How can frogs survive freezing?

19 November 2006



I saw a documentary about frogs, which during the winter literally freeze solid. Come spring, they seem to de-freeze and come back to life. How do they do this?


If you put a human in the freezer, the first thing that would happen is that all of our tissues would freeze. About 60 or 70% of the weight of a human being and most mammals is water. Water forms crystals of ice, and those crystals are often jagged and sharp. These sharp ice crystals destroy the cellular structure, burst holes in the cells and make the tissue fall apart. This is the same reason why when you put a strawberry in the freezer and then get it out again it doesn't resemble a strawberry anymore - it just turns into a sort of mess. Some animals that resist this cellular destruction have managed to evolve a natural antifreeze, which works by stopping crystals forming these big jagged shapes. So that's part of it. They form much smaller crystals that have softer edges. There was a very elegant piece of research published about this time last year in the journal "Science", and they were looking at the snow flea, which lives in Canada. The snow flea makes another form of anti-freeze, and when you zoom in on the body of those animals, which can survive down to about minus 10 or something, you see that the tiny crystals of ice which form in their cells look almost like a grain of rice. They don't look sharp and jagged at all, which means that the cells don't get damaged in the same way. These antifreezes also allow them to resist lower temperatures, which means that their blood doesn't actually turn solid until a much lower temperature than it would do normally. So it works a bit like the antifreeze that you would put into your car. The other part of the survival mechanism is that frogs and other amphibians are cold blooded. So unlike us, where we have to stay warm, or we die, those animals absorb a lot of energy from their environment. Doing a little bit of exercise does put their temperature up a bit but they largely rely on absorbing energy from the environment, and that determines their metabolic rate. So how metabolically active they are can go up or down enormously depending upon the temperature. So if you cool a frog down, it just slows down to near stand still metabolically, and doesn't do anything, until you warm it up again, and they're well adapted to being able to survive like that.


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