The Only Two Frogs That Are Considered Venomous

Over the course of the history of life on Earth, animals have developed all kinds of unique and bizarre defense mechanisms. But the line between "poisonous" and "venomous" is often the most confusing, a distinction blurred to the point where the terms are used almost interchangeably. Scientists draw a critical distinction between the two, however, and understanding it can mean the difference between life and death in the animal kingdom (or kingdom animalia, if you're looking to be technical).

Poisonous animals are dangerous if the animal (and its toxins) is touched or consumed; venomous animals are those who can actively inject their toxins via a biological delivery system. The hundreds of species of poisonous frogs living in Central and South America, for example, have gained international awareness due to their iconic and spellbinding colors as much as their extreme toxicity and risk to humans and other animals.

But in 2015, researchers uncovered something extraordinary: two Brazilian frog species that could weaponize their toxic secretions with precision and intent. Their discovery has changed what scientists previously understood about the prevalence of venomous amphibians in the wild.

The stars of this scientific study, which was published in Current Biology, are Bruno's casque-headed frog (Aparasphenodon brunoi) and Greening's frog (Corythomantis greeningi), two species whose toxins are more potent than even the deadly genus of venomous pitviper snakes known as Bothrops. What makes these two species unique isn't just their toxicity; it's the sophisticated way they are able to deliver their toxins to their enemies. Both frogs possess bony spines around their skull, particularly near their nose, jaw, and eyes. 

When confronted by a predator, the frogs essentially headbutt them, flexing and extending their heads to thrust these tiny bones forward. The bones pick up venom as they pass through toxic skin glands and transfer it to the unfortunate animal before them. A predator on the receiving end of such a sting is in for a bad time. While collecting a specimen of Greening's frog for the study, one researcher was inadvertently injected with a dose of its toxin on the hand, causing "intense pain" to radiate throughout their arm for five hours. Luckily for the researcher, Greening's frog venom is the milder of the two species' weaponry, being "only" twice as powerful as that of Bothrops pitviper snakes. 

The paper's authors noted that these two species may be the tip of the iceberg, and closer study of other poisonous amphibian speciesmay reveal that they should be classified as venomous as well. If you're interested in learning more about rare plant and animal species, check out our explainers on what causes the extinction of plants and animals and how scientists plan to bring the Tasmanian tiger back from extinction.