When Carlos Jared was first ‘stung’ by the venomous face of the Greening’s frog, he didn’t realise what had happened. He had picked up one of the small creatures, and it started thrashing about as if trying to headbutt his hand. At first, it felt like being abraded by rough sandpaper. Then, Jared quickly developed an intense pain, which radiated up his arm and lasted for five hours. And since he was four hours away from any major city, he just had to grin and bear it.
Jared didn’t initially connect the pain to the little frog he had picked up. After all, there was no obvious injury. Only later did he figure out what had happened: The frog had released toxins from its skin, and used the small spines that line its skull to drive those poisons into his hand. He had received a venomous headbutt.
Greening’s frog, also known as the casque-headed tree frog, was first described in 1896. It’s a palm-sized animal with mossy green skin and a distinctive flattened head. The many bones of the top of its skull have fused together, and merged with the lower layers of skin to create a single, sturdy plate.
Partly, this is an adaptation against dehydration. Greening’s frogs (like all of them) depend on water, but they also live in the extremely dry Caatinga forests of Brazil. Their solution is to shuffle backwards into holes and then plug the entrance with their bony helmets. That seals their bodies in humid spaces and stops them from losing too much water.
Since the skull is both flat and well camouflaged, it’s very hard for predators to spot the frogs, let alone pull them out of their hollows. And if any predators are tempted to try, the lip region of the skull is covered in wicked-looking spines. With all the flesh removed, the skull looks a bit like a medieval mace.
Jared experienced firsthand what these spines can do when he started collecting the frogs in Caatinga. He was lucky to only get hours of pain. He soon discovered that the frogs can release a white, toxic mucus from glands in their skin, which can be lethal when swallowed. These glands are especially large in the frog’s face, where they sit over the skull spines. So, when threatened, the frogs can slather their own face with toxic mucus, and then drive it into an antagonist’s flesh using their own skull—a tactic that’s aided by their unusually flexible necks.
So are these frogs poisonous or venomous? Poisonous animals have no way of injecting their toxins, although they might be able to secrete them. That’s the case for most toxic amphibians. “They have a passive defense, only offering their own bodies (or parts of them) to be bitten by the predator or aggressor,” says Jared. “The predator is responsible for its own poisoning.” By contrast, venomous animals can actively deliver their toxins into an enemy or victim using stings, spurs, or fangs. That’s the case for spiders, snakes, and scorpions.
The casque-headed frogs clearly fit in both categories. They can release toxins through their skin—a passive defence. They can also introduce those toxins directly into wounds using their spines—an active defence. Regardless of semantics, what matters is that the defence works.
When tested on mice, the toxins from Greening’s frog proved to be twice as lethal as those of the notorious fer-de-lance snake, and other pit vipers that share the same forests. A second closely related frog—Bruno’s casque-headed frog—is even more dangerous. Its poison glands are smaller and its skull spines are shorter, but its toxins are 25 times more lethal than pit viper venom.
“During all these years that I’ve lived with these animals in their environment, I’ve never seen any sign of predation, or aggression by predators,” says Jared. He imagines that for a snake, swallowing these frogs would be like trying to wolf down a poisoned cactus.
Reference: Jared, Mailho-Fontana, Antoniazzi, Mendes, Barbaro, Rodrigues & Brodie. 2015. Venomous Frogs Use Heads as Weapons. Current Biology http://dx.doi.org/10.1016/j.cub.2015.06.061