Amazing Audible Amphibians - 20 May 2023

Leader Rori Paloski (third from left) explains how to distinguish frog calls. Photo by Glenda Denniston.

On this gorgeous Saturday evening, a stream of excitement flowed into the Preserve. Apart from the smiles, laughter, and crackling weekend fires, however, were the sounds of a different party taking place in the marsh. Seven of us joined DNR herpetologist and UW Madison PhD student Rori Paloski to learn about these boisterous spring gatherings and the amazing audible amphibians behind them.

There are eleven species of frog and one species of toad consistently found in Wisconsin. All of these species can be distinguished by their calls, which are easy to listen for during mating season from early spring into summer. Some species, like wood frogs, chorus frogs, and spring peepers, freeze completely solid during the winter and can start calling soon after melt in early April. Other species, like the green frog and American bullfrog, start later in June and July. This year the American toads had already finished their mating by the time of our field trip on May 20th, but we were just in time for the gray and Cope’s gray tree frogs.

Rori told us that the two tree frog species are practically indistinguishable by appearance alone. The only way to reliably tell them apart is by their calls. The Cope's gray tree frog has a faster, "more insect-like" trill than the slower, "more bird-like" trill of the gray tree frog. Surprisingly, the gray tree frog has twice as many chromosomes as the Cope's gray tree frog, making a blood test the one sure-fire way to identify a physical specimen.

A grey (or Cope's grey) tree frog on its way to the marsh to mate. Photos by Glenda Denniston.

While well-camouflaged from above, this tree frog has bright yellow patches underneath! Photo by Glenda Denniston.

Both gray and Cope's gray tree frogs were calling in the marsh by Lot 60 when we arrived, giving us the opportunity to hear them side-by-side and practice our call discernment skills. One tree frog even hopped right over to us on the path, allowing us to marvel at the bright yellow patches on its underside. While this individual is currently green, these tree frogs can change the color of their skin to best camouflage with their surroundings. Whether this particular frog was a Cope's gray or a gray tree frog we will never know!

Will and Rori looking at tadpoles pulled from a minnow trap. Photo by Signe Holtz.

The second green frog tadpole. Photo by Glenda Denniston.

This green frog tadpole has little legs! Photo by Signe Holtz.

Using a minnow trap provided by the Jessica Hua Lab for the Friends of Amphibians community science initiative, Will Vuyk was also able to find some green frog tadpoles in the Lot 60 marsh. Green frogs overwinter in tadpole form after hatching in July. These ones are likely close to a year old and will soon metamorphose into frogs. One even had visible back legs! Rori explained that the front legs form underneath the skin and can sometimes be felt before they break through. We will be listening close for adult green frog calls in the coming months. The calls sound like the deep thrum of a loose banjo string or large rubber band.

At the Picnic Point marsh we heard more tree frogs, and discovered the results of the recently-finished American toad mating period. Hundreds (maybe thousands?) of little American toad tadpoles wriggled in the minnow trap Will had placed the day before. Unlike green frog tadpoles, American toad tadpoles metamorphose into tiny adult toads the same season they hatch. Soon the Preserve will be hopping in multitudes of these extremely small "penny toads".  Once the tree frogs finish with their mating period, their tadpoles too will join the other toad and frog tadpoles in the Preserve's wetlands. 

Toad tadpoles from the Picnic Point Marsh. Photo by Glenda Denniston.

Will returning captured tadpoles back to the marsh. Photo by Glenda Denniston.

Frogs and toads are abundant and integral members of our native Wisconsin ecosystems. They are also very susceptible to human impacts on the landscape including water quality degradation, chemical pollution, invasive pathogens and urban infrastructure. The more we can learn about our amphibian neighbors the better we can learn to live alongside them. If listening for adult frogs and toads, and searching for tadpoles like we did in this field trip sounds fun to you, take a look at our Citizen Science page to see how you can help with our ongoing "Friends of Amphibians" project!

Report by Will Vuyk. Photos by Glenda Denniston and Signe Holtz.