The Elusive And Amazing Wood Frog

My last column dated March 10 mentioned the old weather saying, “March comes in like a lion and goes out like a lamb,” and that seems to be the way it’s working out for March 2021. The first eight days of this month averaged seven degrees below normal; in like a lion. The thermometer hit 60 degrees on the first day of spring (March 20), and the forecast for the last week of this month has daytime temperatures consistently over 50 degrees. We’re looking at a lamb-like exit.

This past week’s nature sightings included the first piping plover of the year, seen feeding on the beach at Promised Land, Napeague, and the first osprey, one of the pair that nests on the pole at Landing Lane, Accabonac Harbor.

Several other piscivores, or fish-eating birds, were congregating at Accabonac. Common loons and red-breasted mergansers were busy swimming and diving at the harbor’s inlet, joined by a small flock of herring gulls, the latter plunge diving from the air. I’m not certain what fish they were targeting, as the only schooling fish that I’m aware of in the area now are full grown alewives moving toward their spawning grounds. At 10 to 12 inches in length, they would be too large for these birds to handle. It’s possible that the first Atlantic silversides have arrived, as their spawning season is coming up in three weeks.

Returning from Napeague on Sunday, I stopped to check a vernal pool near Fresh Pond for signs of egg masses of spotted salamanders. I’m part of the newly formed Long Island Vernal Pool Working Group that initiated a several year-long inventory of vernal pools this spring. You can find out more about this project at seatuck.org/vernal-pools.

There were no signs of spotteds, but I could hear a chorus of hoarse, duck-like “quacks” emanating from a much larger vernal pool system close by in the woods, and set off to investigate.

This was the distinctive breeding call of the male wood frog (rana sylvatica), a common but rarely seen inhabitant of our forest ecosystem. As usual, upon my approach, the group went silent. I chose a comfortable spot overlooking a portion of the flooded swamp and waited quietly, scanning the surface of the water for signs of the heads of these 2- to 3-inch-long amphibians protruding from the water.

I realized that, when disturbed at their breeding pools, wood frogs not only go silent, but they dive into the leaf litter covering the bottom of the pool and disappear from view. These are not deep dives, as most of the pool I was visiting had a loose collection of leaves settled just inches from the water’s surface. Watching for ripples of water as they re-emerged, the only movements were caused by some type of insect that I could not identify, possibly water striders that utilize water’s surface tension and “walk on water.”

It was a surprisingly long time before I caught sight of the frogs surfacing and repositioning themselves in their preferred calling site, and an even longer time before they began calling again. The males have two vocal pouches on either side of their body, each just above a front leg and behind the eye and tympanum (ear covering). Their black, raccoon-like masks did not stand out as well as I suspected they would, and their various shades of brown, from dark chocolate to a light reddish color, were matched well with the leaf litter. Their breeding season is early, short and intense, lasting less than a week. If you want to hear them, now is the time.

Among the cold-blooded amphibians found in North America, this interesting creature is remarkably cold hardy, with the ability to freeze solid in winter and survive the process of thawing. It is our earliest frog to emerge from winter dormancy and make its way to breeding pools. And its range in North America includes forested sites above the Arctic Circle, farther north than any other amphibian on this continent … incredible!

Wood frog eggs have been the subject of much study. A symbiotic green alga grows on the egg masses, feeding on carbon dioxide and waste nutrients from the frog embryos and providing oxygen to the developing egg. Once hatched, the algae provides a ready source of food for the tadpoles.

Females in a given vernal pool have a habit of laying their gelatinous eggs masses (up to one thousand eggs per mass) in the same spot, referred to as “communal nesting.” The black pigmented embryo absorbs solar radiation, and the clear jelly surrounding it acts like the glass in a greenhouse, trapping the heat inside. The effect of the sun on dozens of egg masses and tens of thousands of eggs clustered together noticeably raises the water temperature in the immediate vicinity of the eggs, as much as 7 degrees to 11 degrees, accelerating development and hatching of eggs.

The solar heating also creates tiny water currents that flow through the egg masses, increasing available oxygen and also assisting in accelerating embryo development. This acceleration can be critical to the survival of tadpoles, as the vernal pool may be dry come late June or July.

Eggs hatch within three weeks. The tadpoles have a flat, sucker-like mouth designed to scrape algae and bacteria off the surface of underwater objects. Over the six- to seven-week-long tadpole stage, they develop and accumulate toxins in their skin, making them distasteful to potential predators. To some degree, this toxin remains as a predator deterrent through their adult stage, although it doesn’t appear to stop raccoons and garter snakes from dining on wood frogs.

As adults, wood frogs are terrestrial residents of the forest floor leaf litter ecosystem, feeding on insects, snails, spiders, ticks and slugs. Their population can be surprisingly dense: up to 240 wood frogs per acre! Despite this, they are rarely encountered. Their perfect camouflage against the leaf litter and their habit of remaining motionless when approached are largely responsible for their going unnoticed.

Their lifespan is short: three years. You can help the local populations of this unique creature by taking responsibility for how you (and in many cases your landscaping crews) steward your property. Is it really necessary to spray pesticides for ticks and mosquitoes? No, it is not. Spraying is very ineffective for both. And is it really necessary to remove every native plant and every leaf from your property? Think about what you are doing to our local wildlife — beautiful leaf litter residents such as the Eastern box turtle and wood frog — as well what you are doing to our rural character.