In the first chapter for my upcoming book Venomous (due out in 2016), I excitedly explain how nearly all the sundry branches of the tree of life have venomous leaves. I’m simply enthralled by the incredible diversity of venomous animals (and plants!) on this planet, from the tentacle-wielding jellies to the spiny scorpionfishes and, of course, the oft-feared and misunderstood snakes, spiders, and scorpions. But until today, there is one group that could not boast a single venomous member: the anurans, commonly known as frogs and toads. While there are plenty of poisonous ones, no one has ever found a venomous frog — that is, until now.
Venomous animals are natural biochemists that take toxic to a whole new level. While it is true that venoms and poisons are both toxins, the two terms are not interchangeable. All toxins cause harm in low doses; Poisons are substances that cause such harm through ingestion, inhalation or absorption. To earn the title of venomous, on the other hand, an animal has to do more than just have toxins — they have to have a means of wounding their intended victims to force those toxins upon them. That “wounding” can be caused by any weapon of choice; jellies and other members of the phylum Cnidaria use specialized stinging cells that shoot out hollow tubes in less than a microsecond to deliver their potent venom. Snakes and spiders use fangs, the venomous fishes use spines, and the newest members of the venomous family — the frogs Aparasphenodon brunoi and Corythomantis greeningi — use their spiky heads.
In a paper published yesterday in Current Biology, Carlos Jared and his colleagues describe two species of frogs which sport specialized bones that they use to deliver toxic skin secretions, making them the first venomous frogs ever described. The frogs themselves are not new species — scientists have known about them for some time — but no one had noticed that they use spiny projections from their skeleton to envenomate would-be predators.
Not suprisingly, such an incredible find has drawn quite a bit of media attention, some of which is pretty well done. But, as you might have guessed by the title, this post isn’t in praise of the paper’s coverage. Come on, journalists — this frog isn’t going to “kill you with a head butt.” They don’t have a “kiss of death.” They aren’t “deadly,” and the toxin they deliver with a fling of their cranium isn’t enough to “kill 80 people.”
Yes, it’s true that the the skins of these frogs contain potent toxic cocktails of proteins. Scientists often measure toxin potency by what is referred to as median lethal dose, or LD50 — the dose which kills half of the individuals (usually mice) who receive it. The LD50 of the venom from of A. brunoi when injected into the bodies of a mice (a route referred to as ‘intraperitoneal’) was 3.12 μg for venom from the head, and 4.36 μg for venom from the body. That was about ten times more lethal than the same venoms from C. greeningi (the LD50s of which were both roughly 50 μg). Since that dose was in mice that ranged from 18 to 20 grams in weight, we can calculate a milligrams per kilogram of body weight toxicity (a standard unit for comparison) which ranges from 0.16 to 0.24 mg/kg for A. brunoi and from 2.5 to 2.9 mg/kg for C. greeningi. Those are some pretty potent venoms, but they’re not anywhere near deadly enough to be of concern to us.
The news reports seem to be extremely excited about the fact that the authors compared the frog venoms to “pit viper venom”, with the journalists harping on the idea that A. brunoi venom is about 25 times as potent as “Bothrops species”. But the authors should have known better than to make such a specious comparison. There are more than 30 different species of Bothrops, each with its own venom potency; it’s not fair to lump them all together and assign some random “mean” value. It’s especially not fair to take a subset of less than half of those species (which doesn’t even include the most potent one!) and call it an average for the genus. Bothrops asper, the most notorious of the genus known by the common name fer-de-lance, has an intraperitoneal LD50 of 0.469 mg/kg — certainly stronger than C. greeningi, though admittedly half the strength of A. brunoi. Meanwhile, Bothrops itapetiningae, the São Paulo lancehead, injects a weak 74.4 mg/kg venom, which is a staggering 437 times weaker than the strongest frog!
Of course, potency isn’t everything. It doesn’t matter if it will take less than 15 milligrams to kill you if you will never, ever, ever get 15 milligrams into your system. One of the weaknesses of this study is that the authors were unable to determine just how much venom is delivered during an encounter. “As you can see of the photo of the live frog some spines are coated with secretion and others don’t seem to be,” explained Edmund Brodie, one of the paper’s co-authors. “The amount of secretion produced is copious but we don’t have an estimate of how much is carried on a spine.” This was due to the method of venom collection, where frogs were massaged while in a container of water to cause them to secrete venom into the fluid. The secretions from multiple frogs were pooled and combined into one crude venom — so there isn’t even a venom per frog estimate. But Bryan Fry, a venom scientist with the University of Queensland who was not a part of this study, estimates the maximum dose from a single frog would be “less than 100 micrograms, and likely significantly less than that.”
Even if we take the high end, and say 100 micrograms per frog, you’d have to be stung by about 128 frogs to receive the LD50 dose from the head of A. brunoi. Even if you give the frogs ten-fold more toxin — an entire milligram — it still would take over a dozen of the animals to reach the LD50. A full-grown Bothrops asper, on the other hand, may have more than 1500 mg of its venom at the ready. Which means while you’d have to pull every drop of venom from dozens to hundreds frogs to hit that median lethal dose, each fer-de-lance is carrying enough venom to inject its LD50 dose into almost 720 people.
But why the comparison to Bothrops, anyway? They’re nowhere near the most potent snake. Oxyuranus scutellatus, the coastal taipan, has an intraperitoneal LD50 of 0.009 mg/kg, and can be milked for almost 900 mg of that venom in one sitting! So they can be slithering around with enough venom to inject more than a thousand people with an LD50 dose. And to put the frogs into perspective of non-snake venoms, the harvester ant, Pogonomyrmex maricopa, boasts an impressive LD50 of 0.12 mg/kg, while both the ant and frog venoms are weaksauce when compared to the 0.01 mg/kg venom of the box jellyfish, Chironex fleckeri.
And if you really want to talk deadly, did you know that tens of millions of people around the world face a toxin every year that’s more than 140,000 times as potent than the strongest venom from A. brunoi? You might have heard of it — it’s one of the nasty molecules from the bacterium Clostridium botulinum, better known by its medical name — Botox. My point is, knowing how strong a toxin is doesn’t tell us as much information as you might think.
Of course, that’s assuming the toxicity is accurate and meaningful to the route of administration, and to that end, there’s something very important to notice in the data. If you look at the experimental protocols and results for the other two studies they did with mice — whether the venoms induced swelling or caused the animals to fuss with an envenomated area — they injected mouse paws with a range of doses of venom and watched for up to 72 hours to see how things progressed. All of which seems perfectly reasonable, until you look closer at those doses: they injected mice with 0.125 to 32 μg. The dose that killed half of the mice they injected, remember, was 3.12 μg — so how did a dozen mice survive 72 hours with 2.5 – 10 times that amount of venom injected into their paw?
According to Brodie, “the effect on mouse mortality is very different depending on the site of injection.” And he’s right — injections under the skin (subcutaneous) can have different LD50s that ones into the body (intraperitoneal). Brodie also noted that “any dose above the LD50 would kill all mice,” thus based on the fact that mice received 32 μg of venom in the paw and survived at least 3 days, the subcutaneous LD50 is over ten times less potent than the intraperitoneal one. Which really makes the comparison to Bothrops even sillier, since the spikes in these frogs are tiny — they’re barely going to break the skin, and certainly aren’t going to allow for venom delivery into the body cavity of any animal, especially not large, thick-skinned ones like us. So what really matters isn’t the intraperitoneal LD50, it’s the subcutaneous one, which Brodie says the team didn’t attempt to figure out.
If you still want to compare to the snakes, I couldn’t find a subcutaneous LD50 for Bothrops asper, but Bothrops jacaraca has a subcutaneous LD50 of 7 mg/kg. The tiger rattlesnake (Crotalus tigris) has a subcutaneous LD50 of 0.21 mg/kg, and the inland taipan, Oxyuranus microlepidotus, is ten-fold stronger with a subcutaneous LD50 of only 0.025 mg/kg!
Regardless of how potent the venoms are, focusing on how many people a gram of the toxin could kill or calling the frogs “lethal” or “deadly” when there’s no evidence that they’ve actually killed anyone completely misses the point. I even asked the authors, and Brodie said there isn’t a single known case of them killing a hopeful predator, let alone a person. “This just gives the impression the frogs are ridiculously toxic,” says Fry, which isn’t why they’re awesome. These are the first venomous frogs known to science, and they’re probably not alone. If their spikey skeletons have gone overlooked for so long, then who knows how many potently poisonous frogs are actually venomous! That’s an incredibly huge finding, or as Fry told The Guardian, “unprecedented would actually be an understatement.”
When Fry spoke to at least one reporter, he cautioned about focusing on toxicity — apparently, his advice fell on deaf ears. “She chose not to include it, feeling perhaps that it detracted from the coolness of the story. Which it very much does not. It is an amazing discovery!” he wrote. Other venom scientists were equally frustrated. “I made exactly the same point!!!” commented Nicholas Casewell, a venom scientist from the Liverpool School of Tropical Medicine. He went on to say that the comparison with Bothrops was the only part of that paper he didn’t like.
Venomous frogs are way too awesome to be bogged down by lazy comparisons and ridiculous hyperbole. These little tree frogs deserve to be celebrated as the fascinating discoveries they are, not vilified as the killers we might wish them to be. They’ve been considered venomous for one day, and already, we’re calling them “deadly” and a suite of other fear-mongering terms. Can’t we talk about venomous animals without making it all about whether they’re going to kill you? And even more to the point, with all the potential pharmaceuticals being discovered in venoms nowadays, these little anurans may turn out to be life-savers rather than murderers. So how about we relax on the toxicity talk and just enjoy them for the wonders they are, eh guys?
Citation: Jared et al. (2015) Venomous Frogs Use Heads as Weapons. Current Biology, In Press, doi: 10.1016/ j.cub.2015.06.061