In the plains of south Texas and west into the Chihuahuan desert, we come across two small greenish "toads" that share can be confused.
The Green Toad (Anaxyrus debilis) is a cute little "true toad" that seldom exceeds three inches in length. They are green with varying amounts of black peppering in their pattern. Although they are bright green in coloration, they are not often seen by locals because they spend much of their lives buried underground or under rocks or other surface cover. The only time they are every really observed in the open is after heavy rains and usually only at night. Then they emerge from their hiding places and move to flooded ponds or ditches to mate.
The other species that can get confused with the Green Toad is the Western Narrow-mouthed Toad (Gastrophryne olivacea). These little "toads" are smaller than Green Toads, have a much smoother skin, a pointy nose and are rarely anything more than a hint of greenish brown. Generally they are just brownish with a few black specks and rarely more than an inch long.
So clearly these two species look nothing like each other so why are they paired as "confusing species'? The problem is their calls. Both species have a prolonged nasal whine as their mating call. Both species tend to come out after heavy summer rains in their dry grassland homes and both species overlap in range in many areas. Here is the call of the Green Toad (Anaxyrus debilis). It is a long high-pitched trilling call -
The difference between the two calls is easier to understand looking at the spectrogram and slowing the calls down. Here is a section of a Green Toad call followed by a Western Narrow-mouthed Toad call with both calls slowed down about 75%.
At this speed the trill quality of the Green Toad sounds noticeably different than the buzzy (faster trill) of the Narrow-mouthed Toad. If you look at the spectrogram for these two calls, you can actually see (and count) the difference. Here is a section of the spectrogram of the two showing 0.25 seconds of each call. You can see that the Narrow-mouthed Toad creates a lot more pulses in that same period of time.
Even though the two species sound very similar, they are pretty easy to distinguish when heard calling together which is fairly common. So if you are out in the area where their ranges overlap and you may just be lucky enough to hear these two tiny "toads" singing into the night!
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).
Western Narrowmouthed Toad call (direct link to call) 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.
Eastern Narrowmouthed Toad Call (direct link to call) 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).
The Western Narrow-mouthed Toad (Gastrophryne olivacea) is a common resident of central and southern Texas but is generally overlooked by most people. This is because it is usually found underground or under rocks and other surface cover. It only comes to the surface to breed after heavy rains. This group of anurans is a clear example of the problem of using common names to categorize animals. The old rule every elementary school child was taught was that "frogs have smooth skin, toads have dry skin" or that "frogs live near the water, toads live further away". Unfortunately, those rules only work for two of the almost 50 Anuran families that are currently recognized. In this case, we have a a family of smooth skinned animals that live away from the water yet some of the members of this family are known as frogs (Sheep Frog) and others are known as toads (Narrow-mouthed Toads).
Narrow-mouthed Toads have a conspicuous fold of skin just behind the eyes and a short, pointy head and fat body. The underside of this species is uniform white or gray, which distinguishes it from its eastern cousin, the Eastern Narrow-mouthed Toad (G. carolinensis). The Eastern species has a black and white mottled belly.
Narrow-mouthed Toads are small frogs. Females reach approximately 1.5 inches while males are even smaller. Males also have a black throat.
Narrow-mouthed Toads tend to call from flooded grassy fields and ditches after heavy rains. The frog shown above was calling from a shallow puddle of water in a flooded lawn in Comal County, Texas. They usually call either floating with their legs extended or standing upright among grassy pond edges as seen here. Their vocal sac is quite large for such a small frog.
Their call is a surprisingly loud, nasal, bleating "waaaaaahhh". It is one of the conspicuous sounds in southern Texas after heavy rains. Here is the recording of the individual in the photo -
Here is the spectrogram from a single call from that frog. You can see that there is a increase in pitch at the beginning followed by the long, wide band bleat.
Large choruses of these little frogs can make a constant buzzing sound. Here's a large chorus from La Salle County, Texas.
Individual calls can be distinguished by the slight upward whistled slur that the frogs make in while in large choruses. This large chorus in La Salle County, Texas shows that pattern.
Here's a short section of that chorus:
and the spectrogram for that section. The sold dark area around 4-5 KHz is the buzzy nasal whine of the Narrowmouthed Toads. The highlighted upward slurs represent where a individual frog starts a new call. You only see/hear this in large choruses of these frogs.
