Nature up close: Boreal chorus frogs

By “Sunday Morning” contributing videographer Judy Lehmberg.

We know the long winter is finally loosening its grip on Yellowstone when we hear the sound of spring: boreal chorus frogs calling from snowmelt pools in the marshy areas near Yellowstone Lake, often heard, but not so easily seen. The frogs sound incredibly loud for a tiny amphibian just over one inch long. Every time I hear them I would swear there were hundreds of them calling, but those hundreds are usually only 10 or 15 frogs.

I have to admit boreal chorus frogs are not high on most tourists’ “must see” list. I don’t blame them. If I had never seen a grizzly bear or a wolf I probably wouldn’t spend time on the frogs, either. But one of the wonderful things about Yellowstone is the more time you spend there, the more you realize you will never see it all. I continue to try, though, peeling away the layers year after year for over 40 years now.

The park rangers know visitors care more about bears, wolves and other “charismatic megafauna,” and exhaust themselves trying to keep a respectful distance between animals and tourists. The last time we photographed the boreal chorus frogs, my husband put on his fishing waders to keep his feet dry, and then carried a lawn chair and his camera/tripod out to the frogs. He looked pretty goofy but he was comfortable and the frogs couldn’t have cared less. They had one thing on their mind and it wasn’t him. After a little while a ranger came by, did a double take, stopped, and asked what we were photographing. I told him, “Boreal chorus frogs.” He laughed and said, “Finally someone is taking a photo of something other than a bear!” I didn’t notice that he took a photo of my husband. Later he told us the park’s herpetologist has a photo on her wall of an old guy sitting in a lawn chair staring intently at the water in front of him. We are very proud that we gave him a laugh and that she knows she isn’t the only one who loves her frogs.

Only male frogs have a vocal sac and can call. They have two calls: one that says “Hey, I am a male frog. All you females come over here and mate with me.” The other call says, “Hey, I am a male frog. Get off of me.” They make their call with their nostrils closed, while pushing lung air over their vocal chords and into the resonating air sac beneath their chin. The opening between the lungs and the air sac modulates air flow, and therefore the sound. The frogs resonating air sac magnifies the sound the male makes to attract a female.

Boreal chorus frogs have as many as eight variations in dorsal coloration in high altitude, montane populations. They can vary in color from tan or brown to olive or green, but all have a dark brown to black stripe from the nostril through the eye and down the side. They also have three, sometimes blotchy, stripes on their back. 

Once a female finds a male frog she watches it call for a little while. If she likes the call she will hang around and they mate.

Frogs have external fertilization. The male grabs the female and that stimulates her to release eggs while the male releases sperm. The eggs hatch out into tadpoles in about two weeks, depending on water temperature. The new tadpoles have a tail, no legs, a two-chambered heart, and gills. As it grows into a frog it loses the tail, grows legs, develops a three-chambered heart, and the gills are replaced by lungs. Really an amazing change! Frog eggs attach to vegetation or float in a mass until hatching into tadpoles.

After a short summer the frogs prepare for a cold winter. This is where it gets really interesting. Many frog species hibernate in mud to avoid cold winters. However, boreal chorus frogs live where the winter temperatures can get down to -40 degrees F, or colder! They can’t dig far enough to avoid those low temperatures. They have evolved an amazing method of surviving those cold temperatures: they partially freeze. 

The tricky thing about freezing is that when water freezes, it expands. When water inside animal cells freezes, they expand to the point they rupture, resulting in the death of the animal. A few species, including the boreal chorus frog, have evolved the ability to control their freezing by producing protein ice nucleators. Ice nucleators are shaped much like ice crystals and therefore act as a location for ice to freeze. The boreal chorus frog’s ice nucleators are located in the intercellular areas of the frog, not in cells, so ice forms outside their cells. The nucleators also control the growth and size of ice crystals to prevent cell damage. Molecules which act as antifreeze are inside their cells and prevent the cells from freezing. Their antifreeze chemicals include glucose and sugar alcohols. Freezing inside their cells would destroy them. As winter begins, the frogs slow their metabolism down until their heart stops and they no longer breathe, all while their extracellular fluids freeze and their cells remain unfrozen.

The frogs defrost in the spring, ready to mate in snow-melt pools before other amphibians can leave ice-covered lakes, their sound resonating over Yellowstone marshes.

Judy Lehmberg is a former college biology teacher who now shoots nature videos.

       
For more info:

• Judy Lehmberg (Official site)
• Judy Lehmberg’s YouTube Channel

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