In the past, there has been little need to physically distinguish between two small, mostly tropical frogs that have been rapidly expanding their respective ranges along the subtropical Gulf Coast. Accordingly, field guides have largely ignored the possibility of confusing these two superficially similar species since they could easily be identified on the basis of their respective geographical distributions: the Greenhouse Frog (Eleutherodactylus planirostris) was limited to peninsular Florida and the adjacent southeastern states; the Rio Grande Chirping Frog (E. cystignathoides) was limited to the more mesic portions of the western Gulf Coast, especially Texas and Louisiana.
More recently, however, the expanded ranges of these two frogs have begun to overlap: the Greenhouse Frog appeared in Louisiana in 1975 (Plotkin and Atkinson 1979); the Rio Grande Chirper was first detected in that state in 1998 (Hardy 2004). The Greenhouse Frog was first documented in southeastern Texas in 1999 with a vouchered specimen in the Texas Cooperative Wildlife Collection (TCWC 83067) from the town of La Marque, on the mainland portion of Galveston County. Since then E. planirostris has also been unofficially recorded in Harris and Nueces counties, all along the Gulf Coast.
Some recent records posted to the Herps of Texas iNaturalist site (https://www.inaturalist.org/projects/herps-of-texas) have suggested that some observers are confusing the newly-arrived Greenhouse Frog with the more established (but similarly introduced) Rio Grande Chirping Frog. Considering that both frogs belong to the same genus (Eleutherodactylus), are similar in size (ca.0.5-1.2 in. [15-30 mm]) and coloration, and share similar behaviors, some confusion is to be expected. Consequently, it seems that now would be a good time to examine some features that should serve to distinguish between these two forms.
Figure 1 (A) The mottled morph of the Greenhouse Frog (Eleutherodactylus planirostris) from “Florida,” photo by Zach Cava, (CC BY-NC). (B) Typical example of the Rio Grande Chirping Frog (Eleutherodactylus cystignathoides) from Atascosa County, Texas, photo by the author.
In the table below (Table 1), I have indicated seven morphological characteristics that, taken together, should serve to distinguish between these two similar frogs. Understand that several of these characters may overlap between the two species, so no single character (other than the presence of stripes) will definitely identify any given frog. However, if a frog has a majority of the characters under either of these species, that should serve as a sort of “gestalt” for one species or the other. [Click on table for larger image]
Table 1. Morphological characters useful in distinguishing Eleutherodactylus planirostris from E. cystignathoides.
Morphs (Figure 2) – The Greenhouse Frog is highly variable in both coloration and pattern. The ground color may vary from reddish to brown to green to almost white, even in the same population at the same location. The dorsal pattern is also variable, but less so than the ground color. Fortunately, the pattern of the Greenhouse Frog may be lumped into two basic categories: a striped morph and a mottled morph (as seen below). Either morph may be predominant at a given location, thus the exact mode of inheritance is uncertain. The Rio Grande Chirping Frog, at least in the United States, is much less variable in coloration and pattern than the Greenhouse Frog. It has only one pattern phase, which is comparable to the mottled morph of the Greenhouse Frog. Even the ground color of the Rio Grande Chirper is a fairly constant khaki shade, varying only in tint, lighter or darker, which may be partially temperature related. Thus, if you encounter a striped Eleutherodactylus frog somewhere along the Gulf Coast, you may be assured it is a Greenhouse Frog. Therefore, the remainder of this article refers to distinguishing the mottled morph of the Greenhouse Frog from the Rio Grande Chirper.
Figure 2. Morphs. (A) The mottled morph of the Greenhouse Frog (Eleutherodactylus planirostris) from “Florida,” photo by Zach Cava, (CC BY-NC); (B) The striped morph of E. planirostris from Alachua County, Florida photo by “gnevans”, (CC BY-NC).
Interocular dark bar (Figure 3) – The Greenhouse Frog almost invariably sports a well-defined dark bar dorsally between the eyes. This bar usually runs from the middle of one orbit directly across to the opposite one. It is typically bordered anteriorly by a much lighter and narrower margin. The posterior edge of the dark bar is irregular and may extend into a triangular or blotch-like shape on the dorsum of the head. If an interocular bar is present at all in the Rio Grande Chirper, it will be weakly-defined, not bordered anteriorly by a much lighter margin, and will typically be broken rather than continuous.
Figure 3. Interocular dark bar. (A) Greenhouse Frog (E. planirostris). Specimen from Jefferson Parish, Louisiana. Photo by James W. Beck (CC BY-NC). (B) Typical example of the Rio Grande Chirping Frog (E. cystignathoides) from Atascosa County, Texas. Photo by the author.
Discrete, prominent skin tubercules (Figure 4) – In general, the Greenhouse Frog has a more roughly textured skin than the Rio Grande Chirper due primarily to the presence of larger individual skin tubercules in the former. This distinction is lessened if the Rio Grande Chirper is suffering from any degree of desiccation.
Figure 4. Discrete, prominent skin tubercules. (A) Greenhouse Frog (E. planirostris). Specimen from East Baton Rouge Parish, Louisiana. Photo by Matt Brady, slightly cropped, used with permission. (B) Rio Grande Chirping Frog (E. cystignathoides) from Atascosa County, Texas. Photo by the author.
Dark shadow from tympanum to nare (Figure 5) – An occasional Greenhouse Frog may have a darkened area on the side of the head, but if present, it is usually irregular and less distinct than that characteristically found in the Chirping Frog. The pattern on the side of the Greenhouse Frog’s head, however, typically resembles the pattern on the remainder of the dorsal surface. In the Rio Grande Chirping Frog, this dusky shadow running from the tympanum through the lower half of the iris to the nare is almost always present and well-defined. It is sharply bordered dorsally by the canthus rostralis and ventrally by the lip.
Figure 5. Dark shadow from tympanum to nare. (A) Greenhouse Frog (E. planirostris). Specimen from Jefferson Parish, Louisiana. Photo by James W. Beck (CC BY-NC). (B) Typical example of the Rio Grande Chirping Frog (E. cystignathoides) from Atascosa County, Texas. Photo by the author.
Light triangular area on top of snout anterior to interocular bar (Figure 6) – In the Greenhouse Frog this area is almost always noticeably lighter than the remainder of the dorsal colora-
tion. In the Rio Grande Chirper the dorsum of the snout is the same shade as the ground color.
Figure 6. Light triangular area on top of snout. (A) Greenhouse Frog (E. planirostris). Specimen from East Baton Rouge Parish, Louisiana. Photo by Matt Brady, slightly cropped, used with permission. (B) Rio Grande Chirping Frog (E. cystignathoides) from Atascosa County, Texas. Photo by the author.
Fine vertebral ridge of small raised tubercules (Figure 7) – Although this feature may be evident in only the highest resolution photographs, it is typically present in most Greenhouse
Frogs and absent or less distinct in most Rio Grande Chirping Frogs. It consists of a continuous fine line of individual very small tubercules running vertebrally from the snout to the urostyle.
Figure 7. Fine vertebral ridge of small raised tubercules. (A) Greenhouse Frog (E. planirostris). Specimen from Monroe County, Florida. Photo by Royal Tyler (CC BY-NC-SA). (B) Rio Grande Chirping Frog (E. cystignathoides) from Atascosa County, Texas. Photo by the author.
Post-axial chevron-shaped blotch (Figure 8) – This marking is usually present in the Greenhouse Frog, creating a generally darker, less random aspect to the dorsal pattern. The dorsal surface of the Rio Grande Chirping Frog typically lacks this feature, displaying instead a dorsal pattern of random dark “squiggles” on a lighter khaki-tinted ground color.
Figure 8. Post-axial chevron-shaped blotch. (A) The Greenhouse Frog (Eleutherodactylus planirostris) from “Florida”. Photo by Zach Cava, (CC BY-NC). (B) Typical example of the Rio Grande Chirping Frog (Eleutherodactylus cystignathoides) from Atascosa County, Texas. Photo by the author.
Acoustic distinctions. Ultimately, however, perhaps the best method of distinguishing between these two species involves using their calls, which, although superficially similar, do have some distinctive characteristics. The calls of both species consist of chirps and trills that are easily confused with birds and/or insects.
The call of the Greenhouse Frog is consistently more “chatty” than that of the Rio Grande Chirper. When this frog is calling, there are very few intervals of relative silence. The call of the Greenhouse Frog may be heard here:
Below (Figure 9) are waveform (top) and spectrographic (bot
tom) analyses of a less noisy commercial recording of the same species (Elliott et al. 2009) made with the Audacity program. The waveform plot indicates the relative amplitude or loudness of a twenty-second portion of the call. The lower plot is an audio spectrograph of the same portion of the call showing the actual frequency distribution (y-axis) of the call components; the portions of the call shown in red are those of greater amplitude; blue represents relative silence.
Notice that there are almost constant softer notes between the louder chirps, creating a “busier” call than that of the Rio Grande Chirper. The spectrograph indicates that the dominant frequency is between 4 and 5 kilohertz, with other tones above and below that range. [Click on graphic for larger view]
Figure 9. Graphic analysis of Greenhouse Frog call. Top: Waveform plot. X-axis = time (sec); Y-axis = relative amplitude. Bottom: audio spectrograph of same call. X-axis = time (sec); Y-axis = frequency (kilohertz). Recorded call from Elliott et al. 2009.
The call of the Rio Grande Chirper, on the other hand, is punctuated by intervals of one to three seconds between chirps and similar spans between trills, although the intervals between call elements may vary with temperature.
The call of the Rio Grande Chirper can be heard here:
Notice that the dominant frequencies of both species are similarly between 3 and 5 kilohertz, but that the Greenhouse Frog call is much busier with various over- and undertones. Nevertheless, it would appear that in a mixed chorus of these two species distinguishing between them by ear would present a challenge. [Click on graphic for larger view]
Figure 10. Graphic analysis of Rio Grande Chirping Frog call. Top: Waveform plot. X-axis = time (sec); Y-axis = relative amplitude. Bottom: audio spectrograph of same call. X-axis = time (sec); Y-axis = frequency (kilohertz). Recorded call from Elliott et al. 2009.
Acknowledgements. I would like to thank those contributors to the iNaturalist citizen science database who have graciously made the photographic documentation of their Greenhouse Frog records freely available via Creative Commons Licensing considerations. Also Thanks to Matt Brady for allowing me to use his photos of an East Baton Rouge E. planirostris specimen.
Audacity. Free, open sourced audio software. [https://www.audacityteam.org/]
Elliott, L.; C. Gerhardt, and C. Davidson. 2009. The Frogs and Toads of North America. Houghton Mifflin Harcourt: Boston, New York. 343 pp. plus audio CD.
Hardy, Laurence M. 2004. “Genus Syrrhophus (Anura: Leptodactylidae) in Louisiana.” The Southwestern Naturalist 49(2): 263-266.
Plotkin, M. and R. Atkinson. 1979. “Geographic distribution: Eleutherodactylus planirostris planirostris.” Herpetol. Rev. 10: 59.