Thermal and metabolic strategies of two marsupials, 'Sminthopsis macroura' and 'Cercartetus nanus': A physiological comparison between daily torpor and hibernation

Many small mammals employ hibernation or daily torpor to reduce energy expenditure by lowing metabolic rate and body temperature. Although it is known that the reduction of metabolic rate is more pronounced during hibernation than during daily torpor, it is unclear whether different physiological processes contribute to metabolic rate reduction during the two patterns of torpor. In addition, differences in relation to behaviour and energy expenditure between hibernation and daily torpor are not well understood. To determine physiological difference between hibernation and daily torpor, a detailed investigation was conducted by using two similar-sized small marsupials, 'Cercartetus nanus', a hibernator, and 'Sminthopsis macroura', a daily heterotherm. Thermophysiological variables, including metabolic rate, body temperature, thermal differential between body temperature and ambient temperature, and conductance of the two species were measured under the same experimental conditions. Interrelations between these variables during both normothermia and torpor were analysed over a wide temperature range. Torpor entry, steady-state torpor, and arousal were distinguished in the analyses. In addition, temperature selection and locomotor activity in relation to the use of torpor were investigated. Energy expenditure in response to duration of torpor and arousal was also compared. ... The thermal and metabolic characteristics of 'C. nanus' and 'S. macroura' indicate that different physiological adjustments are involved in hibernation and daily torpor to achieve energy conservation