The wintery weather has me thinking about strategies that some of our native wildlife have for surviving the winter weather.  The gray treefrog has a pretty incredible trick up its sleeve.  Actually, the trick is in its bodily fluids. It is one of several species of North American frogs that make their own antifreeze.

As the weather gets colder, and other cold blooded animals are finding their way to their underground retreats, the gray treefrog is converting some of its fat to Glycerol.  Glycerol acts like a natural antifreeze in the frog’s body.  It has a much lower freezing point than water and builds up in the frog’s cells. If the frog’s body were to freeze without the glycerol ice crystals would form in its cells and burst them. That would be really bad for the frog. The presence of glycerol means that ice crystals can form outside the cells and not in them. Since the frog is freeze tolerant, it can overwinter in leaf litter or sometimes just tucked into the grooves in tree bark.  I have even heard several accounts of gray tree frogs overwintering in backyard grill covers.

There are two gray tree frog species native to Michigan; the Cope’s gray treefrog and the gray treefrog.  They are what biologists call sibling species.  Which means that they are so closely related that it is impossible to tell them apart just by looking at them.  The Cope’s gray treefrog is more common in the Upper Peninsula and less common in the Lower Peninsula and the gray treefrog has the opposite distribution. They can be differentiated by a subtle difference in the advertisement calls that the two frog species use to attract mates.  Indiana’s Department of Natural Resources has a recording of both, if you would like to see if you can hear the difference (

Here at the zoo, the gray treefrog is one of the few species that we have on exhibit and occurs naturally on grounds. You can see them on exhibit in the Treasures of the Tropics building, or if you come to the zoo in May or June you are quite likely to hear them outdoors.  The native frogs call most nights in May and June from bushes and trees around Monkey Island and in the hoofed stock exhibit between the wart hogs and the lions.  Right now, they are lying dormant under all this snow waiting for the spring to come. 



About Bill Flanagan

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Zookeper at John Ball Zoo for two years, but has been "in the business" for over 20 years.

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