Texas' Narrow-mouthed Toads
- Gastrophryne carolinensis
- Gastrophryne olivacea
- Hypopachus variolosus

Texas has three representatives of the frog family Microhylidae, the Eastern Narrow-mouthed Toad (Gastrophryne carolinensis), the Great Plains Narrow-mouthed Toad (Gastrophryne olivacea) and the Sheep Frog (Hypopachus variolosus).   Although these species only overlap partially in their ranges overall, in Texas many areas are occupied by more than one species.  

(It might be worth mentioning here that the terms "frog" and "toad" are both used to describe the anurans in this family.  The problem is that the terms frog and toad don't have any real biological distinction.  Yes, frogs are generally considered to have moist(er) skin and live in/near water and toads are generally considered to have dryer (wartier) skin and live further from water only returning to the water to breed.  The problem is that that really only works for distinguishing a few of the more typical frogs in the family Ranidae from the more typical toads in the family Bufonidae.  There are Bufonid toads with smooth skin that is pretty moist and there are Ranid frogs that have drier warty skin.  So for the purposes of clarity, we will assume the terms frog and toad are not intended to be diagnostic of the type of anuran we are discussing.  Thus the "Narrow-mouthed Toads" are closely related to the "Sheep Frog" discussed on this entry.  And Narrow-mouthed Toads are sometimes referred to as Narrow-mouthed Frogs!)

In the hand, the US members of this family are relatively easy to tell apart.  All three are small (2-3 inches total length), pointy-nosed little frogs.  They often have a fold of skin on the head/neck right behind the eyes but this is not always evident in all individuals all the time.  They can hop but don't hop nearly as well as more typical "frogs".  They are generally not very well known by most local residents because they spend most of their time underground or under rocks and logs where they can remain moist and safe from predation.  They only come out after flooding rains and move to shallow flooded pools where they breed.

The Eastern Narrow-mouthed Toad (Gastrophryne carolinensis) is a frog of the woodlands and forests of the southeastern US.  In Texas, it is generally found in the eastern 1/3 of the state.  Physically they are variable but typically have mottled dorsal pattern and some characteristic white spots on their relatively dark bellies.  These spots can be seen on the side of the belly in this photo of an individual from Walker County, Texas.  Some Eastern Narrow-mouthed Toads have wide lighter "stripes" that run from the nose to the back legs on the dorsolateral edge of their body leaving them a darker middorsal area.  (The individual pictured here doesn't show that pattern very well).

The Great Plains (or Western) Narrow-mouthed Toad (Gastrophryne olivacea) is very similar to the previous species in that it is a small, pointy-nosed little frog.  It's dorsal pattern tends to be a bit more plain and tends to be more greenish gray generally, but not always, with black spots or markings.  The underside of the Great Plains Narrow-mouthed Toad is what separates them from the Eastern.  The belly of the Great Plains Narrow-mouthed Toad is "plain" whitish or off-white.  It lacks the spotted pattern seen on the Eastern Narrow-mouthed Toad. As the name implies this is also a species that is usually associate with more open, less forested country than the Eastern species.

(It is worth noting that male frogs/toads often have black patches on their throat pouches.  This is true of all three species discussed here, but the rest of the ventral side is generally unmarked in the Great Plains Narrow-mouthed Toad).

The Sheep Frog (Hypopachus variolosus) is different from the other two species in a number of ways.  First off, they are somewhat larger and "chunkier" than the two species of Gastrophryne.  Furthermore, their US range is restricted to the counties in south Texas from about Victoria down into south Texas.  They range widely into Mexico and Central America.  Their dorsal coloration varies from gray to yellow-green, to orange and even brick red in some individuals.  One fairly constant characteristic is a thin pale yellow/white/orange stripe down the center of the head and back.  The Sheep Frog is protected in the state of Texas even though it is not necessarily rare in its limited US range.

So if these three species are easily distinguished by some simple diagnostic characters, where is the confusion?  The answer is in the similarly of their calls.  All three species calls are usually described as a nasal "bleating" like that of a sheep.   And the three frogs calls are similar, but the difference lies in the length and the "vowels" of the call.

The Great Plains Narrow-mouthed Toad has the most easily recognized call to my ear.  It's bleat is a long, high pitched nasal "meeeeeeeee". (Beware the similar sounding unrelated Green Toad - Anaxyrus debilis that often occurs in the same areas).

The Sheep Frog, as the common name suggests has a much more sheep-like call.  It is lower in pitch and shorter than the call of the Great Plains Narrow-mouthed Toad.  It also sounds more like it is built around the vowel "A" rather than "E".  The call is a loud nasal "BAAAAH".

The call of the Eastern Narrow-mouthed Toad is somewhere between the other two.  It is longer than the call of the Sheep Frog, but retains more of the "A" vowel.  It isn't nearly as "meeeee" sounding as the Great Plains Narrow-mouth but rather more of a "meh" sound.  This difference can be difficult to hear at first.

Of course, all three species will occasionally make shorter calls or calls in slightly lower pitch than normal which can sometimes suggest the other species.

The differences between the sounds of these calls can be seen by comparing plots of the different frequencies in their calls.  The three graphs below represent the relative strength of different frequencies in their call.  All three species have buzzy calls containing many frequencies but the graphs allow us to compare where the dominant frequencies are.  I have the three graphs stacked so you can compare the alignment of the frequencies between the species.

The first graph is the frequency distribution for the Great Plains Narrow-mouthed Toad (Gastrophryne olivacea).  You see the width of the graph indicating the many frequencies it contains which is typical of broad "buzzy" calls like this.  The highest peak indicates the dominant (most obvious) frequency of their calls.  In this case the most dominant frequencies are in the 4000-4500 Hz range.  

The second graph is for the Eastern Narrow-mouthed Toad (Gastrophryne carolinensis).   It looks similar to the first graph, however the dominant (loudest) frequencies peak in the low 3000-3500 hz range.  (The short peak above 11000 Hz is an insect that was calling near the frog).  You can see that the peak frequency distribution is shifted slightly to the left and will therefore sound lower than the preceding species.

This third graph shows the frequency distribution for a solitary call of a Sheep Frog (Hypopachus variolosus).  The call is roughly the same length as the others but contains two regions of dominant frequencies.  There is a cluster of peaks around 1000 Hz and then a second broad cluster of peaks around 2500-3000 Hz.  This results in a noticeably lower call.  The roughly equal heights of the lower peaks also makes the call sound "broader" and "more complex" compared to the calls of the two Gastrophryne where one frequency dominates more.

So you can visually see the difference in the frequency distribution of the buzzy calls of these little anurans, but you also hear it.

Sometimes the length of the calls differs slightly.  Eastern Narrow-mouthed Toads have the shortest of the three calls, usually around a second or so but sometimes extended beyond that.   The Great Plains Narrow-mouth and Sheep Frogs have longer calls that are usually 1.5 to 2 or more seconds long.

Here is a recording of a typical call from the three in the order of the graphs above - Great Plains Narrow-mouthed Toad, Eastern Narrow-mouthed Toad and finally Sheep Frog (there are actually two sheep frogs calling).

I have highlighted a section of the dominant frequencies for each species on this spectrogram.  Looking at the spectrogram for those three calls we can see:

1.  The buzzy nature of the calls as indicated by there being many wavy parallel lines at different frequencies.

2.  Although each call has many frequencies, you can see which frequencies dominate by the relative darkness of those lines.  

- For the first call (G. olivacea) you see the dominant frequencies are above 4000 Hz, 
- in the second (G. carolinensis) they are darkest in the range of 2800-3300 Hz 
- in the third (H. variolosus) they are darkest in two ranges, 2500-2800 and 800-1500 Hz.  

You can hear the dominance of each of these frequencies in each species call.

No matter which of these pointy-nosed little frogs it is though, these little microhylids calls are certainly one of my favorite of all frog calls.  They are loud, rather raucous and a little obnoxious.   I wonder why I like them so much? 😉

© Chris Harrison 2016

No comments:

Post a Comment