Lust Among the Bufonids

DON C. FORESTER

March 20, 1991|By DON C. FORESTER

It was nearly midnight, and as I neared home a steady, warm rain had commenced to fall. Its union with the cold earth created a ghostly panorama in which the road seemed covered with mounds of cotton candy swirling surrealistically over the hood of my vehicle.

I had spent three hours vainly searching the rural wetlands of central Maryland for that warty harbinger of spring, the American toad, Bufo Americanus.

As I eased into the carport, the low beams of my aging Toyota cut through the fog like a pair of giant lasers and exposed a container placed squarely in the center of the board designed to catch the oil drips from my car. The diffuse light created a backdrop for the opaque jar and it immediately was evident that it contained at least one life form, maybe more.

Approaching cautiously, I eased up the lid to reveal the jar's contents -- five toads, two pairs engaged in a sexual embrace and one neutral observer.

It was a gift from Bernie, my good-natured neighbor.

For more than a decade he has been engaged each spring in a contention with the toads over his swimming pool. Male toads ascend the steps to the pool each evening to advertise their sexual prowess to unseen females. Too frequently, the plaintive calls are answered and the vinyl pool cover is strewn with sticky strings of toad eggs.

Actually, the eggs weren't so bad. It was the tadpoles that hatched from them that Bernie couldn't abide. Invariably they graduate from the small puddle of rain water trapped on the cover to the expansive pool below. This creates domestic conflict, since the ladies of his family refuse to share the pool with larval toads.

Bernie's epic struggle recalls to my mind my own interest in the American toad.

During the early 1980s I had been studying a small tree frog known as the spring peeper. One of my study sites included an old cistern on the Towson State campus. It had once been the principal source of water for Sheppard Pratt Hospital.

Frequently, as I walked along a gravel road that led to the cistern I would observe toads locked in a passionate embrace right in the middle of the road. More often than not, by the time I arrived this intimate act had degenerated into an orgy, and I would find a writhing mass of toads slowly tumbling along the road.

As a college professor I am somewhat habituated to overt displays of explicit sexual behavior, but I found these toad balls to be particularly interesting. What was the evolutionary significance of this unusual behavior? If females pair up on land, are males wasting their time and energy calling from the water?

With the help of several dedicated undergraduates, I set out to unravel the mysteries of toad reproductive behavior. Now, after three years, the pieces of the puzzle have begun to fall into place.

On or about the vernal equinox toads begin migrating from their terrestrial retreats toward ponds or other suitable aquatic breeding sites. Males arrive first and females follow, a few at a time, over several weeks. Within 48 hours a large site may harbor as many as 500 males.

Although each is preoccupied with procreation, their reproductive strategies are varied. On any given night a few males (usually the largest) position themselves about the margin of the pond and commence to utter musical trills designed to attract receptive females.

While these stationary males vocally debate their relative fitness for all to hear, the remaining males swim erratically about the pond grabbing anything that comes within reach. During these periods of lustful frenzy males often grab one another.

To assure that these errors in sexual assessment don't get out of hand, nature has empowered each male with the ability to vibrate his body in a manner that communicates the simple but unmistable message:''WRONG!''

While the bufonid armada is noisily struggling for naval supremacy, a band of small, shifty-eyed, vagrant males takes up positions around the perimeter of the pond. Refusing to participate in the time-honored tradition of anuran vocal advertisement, these diminutive males sit patiently facing away from the water. Their sole purpose is to intercept females responding to calling males. I have dubbed this male strategy ''gantlet behavior.'' Using chemical and seismic cues to identify their quarry, gantlet males successfully capture 70 percent of the females before they reach the pond.

One important question remains to be addressed. Do males which capture females on land actually fertilize their eggs once they reach the pond, or are they displaced by a larger, more aggressive rival? The answer to this question has me waiting with anticipation for the first rains of spring.

Don C. Forester teaches biology at Towson State University. His personal stationery supplies the border of anurans (having no tail, characteristic of frogs and toads).

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