The Central Texas Chirping Frogs
Eleutherodactylus cystignathoides vs Eleutherodactylus marnockii


Central Texas is now home to two species of Chirping Frogs in the genus Eleutherodactylus.   Historically, the hill country region of Central Texas was the haunt of the Cliff Chirping Frog (E. marnockii).  These frogs reside in the rocky hills and canyons of the Edwards Plateau and west into the mountains of West Texas.   They range south into Mexico but their range is poorly known due to confusion with a very similar species that occurs south of the border*.


Known distribution of the Cliff Chirping Frog in Texas

However, over the last 25-30 years, another species of Chirping Frog has been moving in to the area.  The Rio Grande Chirping Frog (E. cystignathoides) was formerly restricted to the southern tip of Texas in the Rio Grande Valley and south into Mexico.  But this small frog has a propensity for living in gardens and being transported around the state in potted plants.  They have now spread into many cities of Texas and into the rural areas between the cities.  When I was a teenager in Houston in the late 1970s, there were no Chirping Frogs in the Houston area.   By the middle 1980s, they were abundant.  They are continuing to spread and should eventually be documented in all the wooded counties of the eastern half of Texas.  They have spread east into Louisiana and undoubtedly will end up being recorded in OK and AR eventually.**



Native and Introduced distribution of the Rio Grande Chirping Frog in Texas

The gaps in the map for the Rio Grande Chirper in south and southeast Texas are probably due to them not being documented there rather than not being present.  This is a tiny little frog that is easy to overlook and its call is often dismissed as being from a bird or insect.


Here is a map showing the two species ranges together.  The blue counties are those where only Cliff Chirpers are documented, the yellow counties where only Rio Grande Chirpers have been documented and the green counties are counties where both have been documented (although they usually occur in different habitats).   The two red counties represent records for Cliff Chirping Frogs that are questionable based the availability of habitat.  Rio Grande Chirping Frogs occur in both of those counties.




When seen as adults, these two species are reasonably easy to tell apart.  The Cliff Chirper is a noticeably larger frog with larger adults reaching the size of a quarter.  Their overall color pattern is a mottled brown/green on a darker background although they are highly variable.  Some are almost yellow in ground color, others can be very dark.  They usually have a darker line running along the snout to the eye.



Cliff Chirping Frog
Hays County, Texas
Cliff Chirping Frog
Crockett County, Texas


The Rio Grande Chirping Frog has a similar pattern and is again highly variable, but it is noticeably smaller as an adult.  An adult Rio Grande Chirper can sit on a dime and a huge adult would sit easily on a nickel.  Of course, a small Cliff Chirping Frog could be this size as well.


Rio Grande Chirping Frog
Bexar County, Texas

So if you see an adult, you can tell them apart.  But what about their calls?
Unfortunately, both species make a "chirping" call and a sort of "trill".  Here is a chorus of Rio Grande Chirping Frogs from Bastrop State Park.  You can hear both the high "chips" and the "trill" call in between.



Here is a recording of a Rio Grande Chirping Frog from Medina County, Texas - 





Here are a few chips and trills from the Cliff Chirping Frog from Real County, Texas - 



and another series of Cliff Chirpers from Kerr County, Texas -



The obvious difference between their calls is the frequency (pitch) of the calls of each frog.  Possibly because the Rio Grande Chirping Frog is much smaller, the calls of the Rio Grande Chirping Frog are noticeably higher in pitch than the Cliff Chirper.  Although the "trill" part of the call is lower pitched in both species, it is consistently lower in the Cliff Chirper than the Rio Grande Chirper.


Here is a recording of a series of chirps made first by the Rio Grande Chirping Frog and then a series by some Cliff Chirping Frogs.  Even though there is a tiny overlap in the call range, by listening to a series of calls you can distinguish them.  Only the highest of the Cliff Chirper calls approach the lowest frequencies of the Rio Grande Chirpers -



Here is a spectrogram of that recording.  The green line is approximately at 2700 Hz.  You can see that the Rio Grande Chirping Frog calls (the first series) are above that frequency while almost all of the Cliff Chirpers are below that frequency.




Both frogs also make a "trill" as part of their call and once again, the Cliff Chirping Frog's trill is noticeably lower in frequency (pitch) than that of the Rio Grande Chirper.   Here is a few seconds of the Rio Grande Chirper followed by a few seconds of the Cliff Chirping Frog.  The trills are a little tougher to tell apart so it is better to rely on the relative frequency of the "chirp" part of the calls.



Unlike most other Texas Frogs, these species do not breed in water.  The males call from terrestrial sites like cracks and crevices, from bushes or trees or just on the ground.   For this reason, people often don't recognize them as frogs since there may not be significant water nearby.  I have been contacted by people asking what species of bird is chirping in their backyards on moist summer nights.

After breeding the eggs are laid on land in a moist area.  The eggs hatch into full formed froglets rather than tadpoles as the whole tadpole stage is spent inside the egg.  Because they do not need open water for their tadpole stage, they easily adapt to living in suburban backyards, etc. and can be transported to new locations where they can set up and breed.  This is part of the reason for the success of the Rio Grande Chirping Frog in spreading across the eastern parts of Texas.



© Chris Harrison 2015



* Those looking in a field guide to US or Texas Amphibians might note there was another species formerly listed as occurring in far western parts of Trans-Pecos Texas called the Spotted Chirping Frog (E. guttilatus).  Recent research shows that E. guttilatus does not occur in the US and all west Texas Chirping Frogs are the same species, E. marnockii.


** To further complicate matters, there's a new frog in town as well!  The Greenhouse Frog (Eleutherodactylus planirostris) has now been documented in SE Texas near Galveston.  This species is almost identical to the Rio Grande Chirping Frog in size and pattern and their calls are very similar.  It might take some time to document the spread of this frog in SE Texas due to how difficult it is to tell it from the Rio Grande Chirper.

1 comment:

  1. I searched for a long time to identify a sound that I hear in the night when the windows are open. Finally decided it must be some very tiny frogs. Sometimes they are having a conversation. I live in a very dry area on the Edwards Plateau(Austin). Chirps are not as loud as the ones you have here.

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