Frogs in a Frenzy: the Maryland Woodland Rite of Spring


March 20, 1992|By DON C. FORESTER

It was early morning, and a low-pitched, chortling sound was rising from behind a wooded ridge to my left.

To the inattentive it could easily have been mistaken as a flock of ducks frantically squabbling on the surface of a nearby farm pond, but to me it was unmistakable.

It was the sound of spring -- harbinger of longer days, bright flowers and green foliage -- it was the mating call of Rana sylvatica, known to native Americans as ''frog of the woods,'' and to contemporary Marylanders as the wood frog.

A smile pursed my lips as the sound waves slipped through an acoustical window installed in my ear by eons of natural selection; I made a mental note to return that evening.

Locating a breeding congress of wood frogs at night is, in theory, a simple undertaking, a matter of moving toward a point source of sound. Unfortunately, natural and man-made obstacles, such as multiflora rose and barbed wire fences, frequently create a tedious maze for the nocturnal frogophile.

And so it was that evening that a 15-minute walk through the woods became extended three-fold. When I finally arrived at the site, I discovered that it was little more than a pool of rain water trapped in a natural depression between two wooded slopes. But its size and ephemeral nature seemed unimportant, for on that night it was the center of the universe for some three score wood frogs.

Wood frogs are opportunistic breeders, employing a reproductive strategy described by behavioral ecologists as scramble competition. This means that they concentrate their reproductive effort into a few evenings each spring. During these periods, males call incessantly while swimming about eagerly grabbing anything that moves.

Actually, they seem perfectly suited for this type of clandestine activity, for each frog bears a distinctive black marking that covers its eyes like a party mask, and as they swim about frantically seeking a partner, they resemble participants at a large masquerade ball.

In a perfect world, a male wood frog would grab only a female, but in the hectic, confused atmosphere of the breeding pond, they are just as likely to grab another male. When this happens the indiscretion is effectively communicated through a release call and a sharp kick to the face.

In some instances males even embrace inanimate objects such as discarded beer bottles bobbing on the pond surface, but such mistakes, if persistent, have a way of taking care of themselves in as little as a single generation.

Once a male and female frog have paired up, the female carries her mate to a location in the pond where other wood frogs have already laid their eggs. The new egg mass is carefully added to the submerged pile and the adult frogs depart.

Scientists have long been curious about the adaptive value of communal egg laying, for it seems to court disaster by placing all of the population's eggs in a single basket.

Actually the advantages outweigh the risks, for wood frogs lay their eggs in shallow, vernal ponds which are prone to freezing and evaporation. Eggs in the center of a communal mass are insulated from early spring freezes, and in the event of receding pond levels, can survive terrestrial strandings for as long as two weeks.

It was nearly midnight when I finally returned home that night. Muddy and chilled, I had observed nothing during the preceding three hours that I hadn't seen a dozen times before. As I prepared for bed, I knew that the morning would bring stiffness to my middle-aged body, but at the moment, it mattered little.

For my spirit had been lifted from the winter doldrums. It was once again spring in Maryland.

Don C. Forester teaches biology at Towson State University.

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