbandry conditions. The addition of nesting material, in amounts of 8g or more, can reduce thermal stress, freeing up energy to be utilized by other biological systems. This strongly suggests that seemingly mild laboratory temperatures may be more impactful on mice than previously assumed. This research has led to the harmonization of an effective and validated nesting enrichment in Charles River’s North American breeding facilities, potentially impacting the lives of millions of animals.


  1. Cannon, B. & Nedergaard, J. Nonshivering thermogenesis and its adequate measurement in metabolic studies.  Exp. Biol.214, 242-253 (2011).
  2. Gaskill, B. N., Rohr, S. A., Pajor, E. A., Lucas, J. R. & Garner, J. P. Some like it hot: mouse temperature preferences in laboratory housing.  Anim. Behav. Sci.116  279-285 (2009).
  3. Gaskill, B. N., Lucas, J. R., Pajor, E. A. & Garner, J. P. Working with what you’ve got: Changes in thermal preference and behavior in mice with or without nesting material.  Therm. Biol.36, 1193-1199, doi:10.1016/j.jtherbio.2011.02.004 (2011).
  4. Gaskill, B. al. Heat or insulation: Behavioral titration of mouse preference for warmth or access to a nest. PLoS ONE 7, e32799 (2012).
  5. Gordon, C. J. Effect of cage bedding on temperature regulation and metabolism of group-housed female mice.  Med.54, 51-56 (2004).
  6. Ogilvie, D. M. & Stinson, R. H. Effect of age on temperature selection by laboratory mice (Mus musculus).  J. Zool.44, 511-517 (1966).
  7. Greenberg, G. The effects of ambient temperature and population density on aggression in two inbred strains of mice, Mus musculusBehaviour42, 119-130 (1972).
  8. Barnett, S. A., Munro, K. M. H., Smart, J. L. & Stoddart, R. C. House mice bred for many generations in 2 environments.  Zool.177, 153-169 (1975).
  9. Bult, A. & Lynch, C. B. Nesting and fitness: Lifetime reproductive success in house mice bidirectionally selected for thermoregulatory nest-building behavior.  Genet.27, 231-240 (1997).
  10. Gaskill, B. al. Impact of nesting material on mouse body temperature and physiology.  Behav. 110, 87-95, doi:10.1016/j.physbeh.2012.12.018 (2013).
  11. Gaskill, B. N., Garner, J. P. & Pritchett-Corning, K. R. Energy reallocation to breeding performance through improved behavioral thermoregulation.  Am. Assoc. Lab. Anim. Sci.50, 771 (2011).