When nonshivering thermogenesis equals maximum metabolic rate: Thermal acclimation and phenotypic plasticity of fossorial Spalacopus cyanus (Rodentia)

Many small mammals inhabiting fluctuating and cold environments display enh anced capacity for seasonal changes in nonshivering thermogenesis (NST) and thermoregulatory maximum metabolic rate (MMR). However, it is not known ho w this plasticity remains in a mammal that rarely experiences extreme therm al fluctuations. In order to answer this question, we determined body mass (m(b)), basal metabolic rate (BMR), NST, MMR, and minimum thermal conductan ce (C) on a Chilean fossorial caviomorph (Spalacopus cyanus) from a coastal population, acclimated to cold (15 degreesC) and warm (30 degreesC) condit ions. NST was measured as the maximum response of metabolic rate (NSTmax) a fter injection of norepinephrine (NE) in thermoneutrality minus BMR. Maximu m metabolic rate was assessed in animals exposed to enhanced heat-loss atmo sphere (He-O-2) connected with an open-flow respirometer. Body mass and met abolic variables increased significantly after cold acclimation with respec t to warm acclimation but to a low extent (BMR, 26%; NST, 10%; and MMR, 12% ). However, aerobic scope (MMR/BMR), calculated shivering thermogenesis (ST ), and C did not change with acclimation regime. Our data suggest that phys iological plasticity of S. cyanus is relatively low, which is in accordance with a fossorial mode of life. Although little is known about MMR and NST in fossorial mammals, S. cyanus has remarkably high NST; low MMR; and surpr isingly, a nil capacity of ST when compared with other rodents.