The question of which animal has the lowest body temperature is complex, depending on whether the organism is metabolically active or in a state of deep, regulated dormancy. Core body temperature is the operating temperature of internal organs and tissues, which is tightly controlled in most animals. However, some species employ survival strategies that involve intentionally dropping this internal temperature far below the point that would be lethal to most other life forms. Identifying the record holders requires differentiating between the lowest temperature a fully functional animal can sustain and the lowest temperature an animal can survive while in a suspended state.
The Lowest Measured Internal Temperatures
The lowest recorded temperatures belong to animals that undergo controlled biological shutdown, such as deep torpor or purposeful freezing. The Arctic ground squirrel holds the record for the lowest body temperature ever measured in a mammal, reaching as low as -2.9°C (26.8°F) during its nine-month hibernation. This sub-zero temperature is achieved through prolonged torpor, where the squirrel’s heart rate and metabolic activity drop drastically to conserve energy. The animal maintains its body fluids in a supercooled, liquid state, avoiding freezing solid.
Even more extreme cold tolerance is found in certain amphibians, which allow parts of their bodies to freeze. The Alaskan wood frog (Lithobates sylvaticus) can survive with up to 70% of its total body water turned into ice. In this frozen state, the wood frog exhibits no detectable heartbeat, breathing, or blood circulation, essentially entering suspended animation. Wood frogs in natural hibernation have been recorded surviving temperatures down to -18°C (-0.4°F).
Physiological Adaptations for Cryo-Survival
The ability to endure temperatures far below the freezing point of water relies on two primary biological strategies: freeze-avoidance and freeze-tolerance. Freeze-avoiding organisms, such as the Arctic ground squirrel and many insects, utilize supercooling to prevent ice crystal formation within their tissues. They achieve this by removing ice-nucleating agents and producing cryoprotectant compounds like glycerol and other polyols. These compounds act like biological antifreeze, lowering the freezing point of body fluids so they remain liquid even below zero.
The freeze-tolerant strategy, exemplified by the wood frog, involves managing ice formation rather than preventing it entirely. These animals produce massive amounts of cryoprotectants, primarily glucose converted from liver glycogen, and urea, which are pumped into the cells. The high concentration of these solutes protects the cell interiors from dehydration and prevents lethal intracellular freezing. Ice formation is limited to the extracellular spaces, allowing the organism to survive partially frozen until conditions warm enough for thawing.
Defining Endothermy and Ectothermy
The difference in temperature tolerance is rooted in how animals regulate internal heat, defined by endothermy and ectothermy. Endotherms, including mammals and birds, generate most body heat internally through metabolic processes. They maintain a relatively constant, high body temperature regardless of the external environment, a condition known as homeothermy. This steady temperature is necessary for optimal enzyme function but requires continuous, substantial energy input.
Ectotherms, including amphibians, reptiles, fish, and insects, rely on external sources of heat, such as the sun, to regulate their body temperature. Their internal temperature fluctuates with the environment, resulting in a much lower metabolic rate and lower energy requirements than endotherms. This reliance is why ectotherms can more readily achieve the lowest measured temperatures, as their physiological systems are adapted to operate across a broader thermal range.
Lowest Sustainable Active Temperatures
When considering the lowest temperature an animal can sustain while remaining fully active, the focus shifts to certain fish species living in the polar oceans. Antarctic notothenioid fish inhabit seawater consistently near -1.9°C (28.5°F), which is below the natural freezing point of their body fluids, typically around -0.9°C. This poses a constant threat of inoculative freezing, where ice crystals from the environment enter the fish’s body.
These active fish survive by synthesizing specialized molecules called Antifreeze Glycoproteins (AFGPs) or Antifreeze Proteins (AFPs). These proteins circulate in their blood and bind irreversibly to nascent ice crystals, preventing them from growing larger. This mechanism, known as thermal hysteresis, creates a gap between the temperature at which the blood freezes and the temperature at which it melts. This allows the fish to remain liquid and mobile in sub-zero water, ensuring biological processes function efficiently in one of the coldest sustained habitats on Earth.