Frogs of Wisconsin

By Mike Reiter, Polk County Naturalist


     Frogs are members of the Class Amphibia, which makes them amphibians which means “both life forms”. They spend their life both on land and in the water. They have thin, semi-permeable skin which allows them to absorb oxygen and other molecules through their skin. Frogs belong to the Order Anura which means “tailless”. They are again divided into three families, toads, tree frogs and true frogs. They are cold blooded, like reptiles, but unlike reptiles who like the warmth of the sun, they prefer cool, damp places that help prevent them from desiccation or drying out. Most frogs are more active at night which helps them to remain cool and hydrated. The smallest frog species is the Blanchard’s Cricket Frog, which measures just over an inch, while our largest is the bullfrog which may exceed six inches from stem to stern. Cricket frogs live less than a year while the bullfrog can max out at 10 years old.

    Depending on their species, frogs will mate from early spring to summer during three periods dictated by water temperature. They are quite predictable, with the three periods corresponding to 50, 60 and 70 degrees.

     Frogs skin contains glands which help to hydrate their skin and also block pathogens and mold, but their thin skin also allows water and other chemicals to pass through.  Skin coloration allows them to blend into their surroundings and camouflage them against predators.

     Tadpoles eat primarily tiny plants and algae but once they morph into their adult form they turn carnivorous, eating live prey such as insects and small invertebrates. Bullfrogs will take on small fish, mammals and just about anything that looks like a meal!

     Most frogs try to remain undetected to avoid predation, but can “kick” their aggressor if need be. Some emit a foul substance to ward off unwanted aggression. The American toad emits a fluid that creates a burning sensation. Dogs will hyper-salivate when “mouthing” a toad because of secretions from the large glands located on the toad’s back. Toads can also inflate themselves to make it more difficult to swallow.    

     Tadpoles have demonstrated the ability to regenerate their tails if damaged in an attack. This process has generated much interest in the medical field as the area of wound healing and limb regeneration in humans advances.

     Frogs, living in a climate with four seasons, need to adapt to long periods of below freezing conditions during the winter months. Spring peepers and wood frogs will freeze solid during this time. Their cells produce a glycol-like substance similar to antifreeze that protects their cells. In the spring, they literally thaw out and resume normal frog activity.  Other frog species burrow underground or enter the water where they remain submerged taking in oxygen through their skin.

     Wisconsin’s 12 frog species include, the Green Frog, Northern Leopard Frog (most common), Pickerel Frog, the Wood Frog (has dark mask), Western Chorus Frog (has three back stripes), Blanchard’s Cricket Frog (black triangle on head), Northern Spring Peeper (X on back), Eastern Gray Treefrog (can change color), Cope’s Gray Treefrog, Mink Frog (has musty scent), Bullfrog (largest) and last but not least, the American Toad (warty).

     Urban development and draining of wetlands has greatly diminished quality frog habit that once was on the landscape. Because of the way frogs live their lives, they also are considered the “canary” in the coal mine. When water quality is degraded, frogs are one of the first group of animals that are stressed and populations show marked fluctuations. We need to continue to preserve quality wetlands and restore drained and damaged ones to assure we will have these unique animals as neighbors forever! Falling to sleep with the sounds of the frogs and waking up to the sounds of the birds is one of the true joys of nature!