Thermoneutral temperature reduces liver volume but increases fat content in a mammalian hibernator

Hibernators survive challenging winters by entering torpor, which lowers body temperature (T_b) to ∼5 °C for 12-14 days, followed by spontaneous arousals where T_b increases to ∼37 °C for 10-12 h before entering another torpor bout. This T_b cycle is accompanied by significant fluctuations in metabolic rate. Little is known about the role of the liver in lipid metabolism during hibernation. In this study we measured the effect of ambient temperature on liver volume and lipid content in 13-lined ground squirrels (Ictidomys tridecemlineatus). We housed animals at thermoneutral (25 °C) or cold (5 °C) ambient temperatures, with the same photoperiod (12 h light:12 h dark) for an entire year. We determined volume and water-fat ratio of the liver using magnetic resonance imaging (MRI). Ambient temperature significantly affected both liver volume and fat content. From October to August squirrels housed at 25 °C had 25% smaller livers compared to the squirrels housed at 5 °C, but their average lipid content (13.3%) was 37% higher. Because the squirrels housed at 25 °C appeared to continue feeding throughout the winter but did not enter extended torpor, more carbohydrates may have been diverted to lipid stores. By contrast, animals housed at 5 °C did not appear to feed, and carbohydrates would likely be preferentially stored in the liver as glycogen to supply glucose for brain metabolism. These results suggest that the fat burden caused by hibernators preparing for winter can lead to symptoms of metabolic syndrome, but that these symptoms are reversible in the spring.