The question of whether common rats and mice hibernate often arises from observing their inactivity during cold periods. The scientific answer involves understanding torpor, a related but distinct biological strategy. Torpor is a temporary state of reduced metabolic activity that allows warm-blooded animals to conserve energy when environmental conditions are challenging, such as during extreme cold or when food resources are scarce. This adaptation involves precise internal adjustments rodents make to temporarily reduce their energy use.
Defining Rodent Torpor and True Hibernation
Torpor and true hibernation both involve lowering the body’s functions, but they differ significantly in duration and depth. True hibernation is a long-term, seasonal state lasting weeks or months, characteristic of obligate hibernators like ground squirrels or marmots. These animals enter a deep dormancy driven by an internal, annual rhythm, often dropping their body temperature to near-freezing levels.
Torpor is a short-term, shallower state that typically lasts for a few hours or a single day, often called “daily torpor.” Animals capable of this are facultative heterotherms, meaning they can switch between maintaining a constant body temperature and allowing it to fluctuate based on need. This ability is usually triggered by immediate environmental stress rather than a pre-programmed seasonal cycle.
Torpor in Common Rats and Mice
Common house mice (Mus musculus) do not engage in true, seasonal hibernation, but they utilize facultative torpor. This energy-saving tactic is used primarily by wild mice or laboratory strains under duress, allowing them to survive a single cold night or a period of fasting. During a torpor bout, a mouse’s core body temperature can drop from a normal range of about 37°C to as low as 21°C, with the drop being fully controlled and reversible.
The Norway rat (Rattus norvegicus) is much less prone to exhibiting deep torpor. When faced with severe food restriction and cold, rats typically show only a gradual, moderate decrease in their body temperature and metabolic rate, unlike the dramatic suppression seen in mice. Scientists have noted that the ability to enter a deep, regulated torpor state is less robust in the rat compared to its smaller rodent relatives.
The Physiological Science of Torpor
Entry into torpor is a process of metabolic suppression, not simply an uncontrolled cooling of the body. The animal’s metabolic rate can be reduced to as little as 1 to 2% of its active state, which lowers the demand for oxygen and energy. This shift allows the rodent to survive on minimal stored resources for an extended period.
The most noticeable physiological change is controlled hypothermia, which is actively regulated by the central nervous system, specifically the preoptic area of the hypothalamus. As the body temperature drops, the heart rate slows significantly, falling from typical active rates of around 600 beats per minute (bpm) to minimums as low as 150 bpm in a torpid state. The drop in heart rate is complex, being slower during the entry phase than during the rewarming phase at the same body temperature.
The arousal phase involves the rodent rapidly rewarming itself back to its normal body temperature. This process is energy-intensive and is often powered by Brown Adipose Tissue (BAT), a specialized fat tissue rich in mitochondria. BAT generates heat through non-shivering thermogenesis, burning fat to raise the core temperature quickly enough to regain full mobility and function.
What Triggers Torpor in Rodents
The initiation of torpor is a response to environmental signals that indicate a negative energy balance. The primary external triggers are a drop in ambient temperature and a scarcity of food, which is why caloric restriction readily induces torpor in mice. Water deprivation can also contribute to the decision to enter this energy-saving state.
Internal hormonal signals play a significant role in integrating these external cues with the body’s energy reserves. For instance, the hormone Fibroblast Growth Factor 21 (FGF21) is known to rise in circulation after a period of fasting in mice. This hormone promotes the breakdown of fats and predisposes the animal toward entering a torpor bout. Torpor in nocturnal rodents is also under circadian control, most often occurring during the inactive phase of the day.