The wood frog (Lithobates sylvaticus) has the widest distribution of any frog in North America. They are found as far north as Alaska and into the Arctic Circle, through Canada to the Maritimes and south throught the Great Lakes states through the Appalachians to the coast. They are vernal pool breeders and found at most ephemeral wetlands.
Wood frogs are terrestrial creatures spending most of their lives in moist areas including deciduous and coniferous forests, bogs, and swamps. This is a frog you might encounter quite a distance from water while you are walking in the woods. They are aquatic only as eggs and larvae.
Adult wood frogs vary in color from a rosey, almost red, color to a very dark brown. They can be identified by the dark mask that runs from the nose through the eye and the white lip stripe. An adult wood frog is about 1.5 to 2.5 inches in length from snout to vent.
Wood frogs can also be very dark. Yet the eye mask and white upper lip are still visible. Some researchers suggest that males are likely to be dark, particularly in breeding season, and females are likely to be lighter in color.
In spring, wood frogs travel on rainly nights to vernal pools. Male wood frogs float in the pool and make a "quacking" call. Other males join to produce a chorus which is a distinctive sound of spring. Females are attracted to the pool by the chorus.
When a female reaches the pool, she is grabbed by a chorusing male. The male grasps the female in amplexus and waits for her to lay eggs which he fertilizes with his sperm.
Some males, often referred to as satelite males, do not bother to enter the pool and join the chorus. They wait silently near the edge of the pool and grab an approaching female and let her bring both of them to the pool.
Since males respond to movement, they sometimes grab animals other than a female wood frog. This excited male is attached to a green frog.
Males, when seeking to mate, will grab almost anything that moves in the vernal pool. In some cases, many males will cluster around the same female. The female here is the one with hind legs out to the left and her right forearm over the male on the lower right.
A female will produce up to 1500 eggs in one or more masses but most often a single egg mass which is attached to submerged vegetation. A fresh mass might be the size of a golf ball but the gelatinous material surrounding each egg absorbs water and the mass expands to approximately baseball size in a day or so. The large mass above is a wood frog egg mass. The smaller masses are from mole salamanders.
The wood frog egg mass is a cluster of individual eggs. Each egg is surrounded by an individual mass of jelly. Here several masses attached to a piece of vegetation are being lifted from the water. The white eggs to the right are dead, most likely from either not being fertilized or being at the water surface during a freeze.
Wood frogs lay eggs in large clusters in the best area of the pool for attachments sites, warmth from the sun, and protection from predators. A female will produce up to 1500 eggs in one or more masses. After breeding, the adult frogs leave the pool and return to the forest. They are seldom at the pool for more than two weeks.
Some communal clutches are enormous floating rafts of eggs. This raft has over 1200 egg masses and was one of two similar sized rafts in the same pool.
As wood frog eggs develop, the mass is generally colonized by a symbiotic algae (Oophila amblystomatis). The algae utilizes some of the waste from the developing eggs and provides some oxygen for the embryo.The rate at which the embryos develop is dependent upon how warm they are. Egg masses are near the water surface where the sun heats the dark embryos and their surrounding jelly. Eggs may hatch in as little as two weeks.
Upon hatching, the tadpoles swarm about the egg mass as they absorb the egg yolk and feed on the egg mass itself.
When all tadpoles have left the egg mass, the empty mass remains until various vernal pool organisms consume it. The symbiotic algae (Oophila amblystomatis) remains These expended egg masses look like floating masses of algae.
The hatchling wood frogs have external gills for a few days as the body continues to develop into a swimming larva. They attach themselves to the egg mass or other objects as they develop and feed on the algae.
The typical wood frog tadpole begins as a black tadpole but soon becomes brownish with specks of gold and the beginnings of a stripe at the mouth. Tadpoles feed on algae and other plant matter. They also feed on salamander egg masses, toad eggs and dead animal matter in the pool.
Diagnostic characteristics of the wood frog tadpole are the gold flecking on the dorsal side, further flecking on the ventral side and the intestinal coil visible through the transparent skin.
The wood frog tadpole is also identified by the Labial Tooth Row Formula of 3/4 which means there are three rows of labial teeth above the oral disc and four rows below it. In this image, the top three rows are hidden under the upper lip (labium) which hides part of the oral disc. The four lower rows are visible.
Wood frog tadpoles take about seven to twelve weeks from hatching to completely develop into a juvenile frog. When they have four developed legs, they begin the transition (metamorphosis) into an air breathing, carnivorous, terrestrial frog.
At times, large numbers of metamorphosing tadpoles come to surface to gulp air as they develop.
Metamorphosis is complete when the tail is completely absorbed by the frog. The small frog leaves the pool and enters the surrounding uplands to feed and develop into an adult. Complete development takes two or more years.
Wood frogs forage for invertebrate food including insects, spiders, earthworms, caterpillars, snails and slugs. They spend the winter partially frozen under leaf litter and forest debris. With the warm rains of early spring, they emerge and head to a vernal pool for breeding.
Adult wood frogs breed when 2-5 years old. They may breed for a several years. Their breeding success and survival is dependent upon their vernal pool coninuing to exist. Most wood frogs return to their natal pool to breed. All wood frogs that breed in multiple years, return to the first pool they used whether it is their natal pool or not.
Wood Frog Natural History