By Dana Johnson
Slowly but surely the long, hot days of summer give in to the shorter, cooler days of fall. This results in a change in activity for our local frog and toad populations. Their thin skins are very sensitive to temperature changes. They are no longer singing their mating calls of spring, and their insect fodder is becoming less abundant. They are preparing for the long, cold months ahead.
The Minnesota Arrowhead is home to four species of true frogs, including the green, wood, mink, and northern leopard frogs. Also abundant are three species of tree frogs, which are the spring peeper, western chorus, and gray tree frog. Along with these smooth-skinned hoppers are the lumpy, bumpy American toads. Among these eight “herps” (short for herpetofauna-the general term for amphibians and reptiles) are three hibernation patterns, each one fascinating in its own right.
Green, mink, and northern leopard frogs prefer to live along lake shores, streams, and large ponds. When the leaves turn shades of red and gold, and the temperature drops, these frogs take their cue and dive beneath the water’s surface. They will not emerge again until the warm spring sun melts the snow and ice covering their winter habitat. Frogs breathe both with lungs and by absorbing oxygen through their skin, and it’s this keen evolutionary adaptation that allows them to stay submerged for over half of the year. These three species will choose deep waters or flowing streams full of oxygen and cover themselves lightly in debris on the bottom to hide from predators. As the water gets colder they enter a torpid state where they are still able to move, but very slowly. If they freeze, they will die.
The American toad has a very different method of hibernation. Toads are mostly terrestrial amphibians, going to water only to mate in the spring. When the air gets chilly, toads will seek out soft soil and begin to use their back legs as spades, digging a hole behind and underneath their bodies. They will dig down as far as they need to in order to stay below the frost line, which in Minnesota can reach below three feet. Their bodies, like those of the more aquatic frogs, enter a torpid state until the frost recedes and the insects emerge in the spring.
Wood frogs and tree frogs have a third method of hibernation, which is probably the most bizarre of them all. They, in a sense, become “frogcicles.” These frogs will bury themselves as far as they can beneath leaf litter and debris on the forest floor as the cold sets in, but before the first frost. They enter a state of suspended animation where their body fluids partially freeze. They continue to take in oxygen through their skin until their icy encasement melts in the sunlight, and they return to the spring ponds to mate.
For more information on these and other Minnesota herps, please visit:
http://www.dnr.state.mn.us/reptiles_amphibians/index.html