The Little Frog That Could 

by Tom Lott

 

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           Small adult Rio Grande Chirping Frog (Syrrhophus cystignathoides), Atascosa County, Texas

          

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Small adult Rio Grande Chirping Frog (Syrrhophus cystignathoides), Atascosa County, Texas

Among the worst environmental news of the latter decades of the twentieth-century was the revelation that some populations of frogs and toads in various parts of the world seemed to have suffered drastic crashes or to have become extinct altogether. These events are all the more troubling because many cannot be definitely attributed to any obvious environmental disturbance, human-generated or otherwise. The sad fact is that many species of amphibians worldwide are decreasing in abundance and we don't really know why.

Against this gloomy observation, however, is a glimmer of encouragement. Bucking the trend, with no small but unintentional assistance from mankind, is a tiny tropical frog that once just barely entered the state of Texas in the Lower Rio Grande Valley.

Not nearly as long as its scientific name in print (Syrrhopus cystignathoides campi), the Rio Grande Chirping Frog has been leapfrogging (or, more accurately, hitchhiking) its way into a much wider distribution within the state during the last three decades.

I remember my astonishment at finding a single individual at an impromptu dump site in San Antonio in 1969, two-hundred miles north of its known range at the time. However, within the next thirty years they were reported from Wharton, Houston, and even as far north as Tyler and the Dallas-Fort Worth Metroplex. These latter localities are the northernmost reported thus far and suggests that the frogs can probably survive decidedly "untropical" winters.

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Currently known distribution of S. c. campi in Texas by counties

Originally introduced to science by Edward Drinker Cope in 1877, but unknown from Texas until Leonard Stejneger's 1915 description of it as "Syrrhophus campi," this tiny, nondescript frog remains almost as poorly known today as it was then. Except for range extensions, little has been written about it in the herpetological literature since its discovery.

Ironically, we know much more about many rarer frogs, found in exotic, inaccessible localities. I suppose, being human, most herpetologists are drawn to investigate the more "glamorous" species first, ignoring this dowdy little "frog next door." Consequently, much of the natural history of this interesting frog remains equivocal.

We do know that the Chirping Frogs belong to a primarily neotropical group (family Leptodactylidae) that is extremely abundant, in both species and individuals, in more tropical latitudes. Several members of this family have been introduced, mainly via the nursery trade from several Caribbean islands, into southern Florida, where they thrive. Of the five species of Leptodactylid frogs that occur naturally in the United States, all are found in Texas.

For most people, the first indication that this frog is living in their yard is its characteristic "chirp," a sound more insect-like than one would probably expect. The call consists of two or three distinct, evenly spaced chirps that are cricket-like in pitch, lacking the insect's trill, but with much more volume. Chirping may be heard on almost any night during the warmer months, especially when the humidity is high, either naturally or from sprinkling.

The calls also have a ventriloqiual effect, making the frog's location difficult to detect from its sound alone. Even experienced frog biologists often resort to a method called "triangulation" to locate calling individuals. This consists of simply having each of two listeners simultaneously point toward the area from which each thinks the call is coming. Where the imaginary lines extending from their fingers cross, there is a reasonable expectation of finding the frog.

If the Chirping Frogs hold true to generalizations concerning frogs, it is likely that the males alone are responsible for the calls and that they serve both a territorial and a sexual attractant function. The male probably stakes out a site favorable for egg deposition, then commences to call, drawing females to the location. However, considering the numbers of frogs occasionally found together under a single piece of debris, it is doubtful that they are strongly territorial.

One of the traits of the Rio Grande Chirping Frog that allows it to be introduced into new areas is its unusual (for a frog) reproductive cycle. It is much less dependent upon the availability of water than most other frogs for the completion of its life cycle. In fact, it bypasses the free-swimming tadpole stage altogether. The embryo develops completely within the egg, emerging as a minuscule replica of the adult.

This behavior allows eggs laid in potted nursery stock in the Rio Grande Valley to be transported in all directions as the plants are shipped out. What is surprising is that this little frog has not been reported from even more localities. A similar process has allowed the Mediterranean Gecko (Hemidactylus turcicus, a small, wall-climbing lizard native to the Middle East) to invade and colonize much of the southern tier of states in the U.S., at least around human habitations. Gecko eggs, however, are far more resistant to dehydration than those of Chirping Frogs, so I do not expect the frog ever to attain a comparably extensive range to the west.

The possibility that parental care is provided for the developing clutch of eggs has been suggested but has not yet been demonstrated. Several tropical frogs, such as some of the garishly-colored Poison Dart Frogs (Dendrobates), have developed elaborate behaviors where one or both parents care for the eggs and subsequent tadpoles throughout their development.

Cool winter temperatures should also limit this frog's range expansion to the north. Although the San Antonio population apparently survived two unusually cold winters during the 1980s, (one consisting of subfreezing temperatures for a week, the other of thirteen inches of snowfall), I would not expect them to endure much farther north than the Tyler record.

I have also discovered some evidence that the San Antonio population successfully uses cracks in the soil, wood rat (Neotoma micropus) nests (some occupied by their builders!), as well as human-generated surface debris to evade brief interludes of cold temperature.

All amphibians share the problem of water loss through the skin. The Rio Grande Chirping Frog is essentially a "terrestrial" species, meaning that it is not dependent upon standing water, as are the more "semiaquatic" forms, such as Leopard Frogs and Bullfrogs. Terrestrial frogs that live in semi-arid habitats must compensate, either behaviorally or physiologically, to the increased drain upon the water reserve in their tissues that such environments exact.

Extended droughts, although a feature of its "natural" habitat in the Lower Rio Grande Valley, seem, from my field observations, to inflict considerable stress upon the introduced populations I have studied. I suspect that, like other terrestrial amphibians of the area (e.g., the Gulf Coast Toad [Bufo nebulifer]), the Chirping Frog may sustain itself through droughts by moving deep into the cracks that develop in the black clay soil of the San Antonio area.

Of the specimens that I have collected in San Antonio, the amphibians they have been most commonly associated with are the similar-sized Western Narrow-mouthed Toad (Gastrophryne olivacea) and, to a lesser extent, the Green Toad (Bufo debilis). The susceptibility of Chirping Frogs and their eggs to the voracious Imported Fire Ant (Solenopsis invicta) is unknown, although the two associate species named above seemed to have accommodated this environmental insult quite well. I suspect that the Narrow-mouthed Toad even successfully feeds upon fire ants.

The predators of the Rio Grande Chirping Frog are not well known but any of a number of small snakes occupying similar habitat are likely candidates. Several authors have suggested that in the Lower Grande Valley and points to the south, the Black-striped Snake (Coniophanes imperialis) is a significant predator of this species. Garter snakes (Thamnophis) are known to actively pursue the closely related Cliff Chirping Frog (Syrrhophus marnockii) of the Edwards Plateau, so it is probable that they are major predators of the Rio Grande species also.

Frogs typically rely on toxic skin secretions as their first line of defense but, again, little is known about the dermal pharmacology of this species. Indeed, an advantage that introduced species may enjoy in their new surroundings is that they may not be recognized as prey by some indigenous predators.

Alternatively, the introduced species may occupy a niche for which no local predator exists or is specialized to exploit. The Black-striped Snake, for example, would be absent from all of the introduced localities. The Texas Patchnosed Snake (Salvadora grahamiae lineata) is also commonly found in the same kind of microhabitat as the Chirping Frog and constitutes another potential predator.

Introduced species are typically ill-regarded by ecologists, with the implication that introductions, whether deliberate or accidental, are never beneficial for the recipient ecosystem.

Considering the lack of baseline data for the Rio Grande Chirping Frog, the jury may be out for some time on this species. Given that it is mainly edificarian in the areas into which it has been introduced, occupying habitats already grossly altered by human activity, I am inclined to consider this cheerful little amphibian to be, at worst, neutral. Having been documented to feed upon cockroaches, it is certainly welcome around my house!

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Small adult Rio Grande Chirping Frog (Syrrhophus cystignathoides), Atascosa County, Texas

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All text, images, sound bites, etc., are Tom Lott (2007) unless indicated otherwise.