American Bullfrog vs. Pig Frog
Lithobates catesbeianus vs. Lithobates grylio

The Bullfrog (Lithobates catesbeianus) and the Pig Frog (Lithobates grylio) are two of the largest frogs in North America.  Not only are they large, but they are very similar in overall appearance.  They are both big green "typical" frogs with varying amounts of stippling or pattern on their back.  They both have a large tympanum (eardrum) in males.  They occupy similar habitats and are both found in the SE United States.   
In Texas, we don't have to deal with this problem very often as Pig Frogs are restricted to the four southeastern most counties of the state whereas the Bullfrog is found largely statewide.

Bullfrog (Lithobates catesbeianus)
Burleson County, Texas

Pig Frog (Lithobates grylio)

The two species can therefore be difficult to distinguish at sight.  There are differences in the shape of the head (Pig Frog is "pointier) and the webbing  on the hind toes (Bullfrog's toes protrude beyond the webbing).  But these characters can be hard to see if you only have one of the two species or if the species are in the water like the Bullfrog below.

Fortunately, the calls of these two frogs are very distinctive.  Here is the call of the Pig Frog.  As it name suggests it is a series of pig-like snorts.

The Bullfrog not only gets somewhat larger than the Pig Frog, but it has a much larger voice.  Its call is sometimes spelled out as "jug-o-rum" although sometimes it sounds more of a loud "rah-uhm" without the first "jug" syllable.  Here is the call of the Bullfrog.

So if you are in Southeast Texas or elsewhere in the SE United States where these two species occur, give them a listen.  You might just find those big green frogs you are seeing are two species.

© Chris Harrison 2015

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