Big Green Frogs: NC's Delightful Croaking Critters

Come springtime, keep an eye and ear out for these adorable aquatic creatures that are abundant in our local ponds and creeks
by Mike Dunn

Way back when I was a seasonal naturalist at Umstead State Park, I was walking along a creek and heard a loud squeenk sound, followed by a splash. I couldn’t tell what made the noise, but a few feet further it happened again, and this time I saw a frog swimming underwater before quickly hiding beneath some debris.

I looked in my field guide and learned this is an alarm call given by Green frogs as they leap into the water to escape potential predators. So began my acquaintance with one of the most widespread and common frogs in North Carolina.

Found through much of Eastern North America, the Green frog (Lithobates clamitans, formerly Rana clamitans) is a medium-sized frog (2-4 inches) that can be found in almost any aquatic habitat, from the shorelines of lakes and ponds to pools in marshes or the banks of a stream. As its name implies, it usually has some green color, especially on the head and lips. But the body is often a brownish-green to bronze color with faint spots or bars on the back and legs. The belly is usually white and adult males have a yellow throat. A distinguishing feature is a fold of skin, called a dorsolateral ridge, along each side of the back that extends to the middle of the body.

Hoping to attract frogs and aquatic insects, I created my first tiny water garden in Raleigh by burying a plastic concrete mixing tub in the backyard and adding a couple of aquatic plants. I soon had dragonflies hovering around, and it wasn’t long until I saw my first frog, a small Green frog. That pattern was repeated when we moved to our home in the woods in Chatham County, where we created two water gardens using pond liners. Sure enough, after a short time, I saw my first frog, again, a Green frog. No one is quite sure how frogs find new sources of water, but they do move about on rainy nights. One study suggests one species may be able to smell pooled water.

Breeding season for this species begins in April and runs through the summer. We often hear the males giving their advertisement call, a loud c’tunk. Some say it sounds like someone plucking an out-of-tune banjo string. You can easily imitate it with a throaty gulp noise. I admit, I find myself frequently “conversing” with our frog neighbors as I come and go in the yard with an occasional twangy gulp.

When a female chooses one of the calling males, the male clings to her back (a position called “amplexus”) and fertilizes the eggs as she releases them into the water. Green frogs lay 1,000 to 3,000 or more eggs in a loose cluster on the water surface, often anchored to an aquatic plant. The eggs hatch in a few days and tadpoles turn into frogs within a few months (many overwinter and transform the next year). These tadpoles contain chemicals that make them distasteful to most fish, so unlike many of our other amphibian species, they do quite well in aquatic habitats containing fish.

Adult Green frogs eat a variety of invertebrates including beetles, caterpillars and spiders, as well as larger prey such as small fish, tadpoles and other frogs. I think they are a major predator of the treefrogs that use the same pools for breeding. I once saw the hind legs of a Cope’s Gray treefrog sticking out of the mouth of a large Green frog. In turn, they are eaten by numerous predators including raccoons, snakes and the local Red-shouldered hawk, a reptile and amphibian aficionado.

One creature can be a major predator of all our aquatic life when it occasionally shows up in our little ponds: the American bullfrog (Lithobates catesbeianus). I think our ponds might be a bit small for a resident population of America’s largest frog, an adult being 6 to 8 inches in length and weighing in at 1 to 3 pounds. But we do occasionally hear the distinctive call of a male, a deep jug-o-rum, jug-o-rum.

American bullfrogs look similar to Green frogs, but can be identified by their size and the lack of a dorsolateral ridge down the back. Instead, bullfrogs have a fold of skin that starts near the eye and curves around their eardrum, or tympanum. Just as with Green frogs, the tympanum of a male American bullfrog is much larger than its eye (often twice the size), whereas that of a female is about the same size as her eye. Maybe male frogs are better listeners than males of other species (I guess I need bigger ears).

When I worked at the North Carolina Botanical Garden, one of the highlights for our school groups was seeing the huge bullfrog tadpoles in the pond. They can reach lengths of 5 to 6 inches! They are slower to transform than other frogs, with many living two or even three years as a tadpole before changing to an adult frog. Tadpoles feed on algae, organic debris (including dead organisms) and perhaps small invertebrates. Like their cousins, bullfrog tadpoles are also distasteful to fish.

Adults of both species are sit-and-wait predators. Bullfrogs are voracious feeders and will eat anything they can swallow, including many invertebrates and small vertebrates such as other frogs, fish, small mammals and even birds. I once saw a large frog (not sure which species) leap at — but miss — a hummingbird hovering low over one of our water gardens.

In prime habitats, you may hear many bullfrog males making calls to attract females and establish their territories. Territories vary in size, but are roughly 5 to 15 feet across, depending on the quality of the habitat. One spring years ago, we were hiking along a lakeshore where bullfrogs were calling. As I started to photograph one big male, another one came skipping across the water surface and slammed into the first. Frog fight!

After some leg flailing, the frogs locked arms and began a marathon shoving match. I’m not sure about the rules in frog wrestling, but I think the goal is to dunk your opponent until he cries Uncle and makes a hasty retreat out of your prime spot. My original frog seemed to have one primary strategy: shove your nose into the throat of the other guy.

After a couple of minutes of struggle, the throat-shoving proved to be a winning strategy, and, as quickly as it had started, the battle was done. The vanquished frog turned tail and hopped away to fight another day.

Though these battles rarely cause any harm, there is a price to pay for all this posturing. Male bullfrogs tend to be more exposed in their habitat than the reclusive females, and they are more noticeable as they call and move about defending their territories. This makes them more susceptible to predators, of which there are many. That was confirmed later that day when we saw a Great Blue heron snag a large bullfrog (a distracted courting male perhaps?) and gulp it down in just a few seconds.

Bullfrogs hold a special place in American culture, perhaps due to their impressive size and popularity as a game animal and dietary delight. In some areas, like Washington state, where they have been introduced out of their native range, they are cursed as an invasive species that are outcompeting native frogs.

In others, they are praised for their athletic abilities. Inspired by an 1865 short story by Mark Twain, The Celebrated Jumping Frog of Calaveras County, frog jumping contests are still held in many areas across the United States. Distance is measured from the starting point to the point where the frog lands after it jumps three times. The record jump of 21 feet 5 ¾ inches (a little over 7 feet per jump) was set in 1986 at the Calaveras County Fair and Jumping Frog Jubilee by a bullfrog named “Rosie the Ribeter.”

I love the fact that the competitors who bring their own frogs and encourage them to jump are called “frog jockeys.” I’ll never have that coveted title, but I am a frog appreciator!

As you hike near some wetlands or enjoy your own backyard mini-pond these next few months, listen for an army of frogs (that’s the collective term for a group of frogs) and their choruses. And if you don’t mind your neighbors hearing you, go ahead and practice a c’tunk or a jug-o-rum and see if you get an answer.