Lowest body temperature recorded in a mammal

The lowest body temperature ever recorded in a mammal is -2.9ºC (26ºF) for the Arctic ground squirrel (Urocitellus parryii) of Alaska, USA, and north-west Canada. Their body temperatures drop below freezing when in a state of suspended animation during their (up to) nine-month hibernation period in the Arctic winter. Being a warm-blooded mammal, their normal body temperature in the summer months is 37ºC (98ºF).

The study was carried out in Aug 1987 by Professor Brian Barnes of the University of Alaska Fairbanks, who attached tiny temperature-sensitive radio transmitters to the abdomens of 12 squirrels before they entered their burrows for hibernation.

The burrows in which they hibernate are on average -8°C (17.6°F) and can plummet to as low as -26°C (-14.8°F).

The Arctic ground squirrel is able to survive body temperatures below freezing thanks to a range of physiological adaptations. For instance, many of the squirrel's brain cells and synapses are allowed to die off, but are rapidly regenerated as the end of its hibernation approaches. It's also able to "supercool" its body fluids – somehow clearing its blood and other fluids of water molecules to prevent them turning into ice prior to entering torpor, though it's as yet unknown how they achieve this.