October is a time of change for all ecosystems here in Minnesota. The summer visitors begin migrating south lands of eternal warmth. Many of the year round residents who are unable to flee the approaching frosts are forced to adapt to the cold or simply wait it out until the following spring. A vast majority of species choose the latter, with only the heartiest species staying active throughout the long northern winter.
October signals the deciduous trees to drop their leaves, squirrels to create caches of food to last through winter and animal of all kinds to find shelter. Frogs are no exception to this rule, the must find a place to over-winter or risk never rising from their slumber. Unlike many other animals frogs, specifically tree frogs, have the amazing ability to freeze completely solid.
The Eastern Gray Tree Frog (Hyla versicolor) is a common tree frog throughout much of Minnesota. They spend a majority of their lives away from water in forested areas. They return to vernal pool in late spring until the beginning of July. They spend the long winter underneath the leaf litter of the forest floor frozen completely solid. This is no small feat for a complex vertebrate, in fact they are among a very few vertebrates who can tolerate being frozen solid.
Tree Frogs perform this amazing feat by literally turning their blood into antifreeze. They increase the sugar content of their blood to such a degree that as the temperature falls the water in their blood is unable to form destructive crystals. The frog simply freezes solid without the formation of any water crystals that act like microscopic knives cutting through tissue. Humans call this formation of ice in the body frost bite. Severe frost bite can result in complete necrosis of the affected area. Frogs bypass these effects by putting themselves into the equivalent of a diabetic coma.
Eastern Gray Tree Frogs are among my favorite of all frog species in Minnesota. They are amazing not only for their ability to freeze solid but also change color. As the name suggests they are often “gray” but they can also change to a bright green as they transition between bark and leaves. In addition to changing color that can change the texture of their skin, all be to a lesser degree, to match their surroundings.
These unique abilities of this frog species allows them to inhabit everything from the forest floor to the tops of the trees. This diverse habitat allows them to exploit a wide variety of insects to feed on.
It may not look like effective camouflage close up but placed in a more natural habitat it serves its purpose quite well.
Did you find the frog?
Next time you take a mid-winter hike through a snowy wonderland think about all the thousands of insects, frogs, and plants that are patiently waiting in a dormant state for the first sign of spring.