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Field & Street



The Illinois chorus frog is a fossorial amphibian. I think it should be our state fossorial amphibian, but I'll get back to that later.

"Fossorial," which refers to digging, comes from the same Latin root as fossil. Fossorial animals are diggers; they live underground. In the case of the Illinois chorus frog this means completely underground, like moles. None of this hanging out in a burrow during the day and coming up at night to feed. These frogs may come up only once a year, to breed. As soon as they are through with that essential business, they start digging again.

The man who discovered that Illinois chorus frogs are so completely fossorial is Dr. Lauren Brown, a biology professor at Illinois State University. Some years ago he conducted an experiment in which he partly filled three aquaria with sand and then dumped 500 mealworms into each of them. He then added frogs to two of the aquaria and placed metal screens directly on the surface of the sand. The screens allowed air to pass freely between soil and atmosphere, but they kept the frogs underground.

The frogs had been living on a diet of mealworms, but they hadn't been given anything to eat in the days immediately preceding the experiment, so they were seriously hungry when they hit the sand.

After a few days Brown emptied the aquaria and counted the mealworms. There were still 500 in the control aquarium, but only about 450 in the other two tanks. And when he "sacrificed" the frogs (scientists always use that term when they have to kill an animal in an experiment; I imagine them operating with obsidian knives, but they probably use stainless steel) he found the missing mealworms, partially digested, in their stomachs. The frogs had fed underground. They lived without coming to the surface.

I have been in this nature-writing game for many years, but I must confess that I didn't know there was such a thing as a fossorial frog. I knew about toads hopping about on the ground, and tree frogs living in trees. I knew that frogs and toads could dig into the ground to find a good place to hibernate, but all this aside I figured the rest of the froggy race hung about ponds, lakes, and streams, either hiding in the reeds and rushes or sitting on lily pads.

I really shouldn't have been so amazed about this. Frogs are a very successful group of animals. There are around 2,700 species worldwide and around 80 in North America north of Mexico. They all look pretty much like frogs: neckless, tailless, widemouthed, and long-legged. With so many species occupying so many habitats, you would have to say that the basic frog shape is a big success.

Most frogs are hatched as tadpoles, though a few are born as tiny frogs. Tadpoles live in water and look rather like small fish. In my childhood capturing tadpoles at The Pond (we called it that because for me and my buddies it was the only pond in the world) was a regular springtime activity. I always took a few home and kept them in an old fishbowl so I could watch them metamorphose. They grew legs (hind legs first), their tails shrank to nothing, and their mouths got real big. Tadpoles eat a variety of things. If memory serves, I raised mine on goldfish food. Adult frogs are all carnivores. I had no facilities for keeping adult frogs, so mine went back to The Pond as soon as they had completed their metamorphosis.

If I had lived around Illinois chorus frogs, I could have kept them. I could have just filled a box with sandy dirt and dropped in mealworms from time to time. Of course a pet that never comes above ground is not really very satisfying.

Since I began looking into the habits of frogs and toads, I have found that there are quite a few members of the order Salientia that burrow either for safety or to look for food. Most of them dig with their back legs. Spadefoot toads, for example, have a sharp-edged structure on the inside of their hind feet that helps them dig.

What makes the Illinois chorus frog unusual is that it digs with its front legs. And what makes it unique is the motion of those legs. Illinois chorus frogs dig like somebody doing the breaststroke.

There are at least seven species of chorus frogs of the genus Pseudacris in North America. That "at least" is necessary because the question of what populations represent separate species rather than different varieties of the same species is a perennially hot topic among herpetologists. Strecker's chorus frog--Pseudacris streckeri--lives in Texas and Oklahoma. Maps in the field guides show a small, disjunct population in Illinois-Pseudacris streckeri illinoensis. Separate population islands of this variety are also present in the Boot Heel area of Missouri and adjacent areas in northeast Arkansas.

It is possible that our chorus frogs should be a separate species, but we really don't have enough information to make a judgment. Lauren Brown says the mating calls of male Texas frogs are noticeably different from the mating calls of male Illinois frogs, but we don't know if the females would regard them as different. The question could be resolved by playing recordings of the males for an audience of females and noting which ones they respond to. If Illinois females do not respond to Texas males, then our Illinois frog could be regarded as our own special full species.

Did I mention that the Illinois chorus frog is on the endangered list in the state it is named after? And that it is among the species being considered for federal listing? I should note here that Dr. Brown asked me to mention that when he sacrificed those chorus frogs they were not yet on the endangered list.

The problems of the Illinois chorus frog are very closely related to its mode of locomotion. Since it spends its life digging in the earth, it needs earth that is easily dug. In fact, it is found only in very sandy soils. Ellen Beltz, who just completed a population survey of the species along the lower Illinois River, thinks it is tied almost exclusively to Parkland sands, a particular kind of very fine sandy soil found on the floodplain of the Illinois River.

However, there may be populations of this frog in other counties in southern Illinois in sandy soils that are not Parkland sands. At this point we don't know for sure.

Beltz did her study in the very early spring of 1991. March is really the only time of year when it is possible to survey chorus frogs, because it is then that they leave their burrows and migrate to the nearest pond. The males' song is a single ringing note, repeated frequently, and audible on quiet nights as much as a mile away. if you have ever heard the single note of the tree frog called a spring peeper, you have a general sense of the song's sound. The major difference is that spring peepers are sopranos; chorus frogs are baritones.

Also, spring peepers are scattered. Chorus frogs gather at ponds and sing together. The size of the chorus, the number of individual males joining their voices together, may influence the female's choice of which pond to visit in search of a mate. Big choruses suggest rich ponds.

These days, the choruses tend to be small. Dr. Brown says he has rarely heard as many as 15 frogs in a chorus. The usual number is six or less. Species with large, healthy populations might muster thousands of individual singers.

Beltz surveyed her frogs on cold March nights, driving down back roads that sometimes vanished beneath her car and left her stuck in the sand. She drove about six miles an hour with the windows open, and listened. It was cold, arduous work, and the fact that she was doing it suggests how little we know about this frog. We are still in the stage of learning where it lives. We know almost nothing about how it lives.

Two hundred years ago the valley of the lower Illinois had a broad floodplain that was inundated practically every spring. The floods created shallow seasonal wetlands where frogs--and many other creatures--could breed. After settlement we built levees along the river to hold back the floods and started growing corn and soybeans on the drained lands. A line of bluffs defines the edge of the plain. Natural springs flow from these bluffs. Long ago the water from these springs followed braided, irregular channels down to the river. Today the spring water flows in ditches to the riverbank, where electric pumps pull it over the levees and into the channel. Meanwhile the dry weather of the past few years has stimulated farmers to dig more wells to feed water to sprayers that irrigate the fields.

So we have electric pumps carrying free water away from the fields, while more electric pumps bring up groundwater to irrigate the fields. The irrigation wells are lowering the water table, which reduces the size of the remaining wetlands and therefore decreases the available breeding habitat for chorus frogs.

We don't know exactly what to do to protect the remaining chorus frogs because we know almost nothing about their ecological requirements. We see them in March when they are mating, and then they disappear for a year. Where do they go? What kind of reserves could we set aside for them? Answering the latter question would require a serious ecological study. It would be difficult (how do you fasten a radio transmitter to a one-inch frog?) and expensive--and no money is available.

So maybe we should start lobbying the legislature to make the Illinois chorus frog the official State Fossorial Amphibian. There aren't many animals named after our state, and we should take care of any we do have. Once the Illinois chorus frog achieves official status, maybe we can prevail upon Springfield to come up with a few bucks for a serious ecological study. Indiana got a bat on the federal endangered list. I say we should get a frog.

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