A forecast that calls for snow and temperatures in the 20s hardly seems like weather a frog would enjoy, but one species likes to get a head start on spring. Last week when the thermometer reached 60 degrees, I heard wood frogs singing. Though spring still seemed a distant promise, the familiar voice reassured me that warmer weather is, indeed, on the way.
The wood frog's voice is not at all froglike. It's not a croak, a "ribbit," or even a peep. It's raspy. I imagine the conversational "putt, putt, putt" of a flock of turkeys. One field guide describes the wood frog's call as, "like a flock of quacking mallards."
A wood frog's behavior isn't very frog-like, either. They emerge in late winter. Ice frequently rings the edges of puddles, ponds, and streams when the wood frog chorus begins. That's not too surprising, though, for a species whose range extends north of the Arctic Circle.
Here in the temperate zone, wood frogs inhabit damp forests and spend most of the year far from water. They are active during the day and live a solitary life, except during the brief mating season. In February and March, they gather at ponds and puddles to mate.
Wood frogs can measure three inches from nose to tail, but most are smaller. They're best recognized by the prominent black "robber's" mask that extends from in front of the eyes back to the shoulder. When Emma was a little girl, she called it a "frog raccoon." Otherwise, a wood frog's nondescript tan body blends in perfectly with the decomposing leaf litter on the forest floor.
Late winter rains signal the onset of the breeding season. When the air temperature reaches 50 degrees Fahrenheit, male wood frogs move to small ponds and low lying spots that collect snow melt and run-off.
Here the males gather in groups to sing. Within a day or two, females arrive. I suspect no sound in the woods is sweeter to a female wood frog than the raspy quack of a sexy male.
Wasting little time, males clasp receptive females in a mating embrace herpetologists call amplexus. Females lay between 500 and 700 eggs, which the male fertilizes as she releases them. Typically females release their eggs at communal nesting sites, so masses of thousands of eggs are not uncommon.
Individual eggs are small black spheres (the embryos) surrounded by a clear gelatinous mass. An egg mass looks a lot like clear tapioca pudding. The eggs incubate in the water for up to a month, though hatching can occur in less than two weeks when temperatures are unseasonably warm. At 45 degrees Fahrenheit., for example, incubation takes about 20 days.
Wood frog eggs can survive a late winter or early spring freeze. The embryos simply stop growing and wait for warmer temperatures to return. Then development resumes.
Eggs located in the center of the mass have a distinct advantage, and this explains why wood frogs are in such a rush to breed. The temperature in the middle of an egg mass may be as much as 12 F warmer than the ambient water temperature, so these eggs are more likely to hatch than those on the perimeter. Therefore, the first eggs laid occupy the center of egg masses and are thus most likely to hatch first. Furthermore, older, larger wood frog tadpoles eat smaller, younger tadpoles. So eggs that are laid first and end up in the center of the egg mass enjoy several advantages that promote survival.
Wood frog tadpoles measure less than a quarter-inch long when they hatch, and grow to more than two inches over the next 12 to 14 weeks. Then, they transform into the adult form. Throughout the late summer and into the fall, juvenile wood frogs live on the forest floor near the breeding pool. Females require three years to attain sexual maturity, while males can breed in their second year.
In November, the wood frog's life cycle comes full circle. Shorter days and cooler temperatures drive them into underground burrows where they hibernate until awakened by the longer days and warmer rays of February and March.