The Plains Leopard Frog (Lithobates blairi) is a frog of the Great Plains of the United States whose range comes down into the pandhandle of Texas and south to about central Texas (geographic central TX, that is).
The Rio Grande Leopard Frog (Lithobates berlandieri) meets the Plains Leopard Frog in the southern plains and then continues south through south Texas and down into Mexico. It also is found in the desert areas of the Trans-Pecos.
The Southern Leopard Frog (Lithobates sphenocephalus) is a frog of the southeastern United States found from the east coast all the way to Central Texas, ending roughly at the Balcones Escarpment.
Here is a map of their ranges in Texas based on known county records. (I based this map on the county records shown on the herpsoftexas.org website and put together using the diymaps.net website)
On this map, the blue counties represent those counties where only the Plains Leopard Frog is documented, the red counties are those where only the Rio Grande Leopard Frog is documented and the yellow counties are those where only the Southern Leopard Frog is documented.
Purple counties represent those counties where both Rio Grande and Plains Leopard Frogs have been documented and orange counties are those where both Rio Grande and Southern Leopard Frogs have been documented.
The few green counties represent either where both Southern and Plains Leopard Frogs have been documented or in a few cases where all three species have been documented in the same county (the diymaps.net site only allows 6 categories :-().
The few empty square represent areas where a Leopard Frog species undoubtedly occurs, they just haven't been documented there yet (or I messed up making the map?).
The problem is that all three of these species look pretty much the same! They are tricky to tell apart, even when you have seen a lot of them. One character that can be used (sometimes) is the presence of a break in the posterior part of their dorsolateral fold (see my discussion/diagram of this character in my comparison of Central Texas Leopard Frogs). In theory, Rio Grande and Plains Leopard Frogs have this break and Southern's don't. But it isn't always an easy character to see. Even with the frog in hand, you can sometimes be unsure if that is "enough" of a break.
The other character used to help tell them apart is the amount of white striping on the upper lip between the nose and eye, but once again I find this character a bit confusing.
So the field guides tell you that:
Southern Leopard Frog has an unbroken dorsolateral fold.
Rio Grande Leopard Frog has a broken dorsolateral fold but with a less prominent lip stripe.
Plains Leopard Frog has a broken dorsolateral fold with a prominent lip stripe.
All three species have a tan/brownish ground color than can show varying amounts of green. While Southern Leopard Frogs are more green on average than the other two species, all three can be quite green or show very little green. This a greenish Rio Grande Leopard Frog.
And here is a Southern Leopard Frog with almost no visible green -
Most Leopard Frogs can be identified by those characters (and range), but I see a few that seem to fall through the cracks. I actually question some of the records on that herpsoftexas.org map because they can be tough to ID to species by those two characters alone.
However, if you hear them call, you can very quickly see why these three similar frogs are regarded as separate species. Their calls are quite distinctive.
The call of the Southern Leopard Frog (Lithobates sphenocephalus) is a chuckling call -
In contrast, the call of the Rio Grande Leopard Frog (Lithobates berlandieri) is more of a snoring call (with an occasional whimpering sound) -
The call of the Plains Leopard Frog (Lithobates blairi) sound a little like someone plucking a tight cord.
So, if you are having trouble with Leopard Frog identification in your area, forget the field guide and just give them a listen. With a little experience, they can be pretty easy to tell apart!
* I guess I should mention that there are historical records of the Northern Leopard Frog (Lithobates pipiens) along the Rio Grande in El Paso County in far western Texas. However that species appears to have been extirpated from the state. The most recent record is half a century old and Rio Grande Leopard Frogs have now apparently replaced them in that area.
© Chris Harrison 2015