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Many species of frog are known to be poisonous, such as the often colourful poison dart frogs. But although incredibly rare, there are species that are venomous too.
There are currently only two species of frog known to be venomous: Bruno's casque-headed frog (Aparasphenodon brunoi) and Greening's frog (Corythomantis greeningi).
Both are found in Brazil and produce skin secretions like other poisonous frogs - but these species have spiny bone protrusions that make their poison venom.
In the video below, watch Dr Jeff Streicher explain how headbutting is an effective defence for these frogs.
In 2015, a team of scientists reported that the Greening's frog and Bruno's casque-headed frog, both endemic to Brazil, have a significant defensive advantage when it comes to transferring their sticky, toxic skin secretions. Unlike other toxin-secreting frogs such as poison dart frogs, these two species have bony spines on their skulls.
When pressure is applied, the spines pierce their skin. The frogs are considered venomous as the toxic skin secretions that coat these spines can inject venom via a wound in the skin of would-be predators - including humans.
One of the scientists involved in the study of the frogs was injected while collecting the animals. They felt the pain radiate through their arm for five hours. Fortunately they were handling the less toxic of the two: C. greeningi.
Such an intense response in a human victim led the scientists to believe that the venom would be all the more effective if injected into the mouth lining of a predator.
When restrained, both frog species were found to have the ability to flex and twist their heads in ways that other species can't, which means more opportunities to inject their captor.
The venom of Bruno's casque-headed frog is an estimated 25 times more potent than that of Fer-de-lance pit vipers from Central and South America (genus Bothrops).
The venom of the Brazilian Greening's frog is twice as powerful as Bothrops vipers. But this species' head spines are more developed and its skin glands are larger, allowing more venom to be secreted and injected.
Most frogs are known to produce toxic secretions, as do some salamanders and newts. These animals mainly use these poisons as a defence mechanism against predators, making them dangerous to be swallowed.
But the authors of the 2015 study suggested that there may be some poisonous amphibians that, if studied more closely, may actually be venomous.
Echinotriton salamanders were among the suggested species. They have spiny ribs that protrude through their skin, so their poisons may be imparted through a wound as venom.
The authors also suggest that other frogs with spines in the head region could be studied to identify whether they too are venomous. These include the spiny-headed tree frog (Anotheca spinosa) and Ranwella's horned tree frog (Polypedates ranwellai).
P. ranwellai belongs to a genus of frog known to have deadly tetrodotoxin in its skin secretions - although to date the Ranwella's horned tree frog has not been tested for this toxin.
So although uncommon, there may be more venomous amphibious species yet to be formally identified.