True frogs and ‘bull’ frogs
A month or so back, I was on a bird walk and we passed a small shallow pond. A frog was spotted on the far side of the pond, a little too far for a close look with binoculars. It looked like it could have been a green frog. Someone said, “That’s a bullfrog, it’s got a green head and brown back!” Was he right or wrong? Read on for an insight on how to tell the “green” from the “bull.”
Green frogs are probably the most abundant frogs in the region and are two to four inches in length. Even if not breeding, their occasional “gulp” call can be heard through the summer. They usually have a green snout and head, and may have green patterns on the dorsum, or back. However, many individuals may have a green head with a brown dorsum.
Bullfrogs are abundant in the region and are the largest frogs you will see; they can grow to over eight inches long. The call of a bullfrog is very distinct low-pitched “rumm… rumm… rumm” and is usually loud enough to carry across a good-sized lake. Their color patterns can vary from a green to a mottled green and brown pattern that is similar to the green frog.
The best way to tell the difference between these two species is to look on their backs, starting just behind each eye. Glandular folds of skin are present on both species; they are very prominent ridges and easily seen in close or with binoculars. The green frog has folds running from behind each eye to the base of each hind leg (on a green frog, these are called “dorsolateral ridges”). Bullfrogs have the glandular folds behind each eye, but they only hook around the rear of each tympanum (ear).
Both species are included in the same genus; they are members of the lithobates, or “true frogs.” (There was a disagreement over the genus, so old field guides might have Ranas clamatans for a green frog, for example, and a modern guide may have Lithobates clamatans for the same species). Other frogs, such as pickerel frogs, are in this genus, but have a distinct pattern not easily confused with the two mentioned species here. After a look at the captions for this column, you will be able to tell the “green” from the “bull.”