Maximum rates of sustained metabolic rate in cold-exposed Djungarian hamsters (Phodopus sungorus): the second wind
Djungarian hamsters (Phodopus sungorus) tolerate short-term exposure to ambient temperatures (Tas) down to −70°C, but surprisingly, previously appeared to reach maximum sustainable metabolic rate (SusMR) when kept at Tas as high as ≥−2°C. We hypothesized that SusMR in Djungarian hamsters may be affected by the degree of prior cold acclimation and temporal patterns of Ta changes experienced by the animals, as average Ta declines. After cold-acclimation at +5°C for 6 weeks, hamsters reached rates of SusMR that were 35% higher than previously determined and were able to maintain positive energy balances down to Ta −9°C. SusMR was unaffected, however, by whether mean cold load was constant or caused by Tas cycling between +3°C and as low as −25°C, at hourly intervals. At mean Tas between +3 and −3°C hamsters significantly reduced body mass and energy expenditure, but were able to maintain stable body mass at lower Tas (−5 to −9°C). These results indicate that prior cold-acclimation profoundly affects SusMR in hamsters and that body mass regulation may play an integral part in maintaining positive energy balance during cold exposure. Because the degree of instantaneous cold load had no effect on SusMR, we hypothesize that limits to energy turnover in Djungarian hamsters are not determined by the capacity to withstand extreme temperatures (i.e., peripheral limits) but are due to central limitation of energy intake.