Froggie Went A Courtin

For weeks I've been telling Pat that spring is coming. She just laughs at me, explaining that if and when winter reappears, I will blame her. She is probably right. But as Bullwinkle the Moose used to say when trying to pull a rabbit out of his hat: "This time for sure". Of course, he always got a rhinoceros, but that's beside the point. This time for sure. The signs are getting too obvious: the front yard down the street is full of pale crocuses; the skunk cabbage is fragrantly blooming in wet places, using its perfume of cadaverine and other assorted rotted meat stinks to attract scavenging flies to pollinate it and to lay their eggs which hatch into hopeful maggots, only to starve in the warm depths of the spadix.

Today, spring manifested itself beyond doubt. Long skeins of Canada Geese are headed north. Aldo Leopold in Sand County Almanac, described the "March goose who is staking two hundred miles of black night on the chance of finding a hole in the lake with the conviction of an Old Testament prophet who has burned his bridges." Leopold lived in Wisconsin, on the edges of the frozen tundra; just a hop, skip and jump from Hudson Bay, the main breeding ground of most of the migratory geese east of the Rockies. The lines of geese I saw this morning, undulating and shifting like some aerial version of crack-the-whip, will probably move north until they find the last open water and wait out the next part of the thaw, leapfrogging until they get to ice-free territory.

The thing that clinched it for me, though, was the chorus of spring peepers and wood frogs I heard last night at a low swampy area a few minutes from home. The peepers were making their sleigh bells racket at full blast but underneath you could hear the lower, almost conversational duck-like quacking of the wood frogs. Wood frogs are one of the joys of backyard biology (although as E.O. Wilson, one of the prominent ecologists of our day, said: "Even the most humble roadside weed has a fascinating ecological history"). Wood frogs are one of those creatures that, if found only in Borneo or Tierra del Fuego, would rate an hour on Animal Planet. As it is, they are all over North America, from Georgia to the Arctic Circle and beyond. And that is what makes them so cool (in more ways than one). When winter comes, wood frogs can freeze solid, turning into little froggy Popsicles. As long as the temperature drops slowly enough, wood frogs can pull water out of their organs and muscles, sequestering it in their stomach and bladder where it can expand (water expands as it freezes, thus potholes in pavements) without rupturing cell walls and killing them. Slow chilling also allows the frog to produce anti-freeze glucose and other sugars helping to keep ice crystals from forming within their cells. Breathing stops, heart beat ceases, and the beast basically solidifies. Come the thaw, the frog reabsorbs the water, rises from its cocoon of ice, comes back to life, and starts looking for love.

Wood frogs breed in vernal pools. In fact, they are what ecologists call "obligate vernal pool species", meaning they breed nowhere else. Vernal pools are temporary snow melt and rain pools, forming in late winter and early spring. They may last for a few weeks to several months but they all eventually dry out. For wood frogs and other obligate vernal pool species, such as spotted and Jefferson salamanders, this is good and bad. Temporary water means no fish predators but it also means eggs must develop and hatch into larvae, larvae turn to adults, and adults get the hell out of Dodge before the whole shebang turns into cracked drying mud.

Wood frogs are "explosive breeders" meaning all the frogs in an area move to the pools and engage in a mating orgy. Since females may spend up to forty percent of their body weight making eggs, they need an additional year to build up body fat to make the next clutch - males outnumber females in a pool by two or more to one. Males lay on the surface, making their duck calls (the lower frequencies of which may propagate underwater) and attract interested females. When a female responds, the male climbs on her back and holds on, a behavior called amplexus. When the female releases her eggs, he fertilizes them as they come out. Eggs are covered in jelly, absorbing water until the egg mass forms a tight golf ball- sized mass. Fights break out as additional males try to dislodge the breeder or just pile on, hoping to add their sperm to the mix when the females drops her eggs. Males will attempt to mate with anything floating: females, sticks, and other males until they kick them off. Enough males piling on to a female will drown her, but even that won't stop them. Wood frogs make the old Roman Bacchanals look like a Sunday ice cream social.

Assuming everyone survives intact, females tend to clump their eggs with existing egg masses. The trick is to get your eggs out as early as possible so your clump is near the middle of the blob. The jelly-covered egg masses do hold moisture but those on the outside are at higher risk of drying out if and when water levels fall. The jelly acts as a solar panel, focusing and concentrating warmth towards the center, so eggs in the middle develop faster and turn into tadpoles earlier. Once a tadpole makes it that far, its troubles are just beginning. Here you are, stuck in a rapidly shrinking pool, with a gazillion other tadpoles, all trying to eat enough, fast enough to get out before the hammer falls. Even though there are no fish, a whole host of predators inhabit vernal pools and like nothing better than tadpole du jeur. Voracious dragonfly larvae stalk the bottom, slurping down anything that won't eat them first. In addition, carnivorous Spotted and Jefferson salamander larvae patrol the pools like tiny sharks. Even more fearsome are the larvae of Marbled Salamanders. Marbleds get a big head start on the other predatory salamander species by breeding in the same pools in late September on a wet night and placing their eggs under damp leaves. The eggs remain throughout the winter in stasis until the pool fills. They hatch in less than two days, becoming the alpha predator, the Great White Shark, in the vernal pool food chain. And don't forget, while the tadpoles are trying not to become lunch, the pool is steadily drying out. If the rate of drying hits a threshold level known only to the tadpoles, they change from happy little algae grazers and filter feeders into cannibals, chowing down on anything smaller, including brother and sister tadpoles and small larval salamanders, that crosses their path. Big eaters develop faster and get out sooner.

Animals living in vernal pools tend to share certain characteristics. They tend to be long-lived. Wood frogs live three to five years and salamanders up to twenty. Long life spans are conducive to "boom and bust" reproductive strategies - the pool may dry out early one year, dooming all the tadpoles, but next year it may last longer than usual and more frogs than average emerge. If you are an adult, all you need is two eggs to make it to breeding adults to have moved your chromosomes to the next generation (Darwin's definition of "fitness"). Vernal pool animals tend to be explosive breeders with everyone mating with everyone. Once the eggs are in the water, the adults quickly return to the forest and go back to whatever it is they do for the rest of the year. In the case of wood frogs, living in the leaf litter and eating small invertebrates, in the case of salamanders, going back into their burrows and doing who knows what for the next fifty weeks. But that is another story.