Great Plains Narrowmouthed vs Green Toad:Gastrophryne olivacea vs. Anaxyrus debilis

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 -

Green Toad call mp3

Here is the call of the Western Narrowmouthed Toad

Western Narrow-mouthed Toad call mp3

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!


© Chris Harrison 2016