First known venomous frogs stab with toxin-dripping lip spikes

Brazilian amphibians carry venoms more deadly than those of pit vipers

yellow-skinned frog

WORST HEAD-BUTT  When this yellow-skinned frog curls back its plump upper lip, bony spikes underneath stab toxins into foes. 

Carlos Jared/Butantan Institute

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Carlos Jared discovered the first known venomous frog by accident. And it took him a long time to connect his pain with tree frogs that head-butted his hand.

Jared, now at the Butantan Institute in São Paulo, got his first hint of true venom when collecting yellow-skinned frogs (Corythomantis greeningi) among cacti and scrubby trees in Brazil’s dry Caatinga region. For hours after grabbing the frogs, intense pain radiated up his arm for no obvious reason.

He knew frogs have no fangs to deliver toxin. Many frog species can poison an animal that touches them, but they’re poisonous.  True venomous animals actively deliver toxins.

Jared realized head-butting delivers venom only when he saw the frogs’ upper lips under a microscope. Bone spikes erupted near venom glands that looked “giant,” he says. As a frog’s lips curl back, glands dribble toxins onto spikes sticking out from the skull and the frog pokes them against foes.  

Gram for gram, the frog venom is almost twice as dangerous to mammals as typical venom of the feared Bothrops pit vipers, Jared, Edmund Brodie Jr. of Utah State University in Logan and their colleagues report online August 6 in Current Biology.

The researchers also report a second spiky-skulled venomous frog, Aparasphenodon brunoi, which is a forest species not very closely related to yellow-skinned frogs. It head-butts toxins 25 times as powerful as typical pit viper venom, a phenomenon luckily not discovered by handling.

Accidents are how most venomous animals first come to scientific notice, Brodie says. Early in his career, he discovered details of fire salamander venom by tickling a new specimen with a piece of grass. He was showing students how toxins ooze from its skin and “it sprayed me right in the eye,” he says. “I was immediately blinded.”

“I ran to the sink and ran water in my eye for about 20 minutes,” he says. “The toxin isn’t water soluble, so it didn’t help much. It was extraordinarily painful,” he notes in mild tones. Also, “the first time you observe something like that, you’re not sure it’s temporary blindness.” It was.

Venomous amphibians may be more common than people expect, Brodie says. Now that the researchers know about bone points for venom delivery, they want to investigate some salamanders with ribs that punch through the skin. And at least three more frogs grow suspicious spines around their heads. “It’s not Kermit anymore,” he says. 

Editor’s Note: This story was updated on August 13, 2015, to clarify the habitat differences between the two venomous frogs.

WHO NEEDS FANGS An edging of bony spikes along the front turns the skull of a yellow-skinned frog into a weapon for venomous head-butts. Carlos Jared/Butantan Institute
MAKING POINTS A close-up of the yellow-skinned frog’s attacking edge shows the bony spikes covered by skin that’s turning yellow with toxin ready for the stabbing. Carlos Jared/Butantan Institute

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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