Key indicators of metabolic activity in the long-tailed ground squirrels, (Urocitellus undulatus) in different functional states.

The aim of this study is to examine the seasonal strategies employed by obligate hibernators, such as long-tailed ground squirrels (Urocitellus undulatus), for metabolic maintenance at various levels of organization. To this end, we conducted a comprehensive evaluation encompassing respirometry, histological and biochemical analysis of the thyroid gland, examination of neutral and phospholipid content in brown adipose tissue, and biochemical assessment of the blood lipid profile. Our results revealed pronounced variations in metabolic parameters on a seasonal basis. In the fall (pre-hibernation), U. undulatus exhibited an increase in respiratory metabolism ranging from 1.4 to 1.7 times compared to the summer control group. It is accompanied with an augmentation in the functional activity of the thyroid gland, an increase in all fractions of phospholipids in brown adipose tissue, and an elevation in the level of lipase in the blood. Concurrently, there is a decline in di- and triglyceride levels in brown fat. In a state of torpor, there was an observed increase in the growth of certain indicators (lipids, thyroid hormones, etc.), a particularly significant rise is observed for β-hydroxybutyrate. In the interbout euthermia, a recovery of nearly all indicators to summer-fall values was observed. These findings hold potential relevance for a better understanding of metabolic disorders and adaptations to extreme conditions.