American Toad vs. Southern ToadAnaxyrus americanus vs. A. terrestris

When I drove to Florida recently, one of my target species to record was the Southern Toad.  I hadn't really learned the call in advance but I figured I would recognize it when I heard it since I knew most of the other species in the state.   On the way, we stopped one evening in southern Mississippi to do some herping and I heard a long trilling call.  At first I thought it was an American Toad and recorded it without thinking.  A few minutes later we found a toad on the road, but it was a Southern Toad.  That's when I realized I was too far south for American Toads...so what was that call?  I pulled up my Lang Elliott recording of US Anurans on my Ipod and listened.  It was indeed, the Southern Toad.  I had no idea they sounded like American Toads!  

Actually, there are several species of similar looking Anaxyrus species in the Southeastern US; Fowler's Toad, American Toad, and Southern Toad.

Fowler's Toad (Anaxyrus fowleri) looks similar to the American Toad (Anaxyrus americanus) but is easily separated by call.  I have dealt with those two species in a separate blog post.  Southern Toad (Anaxyrus terrestris) is easily separated from American and Fowler's Toads by the enlarged posterior extensions on its cranial crests.

Southern Toad
Florida

American Toad
Arkansas

So why a comparative post?  Because although Southern and American Toad only overlap in a small part of their ranges, as I just learned, their calls are very similar.  So how do you tell them apart?

Where they do come near each other, the American Toad is found in the mountains and Southern Toad is found in the below the fall line in the Coastal Plain.  But there are areas where both toads can be found in the same areas.  Here is an approximate range map for the two species in the Southeastern US showing the possible areas of overlap (this map is not meant to be accurate county by county, just an approximation).  Red represents the range of American Toad, yellow represents Southern Toad and the orange the possible zone of overlap.

Both species have a call that is a long high pitched trill.

Here is the American Toad (Anaxyrus americanus)

American Toad call

and here is the Southern Toad (Anaxyrus terrestris)

Southern Toad call

Looking at the spectra of the two calls, we see that the Southern Toad call is a noticeably higher in pitch than the American Toad.   In this example we see the American Toad with a peak frequency of around 1900 Hz while the Southern Toad is visibly (and audibly) higher with a peak of 2350 Hz.

So although these toads barely overlap in range, their calls are similar enough sounding that in the zones of overlap, it might be worth listening carefully or recording the call just to be sure.

EDIT - May 2019 - I started trying to use my understanding to identify some toad recordings on iNaturalist.org.   As I did, I wondered if my conclusions held for all American and Southern Toads, so I did a little research.

I found and listened to a few known recordings of each species and measuring the carrier (peak) frequency for each species trills.   I used some recordings from Lang Elliott's excellent Frogs and Toads of North America CD, some recordings I had, some recordings from reputable state websites and some recordings of calling toads from YouTube (where I could confirm the ID of the caller).

For American Toads, I measured nine samples that had a carrier frequency range from 1452 to 1877 Hz.   The mean value was 1658 Hz.
For Southern Toads, I measured ten samples that had a carrier frequency range from 1901 to 2284 Hz.  The mean value was 2095 Hz.
There was no overlap between the 19 calls between species.

Just to help visualize the difference, I made a quick box plot in Excel.

On the box plot, you can see the difference in range and lack of overlap.   So I feel more confident that these two similar sounding species can be differentiated by call, even if it does take a recording and a bit of analysis with Audacity (the excellent, free, open source audio editor)

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© Chris Harrison 2018 & 2019