From The Staff

The Giant Tadpole That Never Got Its Legs

A record-breaking, 10-inch-long whopper of a bullfrog tadpole was discovered by a crew of ecologists in a pond in Arizona.

August 14, 2018

From The Staff Biology Embryology Natural History Zoology

The biggest tadpole ever found—at a whopping 10 inches long—was discovered by a crew of ecologists in a pond in the Chiricahua Mountains of Arizona. Alina Downer, an intern at the American Museum of Natural History's Southwestern Research Station, came across the monster bullfrog tadpole as her crew was draining a manmade pond as part of a habitat restoration project for the endangered Chiricahua leopard frog.

As the water level lowered, Downer and her colleagues were assessing what organisms were left in the muddy shallows that she likened to “chocolate soup.” Downer says, “I was fishing around with my hands while walking in the water, and I felt something large, smooth, and wriggly—which was unexpected, since the only other fish in the pond were about an inch long.”

As an avid naturalist, Downer’s first instinct was curiosity. “At first I thought it was a giant catfish,” she says, grinning at the uncanny memory. “Whatever it was, I knew I had to grab it.” She herded the slippery creature into shallower water until she could capture it. To her surprise, it turned out to be “an enormous monster of a tadpole”—so big she had to hold it with two hands.

Photograph here and in header above courtesy of Alina Downer.

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At a length of 257 millimeters (10.1 inches) and counting, the gigantic pollywog beats out the previous recordholder for bullfrog (Rana catesbeiana) tadpole size by 67 millimeters (2.5 inches) [scratch that: 60 millimeters (2.4 inches)], and the largest record for an African clawed frog (Xenopus laevis) tadpole by 37 millimeters (1.4 inch). It is larger in circumference than a can of soda. Its head is around the size of a big adult bullfrog, but is round and blunt-faced like a tadpole, with fishy tadpole lips rather than a frog mouth. David Pfennig, a biologist at the University of North Carolina with expertise in frog development, has taken a look at the tadpole and says, “It has the features of a very young tadpole—one that might have normally only recently hatched out from the egg. But of course, it is many times the size of a newly hatched or even mature tadpole.”

No one knows how old the tadpole is. Most bullfrog tadpoles live two to three years before metamorphosing into adult frogs, sprouting legs after about a year. This tadpole either vastly outpaced its cohort in growth, or it has stayed a tadpole long after its cohort sprouted legs and moved on. The largest African clawed frog tadpoles never matured into frogs and lived as long as eight years.

Photograph on the left courtesy of Michele Lanan, and photograph on the right courtesy of Alina Downer.

Pfennig notes that it’s normal for a tadpole that matures more slowly to grow more rapidly, but even so this one is a definite outlier. “I’m guessing that this guy will never transform into a frog,” he says. This stunted development and impressive growth could be because something has gone wrong hormonally. Pfennig says, “The normal developmental hormones, such as thyroid hormones, may be ‘turned off’ or downregulated, while the growth hormones may be ‘turned on’ or upregulated.”

Photograph courtesy of Michele Lanan.

After its discovery, the tadpole has become a popular photo op at the Southwestern Research Station, where it lives in an aquarium and is regularly fed its favorite algae. Downer says, “I'm curious if it's going to keep getting bigger, and what the life span will be like.” Although the tadpole's growth rate has slowed down since it was found in the spring, all-in-all it seems quite robust.

Downer, Pfennig, and other researchers at the Station plan to continue to monitor the monster tadpole to answer some of their remaining questions about its life history, genetics, and development. Pfennig says he will take some tissue samples back to his lab in North Carolina to learn more. Although the absurdity of this one-of-a-kind tadpole has inspired plenty of laughter, it also is inspiring some pretty serious science.

This story was created with travel support from the American Museum of Natural History’s Southwestern Research Station. Thanks to SWRS research and education director Michele Lanan for providing references for fact-checking.

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