Species Spotlight: Wood Frogs



All across North America, the sounds of frogs ring in the start of spring. Listen to a recording of an early spring chorus with several species and you may be able to figure out roughly where the recording was made as every region has its own unique spring mix of amphibian vocalists. In Vermont where I live, Spring Peepers and Wood Frogs will be the first to call, usually between mid-March and mid-April, with Northern Leopard Frogs, Pickerel Frogs, and American Toads chiming in shortly after. Head to the southeast and while you may hear some of those same species, the local frog choirs are more likely to be dominated by a variety of chorus frog species or even Spadefoot Toads. Up in the northwestern United States, Pacific Treefrogs and Red-legged Frogs are among the first to sing. While the specific cast of characters varies from one spot to another, there is only one species that can be heard from coast to coast spanning from Maine and Nova Scotia all the way to the rocky shores of British Columbia and Alaska. Soon, people stepping outside their front doors in the northeastern US, Alaska, and almost every square mile of Canada south of the Arctic tree line may be greeted by the same sound – the gentle clucking of Wood Frogs eager to mate.

Reminiscent of quacking or clucking ducks, Wood Frogs are among the first frog species to begin calling in the northeastern United States and most of Canada.

A woodland species topping off at less than 3-inches in length and blending in easily among the leaflitter of the forest floor, Wood Frogs are mostly brown in color, ranging from a light tan to almost red. Males tend to be darker than females, but regardless of age or sex, they all have two distinct lines/folds of skin called dorsolateral ridges running down their back, sometimes with an additional light stripe down the center. Their throat and upper lips are white, and the skin behind their eyes is a dark brown, almost black, eliciting comparisons to a bandit mask. Although no other North American species has this mix of field marks, they can sometimes be confused with Spring Peepers, which breed in similar habitat and at the same time of year, but peepers lack dorsolateral ridges, are much smaller, and typically have a light X across their back.

This photo taken by Cindy Sprague portrays the wide range of tan, brown, and red colorations that Wood Frogs can have.

As soon as the snow begins to melt, vernal pools gradually fill with water, and permanent wetlands begin to open back up after a winter locked in ice. This is when Wood Frogs emerge from their winter refugia under the leaflitter and head to their preferred breeding grounds. Although they can breed in a wide variety of wetland types, Wood Frogs seem best adapted to breeding in vernal pools, which usually dry up in the summer, but are free of fish that prey on frogs and their eggs. While the lack of fish is a huge advantage for breeding amphibians, the fact that vernal pools dry up in the summer pits Wood Frogs against the clock – they need to get to the pools and lay their eggs as soon as possible in the early spring, even if the pools still have some ice or are partially surrounded by snow, otherwise their young may not develop in time to survive on land before the water disappears. Wood Frogs are freeze tolerant, so they can overwinter under the leaflitter rather than burrow underground, which allows them to wake up and move towards water while other amphibians are still tucked away beneath the forest floor. 

Vernal pools that fill with snow melt and rainwater in the spring, but dry out in the summer, are ideal breeding habitat for species such as Wood Frogs since predatory fish are absent from such wetlands.

Not only are Wood Frogs one of the most widespread reptiles and amphibians in North America , but through much of their range they are also one of the most abundant (this is especially true in extreme northern locations where Wood Frogs are the ONLY amphibian). They owe much of their success to their extreme tolerance of bitter cold conditions. You can read much more about how Wood Frogs and some other amphibians can survive freezing in another recent blog post of mine here, but the short version is that Wood Frogs produce high concentrations of sugars in their bodies which draws water out of their cells, minimizing damage caused when water expands as it freezes. By concentrating more sugar in their core and vital organs, the frogs freeze from the outside in, with all but their heart and liver freezing mostly solid. When they thaw, they do so in reverse, allowing circulation to be restored one organ at a time until they spring back to life and head to their breeding grounds. In most areas, they can only survive freezing down to about 23 degrees F for up to a couple weeks at a time, but a recent study found that Alaskan Wood Frogs can withstand freezing down to 0F for up to 200 consecutive days without harm – a feat rivaled only by the Siberian Salamander among the vertebrate world. 

Wood Frogs have one of the most widespread ranges of any amphibian and are the only species found from coast to coast in the north.

After mating and laying eggs, the adults venture back into the woodlands to forage on small insects, but usually stay within about 1500 feet of their breeding wetlands. Their eggs, which are laid in gelatinous masses of between 500 and 2000 eggs attached to branches and vegetation in the water (read more about egg identification here), typically hatch within a couple weeks or so, and after feeding primarily on algae and carrion for a 2-3 months, juvenile frogs resembling adults emerge. After one to two years foraging on the forest floor alongside adults, most that survive return to the same pools where they were hatched to repeat the cycle. 

Adult Wood Frog on the forest floor.
Wood Frog egg mass.

While Wood Frog populations remain strong through much of their range, certain threats pose a growing concern. Much of their habitat is fragmented by roads, which is especially problematic when roads cut between their breeding wetlands and foraging habitat. And much of their habitat has been lost to development and agriculture, leaving much less land for the frogs and separating what remains by even greater distances. The introduction of foreign pathogens, such as chytrid fungus, also have the potential do devastate populations and may represent the single greatest threat to most amphibian species. At the local scale, however, one of the most important things landowners can do to protect Wood Frogs and other amphibians to establish a 100-ft buffer around vernal pools where the habitat is left alone, and to minimize land clearing and vehicle use to the greatest extent possible within 750 to 1500-ft of the pools. You may even be able to make such conservation measures permanent with a conservation easement through a land trust. Although many states have laws protecting vernal pools from draining and development, they are small and difficult to detect in aerial images, so those laws can be difficult to enforce. If your state maintains a database of vernal pools, you can reach out to conservation agency biologists to inquire about how to properly document the habitat on your land so they are afforded at least some protections from development by future landowners. You can also help prevent frogs from being crushed on roads by staying home on rainy spring nights or avoiding driving in places where you have seen amphibians crossing roads in large numbers on rainy nights in the past. 

Amphibians can use all the help they can get, even common species ranging over vast areas. Keeping a common species common is much easier than bringing them back from the brink of collapse, so anything you can do to protect their habitat or reduce the number killed by cars during their breeding season is a noble effort to be commended. Ecosystems with the greatest biodiversity are the most resilient to stress and disturbance, so let’s try not to take Wood Frogs for granted.