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Toxic Toads and Tyrannical Treefrogs

Florida's first known amphibian invader was the inch-long greenhouse frog (Eleutherodactylus planirostris), a soil-colored little creature that likely made its way over on plant shipments from Cuba in the mid-1800s. Greenhouse frogs are now an inconspicuous but widespread addition to the state fauna. The state's two other well-established exotic amphibians—the cane toad (Bufo marinus) and Cuban treefrog (Osteopilus septentrionalis)—make a far greater impression on Floridians. The toad grows to the size of a cow patty and eats almost anything that moves, including native snakes and frogs and cockroaches and beetles that it laps up while sitting beneath street lights at night. Unlike most amphibians, it scavenges too, eating pet food and sometimes even dog feces.

After decades of introductions, the cane toad, a native from southern Texas south well into South America, is now the world's most widely introduced amphibian (followed closely by the American bullfrog). The University of Florida and sugar cane growers intentionally released them in Florida several times to control insects that were eating cane crops, but an accidental release at Miami International Airport, likely in the 1950s, really helped them take off. Their impressive size and distribution aside, these flabby invaders are also highly toxic. "Every life stage, from egg to tadpole to adult, is toxic to some degree," says Kevin G. Smith, a University of Tennessee Ph.D. candidate who has studied the toxicity and Florida distribution of both cane toads and Cuban treefrogs.

The toads seem to know they are dangerous. "They'll lean and point their glands at you," says Smith, who adds that while the parotid glands behind the animals' eyes hold a concentration of the toxin, agitated toads exude small amounts all over their skin. "It looks like they're sprinkled with milk," says Smith, who has caught many a toad for his studies. Similar to digitalis, a cardiac stimulant made from the dried leaves of foxglove plants, the toad toxin attacks the heart of a predator. "Basically, it can induce a heart attack. With many dogs, it happens right away," says Smith. "Not all dogs die after biting them," says Florida herpetologist Kenneth Krysko, "but I recently had an 80-pound rottweiler die in my arms."

Australian conservationists are quaking in their boots as they document the cane toad's march across tropical northern Australia and into some of the country's most biologically diverse wild areas. Many fear that populations of native snakes and other predators will die out when they attempt to prey on the toads. Australian monitors have already been among the casualties. Back in the States, toxic toads are probably not as much of a menace to native wildlife. Red-shouldered hawks and crows have been seen flipping them over and tearing into their underbellies, apparently without being poisoned. "In Florida, predators are used to avoiding toads period, or some may have innate immunity to them. There are no native toads in Australia. Predators there have never experienced toad toxin like in the U.S.," says Smith, who notes that most native U.S. toads, when bitten, extrude toxins.

Cuban treefrogs have a toxin in their skin too. "People have been sent to the hospital after handling them and then rubbing their eyes," says Krysko. But the Cuban treefrog toxin cannot match that of the cane toad. "They're best described as distasteful," says Smith. But apparently not to all wildlife: Some native snakes, hawks, owls, and crows have been seen eating them with no ill effect.

Cuban treefrogs make a noticeable impact on native species through competition and predation. While scientists work on figuring out the extent to which large adult Cuban treefrogs prey upon smaller native frogs, Smith, funded by the U.S. Geological Survey, has been studying what happens just after the frog eggs hatch. Cuban treefrogs often lay their eggs in ephemeral pools that pop up after thunderstorms—small microhabitats free of fish and many other aquatic predators—that are also used by native frogs. Packed into close quarters, tadpoles must compete for limited food. Smith found that "Cuban treefrogs are outstanding competitors as tadpoles and they really do a job on native species." In the presence of the exotics, many native tadpoles never reach the frog stage of metamorphosis. Many of those that do turn out to be smaller than usual due to lack of food.

—Howard Youth

ZooGoer 34(3) 2005. Copyright 2005 Friends of the National Zoo.
All rights reserved.

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