Maher RL, Barbash SM, Lynch DV, Swoap SJ. Group housing and nest building only slightly ameliorate the cold stress of typical housing in female C57BL/6J mice. Am J Physiol Regul Integr Comp Physiol 308: R1070 -R1079, 2015. First published April 15, 2015 doi:10.1152/ajpregu.00407.2014.-Huddling and nest building are two methods of behavioral thermoregulation used by mice under cold stress. In the laboratory, mice are typically housed at an ambient temperature (T a) of 20°C, well below the lower end of their thermoneutral zone. We tested the hypothesis that the thermoregulatory benefits of huddling and nest building at a T a of 20°C would ameliorate this cold stress compared with being singly housed at 20°C as assessed by heart rate (HR), blood pressure (BP), triiodothyronine (T3), brown adipose (BAT) expression of Elovl3 mRNA, and BAT lipid content. A series of experiments using C57BL/6J female mice exposed to 20°C in the presence or absence of nesting material and/or cage mates was used to test this hypothesis. Mice showed large differences in HR, BP, shivering, and core body temperature (T b) when comparing singly housed mice at 20°C and 30°C, but only a modest reduction in HR with the inclusion of cage mates or bedding. However, group housing and/or nesting at 20°C decreased T3 levels compared with singly housed mice at 20°C. Singly housed mice at 20°C had a 22-fold higher level of BAT Elovl3 mRNA expression and a significantly lower triacylglycerol (TAG) content of BAT compared with singly housed mice at 30°C. Group housing at 20°C led to blunted changes in both Elovl3 mRNA and TAG levels. These findings suggest that huddling and nest building have a limited effect to ameliorate the cold stress associated with housing at 20°C. ambient temperature; blood pressure; brown fat; nesting THE MOUSE (MUS MUSCULUS) IS an important model system for mammalian studies. Its genetic tractability with a short reproductive time, coupled with advances in the miniaturization of physiological tools, create an increased ease for the merging of physiological studies and "omic" studies. The ambient temperature (T a ) of animal housing facilities is within a standard range that, while comfortable for human caretakers, falls well below the mouse thermoneutral zone (TNZ), the range of T a s in which endotherms do not elevate their metabolic rate to maintain core body temperature (T b ) (19, 23). The T a at which metabolic rate begins to rise to offset heat loss (i.e., the lower critical temperature) is ϳ29 -30°C for mice (18). Housed at temperatures below this zone, mice are under cold stress (8).To elevate metabolic rate and minimize heat loss when exposed to cooler temperatures, endotherms engage one of many behavioral changes and/or several physiological adaptations, which vary across species and which are not mutually exclu-