Natural Highlights: Bullfrog or Green Frog?

Amphibians are especially sensitive to water quality and are among the first organisms harmed when conditions decline. Their presence is thus an indicator of good water quality. Thanks to the conservation efforts of the Wolf River Conservancy and other organizations, the wetlands of the Wolf River support healthy populations of many amphibian species.  The calls of several frog species are frequently heard along the urban Wolf River and near ponds and lakes in Memphis in the spring and summer - Chorus Frogs, Cricket Frogs, Green Tree Frogs, Fowlers Toads, American Bullfrogs, Green Frogs, and others  – signifying the water quality we all celebrated and worked to improve and protect during our big World Water Day project on March 22.

While many of us can easily identify the deep jug-o-rum courting call of the American Bullfrog, we might not be aware that it has a lookalike doppelganger which is often active at the same time. The Green Frog has an unmistakable call, completely different from the Bullfrog’s, sounding something like a broken or loosely wound banjo string when plucked.  But if you’re lucky enough to see one, deciding whether it’s a Green Frog (Lithobates clamitans) or a Bullfrog (Lithobates catesbeianus) can be a challenge if it doesn’t happen to be calling at the time.

Both Bullfrogs and Green Frogs come in shades of green, brown, or greenish brown with no consistent markings, and both can survive in larger bodies of water along with fish.  They are both big frogs, though Bullfrogs can grow much larger than Green Frogs, which tend not to exceed 4 inches long from nose to tail.  Bullfrogs can reach 8 inches from nose to tail, large enough to consume an adult Green Frog and many other organisms, including smaller Bullfrogs.  Fortunately, since there are plenty of young Bullfrogs about the same size as many Green Frogs, there is one easily observable difference between the two species, especially useful for identifying skittish frogs in lowlight conditions. Green Frogs have a dorsolateral fold or ridge which continues past their eyes along each side toward the rear, while in Bullfrogs this fold stops at the eye.  There are more subtle differences – Bullfrogs have a bigger head and more rounded snout while Green Frogs have a sharper snout, Bullfrogs’ hind feet have more webbing between the toes, Bullfrogs tend to be greener in color – but the dorsolateral fold down the sides of Green Frogs is the best feature to check.  

By the way, the impressive eardrum (the tympanum, or the big circle right behind each eye) which is clearly visible on both Bullfrogs and Green Frogs can be used to distinguish between males and females only, but not between species. The tympanums of males are very large –   larger than the eye – and the tympanums of females are the same size or smaller than the eye.  This is true of both Bullfrogs and Green Frogs.  

We hope you see and hear many of both species as the weather turns warm and these frogs get busy!  Use these links for videos of calling Bullfrogs and Green Frogs.

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