Read more: Extreme survival: The toughest life forms on Earth
More than 80 per cent of the habitats on Earth are colder than 5 °C – but there is no shortage of species that can cope with the chill
Get below 5 °C, and enzymes, the biological catalysts that facilitate all of life’s chemistry, work painfully slowly. Below freezing, matters get even worse. Ice crystals start to form in and around cells, sucking water out of them and cutting their membranes and contents to shreds.
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Nevertheless, more than 80 per cent of the habitats on Earth are colder than 5 °C. And there is no shortage of species that can cope with the chill.
For once, microbes don’t come out on top: they stop growing at around -15°C. More complex animals take this record, employing an array of tricks to survive the freeze.
Mammals and birds have a head start, as they generate their own heat as a by-product of metabolism. They also insulate their bodies with fur, blubber and, in the case of emperor penguins, by huddling en masse against the icy Antarctic winds that can bring the air temperature down to -60 °C.
Plenty of animals that lack their own internal heating can still survive such temperatures. Take the insect-like creatures called Arctic springtails (Megaphorura arctica). These lower the freezing point of their body fluids as winter approaches by synthesising antifreeze molecules and getting rid of anything that could act as a nucleation site for ice crystals to form around, such as gut contents and bacteria. They also manufacture cryoprotectants – sugars or glycols that protect cells from being damaged by the freezing process.
Some animals can even survive being frozen solid, including many insects, western painted turtle hatchlings (Chrysemys picta bellii) and several North American frogs, such as the wood frog, Rana sylvatica. These animals use antifreeze to protect the body parts that matter most. In less crucial parts, such as their body cavity and the lenses of their eyes, they encourage controlled freezing by producing proteins that act as ice nucleation sites or encouraging the growth of ice-nucleating bacteria.
The woolly bear caterpillar, Gynaephora groenlandica, of Ellesmere Island, Canada, is an extreme example. It hibernates in temperatures of -70 °C, surviving by letting its gut contents, blood and any other extracellular liquid freeze. The Antarctic nematode, Panagrolaimus davidii, can go a step further, allowing freezing of its cells’ cytoplasm – the liquid component – keeping only its cell nuclei and other organelles unfrozen. No one knows how they cope with this bizarre state, but they do synthesise a cryoprotectant that may smooth the edges of ice crystals, preventing damage.
