During a recent lecture on evolutioin I had to explain the differences between three different processes known as geographic, temporal and behavioral isolation. Geographic isolation is the easiest of these concepts to understand because it involves two different populations of the same species of organism that are separated (or isolated) by a landscape feature of some sort. Streams, rivers, highways, mountain ranges, deserts and oceans are all good examples of features that could do this.

The next-easiest one of these processes to try to explain would be behavioral isolation. This one involves two populations of the same species that are kept separate from one another due to some sort of behavior, the most obvious of which is song (or language). When I mentioned the idea that different birds sing different songs it seemed to resonate in the minds of my students. But then I mentioned the idea that different frogs also sing different songs and that produced blank stares and puzzled looks. One student even asked, “Don’t frogs just say ribbit?”

After a slight internal wince I then moved on to the final mechanism of isolation known as temporal isolation. In this mechanism it is the timing of events that can separate or isolate two populations. I mentioned the blooming times of the same species of flowers found at different altitudes on a mountain. And, again, I mentioned the different times that our local frogs sing their songs. More blank stares.

So, I asked if anyone had heard Spring Peepers sing. No one seemed to know what I was talking about. Then I asked if anyone knew the names of any of our local frogs. Silence. When I mentioned that there are seven species of frogs that can be found in this region there were a few flickers of interest, but after a while I realized that almost none of my students knew anything about this group of amphibians. I was a little depressed by that.

I decided to fire up the classroom speakers and play some frog songs for the kids. A few recognized the spring peepers and confessed that they thought it was the sound of birds. Most kids recognized the song of a bullfrog, perhaps from TV and movies, but none had ever heard the songs of wood frogs before. Once again, a student in one of my classes put a spotlight on the whole affair by saying, “Those sound like chickens.” I knew then that I needed to do a little work on frogs.

There are seven species of frogs that can readily be seen and heard in western Massachusetts. They are the spring peeper, the wood frog, the gray tree frog, the leopard frog, the mink frog, the green frog and the bullfrog. The first of these species to start singing are the spring peeper and the wood frog, and I officially heard both in my own yard last week. And there is a good scientific reason for this, too.

Both species breed in “vernal pools.” Simply put, these are small bodies of water that form in wooded areas that fill with the water from snowmelt and April showers. The pools are always in the same places, so the local frogs know exactly where and when to find the water that they are absolutely dependent upon. They have to get an early start because once the trees put out their leaves and photosynthesis really gets going, the pools will start to draw down and dry up. Any pollywogs not ready to leave the water before the water disappears will die.

So, these two small species of frogs are already at it and their voices will drift across the landscape for the next few weeks. The spring peeper song is a very high-pitched single note that is repeated over and over by each male. A large group of peepers in the same place can make a very loud noise when they all sing at the same time and the sound is very easy to identify. Wood frogs also sing songs that are easy to identify, but you first have to know that the song is that of a frog.

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This sound is much lower in pitch than that of the spring peeper and it does actually sound like a chicken clucking. I’ve also thought of it as sounding like little animals barking, or a duck choking and sputtering, but it is difficult to really describe well. So, if you have the time, just do an internet search for wood frog songs and you should find several examples instantly.

Male wood frogs are singing to attract mates and when a female arrives she will get a lot of attention. A male wood frog will grab her around her waist (a position known as amplexus) and when she releases her eggs into the water the male will fertilize them. This external fertilization is also found in fish, salamanders and toads. A single female can deposit 1,000 eggs in masses that may either rest on the bottom of a pool, or can be attached to sticks. Then the adults will retreat back into the forest, where they spend the rest of the summer, fall and winter. These frogs only need standing water for reproduction.

On a warm spring evening, either on the way to, or back home from, an ice cream stand, consider taking the back roads and rolling the windows down. If you approach an area with a vernal pool you may be surprised how loud the little amphibians can be and you might also find the world in which you live to be just a little more interesting than it had been before.

Bill Danielson has been a professional writer and nature photographer for 26 years. He has worked for the National Park Service, the US Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more in formation visit his website at www.speakingofnature.com, or go to Speaking of Nature on Facebook.