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.