Projected on the screen is a slide of two bullfrogs mating, a head-on view. By their expressions, the animals don`t particularly seem to be enjoying the encounter, though with frogs it`s hard to tell.
”Female on bottom, male on top, but fertilization is external,” notes naturalist Nan Peters, as the audience of 30 families ponders the picture.
”They`re in the pond. The female releases her eggs,” Peters goes on.
”The male pours his sperm over the eggs. You`ve all seen the eggs–long strings, or clumps, floating or attached to leaves. Frogs lay huge masses of eggs because only a relative few will survive and become tadpoles, and few tadpoles ever make it to froghood.”
The incident on the screen, representing 350 million years of natural selection, underscores what has become a popular–if unpretentious–rite of spring in the Chicago metropolitan area: The annual Frog Walk in Deerfield`s Ryerson Woods.
Frogs and toads are most active in the springtime, or at least they seem to be because we hear them. From April through June, males croak to attract females and conduct their frantic courtship under a safe blanket of darkness. And on two recent nights as many as 50 humans attended a talk at Ryerson Woods and a twilight tour to listen to the frogs.
”Melting snows and spring rains create ponds in the woods,” Peters explains. ”The ponds draw the frogs and toads. The animals you will hear tonight are incredible. Many of them are no bigger than your thumbnail, yet we can hear them several blocks away.
”There`s real urgency: `C`mon, over here!` the males are calling. `Here I am! Tonight`s the night!”`
And Ryerson`s the place. Wildlife long ago learned of this remarkable 550-acre sanctuary off Riverwoods Road.
The Edward L. Ryerson Conservation Area is a virgin maple/basswood forest, a landscape mostly untouched by man and basically unchanged since the last Ice Age some 12,000 years ago. The Lake County Forest Preserve District, while coping with 10,000 annual Ryerson visitors who come for a host of quiet programs, is determined to keep things precisely as they were.
These days the forest practically explodes with plants and wildflowers, migrating birds and the renewing of life along the Des Plaines River. Deer abound, as do mink, fox, beaver, woodchuck, raccoon, opossum, weasel, the ubiquitous chipmunks and squirrels, plus a host of amphibians and reptiles
–even the rare Massasauga (or pygmy) rattlesnake.
At the first signs of warmth, throngs of male frogs emerge from hibernation here and all over the Chicago area. They hop to breeding ponds and begin to call, buoyantly with hope in their hearts, seething with desire. Their screams lure females for courtship activity so fascinating that scientists keep slogging through the world`s swamps trying to figure it all out.
Most people would dismiss a frog chorus as merely generic ”crickets.”
But when you listen closely, the amazing polyphony sounds like a bunch of different radio stations all blaring simultaneously.
There are groans and chuckles, metallic clicks and ducklike quacks. There is mewing and wailing, trilling and bleating, squawking and shrieking, belching and whinnying. Each species has its own tunes, sex strategies and schemes, and each individual has his own aria and sense of style.
Such choruses probably were the first voices ever heard on earth.
Amphibians, by 200 million years ago, had evolved the lungs and abilities necessary to live on land as well as water. A transition between fishes and reptiles, they still prefer damp areas and must return to water to lay eggs. Ancient amphibians preyed on worms, insects and other invertebrates that had preceded them on land, and their mating songs must have drifted across a silent planet that, until then, had heard nothing but the chirps and whirrs of insects.
The most numerous amphibians on earth are called anurans, or ”tailess ones.” The 2,600 different species are divided into two main types. Those with smooth, thin, porous skins requiring water to sustain life, we call frogs. Those with dryer, thicker, rougher and more warty skins that enable them to live on land are the toads.
”Frogs and toads defend themselves by leaping suddenly, by concealment, camouflage, and, in the case of toads, by tasting bad. Toads have huge bumps behind their eyes, special glands (paratoid glands) that produce a noxious secretion. If you`ve ever seen a dog pick up a toad and spit it out, that`s why.”
Other toads display more aggression. Upon encountering a snake, the European toad instantly inflates its entire body and bravely stands on tip-toes. An impressive sight, this, and one that usually gives the snake pause.
The fire-bellied toad, on the other hand, suddenly flips over on its back exposing an underside ablaze with yellow and black, a color pattern widely recognized in nature as a warning. Nor is the toad bluffing: glands on its belly produce a bitter-tasting poison.
Frogs, it has long been noted, behave as if they divide the world into three categories: 1. If an object is small, eat it. 2. If an object is big, hop away. 3. If an object is in between, try to mate with it.
In order to meet females, male frogs and toads rely on sound. Their lungs, which blow air through vocal cords, are small and weak, but huge swelling throats or resonating sacs bulging from the corners of jaws enable the animals to amplify their voices. A pondside assemblage of calling frogs can create such a racket that a shouting human voice cannot be heard. Stand just a few feet away from a male cricket frog and it hurts your ears. The tiny creature is pumping out a painful 115 decibels.
The Chicago area plays host to an interesting variety of frogs and toads. The American toad (Bufo americanus) grows to a length of four inches, with a chest heavily spotted by black or gray. Its call is a long, pleasant, musical trill lasting up to 30 seconds. The rare Fowler`s toad (Bufo woodhousei fowleri), whose chest is white and unspotted, prefers sandy areas and has a loud, nasal bleat, sounding something like a human baby.
Then there are tree frogs and true frogs. Tree frogs, exclusively nocturnal, have toe pads that act as suction cups to aid in climbing. Most are small and have very long legs.
Most common is the western chorus frog (pseudacris triseriata), which grows to 1 1/2 inches and is tan or brown above with three dark stripes down the back and another dark stripe down each side, passing through the eye. Springtime`s first caller, these frogs may be found by the hundreds at temporary ponds. They rarely climb more than a few feet. Their distinctive call may be imitated by running a fingernail down the last 20 teeth of a fine- toothed comb.
Also very common is the spring peeper (hyla crucifer), which reaches 1 1/ 4 inches in length, is tan or brown with a large dark irregular ”X” on the back, and has large toe pads. Its loud, high-pitched ascending ”peep” can resound for a half mile.
Among true frogs, the northern leopard frog (Rana pipiens) probably is the kind you dissected in biology class. Reaching 1/2 inches in length, they are green or tan above, with large dark spots with light borders. They emit a deep irregular snore that may be imitated by rubbing a thumb over an inflated balloon.
The call of the green frog explodes with just a single note akin to the sound of a banjo string breaking. Four inches long, this common frog has green or olive brown above, usually has bright green on its upper lip, and prefers permanent water.
The top frog hereabouts is Rana catesbeiana, the bullfrog. These animals grow to 8 inches in length, have teeth, and will chase anything that moves
–”I`ve seen them eat mice,” notes Nan Peters. The bullfrog`s call is not the stereotyped ”ribbit,” but more like a bovine bellow–”jug-o-rum.”
Male bullfrogs are highly territorial and fight to seize spawning areas. They prefer places least affected by extremes in temperature, parts of a pond that also favor embryonic development and offer protection from leeches. The biggest, toughest male bulls establish their turf, but grudgingly accept other bullfrogs as neighbors and join together in ad hoc juntas to run off invaders. Bullfrogs have two important calls. The first is the distress call, a horrible and unforgettable high-pitched wail made when the frog is grabbed by a raccoon or other predator. Whether the call is emitted in terror, or to warn other frogs, or as an attempt to startle the predator is not known.
The release call, though, is well understood.
Male bullfrogs have a particularly tough time determining the sex of strangers in the night. Should a strange frog bump into a calling bullfrog, the bull will quickly clasp it, hoping it is female.
If the male is correct, the female emits no sound and nature takes its course. But should the clasped frog be a male, or a female unwilling to mate, it will emit a distinct release call lasting several seconds. Whereupon the bull lets go. Many frog species breed this way, indiscriminately grabbing without choosing specific mates.
The significance of the release call was proven a few years ago by researchers at the American Museum of Natural History in New York City. After partially filling balloons with water so they would float, scientists tied strings to the balloons and dragged them near calling bullfrogs. The frogs grabbed the balloons. Because no release call was possible, the bullfrogs sat there all night embracing their balloons and looking ridiculous.
Researchers have shown that frog eardrums are tuned to detect only the mating call of their own species. A bullfrog`s ear, say, is tuned to a different frequency than that of a tree frog. Within members of a species, the tuning may be extraordinarily fine. Among cricket frogs, a female is tuned to her local station only. If she is from northern Illinois, she won`t mate with a male from Downstate.
Tiny cricket frogs, no bigger than 1 1/2 inches, are among the many frog species that don`t mate at random but become highly selective. A female cricket enters a pond. She discerns a mayhem of characteristic ”gick-gick-gicks.” How to single out that special voice in the crowd?
Many females appear to favor older males, perhaps because their very existence indicates good genes for survival. Because older frogs are larger, their voices resonate more loudly and deeply. In this way, they stand out.
Tempted, the female cricket frog will sit a few inches away from her swain, listening intently to his calls for an hour or more. When she finally has decided this is the frog for her, she swims the last few inches and touches him. He grabs her and they spawn, for as long as three hours.
Frog courtship is made even more complex because ponds are sure to contain males that are smaller but no less ardent. Among cricket frogs, such sneaky ”satellite” males often will hide near a calling male, but won`t utter a sound lest they get mugged by the alpha frog.
As the female swims by, a smaller frog snatches her. Sometimes the bigger male catches on and there`s a fight; occasionally the female is able to shake off, or otherwise elude, the rapist. Female bullfrogs, for instance, often dive under lurking males to reach the caller they choose. Among natterjack toads, any silent male that approaches a female will cause her to flee. But the fact of life among frogs is that a determined courtier, no matter his size, often gets the girl.
One North Carolina toad has developed a particularly ingenious mating strategy: Small toads have been observed sitting in deeper, colder water in apparent attempts to lower their voices. Some scientists challenge these observations, however, noting that it would take a huge temperature drop to change pitch significantly. Frogs are cold-blooded (poikilothermic) animals. Their body temperatures depend on the temperature of the surrounding air, which is why they hibernate in winter, and move sluggishly in cool weather.
Cool nights at a pond mean slower, fainter, lower-pitched calls. Female frogs, scientists believe, can detect such temperature frequency changes. The females swim toward hardier woodsmen in such situations, males that probably are sitting in water that`s warmer and nicer for mating.
Mating calls afford infinite variations and tricks. Male frogs in Sri Lanka, for instance, methodically try to match each other`s calls. Such dueling frogs may battle for many minutes. If neither contestant backs down, the contest ends in a brawl.
Panamanian tree frogs use complicated two-part calls consisting of whistles followed by clicks. A male whistles. His challenger also whistles, and raises him one click. The first frog calls, with a whistle and a click, then raises another click. And so goes the game, as a female sits idly by and ultimately chooses the frog with the best hand.
But eavesdropping males are quick to confuse the stakes. One may toss in a click, while the two players are whistling. Or, he may rhythmically add clicks to the concluding crescendo, trying to take credit for the call, much as a bogus marathoner occasionally pretends to win a race.
At Ryerson Woods, the frog walk is esthetic as well as scientific. After the lecture, the group hikes for 15 minutes over near Riverwoods Road. As cars incongruously thunder by above and behind them, the visitors creep into the forest until the ground becomes muddy. There, in the dark, they listen to tonight`s program: the chorus frogs and peepers playing out their destinies in the bog.
”This is great,” says Nan Peters. ”We do this only two nights, and we got skunked last year. We couldn`t hear anything.”
”A pretty sound,” someone says.
”A spring sound,” someone adds. ”Reminds me of sitting in a rocker on the porch.”
Just then comes a crack of thunder.
”Wonder how close that storm is?”
Very close, it turns out. The frogs are much too busy to notice, but the soaking rains send the frog walkers scurrying back to their cars. Yet nobody seems to mind.
The rains are warm.
