Amphibians
Carpenter Frog
Lithobates virgatipes · also called sphagnum frog
Stand at the edge of a sphagnum bog in the New Jersey Pine Barrens on a spring night and you will hear what sounds like a construction crew working in the dark. It is the carpenter frog, and its knocking, hammering call is the reason for its name.
Overview
Lithobates virgatipes lives only in the acidic sphagnum bogs and cedar swamps of the US Atlantic coastal plain, from New Jersey to northern Florida. It is a slender brown frog, 4–7 cm long, with four yellowish lengthwise stripes, and it stays in or beside the water at all times.
The hammering call
The male’s call is a rapid double or triple “clack” that carries across a bog. A chorus, with each male slightly off the beat, sounds exactly like several carpenters hammering nails.
Life in acid water
It breeds in tannin-stained bog water with a pH as low as 3.6 to 4.5 — harsh conditions that keep fish and most other frogs out and give its slow-developing tadpoles a refuge. The tadpoles usually overwinter and take about a year to transform.
Status
Least Concern overall and locally common in good bog habitat, but a conservation concern in the north, especially New Jersey, where coastal bogs have been heavily developed.
Carpenter Frog: common questions
Does the carpenter frog really sound like hammering?
Strikingly so. The male's advertising call is a short, sharp, wooden-sounding "pu-tunk" or "clack", repeated two to four times in quick succession, and it has a percussive, knocking quality quite unlike the trills, chirps, and booms of most frogs. When several males call together across a bog, with each one slightly out of phase with the others, the overall effect is a rhythmic, arrhythmic clatter that people consistently describe as sounding like carpenters at work, hence the name. It is one of the easiest frog calls to recognise once you have heard it.
How can a frog live in water that acidic?
Highly acidic bog water, stained brown by tannins from sphagnum moss and decaying plants and often below pH 4.5, is lethal to the eggs and larvae of many amphibians, which is exactly why the carpenter frog does well there. Its eggs and tadpoles are physiologically tolerant of the low pH and the low oxygen and nutrient levels that go with it, and by breeding in this harsh water it largely escapes the fish and the competing and predatory tadpoles of other frogs that cannot survive there. The trade-off is slow development in cold, poor water, so the tadpoles take about a year to transform.
Where does the carpenter frog live, and is it common?
It is restricted to the acidic wetlands of the Atlantic coastal plain, from the pine barrens of New Jersey south to northern Florida, and it does not occur inland or west of the Appalachians. Within that range it can be locally abundant in a good sphagnum bog or cedar swamp, with dozens of males calling from a single wetland, but the habitat itself is naturally patchy and has been much reduced by drainage, filling, and development, especially in the north. As a result the species is considered secure overall but is a conservation concern in New Jersey and at other northern sites.
What eats carpenter frogs?
Wading birds such as herons and bitterns, water snakes, snapping turtles, raccoons, and larger frogs including bullfrogs and green frogs all take adult carpenter frogs, and the tadpoles are eaten by dragonfly larvae, diving beetles, and aquatic salamanders, though the acidic water excludes many of the fish that would otherwise be the main tadpole predators. The carpenter frog's own defences are limited to its brown, striped camouflage among the bog vegetation and a fast, splashing dive into the water and down among the sphagnum when a threat appears.