Body size influences the capacity to cope with extreme cold or hot temperatures in the striped hamster.

Body size of organisms is a key trait influencing nearly all aspects of their life history. Despite growing evidence of Bergmann's rule, there is considerably less known about the links between body size and the maximum capacity to thermoregulate of an animal in response to extreme cold or hot environment. Thermal characteristics such as resting metabolic rate (RMR) and non-shivering thermogenesis (NST), and the upper- and lower-critical temperatures of the thermal neutral zone (TNZ) were investigated in small and large body sized striped hamsters (Cricetulus barabensis). The maximum capacity to thermoregulate in response to extreme cold (-15 °C) or hot temperature (38 °C) was also examined, where both, different sized hamsters had similar RMR and NST regardless of temperature exposure. The large hamsters had 29.9% more body mass compared to small hamsters. The large hamsters showed a wider TNZ, with lower, lower-critical temperature, and showed considerable hyperthermia at the end of a 17-h hot exposure. In contrast, the small hamsters showed hypothermia following a 17-h cold exposure relative to large hamsters. In addition, the large hamsters showed 17.2% lower basal thermal conductance, and 14.9% lower maximum thermal conductance than the small hamsters after cold exposure, and 22.6% lower thermal conductance following heat exposure. Several molecular markers indicative of thermogenesis and oxidative stress did not differ significantly between the large and small hamsters. These findings suggest that individuals with larger body sizes have greater capacity to thermoregulate to cope with extreme cold, and a reduced capacity in response to extreme hot. In contrast, smaller individuals demonstrated the opposite trend. Body size may decide the capacity to thermoregulate to cope with extreme cold and heat, within which body heat dissipation is likely more important than heat production.