Abstract:Cryptotis parva exhibits a geographic range and ecological requirements unique among North American soricines: it possesses a latitudinal distribution, metabolism and communal nesting pattern more like the crocidurines of the eastern hemisphere. We utilized oxygen consumption (VO 2 ) techniques to examine metabolic shifts and video to document activity patterns and dynamics of solitary and group nesting C. parva. Between ambient temperatures of 4°C and 34°C, solitary C. parva demonstrated an inverse relationship between ambient temperature (T a ) and resting metabolic rate (RMR); thermal neutral zone (TNZ) was very narrow, between a T a of 34°C and 36°C. VO 2 was measured in groups ranging in size from one to eight at T a s of 4°C, 14°C, 24°C and 34°C. The group size had a significant effect on the median RMR and median predicted Kleiber value and was more effective at reducing metabolic cost at a lower T a . In a second experiment designed to assess the effects of huddling group size and incubator T a on the T a of the nest chamber, both had significant effects. Group size had significant effects on the T a of the nest chamber at incubator temperatures of 5°C, 10°C, 15°C and 32°C, but not at 25°C. We found no behavioral or physiologic evidence of heterothermy.