Chubby Little Frog, Skinny Little Window

It was the second day of March back in 2008, and a warm front was roaring up out of the south, bringing scudding clouds and rain. Up to that point, spring had not put in much of an appearance, with temperatures sometimes twenty degrees colder than normal. With this dramatic change in the weather, here at last was a window of opportunity to go look for a little frog that spends much of its life tucked out of sight.

The winds were strong enough that I burned almost an extra quarter tank of gas while driving down to the bottom of Illinois. Close to sunset, I met up with Scott Albert and Mike Steffen at a local gas station. Scott and Mike were third year students at SIU Carbondale, and back then they got a lot of herping done in the southern portions of the state. “Frogs are already calling,” said Scott. That sounded promising, given that it wasn’t dark yet.

We drove out on a series of small farm roads, and pulled over next to an uncultivated field. There was water out there somewhere; I could hear spring peepers calling, and some other frogs as well. Scott and Mike pulled on waders and I put on my rubber boots, hoping the pond wouldn’t be too deep. I like my boots, but I could see the advantage waders provided even in shallow water – one can kneel and take photographs without getting soaked in chilly late winter water.

We waded out into more of a small pool than a pond, with clumps of last year’s tall grasses and old cornstalks scattered throughout. The water was perhaps six to eight inches in depth, and more clear than muddy. Spring Peepers (Pseudacris crucifer) were calling, as were small numbers of Midland Chorus Frogs (Pseudacris triseriata), with their almost mechanical clicking trills. Off in the distance we could hear the squawky chuckle of Southern Leopard Frogs (Lithobates sphenocephalus) calling at another pond.

At first, the spring peepers quieted as we approached, but as night continued to fall, they became louder and louder as more males took up the chorus. It was almost deafening, and clearly we were visiting during Peak Peeper, with a full congress of males and females present. Underneath this chorus, we could make out another call – shorter in length, and just a little lower in pitch. This was the frog we had come to see, and now all we had to do was find them. It was hard enough spotting peepers and chorus frogs, which tended to call from underneath vegetation for the most part.

We moved slowly, shining our lights at the clumps of vegetation. It was frustrating, hearing so many frogs while only spotting the occasional peeper. But then I spotted a chubby little frog, almost standing upright in the middle of a clump of grass. “I think I’ve got one here!’ I said, pointing. “Congratulations, Mike,” said Scott, “there’s your first Illinois Chorus Frog.”

With some animals, you can look at a hundred pictures, but it isn’t until you actually see one that your brain can assemble a working search image. Perhaps pictures of frogs and real frogs are processed in different parts of the brain, or perhaps it’s just my brain that works that way. At any rate, once I spotted one Pseudacris illinoensis, more jumped out at me, so to speak, and for Scott and Mike as well. It also didn’t hurt that it was completely dark now, and the little frogs had lost all concern for us in their quest to make a date.

It didn’t take long to figure out that some of the male Illinois Chorus Frogs tended to ‘stand upright’ when calling, grabbing the grass stalks with their strong, stubby forelimbs. If you’re a little taller, maybe you can call a little farther, although there’s always risk of predation when you’re exposed out in the open. As long as we didn’t keep a light trained on them, we were able to take flash pictures and catch them in mid-call. I was glad I remembered to bring my flash bracket rig along; that and a head lamp made the process a lot easier.

Here and there we came across females, floating in the water with their heads protruding, presumably listening to the staccato love songs all around them. By now the combined chorus of crucifer and illinoensis was nearly deafening. The peepers cranked out short little high notes, with the chorus frogs just a tad lower and coming a little faster. With so many peeps and chirps coming so fast, it seemed like my ear and brain couldn’t process them fast enough, resulting in me hearing a tinny “eeeeeeeeeeeeeeeee” instead of individual notes.

I think it was Mike who commented on what a perfect night this was, and I agreed wholeheartedly. It was still 71 degrees at eight o’clock, thanks to the warm and steady southern wind. Above us, the clouds had moved off, and the clear sky held a spray of stars…it was a perfect night for frogs and froggers alike to strike while the iron was hot.

Finally it was time to slosh back to the cars and leave the frogs to their urgent business. During the next few days the wind would turn, and cold weather would creep back. It’s a small window of activity for Illinois Chorus Frogs – few short weeks to call, to mate, and to forge the next link in a long chain of frogs that started long ago, before returning to a quiet, hidden life in the sandy soil of a farm field.

Pseudacris illinoensis belongs to the “fat frog clade” of chorus frogs, along with Strecker’s Chorus Frog (P. streckeri), which occurs in the southwest, and the Ornate Chorus Frog (P. ornatus), found in the southeast. These three species are characterized by squat, toad-like bodies, and short, thick forelimbs, and widthwise and lengthwise, they are larger than other species of Pseudacris. Their digits lack the adhesive disks present in other hylid frogs; this group doesn’t climb, but instead burrows into sandy soil. More than likely, their large, rounded bodies aid in reducing water loss during dry periods underground.

There are a number of frog families adapted to burrowing into the substrate, and some, like spadefoot toads for example, back their way into the ground using their hind legs and feet. The Fat Frog Clade does things differently – they use their short-but-strong forelimbs to dig into the sandy ground head-first. The synchronized digging of the forelimbs have been likened to a swimmer’s breast stroke, with the back legs occasionally kicking sand backward. Forward burrowing may have several advantages, such as avoiding obstacles and facilitating underground feeding (stay tuned for more on that).

In all three species in this group, adults emerge from their burrows in response to the rainfall that kicks off the breeding season – P. ornata during the midwinter months, and P. streckeri and P. illinoensis in late winter or early spring. For all three, it’s not clear how often adults emerge from their burrows after the breeding season is over – they are infrequently encountered during the summer months. And all three have serious data gaps with regard to the egg, larvae, and juvenile life stages. Living underground with only brief appearances on the other side of the sand prolongs the Fat Frog mystery.

The Illinois Chorus Frog was first described from Illinois in 1922, and was later assigned as a subspecies of Pseudacris streckeri, where it stayed for decades until being elevated to full species status. This is still a point of argument among some taxonomists, but I tend to agree that P. illinoensis is its own species.. Populations of ICFs occur in west-central and southwestern Illinois, in southeastern Missouri, and in northeastern Arkansas. From genetic marker studies, it is likely that ICFs originated from Pseudacris streckeri populations that migrated towards the northeast about 1.4 million years ago, and were later isolated by changes in climate, with Pseudacris illinoensis fully diverging sometime in the Pliocene.

Pseudacris illinoensis are sand prairie frogs, existing in Pleistocene sand deposits laid down during the retreat of the Wisconsinan Glaciation. Many of the scattered and sometimes isolated populations of these frogs exist in suitable habitat along the Illinois and Mississippi rivers. Aside from emerging in late winter or early spring to breed, adult ICFs spend much of the year buried under the sand, but how much is a bit of a mystery. “They must come up to forage for food at night,” is something I and others have long thought, but when, and how much, is not really known. While ICFs have been observed on the surface at night during the summer months, there have not been enough observations for us to fill in the blanks on that part of their natural history.

A behavior which may explain why we don’t regularly see ICFs above ground at other times of the year is their unique ability to feed while in their underground burrows. This behavior has been observed in captive illinoensis, and stomach-content analysis of road-killed frogs tends to support this idea, and it appears their underground snacking is largely on several species of cutworms. Because of the sandy nature of their subterranean abodes, they don’t use tongue propulsion as per usual. Instead, prey items are stuffed into the mouth with the forelimbs, which is probably an easier approach in close quarters, and certainly reduces the amount of sand particles taken into the mouth while securing prey.

Later in spring, when Pseudacris illinoensis tadpoles emerge from the water as young froglets, they don’t immediately burrow into the ground as adults do, but spend the spring and summer topside, feeding and growing. Only in autumn do they begin to dig burrows, and it may be that as freshly formed frogs, they lack the musculature and bone development (including bones of the skull) needed to dig. It’s not clear how deep ICFs burrow during the summer months, but during the winter, they certainly need to tunnel deep enough to go below the frost line.

In late March of 2010, similar weather conditions prompted Jake Scott and Dick Bartlett to drive up from central Florida to see the breeding congregations with Scott Albert and I. You may be surprised that someone would drive that far to see one chubby little frog, but that’s part of the herper lifestyle, and with such a small window of opportunity, you have to strike while the iron is hot, just like the frogs.

It so happened that we went to the same small pond, with much the same results. Jake and Dick were able to witness and photograph this singular event, and make the long drive home with a mark of success. As a whole, seeing these frogs is a bit of a crapshoot. The location of the ponds may vary from year to year, and in some years, the warm fronts and late winter rains may not happen at all. The frogs may have to wait for another year for the chance to carry on the species, and frog enthusiasts may have to hang on for another year as well.

It’s hardly surprising that Pseudacris illinoensis is not doing very well in this day and age, what with climate change, the expansion of agriculture, river channeling, and the direct destruction of habitat by the construction of warehouses, Walmarts, and Subway sandwich shops (I am being specific because these are actual occurrences). Illinois seems to have the strongest populations, followed by Missouri, while most populations in Arkansas have been extirpated. Illinois Chorus Frogs manage to hang on through the hot and dry months of the year; I can only hope they can hang on as a species until kinder times prevail.

Thanks for reading this.