News & Events

Hartwick's Sessions, Collaborator Solve Frog Leg Mystery

June 30, 2009

More than 20 years ago, Hartwick College Professor of Biology Stanley K. Sessions began studying frog legs--not their culinary traits, but their biological ones, as he sought to unravel the mysteries of development and evolution. With a recently published paper, Sessions and a colleague have finally solved an environmental puzzle that has confounded scientists for over a decade: Deformed amphibians.

Nearly 15 years ago, the presence of deformed frogs with missing or extra limbs became a major international topic of concern, as many thought these deformities were the result of UV irradiation or chemical pollutants seeping through the skin of the pond dwellers. Many years and millions of dollars have gone into identifying the chemical pollutant that might have caused these deformities, but without success.

During his postdoctoral work at the University of California's Berkeley and Irvine campuses, Sessions first took up the search for the cause. Shortly after coming to Hartwick in 1990, he solved the first bit, publishing his research indicating that extra limbs on frogs were caused by parasitic flatworms known as trematodes. While they were certainly more grotesque, the extra-limb deformity was far less common than frogs with missing limbs with no evidence of trematodes. What remained unanswered was what turned out to be the stickier question--what was the explanation for frogs with missing legs?

"People said, 'Okay, you explained the frogs with extra limbs, but what about these?'" Sessions said. "We thought it couldn't be predation because predators eat frogs, usually killing them in the process, and these frogs were otherwise healthy and intact. It turned out the missing limbs were much more difficult to explain than extra limbs, but I think we've finally cracked it!"

What Sessions, his students, and his collaborator Brandon Ballengée discovered was that the missing limbs were, in fact, caused by a predator--dragonfly nymphs, to be exact.

"What we've found is that these predators grab tadpoles and almost surgically remove their tender hind limbs with their mandibles," he explained. "They then let the tadpole swim away and it can metamorphose into a frog that's missing a hind limb or part of hind limb. We call this phenomenon 'selective predation" since the predators consume only selected parts of the prey. It seems pretty obvious in hind sight, but this solves the whole rest of the problem."

Sessions and co-author Ballengée of the University of Plymouth, UK published their paper "Explanation for Missing Limbs in Deformed Amphibians" in the most recent edition of the Journal of Experimental Zoology--the same journal in which Sessions' research on frogs with extra limbs appeared in 1990.

Throughout his Hartwick career, Sessions has worked extensively with undergraduate students in his laboratory. A recently completed National Science Foundation grant, for example, provided funding for six students each summer for four years, all of whom performed critical roles in Sessions' lab. In his time at Hartwick, Sessions has published about a dozen papers on deformed amphibians. Hartwick students have been co-authors on about half.

While this significant breakthrough answers at least one lingering question, namely what is actually causing the deformities, Sessions noted the results of this research do not eliminate the potential for at least the indirect involvement of environmental pollutants in causing these deformities.

"Rather, we see our results as underlining the leading hypotheses to be excluded when confronted with deformed amphibians, at least those featuring extra limbs or missing limbs," he said. "Are parasites sufficient to cause extra limbs? Yes. Is selective predation sufficient to cause loss or reduction of limbs? Yes. Are chemical pollutants necessary to understand either of these phenomena? No."

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Hartwick College is a private liberal arts and sciences college of 1,480 students, located in Oneonta, NY in the northern foothills of the Catskill Mountains. Hartwick's expansive Liberal Arts in Practice curriculum merges traditional liberal arts study, personalized teaching, and experiential learning approaches to emphasize Connecting the Classroom to the World. Add to that a wide range of off-campus internships, collaborative research, study-abroad opportunities, and a unique January Term, and Hartwick prepares students for the world ahead. Strong financial aid and scholarship programs keep a Hartwick education affordable.

Contact: Christopher Lott
Phone: 607-431-4030