Green frogs (Lithobates clamitans), as you might guess, can be greenish – most often, they have a greenish head, but their bodies can be brown. They are about 3 inches long and live throughout much of eastern North America, usually on riverbanks or the edges or ponds. They spend most of their hours waiting patiently for food to wander within reach of their lunging leap and long tongue. Their eyes cannot spot objects close to them, but they have the remarkable ability to pinpoint objects farther away. This is even more remarkable when you realize that frogs close their eyes when they stick out their long tongue. They will eat almost anything – spiders, flies, grasshoppers, caterpillars, butterflies, crayfish and even their own cast-off skin! (They shed their skin every couple of months.)
Although these frogs live mostly on land, if you approach one it will jump into the water, often letting out a loud cry – these amphibians were once called “screaming frogs.” In May, adults emerge from their winter hibernation and travel to breeding sites. Males arrive first and establish their territory, which they defend vigorously. If another male frog gets too close, the established male splashes, jumps up and down or kicks his legs to encourage the other to move along. If this does not work, the two males wrestle, kick, bite and push until one frog falls on his back. The other frog may hold the fallen frog under water until the fallen frog gives up. The winner claims or reclaims the territory.
Females arrive later, when they are full of eggs and ready to breed. Females are choosy and select a mate only after spending a few days visiting all the territories. They prefer males who can hold onto a territory and who have a deep mating call. Larger, stronger males tend to have deeper calls, so the females are choosing males that are the most fit. Once a female selects a mate, she backs her body into his, and the smaller male clasps her around the waist to induce her to release her eggs. The male covers these eggs with sperm as they are released.
Females lay from 1,000 to 7,000 eggs on the water’s surface, attached to plants in shallow water. The eggs, often laid in June, hatch in a few days, and tadpoles (also called pollywogs) emerge. Both the terms tadpole and pollywog originated in middle English. Tad is from a 15th century word meaning “toad,” and pol means “head.” Likewise, pollywog comes from pol meaning “head,” and wiglen meaning “to wiggle.” So whether you think they are wiggleheads or toadheads, it’s clear the head is the most prominent feature.
Females sometimes lay a second set of eggs later in the season. These later tadpoles usually overwinter as tadpoles, while early season tadpoles may complete their metamorphosis into adult frogs before winter arrives.
New tadpoles are small and do not have eyelids or arms, legs or lungs. They have external gills and tails. Soon they develop hind leg buds, which grow into legs. Then the front legs emerge, and the tail is reabsorbed. Jaws and teeth, eyelids and mucous glands develop. The lungs begin to develop and are used as an extra breathing organ. Tadpole tails are fragile, which allows them to tear free if a predator grasps them by the tail.
Tadpoles are like tiny vegetarian piranhas that eat continuously, for they must store enough energy to transform into a frog. They have teeth in their upper jaw and scrape along the bottom of a pond with these teeth. But just before and during metamorphosis, hormones signal them to stop eating altogether. During this phase, their intestines and internal organs reorganize. Their ears and eyes develop, their skin becomes thicker. These creatures are most vulnerable at this time because their tail is being reabsorbed to help the bony limbs grow, but without either a long tail or long limbs, they cannot escape from predators quickly.
The new adult frogs move out of the pond to live on land near water. The former vegetarian now eats insects and worms, spider and slugs, snails and snakes, fish and frogs.
Green frogs (both adults and tadpoles) overwinter in riverbeds or in the muck at the bottom of ponds. They find a spot with enough water flow that it stays unfrozen throughout the winter – unlike some frogs, green frogs cannot survive if they freeze. Hiding in the insulating mud, they are still able to get oxygen through their skin. Their metabolism slows way down, though, and they live off their fat reserves. During a mild winter, they may become active for a time.
Green frogs adapt to their environment. Because they are cold-blooded amphibians, their skin becomes darker when the temperature cools. This darker skin absorbs more heat and keeps them warmer. They can also hear very well whether they are above water or below. They do not have ears, but you can see their eardrums: the large round area just behind the eye. Males have larger eardrums than females.
Scientists have learned that up to 16 percent of green frog tadpoles reverse their sex. There is no doubt that some pollutants and pesticides can cause sex reversal in green frogs, but studies have also shown that this happens in some green frog tadpoles even in pristine environments. This leads scientists to believe the sex reversal is sometimes a natural process, caused by temperature or other unknown features of the water. According to National Geographic, sex reversals can increase genetic diversity by helping rid the species of bad mutations in sex-linked chromosomes.
Although green frogs are one of the most widespread frogs on the planet, there is still much we do not know about them. Look for these common (yet elusive) amphibians the next time you visit a pond or wetlands.
Reader photo
John Barnett spotted this orchard orbweaver in Spring Hill, Somerville, on July 15, 2023.
Have you taken photos of our urban wild things? Send your images to Cambridge Day and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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