The American Bullfrog: The Frog that Eats Its Own

I wrote a story about a frog… and it is honestly one of my favorite stories out of everything that I have ever written! It’s about the American Bullfrog and, without giving away too much detail, the scientists that I encountered while writing this piece were just as peculiar as the frogs themselves.

Imagine brief, hot summer rains in the American southwest. The hum of cicadas hiding amongst the trees, the howl of a pack of coyotes – and then an obnoxious honk. Along the Atlantic American coast, this honk would be commonplace and multiplied by the hundreds. But in the southwest, the noise alerts local wildlife of a dangerous predator.

The honk also inspires Dr. Cecil Schwalbe, a former herpetologist with the University of Arizona and United States Geological Survey, to grab his gun and go frog hunting. The call comes from the American Bullfrog, which is a dangerous invasive species in the southwest and a predator of native desert wildlife.

The American Bullfrog (Image from the National Aquarium)

The American Bullfrog is olive green in color, with tan or brown spots on its back and larger spots or stripes along its hind legs. It can be equal to or larger in size than the average human palm. It is also common to find one with a foot sticking out of its’ mouth.

“There’s not a single food class they haven’t attacked,” Schwalbe said. “The diet of the frog in Arizona is amazing. We found, in the stomach of frogs, every vertebrae class that occurs in Arizona. There were mammals in there, rodents, even the bats that they ate. They eat snakes, they eat lizards, they even eat baby rattlesnakes; they eat turtles – they eat baby Sonoran Mud Turtles. One of their favorite foods are tarantulas.”

The bullfrog also cannibalizes weaker bullfrogs, along with other native toads and frogs.

Don Swann, a former student of Schwalbe at the University of Arizona and now a researcher of the Chiricahua Lowland Leopard Frog with the United States National Park Service, observes the firsthand effects of the bullfrog on local desert ecosystems.

“The bullfrog eliminates native aquatic animals [and] devastates local ecosystems,” Swann said.

The bullfrog is a major threat to the leopard frogs that Swann researches. His work started in long-term monitoring of local desert riparian areas and the reptiles and amphibians that reside within them, which then turned into a lifelong project.

“They eat them,” Swann said, regarding bullfrogs and leopard frogs. “They’re not evolved with bullfrogs. The southwest was not prepared.”

Swann grew up along the east coast of the United States, where bullfrogs are plentiful. He has learned to love the frogs and their call, which he associates as the bullfrog trying to run away from other predatory fish and wildlife.

That is one of the issues that the southwest faces: there are not many predators of the gigantic bullfrog. Birds of prey and larger mammals can prey upon the bullfrog, but their populations are minor compared to the number of small rodents, amphibians, reptiles, and insects that the desert houses. This means more food and opportunity for the bullfrog.

“But you can’t blame the individual bullfrog, because they’re just doing what bullfrogs do,” Schwalbe said.

But native desert species do have one adaptation that the bullfrog has not yet acquired: extreme heat and weather resistance. Swann explained that the bullfrogs cannot survive without seasonal flooding, not extreme flooding, in order to travel throughout dry desert landscapes to catch prey and expand their populations.

In fact, the American Bullfrog would not even be located in the southwest if humans had not introduced it.

“During the gold rush, humans put bullfrogs in boxes on trains and shipped them to California, and introduced them into ponds there,” Schwalbe said. “The miners out there living on the land had caught and eaten many of the California Red-Legged Frog, and so California thought since the bullfrog was bigger and it was introduced there, it will give more food for the miners and rural people.”

“They didn’t think at the time about the catastrophic effect it will have on the local ponds,” Schwalbe said.

And so, with his assistants in tow, Schwalbe tracked the invasive bullfrog to protect local ecosystems. While also using his gun, Schwalbe used pond draining, trapping, hand grabbing, and spearing to catch and remove the bullfrogs and their eggs. He would locate the bullfrogs in a landscape and systematically remove them from the area. In two days, Schwalbe and his companions would remove upwards of 5,000 bullfrogs. Their efforts have eradicated the American Bullfrog from local parks, public lands, and downtown Tucson area, and forced the bullfrog to travel to the highlands.

Biologists like Schwalbe first began to hunt bullfrogs from wildlife parks in 1985, when the United States Fish and Wildlife Service stopped stocking ponds with the bullfrogs for game. By then the bullfrog populations began exploding out of control and Schwalbe explained that if researchers had not been exterminating bullfrog populations, especially as heavily as they did in 2010, then they would still be a problem today.

“Our efforts were highly successful but requires constant vigilance,” Schwalbe said. “All the bullfrog requires in Arizona is a pool of water.”

The American Bullfrog eradication comes as a stark contrast to southwest conservation ideals. Preserving biodiversity of desert flora and fauna is a goal of biologists like Schwalbe and Swann. As temperature rise annually and there are more erratic weather patterns, many native species are finding it difficult to adapt to this change in global climate. Yet somehow the bullfrog continues to thrive.

“We ended up calling them the Great White Shark of the inland waterways,” Schwalbe said.

Dr. John Wiens, a University of Arizona herpetology researcher, agrees with Schwalbe’s “Great White Shark” theme and has a very strong opinion of the American Bullfrog.

“I don’t like them in Arizona because they’re not native and they are potentially disruptive to other species that are native,” Wiens said. “I don’t like them at all.”

Wiens’ research involves a phylogenetic, or the study of evolutionary history and relationships between organisms, perspective on reptiles and amphibians using research and hypotheses.

But no matter their faults, Swann and Schwalbe still find space in their hearts to appreciate the American Bullfrog.

“I really like them. I think that they’re interesting animals,” Swann said.

“I have incredible respect for the American Bullfrog,” Schwalbe said, “The bullfrog is just a frog out of place.”


This article was originally written in the fall of 2016.


Thank you for reading! What are your thoughts on the American Bullfrog? Or have you ever tried frog legs?! Let me know in the comments below and like this post if you want more like it!




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