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Frogs on the Move: A Living Fossil Offers New Insight into the Evolution of Jumping in Frogs

 Frogs on the Move

Most of us can probably recall learning about frogs at some point in a science class, perhaps during discussions of tadpole metamorphosis or through frog dissection. These nearly universal experiences attest to the importance of frogs as model organisms in biology.

Given this prominence, you might think that biologists would have a good understanding about something as fundamental as their movement. Surprisingly, that is not the case.

Many components of the unique musculoskeletal system of frogs remain an enigma. Moreover, most of what biologists do know about locomotion in frogs is based upon a few highly specialized species that are not representative of the majority of frogs. Dr. Richard Essner, associate professor of biological sciences, is intent on figuring out not only how frogs move, but also how they evolved their impressive locomotor repertoire from generalized salamander-like ancestors.

Essner’s interest in frogs began at an early age. “I am a lifelong herp nerd,” he said, referring to his passion for amphibians and reptiles. “I spent countless hours outdoors as a kid searching under rocks and logs.” Later as an undergraduate, he became interested in the relationship between animal form and function—an area of biology known as functional morphology.

He recalls taking a herpetology course and being especially amazed at the diversity of frogs. “I couldn’t believe that there were more frogs than mammals—over 5,000 species! It was clear to me, even then, that the key to the evolutionary success of frogs is locomotion.” It was during this same course that he learned about and became fascinated by one particular frog species, the Tailed Frog, widely considered to be a living fossil, due to their possession of an array of primitive morphological features.

Tailed frogs are found only in the Pacific Northwest of the United States and Canada and belong to an ancient group (Family: Leiopelmatidae) that diverged from all other frogs more than 200 million years ago. Essner and his students have been conducting detailed studies of Tailed Frog locomotion in the lab using high-speed video. They compare their locomotion with that of more advanced frogs, in hopes of gaining insight into the evolution of jumping.

Their work recently received media attention, following publication of an article about the landing behavior of frogs. Their research was given a press release and the cover of the German science journal, Naturwissenschaften. The study demonstrated that leiopelmatids are unique among frogs in their lack of controlled landings. Essner and colleagues from Ohio University and the University of Otago in New Zealand found that leiopelmatid frogs do not land on their limbs like other frogs, but instead do an aquatic-style bellyflop landing, even in terrestrial environments. Their findings suggest that jumping evolved as a two-step process—with the launch phase of jumping appearing early in the evolutionary history of the group and controlled landings appearing later, following the divergence of leiopelmatids from all other frogs.

Essner is continuing his collaboration with the Reilly lab and the Ohio Center for Ecology and Evolutionary Studies (OCEES) at Ohio University. The team will investigate other aspects of frog locomotion, including musculoskeletal function. Their goal is determining the mechanisms responsible for differences in locomotor behavior among frogs. “We have been using electromyography to investigate muscle function, as well as high-speed 3D x-ray cineradiography to observe the internal movement of the skeleton,” Essner said.

They are especially interested in understanding how the pelvic region of the frog functions during locomotion. Frogs have unusual pelvic anatomy compared to other terrestrial vertebrates, with a greatly elongated pelvis, complete rearrangement of pelvic musculature, and fusion of tail vertebrae into a novel rod-like structure known as the urostyle. “The pelvic anatomy of frogs is strange. We really know very little about how frogs use their pelvis to engage in a diverse array of locomotor behaviors such as jumping, swimming and crawling,” Essner said.

Essner is teaching a field course during summer 2013 in the Idaho Panhandle, where he has previously collected Tailed Frogs. He hopes to learn more about their behavior in the wild and plans to have students conduct an ethological study of Tailed Frogs in their natural habitat. His current research has received funding from the SIUE Competitive Application Resubmission Initiative. An article about his frog research, geared toward children, will be featured in the September 2013 issue of Highlights magazine.

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