Winter Is Coming: Where do the frogs go?

Winter is coming, what happens to the animals that have no fur and can’t produce their own body heat?  Frogs and other amphibians fit in this category.  Frogs are ectothermic, which means that they can’t produce their own body heat.  Some people refer to them as cold-blooded, but this is inaccurate.  Since frogs can’t produce their own body heat, they rely on the outside temperature.  If it is hot outside the frogs are actually warm and if it is cold outside then, yes, frogs are indeed cold.  What happens when it is so cold outside that the world turns into a frozen wonderland? 

Buried Alive

Many frogs, toads, and salamanders will bury themselves under the ground where it doesn’t freeze and they will stay there until the temperature warms up. 

Easter Newts in the winter
Eastern Newt (Notophthalmus viridescens) overwintering under a rock in January in Virginia.

Frozen Alive

Other frogs, like the wood frog and many tree frogs, will actually freeze almost solid during the winter (1,2,3,4)!  Usually the frogs burrow themselves under some leaves and sticks on the forest floor to stay out of sight while they are frozen and unable to escape anything (1).  I have never found a frozen wood frog but I’ve seen pictures and YouTube videos of them and they look like little frog shaped cold rocks.  

How can a wood frog freeze solid and still live to hop away?  Wood frogs, and other frogs that freeze during the winter (including many treefrog species) produce glycerol (basically DIY antifreeze).  When the frog comes into contact with ice in nature this will trigger an increased production of glycerol.  Their heart will start pumping a lot faster to distribute this glycerol throughout the body.  By biochemical methods, the glycerol will turn into glucose, also DIY antifreeze (1,2,3,4).  This glucose makes the freezing point lower within the organs so that water starts to freeze in the spaces between the organs instead of inside the organs.  The glucose also slows down freezing which prevents ice crystals from forming.  Ice crystals have pointy edges: pointy edges and soft thin delicate organs are not a good mix as the ice crystals would puncture the frog’s organs: definitely not a benefit to living.  The glucose will concentrate in organs like the brain, heart, and liver first, and organs like the skin will contain the least amount of glucose (2).

Wood frog (Lithobates sylvaticus) in summer on moss. I don’t have any pictures of them frozen in winter.

Osmosis is the process of water moving from a higher concentration of water to an area of a lower concentration of water in order to balance the water between the two areas.  All those memes about people sleeping with their head on a book so they can learn through osmosis are just wrong!  Osmosis is strictly the movement of water; those memes should actually refer to diffusion… magical diffusion.  Anyway, as water freezes in the space between the organs the amount of liquid water in this area decreases because the frozen water doesn’t count.  The water in the organs then moves out to these spaces to try and balance out the liquid water between the organs and the space.  After the water has entered the space between the organs, it too will slowly freeze.  This will continue to happen until all of the water is drawn out of the organs and into this space between the organs where the water then freezes.

Wood frog (Lithobates sylvaticus). Looks like it has some attitude.

The heart will slow down after freezing and eventually stop (3). The blood will pool above the heart until it thaws back out again (2).  Some papers will report that the heart rate does not stop completely but instead it will beat approximately once every two minutes.  As I have not personally researched freezing frogs, I wanted to provide both conclusions.  Actual location of the frog could determine if the frog’s heart completely stops or not.  If the frog is further north where it is below freezing for many months the heart may completely stop compared to somewhere that may only freeze for a few weeks (1).

Well that’s a wrap for today. While some frogs do bury themselves to avoid the cold, others will freeze almost solid! How do you avoid or tolerate winter? Me, I moved to the desert.

Citations

  1. Layne, J.R. and R.E. Lee (1995). Adaptations of frogs to survive freezing. Climate Research 5:53-59.
  2. Pinder, A.W., K.B. Storey, and G.R. Ultsch (1992). Estivation and Hibernation. Environmental Physiology of the Amphibians. 265-268. 
  3. Layne, J.R., R.E. Lee, and T.L. Heil (1989). Freezing-induced changes in the heart rate of wood frogs (Rana sylvatica). American Journal of Physiology – Regulatory, Integrative, and Comparative Physiology 257:1046-1049.
  4. Sinclair, B.J., J.R. Stinziano, C.M. Williams, H.A. MacMillan, K.E. Marshall, and K.B. Storey (2013). Real-time measurement of metabolic rate during freezing and thawing of the wood frog, Rana sylvatica: implications for overwinter energy use. Journal of Experimental Biology 216:292-302.

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