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Technically speaking, almost all frogs are poisonous in the sense that they have glands in their skin that secrete noxious substances. These substances evolved to discourage predators. Whether they're toxic to human beings is another matter, and depends on the particular frog, the type and concentration of glandular substances, and how they're delivered. South Africa is home to several frog species that are dangerous to human beings.
Don't Eat the Toads
Toads are a specialized group of frogs whose primary habitat is terrestrial. All but a single known species are poisonous. South Africa is home to at least five species of the Bufo genus, the so-called "true toads": the gutteral, flat-backed, olive, raucous and northern pygmy toads. All bufids secrete a milky white psychoactive poison called bufotoxin from their parotid glands. South Africa has an additional toad, Schismaderma carens, the red toad. Red toads have no visible parotid glands, yet potentially lethal cardiac toxins have been isolated from their bodies and eggs.
Don't Eat the Frogs, Either
Unfortunately for the African bullfrog, Pyxicephalus edulis, he's considered a culinary delight among human beings who share his region -- his scientific name even means "edible" -- so you might reasonably assume he isn't poisonous. Yet the jury is still out on P. edulis' toxicity. Eating pre-pubescent frogs can cause extremely painful urinary inflammation, but it's unknown whether this is related to poison or to a bacterial infection youthful frogs may carry. A similar situation exists for the Hyperolius genus, the water-lily, tinker reed, and painted reed frogs. These guys have bright warning coloration and apparently kill livestock specimens that eat them accidentally, though their specific toxins have not yet been identified or confirmed.
Touching May Be Bad News, Too
South Africa's Phrynomantis bifasciatus, the red-banded or banded rubber frog, is definitely best admired from afar. Distressed frogs secrete a cardiac toxin lethal to other frogs -- and to human beings if it comes in contact with broken skin or a mucous membrane.
Even for Other Frogs
Some frogs defend their habitat by poisoning competitors. Cacosternum boettgerri, the common caco, along with other members of her genus, secretes a poison toxic to other species of frogs.
Eye ... Er, Mouth ... of the Beholder
Assessing whether or not a frog is poisonous is no easy task -- unless you learn the hard way. Scientists don't yet know about all the compounds frogs secrete or their ultimate purposes. Test results can be surprising. For example, Xenopus laevis, the common platanna or African clawed frog, is a common research animal and aquarium pet, considered completely nontoxic. Tests of their skin show it produces 5-hydroxytryptamine: serotonin. Why? We don't know yet. Even though serotonin's a human mood elevator, please don't put the frogs in your mouth.
- Amphibian Specialist Group: Ensuring a Future for South Africa's Frogs - A Strategy for Conservation Research
- Eco Travel Africa: The Frogs and Toads of South Africa
- Animal Diversity Web: Hyperolius Viridiflavus
- South African Medical Journal: Noxious Toads and Frogs of South Africa
- Science in Africa: The Delicacy of Giant Bullfrog Eating in Namibia
- African Amphibians Lifedesk: Cacosternum Capense
- Cell and Tissue Research: Development of Xenopus Laevis Skin Glands Producing 5-Hydroxytryptamine and Caerulein
- Hemera Technologies/PhotoObjects.net/Getty Images