The Diaspora of the Rio Grande Chirping Frog, Eleutherodactylus cystignathoides (Anura: Eleutherodactylidae) in the United States
by Tom Lott

Figure 1. An adult Rio Grande Chirping Frog (Eleutherodactylus cystignathoides) from Atascosa County, Texas. Photograph by the author.
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 had suffered drastic crashes or had become extinct altogether. Due in part to the typical amphibian lifestyle that requires an aquatic stage, many frogs have fallen victim to the rapid pace at which wetlands are being “reclaimed” and/or contaminated. Others, even though occurring in relatively pristine habitats, have been devastated by a host of fungal, viral, and other pathogens only recently identified. Worldwide, it is estimated that 168 amphibian species have become extinct during the past two decades and fully one-third of all known species are considered to be threatened with extinction (Dodd, 2013).
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 (Eleutherodactylus cystignathoides), the Rio Grande Chirping Frog has been leapfrogging (or, more accurately, hitchhiking) its way into a much wider distribution within three Gulf States during the last five decades.
I recall my astonishment at finding a single individual at an impromptu dump site in San Antonio, Texas, in 1969, two-hundred miles north of its known range at the time. However, within the next forty years they were reported from Corpus Christi, Houston, and even as far north as the Dallas-Fort Worth Metroplex and Shreveport, Louisiana. These latter two localities are the northernmost reported thus far and suggest that the frogs, though tropical in origin, can probably survive decidedly “untropical” winters.
Chirping Frogs belong to a primarily Neotropical group (family Eleutherodactylidae) 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. At the generic level, the exact placement of the Chirping Frogs remains somewhat unresolved and they tend to bounce back-and-forth between the genera Eleutherodactylus and Syrrhophus, according to the most recent whim of taxonomists and phylogeneticists (one should be aware, however, that much of the limited literature on the group is found under the latter name). Of the three species of Eleutherodactylid frogs that occur naturally in the United States, all are found in Texas.
The Rio Grande Chirping Frog was originally introduced to science by the Smithsonian’s Leonard Stejneger's (1915) description of it as a new species, “Syrrhophus campi,” based on specimens from Brownsville, Texas. It remained thus classified until John D. Lynch’s (1970) review of the genus placed it as a northern subspecies of a frog, Syrrhopus cystignathoides, that had been described earlier by Edward D. Cope (1877), using specimens from southeastern Mexico. 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.
Even with range extensions, it has become apparent that the accepted method of establishing the distributions of reptiles and amphibians (involving depositing physical voucher specimens in recognized museum collections, awaiting expert taxonomic verification, etc.) is inadequate when dealing with species experiencing rapid, even explosive range expansions.
For example, the first “official” record for Eleutherodactylus cystignathoides from Brazoria County, Texas (which is adjacent to the dense, long known Houston population) appeared only in 2005 (McCoid, 2005). Evidence indicates, however, that the Rio Grande Chirping Frog was present in Brazoria County almost 30 years earlier. A specimen from the town of Brazoria, collected by one of William K. Davis' field biology classes from Southwest Texas State University in 1976, resided in a study collection at the university (pers. obs.). When asked about the specimen, Davis replied that it was genuinely from that locality and that he had intended to publish a note about it, but had never gotten around to it (Wm. K. Davis, pers. comm.). This was three years prior even to the “official” announcement of the Houston population (Quinn, 1979).
Similarly, the first record indicating that E. cystignathoides could have established a population outside the Lower Rio Grande Valley, in San Antonio, Bexar County, was published in 1976 (Mather and Dixon, 1976). However, almost seven years prior to that, on 7 November 1969, I collected a single adult specimen of Eleutherodactylus cystignathoides just a few meters inside the southern limits of that city, indicating that the taxon had likely been present in Bexar County for some time before its announcement to science. The failure of both Davis and me to make timely notice of our respective discoveries is symptomatic of a “time lag” that is practically built into the traditional procedure for documenting range extensions and one which is counterproductive to our ability to accurately determine the up to date distribution of dynamically expanding species such as the Rio Grande Chirping Frog.
Consequently, the range map included here (Fig. 2) is based upon all “reliable” available records, such as VertNet, literature records, reviewed citizen science sites (e.g., iNat, HERP database, etc.), as well as personal observations of trusted observers. It is believed that this method, similar to that employed by the birding community, yields a slightly more current and ephemerally accurate impression of the species’ distribution. It should be understood, however, that by the time this reaches publication it will be rendered somewhat obsolescent by new discoveries.

Figure 2. Map of the current distribution of the Rio Grande Chirping Frog (Eleutherodactylus cystignathoides) based on available records. The stippled areas represent county records since 1976. The two counties in black at the southern tip of Texas (Cameron and Hidalgo) represent the presumed original (natural) range of the species in the U. S. [Click on image to enlarge]
For most people, their first encounter with this frog will be hearing its characteristic “chirp” in their yard; a sound more bird or 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 some of 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 landscape irrigation.
The calls also have a ventriloquial 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.
In keeping with generalizations among frogs, it appears that male chirping frogs 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 has apparently allowed eggs laid in potted nursery stock in Texas' 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 where it now almost certainly occurs. 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., mostly around human habitations. Gecko eggs, however, are far more resistant to desiccation than those of even terrestrial frogs, so I do not expect the chirping frog to ever attain a comparably extensive range to the west (although there are persistent rumors of an established population existing within a well-watered yard in Phoenix, Arizona!).
Ironically, despite its ubiquity where it occurs, we know much more about many rarer frogs, found in exotic, inaccessible localities. Being human, most herpetologists are drawn to investigate the more “glamorous” species first, ignoring this dowdy little “frog next door.” Consequently, much of its interesting natural history remains equivocal.
However, our most detailed and non-anecdotal knowledge of the reproductive and developmental biology of this little frog has resulted mainly from the efforts of Louise Hayes-Odum, who studied the introduced population on the grounds of the Houston Zoo in 1984. One of the initial problems she encountered was how to determine the sex of individual specimens, which superficially appear identical. Hayes-Odum solved this problem by relying upon the presence of developing eggs in adult females, which are visible through the abdominal wall, as well as their slightly larger size (>22mm, snout-vent); nocturnally vocalizing frogs were presumed to be males. She found frogs calling from perches as high as 22cm (8.7 in) above the ground during the night and at air temperatures as low as 20.5oC (68.9oF), but observed that they retreated towards lower perches near dawn. Most calling heard after sunrise was from frogs located within tunnels in the ground. A clutch of 5 eggs dug up (close to the surface) at the study site averaged ca. 5mm in diameter and were unpigmented, even though at advanced stages of development. Wild hatchlings (ca. 6mm, snout-vent) were first observed on 6 June.
Since, despite her efforts, Hayes-Odum failed to observe reproductive behavior in the field, she stimulated it among a captive group by the injection of a small amount of synthetic gonadotrophin-releasing hormone into both sexes. Interactions between the frogs appeared to be territorial, and on one occasion a female seemed to be soliciting amplexus from a presumed male. Incubation took from 14 to 16 days at temperatures from 27-33oC (80.6-91.4oF). Three clutches of eggs were observed, numbering 5, 10, and 13. (Hayes-Odum, 1990)
Cool winter temperatures should 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 Fort Worth-Shreveport records indicate. They are, however, rapidly expanding their domain eastward along the Gulf Coast, aided no doubt by the more subtropical climate to be found there. The Rio Grande Chirper is currently known from five parishes in Louisiana and their easternmost extent, as of this writing, is Mobile County, Alabama.
Interestingly, in Alabama and in at least three of the Louisiana localities where they have been found, the Rio Grande Chirper is occurring sympatrically with a very similar, congeneric, exotic species, the Greenhouse Frog (Eleutherodactylus planirostris), which has itself been moving northward and westward from its apparent introduction point of Key West, Florida in 1863 (Lazell, 1989).
Initially, it seems that most authors expected that these introduced populations of E. cystignathoides would remain more or less restricted to edificarian or other human-altered habitats. However, observations from southeastern Texas and from localities as far west as Victoria and Bexar counties suggest that this species may be carving out a niche for itself in the natural environment, becoming less dependent upon human disturbances for its survival in those locales. In general, I would speculate that virtually every county in Texas east of or along I-35 likely harbors at least one or more localized populations of E. cystignathoides. The virtual absence of records for this species to the west of the I-35 corridor, except for a slight penetration onto the southeastern Edwards Plateau, is puzzling but could merely represent a sampling anomaly.
The existence of the very similar and naturally-occurring Cliff Chirping Frog (Eleutherodactylus marnockii) along the Balcones Fault Zone and on much of the Edwards Plateau presents the possibility of interaction between it and E. cystignathoides. I am presently unaware of any reports of suspected hybridization or even competition between these two species where they occur sympatrically, although Wallace (2005) opined that such contacts could detrimentally introduce hybridization, competition, and novel pathogens into the native E. marnockii populations. He further stated that steps should be taken to prevent further introductions of E. cystignathoides, but did not suggest any practical means of doing this nor of eradicating currently well-established colonies of Rio Grande Chirpers. .
Both the native Cliff Chirping Frog of the Edwards Plateau of Texas and the locally introduced Rio Grande Chirper have very similar vocalizations, consisting of several distinct chirps alternating with a similar number of trills, but there does not appear to be any diagnostic pattern in the sequence of these variations. The calls of the slightly smaller Rio Grande Chirper seem to be somewhat higher in pitch than the larger Cliff Chirper, but this is likely a function of relative body size and ambient temperature, with some overlap between the two species. Consequently, extra care should be exercised in the evaluation of aural records from localities where both species may occur as their vocalizations are distinguishable only to the acutely sensitive and well-trained ear.
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 “semi-aquatic” 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 environment 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 [Incilius nebulifer]), the chirping frog may sustain itself through droughts by moving deep into cracks that develop in the black clay soil of the San Antonio area. I have also discovered some evidence that the San Antonio population occasionally uses wood rat (Neotoma micropus) nests( some still occupied by their builders!), along with pocket gopher and mole tunnels, as well as human-generated surface debris to evade brief interludes of cold temperature.
Of the specimens that I have found 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 (Anaxyrus debilis). The susceptibility of chirping frogs and their eggs to the voracious Imported Fire Ant (Solenopsis invicta) is unknown, although the two associated species named above seemed to have accommodated this environmental insult. 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 (e.g., Conant, 1955). Garter snakes (Thamnophis sp.) are known to actively prey upon the closely related Cliff Chirping Frog (Eleutherodactylus marnockii) of the Edwards Plateau, so it is probable that they are major predators of the Rio Grande species also. Larger spiders are also known to prey upon small frogs: Chris Harrison photographed an undetermined species of tarantula attacking a frog of this species in Tamaulipas, Mexico in1999 (Harrison, 2010). .
Frogs typically rely on toxic skin secretions as their first line of defense against predators but, again, little is known about the dermal pharmacology of this species. At least one hobbyist has anecdotally alleged that Rio Grande Chirpers were toxic to water snakes (Nerodia) that consumed them as well as to other amphibians with which they merely shared enclosures. Another, however, routinely used them without problems to entice feeding in hatchling Green Tree Pythons (Morelia viridis).
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 largely 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!
Literature Cited
Conant, R. 1955. Notes on three Texas reptiles, including an addition to the fauna of the state. Amer. Mus. Novit. 1726: 1-6.
Cope, E.D. 1877 (1878). A new genus of Cystignathidae from Texas. American Naturalist 12:253.
Dodd, C.K. 2013. Frogs of the United States and Canada. Johns Hopkins University Press. 1032 pp.
Harrison, C. 2010. Noted and photographed a tarantula preying upon an E. cystignathoides in Tamaulipas, MX. [http://www.naherp.com/viewrecord.php?r_id=58440]
Hayes-Odum, L.A. 1990. Observations on reproduction and embryonic development in Syrrhophus cystignathoides campi (Anura: Leptodactylidae). Southwest. Nat. 35(3): 358-361.
Lazell, J. D., Jr. 1989. Wildlife of the Florida Keys: A Natural History. Washington, D. C.: Island Press. 254 pp.
Lynch, J.D. 1970. A taxonomic revision of the leptodactylid frog genus Syrrhophus Cope. Univ. Kans. Publ. Mus. Nat. Hist. 20(1): 1-45
Mather, C.M., and J.R. Dixon. 1976. Geographic records of some Texas amphibians and reptiles. Herpetol. Rev. 7(3): 127.
McCoid, M.J. 2005. Geographic distribution. Eleutherodactylus cystignathoides. Herpetol. Rev. 36(2): 199.
Quinn, H. R. 1979b. The Rio Grande chirping frog, Syrrhophus cystignathoides campi (Amphibia, Leptodactylidae), from Houston, Texas. Trans. Kans. Acad. Sci. 82(4):209-10.
Stejneger, L. 1915. A new species of tailless batrachian from North America. Proc. Biol. Soc. Wash. 28:131.
Wallace, J. Eric. 2005. Eleutherodactylus (=Syrrhophus) marnockii (Cope, 1878a). Cliff Chirping Frog. in Lannoo, Michael (ed.), Amphibian Declines: the conservation status of United States species. Berkeley: Univ. of Cal. Press, pp. 496-499.
[This article first appeared in the March 2019 issue of the Bulletin of the Chicago Herpetological Society.]