In our area, there are only two large frogs that you are likely to find in the warm waters of a lily pond — the green frog (Rana clamitans melanota) and the bullfrog (R. catesbeiana). The biggest problem with identifying these two frogs is the simple fact that although they are different species, they can look quite similar.
Color is a problem because both species have the ability to change the color of their skins. This helps with camouflage, but it is mainly done to assist with the absorption of heat. A cold frog is darker in color, and as a result a green frog can be mostly brown and a bullfrog can look as black as mud.
Size is also problematic. A mature green frog may reach a length of 4 inches, but that is quite small compared to a mature bullfrog, which can reach 8 inches in length. A truly gigantic frog is probably a bullfrog, but assessing size can be tricky without a ruler.
So if color and size aren’t going to help, what do we do? Well, as it happens, we are in luck. A structure known as a lateral ridge is present in green frogs, but absent in bullfrogs. This — and this alone — will help the novice frog watcher distinguish between the two large green frogs that inhabit our lakes and ponds.
The lateral ridge extends from the back of the eye down to the beginning of the pelvic girdle. It looks like a fold of skin that has been rolled and sewn like the piping on old car seats.
Both species have a ridge of skin that curves around the eardrum (known as the tympanum), but only the green frog has the raised, continuous lateral ridge. The skin on the back of a bullfrog is smooth.
The ears have it
There is one additional physical feature that can be fun to look for when frog watching. Unlike mammals, reptiles, or birds, amphibians have no ear canal whatsoever. Thus, while our eardrums are safely hidden within our heads, the eardrums of frogs are out on the surface and plainly visible behind the eyes.
In both species, the tympanum of a male frog will be much larger than the size of the eye. In females, on the other hand, the tympanum will basically be the same size as the eye. So once you get good identifying the species you will also have a chance to determine the sex of the frog you are looking at.
Since the eyes and ears of frogs are prominently located on the top of the head, it is actually easier to identify the sex of an individual before determining its species.
When you’re just listening to frogs, identification is much simpler. The song of the male green frog sounds like the short, loud note that an amateur might produce with a banjo. Humans can imitate this vocalization by making high-pitched gulping sounds with an open mouth.
Bullfrogs, on the other hand, make much louder noises that are sustained for a longer period of time. Deep, sonorous calls coming from any wetland area are the songs of amorous males attempting to woo any female that might be listening. On a summer evening, the song of a distant bullfrog is a wonderful thing indeed.
So here is a challenge for you. Can you identify the sex and species of the frog in this photo? Send me an email at firstname.lastname@example.org and I’ll let you know if you’re right.
Bill Danielson is a professional nature photographer and author living in Altamont. Contact him at www.speakingofnature.com.