Last week, while out checking my moth sheet at my parents’, a frog came up and found me. He hopped right up and on to the white sheet, where I couldn’t miss him. Perhaps he was jealous of all the attention the Gray Treefrog got a few weeks earlier, but he certainly seemed to want his own feature story. What could I do but oblige?
He was small, and his colours seemed underdeveloped, which made me wonder if he was still an immature, not too far removed from his water-loving days. Still, despite the indistinct markings, they were clear enough for me to confidently identify him as a Wood Frog, Rana sylvatica. Adult Wood Frogs will have a dark chocolate-coloured mask, which has contributed to their colloquial names of bandit frog or masked frog, though these names are rarely used. They’re the only species in eastern North America with this facial marking. The top of their eyeball is golden while the bottom is dark, blending in with the mask pattern that crosses it. They have two lines that run laterally down the length of their back, a white “moustache” along their upper lip, and a small dark patch at the front of each shoulder, but generally otherwise they’re brown and fairly nondescript. If the mask isn’t well-developed, as in this individual, they’re not very distinctive.
They vary in size according to age and sex. Males are smaller than females, and the small size of this little individual may also have meant it was a male. In breeding season the males will develop a swollen “thumb” that they use in gripping the female during mating, but it wouldn’t be apparent at this time of year. Wood Frogs may live up to 4 or more years; males begin breeding their first summer as an adult, but a female doesn’t breed until her second.
Wood frogs are North America’s most northern-occurring species, and the only one found above the Arctic Circle. They range from coast to coast in Canada, found from the Aleutian Islands in Alaska, through northern and interior British Columbia and the three territories, east to Labrador and the Maritimes. In Ontario, they’re found right up to the Hudson Bay coast. They are also Canada’s most wide-ranging species. They’re less widespread in the states, found mostly in the northeast, with small local populations in Colorado and Wyoming.
Part of the reason for this distribution is their choice of habitat. They’re found in the boreal forest and the deciduous forests of the northeast. Although individuals may move up to a kilometer during their summer foraging activities, they’re rarely found far from water. More specifically, they must be near vernal pools, small ponds and wet depressions that dry up partially or completely in the summer, their primary breeding habitat.
Even before the Spring Peepers start peeping, the Wood Frogs emerge and congregate at vernal pools to breed. They are quite often the first frogs heard calling just as spring starts to arrive. Males will gather at the breeding site, calling loudly and sounding a little like a duck quacking. However, they only call until the females arrive, which may be right away or up to two weeks later, after which they’re infrequently heard.
Females may lay up to 2000 or more eggs, secured to the base of a submerged shrub or other vegetation. Eggs are laid in a large mass, and later females may add their eggs to the initial groups. Laying eggs together like this creates a large, dark ball, which warms up in sunlight more quickly than individual or a string of eggs would, important in early spring when the water can be very cold. Mating takes place in early to mid-April, as soon as the pond’s water is accessible. Because the pond is likely to completely dry up in the heat of summer, it is mandatory that the frogs’ tadpoles get as early a start on life as possible.
Although the tadpoles in vernal pools are under constant threat of their habitat drying up before they’re ready to leave, they’re also protected from fish and many other predators found in larger water bodies that can’t tolerate the drying out. It takes a tadpole anywhere from about 45 to 85 days to go from egg to frog, with the wide range depending largely on water temperature. Warm conditions can speed up the process considerably (which is good, because warm conditions can also speed evaporation). Usually the tadpoles are out well before the water dries up completely.
Newly-metamorphosed froglets and adult frogs that have finished breeding will spend the rest of the summer hopping around the woods surrounding the vernal pools, foraging on typical frog fare: crickets, earthworms, slugs, spiders, beetles, and whatever other similarly-sized invertebrates they happen to come across. During this period they may roam hundreds of meters from their home pond, and about 20% of young froglets will disperse to other nearby ponds, maintaining gene flow.
When fall rolls around, the frogs return to the upland forest area near their pond and start preparing for the winter. Part of this involves converting the glycogen in their bodies into glucose. Glucose acts as a sort of natural “antifreeze”, preventing the fluid inside their cells from developing ice crystals which will puncture the cell walls. The frog then buries down under the leaf litter, but, unlike many other frogs that will burrow down below the frost line, the Wood Frog remains near the surface. Slowly as the weather turns cold it stops breathing, its heart stops beating, and its brain shuts down – for all intents and purposes, becoming clinically dead. The internal (but extracellular) fluids of the frog freeze almost solid. The frog remains protected from the elements by the surrounding insulation of the leaf litter and snow, but still able to withstand ambient temperatures of as low as -6 C (21 F).
With the melting of the snow and the warming weather, the process reverses. The sun’s warm rays hit the dark leaf litter, warming the ground and the frog hidden within it. Slowly the frog’s internal fluids melt to become liquid again. About an hour later its heart then begins beating again, and it starts to breath. Brain function returns to normal, and the frog’s body begins to convert the glucose back to glycogen again. Within a day the frog can resume normal activity.
Several species of frogs have evolved the ability to survive such winter freezing, nearly all being early spring breeders. Spring Peepers, Boreal Chorus Frogs and Gray Treefrogs all freeze during the winter. The advantage to such an adaptation is that it allows the adults to start breeding as soon as temperatures rise above freezing. Virtually all of these species are terrestrial forest species that take advantage of vernal pools and need to lay their eggs as early as possible.
Naturally, scientists are very interested in this amazing ability, since it would have application in tissue and organ preservation, and the practice of cryonics (freezing whole living organisms to be resuscitated later). Given that a frog may live to age four or more, it may go through this freezing process three or more times in its life. Check out this documentary clip (from YouTube) to watch a frozen frog come back to life: