Although most amphibian species produce or sequester noxious or toxic secretions in the glands of the skin to use as antipredator mechanisms, they have been considered poisonous rather than venomous because delivery mechanisms are absent.
The frogs in question – the Greening’s frog (Corythomantis greeningi) and the Bruno’s casque-headed frog (Aparasphenodon brunoi) – produce potent toxins and also have a mechanism to deliver those harmful secretions into another animal using bony spines on their heads.
“Discovering a truly venomous frog is nothing any of us expected, and finding frogs with skin secretions more venomous than those of the deadly pit vipers of the genus Bothrops was astounding,” said Edmund Brodie, Jr. from Utah State University, a team member and a co-author on the study.
The Greening’s frog and the Bruno’s casque-headed frog have both been known for many decades, if not centuries. But scientists have known little of their biology.
The team’s calculations suggest that a single gram of the toxic secretion from the Bruno’s casque-headed frog would be enough to kill more than 300,000 mice or about 80 humans.
“It is unlikely that a frog of this species produces this much toxin, and only very small amounts would be transferred by the spines into a wound. Regardless, we have been unwilling to test this by allowing a frog to jab us with its spines,” said lead author Dr Carlos Jared from the Instituto Butantan in São Paulo, Brazil.
The researchers, whose findings are published in the journal Current Biology, only discovered they were venomous while collecting amphibians for study. Jared was injured by a spine from Corythoimantis greeingi while handling it, leading to intense spreading pain in his hand for around five hours. Fortunately for him, the frog was the less toxic of the two. He said: 'The action should be even more effective on the mouth lining of an attacking predator.'
The researchers say they have still to find out exactly how much toxin the frogs can deliver in one go. However, they believe there may be other species of frog that are also venomous. Dr Brodie added: 'It is unlikely that a frog of this species produces this much toxin, and only very small amounts would be transferred by the spines into a wound. 'Regardless, we have been unwilling to test this by allowing a frog to jab us with its spines.'
“The new discovery is important for understanding the biology of amphibians and their interactions with predators in the wild,” the scientists said.
Jared et al. 2015. Venomous Frogs Use Heads as Weapons. Current Biology, doi: 10.1016/j.cub.2015.06.061