Animals have all sorts of strategies for coping with cold temperatures, and some are pretty extraordinary
LET THE BODY FREEZE
If a human were to let their body freeze, the result would be all-over frostbite and probably death. But there are creatures whose tissues are able to freeze solid in cold weather, then defrost in the spring, with no adverse effects. Take the wood frog, a native of North America, which hibernates over the winter hidden beneath leaves. It has ‘nucleating proteins’ in its blood that cause water to be drawn from the frog’s cells into its blood, where it can safely freeze. This spares the cells from the damage that would be caused by ice crystals forming in them. The water in the cells is replaced by urea and a sugary liquid produced by the frog’s liver.
While in this frozen state, the frog displays no heart or brain activity. Research carried out at Carleton University in Ottawa, Canada, showed that when temperatures begin to rise, it takes around 20 minutes for the ice to melt in the frog’s circulatory system and a further 20 or 30 minutes for its heart to start beating. It can go through this cycle several times in a year. Freezing has become an essential part of the frog’s life. If temperatures remain too warm, it doesn’t freeze and requires food throughout the winter. Food is scarce at that time of year, so the frog is more likely to die from starvation than if it were in a state of suspended animation.
USE AN ANTIFREEZE
The crocodile icefish, which lives in the freezing waters around the South Pole, has a natural antifreeze in its blood. Saltwater freezes at a lower temperature than freshwater, and the icefish live in seas as cold as -2 degrees Celsius – conditions that would kill most fish species. As with the wood frog, it’s a protein that protects the crocodile icefish. Known as an ice-structuring protein, it binds to ice crystals as they form in the fish’s blood and prevents them from growing any larger while also inhibiting the growth of new ones. It can do this because ice crystals have flat surfaces it can stick to – something not usually found in blood cells.
Icefish blood also has fewer red blood cells than most other species. This means its blood is less viscous and able to keep flowing in extremely cold temperatures. The crocodile icefish has so little haemoglobin that its blood is yellow. These fish are able to survive because their Southern Ocean home is high in dissolved oxygen, the solubility of which is increased at low temperatures – enough to be carried in blood plasma. The icefish is not the only living thing to fight the col
