An extreme sleeper
In the high-altitude steppes and alpine meadows of Central and Eastern Anatolia, a small rodent has mastered the art of energy conservation. The Anatolian ground squirrel (Spermophilus xanthoprymnus) is an obligate hibernator, spending the majority of its life in an underground burrow. This species engages in one of the longest hibernation periods known for a mammal, lasting from late summer until the following spring, a period that can stretch to nine months.
This long slumber is not a single, unbroken sleep. It is a series of controlled metabolic shutdowns called torpor bouts. During these periods, which can last for several days to weeks, the squirrel's physiology undergoes a dramatic transformation. Its body temperature plummets from a normal 37°C to just above the ambient temperature of its burrow, sometimes approaching 0°C. In some related ground squirrel species, body temperatures have been recorded as low as -2.9°C without the animal freezing. To achieve this, the squirrel's metabolic rate slows to as little as 1-2% of its active rate. The heart rate of a similar-sized ground squirrel can drop from over 200 beats per minute to just five. These torpor bouts are interrupted by brief, periodic arousals where the squirrel rapidly warms itself back to a normal body temperature for a short time before re-entering torpor.
A flexible metabolism
Surviving such a prolonged period without food requires precise energy management. The Anatolian ground squirrel accomplishes this with a flexible metabolic system. Throughout most of the hibernation season, the squirrel relies almost exclusively on burning stored body fat, specifically lipids from white adipose tissue. This is an efficient energy source, and a respiratory quotient of around 0.7 during torpor confirms that lipids are the primary fuel.
As hibernation progresses, relying solely on lipids can cause cellular stress. Studies on other hibernating species, like the arctic ground squirrel, show a metabolic shift as the season wears on. When conditions change or lipid stores are managed, the animals begin to use nonlipid fuels like carbohydrates. This metabolic switching is an adaptation that allows the squirrel to endure the immense physiological challenge of its nine-month fast. The squirrel's diet during its short active season is important to this process. It feeds intensively on seeds, leaves, and shoots, building up the fat reserves necessary for the long dormancy. This brief window of activity, from March to September, is a frantic period of eating and reproduction before the deep sleep begins again.