Well, my headline might be a bit misleading, as we’re not sure whether the unusual spines of these frogs are used as (or evolved to be) deterrents to predators, but it’s highly likely. A researcher who grabbed one of the frogs I’m about to describe reports this sensation:
One of us (C.J.) was injured on the hand by the spines of C. greeningi while collecting frogs, causing intense pain radiating up the arm, lasting about 5 hr.
But I’m getting ahead of myself. What I’m reporting here is the appearance of something hitherto unknown: venomous amphibians. While toxic or poisonous frogs are well known (the most famous examples being the aposematically colored poison arrow frogs of Central and South America; see below), the biological definition of a “venomous” animal is one that is not only poisonous, but has a special delivery system to get the poison into either the predator (saving the animal’s life) or prey (killing for noms).
Frogs like the beautiful one shown above are considered toxic but not venomous, as the toxin is spread over the skin, and thus the animal lacks a “delivery system.” (One might quibble here and say that the skin is the delivery system, but I won’t pursue that issue.)
But now two species of truly venomous frogs have been discovered, both described in a paper by Carlos Jared et al. in the latest issue of Current Biology (reference below; apparently a free download). The species, Corythomantis greeningi and Aparasphenodon brunoi, collected in Brazil, have bony spines on the skull that protrude through the skin, and are surrounded by head glands that secrete a very powerful toxin. Since these frogs wouldn’t need such an adaptation to catch prey, this almost certainly evolved to deter predators. The salient results:
- The frogs have bony spines on their heads that protrude through the skin. These spines certainly get coated with the toxin secreted by glands on the head, injecting the poison when the head contacts a predator.
- The poison in these frogs, while not chemically identified, appears to be one that degrades proteins, and is extremely toxic. In A. brunois it’s 25 times more toxic than the venom of the neotropical pit viper Bothrops, a very dangerous snake; and in C. greeningi it’s twice as toxic as Bothrops venom. This was determined by injecting the toxin into mice, an experiment that I consider ethically dubious but obviously necessary to test the hypothesis. (Seriously, how many mice need to die in terrible agony to satisfy the curiosity of the researchers?) The LD50 of A. brunois venom, for instance, is only about 3-4 micrograms (millionths of a gram) when injected into a mouse.
Here are photos of the two frogs and the spines on their heads, with captions from the Current Biology article:
- The frogs also have a special behavior that facilitates delivery of the toxin to the predator. As the paper notes, “These frogs have an unusual ability to flex the head laterally and vertically, as compared to most other frogs, thereby facilitating contact between the spines in the rostral and posterior margin of the head and the hand grasping the frog. One of us (C.J.) was injured on the hand by the spines of C. greeningi while collecting frogs, causing intense pain radiating up the arm, lasting about 5 hr. This action should be even more effective on the mouth lining of an attacking predator.”
Here’s a scanning electron microscope picture of the head spines and surrounding glands:
That explains why the venom, which takes a while to kill a mouse when injected into the belly, could work almost instantly when the predator grabs it. Before a fatal bite would be inflicted, the predator would experience serious mouth pain. That would cause predators to learn to avoid these frogs, at least one of which (A. brunoi) appears to have aposematic “warning coloration.” If the toxic frog was always killed by the predator and only subsequently sickened or injured that predator, there would be little advantage for an individual to evolve toxicity, for a toxic frog wouldn’t leave more genes than a nontoxic frog. In such cases, though, the predator could still evolve to avoid the frog, for genes for recognition and avoidance would be advantageous in predators.
The authors conclude, reasonably, that venomous amphibians may be more common than generally thought. Over two years ago (was it really that long?) I reported about a salamander coated with a seemingly toxic slime, and which also could poke its ribs through its body wall. That might be considered a venomous amphibian in the same way as these frogs, as the ribs could possibly inject a predator with the slime. But Jared et al. note that the toxicity of that slime has not yet been demonstrated. So, for the nonce, we have two pretty solid cases of a venomous amphibian.
h/t: Dom
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Jared, C. et al., 2015. Venomous frogs use heads as weapons. Current Biology, online, DOI: http://dx.doi.org/10.1016/j.cub.2015.06.061
Interesting – and frogs always seem so placid looking. It’s all a trick!
Anti-nom-venom vs non-nom-venom.
Non-nom-venom.
I have sticky fingers today…
My first thought is that field biologists specializing in South American frogs ought to know better than to grab them bare-handed.
My second thought is to wonder whether the venom has antibiotic properties. Otherwise it seems like bony spines poking through the skin would leave the frogs vulnerable to infection.
I look at that illustration (1C) and think “that looks an awful lot like the skull structure of some early amphibians and late fishes”. which is going back a long way, but would also imply that despite these organisms being generally considered apex predators in their ecologies, actually had something else to worry about.
Possibly sharks – they were around, and large at the time.
I can’t remember the name of the relevant group. It’ll come back to me.
Then again, it could also be a dermal expression of a sensory system. That has been a common idea too (see Fig 27 here).
Field biology at its most interesting! I wonder if the toxin is derived from the frogs’ diet, as it is in the dart frogs. (And if native human populations found a way to use it for hunting!)
The case for Pleurodeles waltl using its sharp ribs as a venom delivery system is just as good as the case for these frogs. It too has special behaviors (which in its case bring the ribs to bear against attackers), and the toxicity of its skin secretions to mice was shown in essentially the same way as for the frogs. This has all been known since 1978, so these frogs may be the first known venomous frogs, but they are not the first known venomous amphibians. What’s a little curious is that Butch Brodie was an author of the 1978 Pleurodeles paper, and is also a coauthor of the one just published on frogs!
Nowak, Robert T., and Edmund D. Brodie, Jr. 1978. Rib penetration and associated antipredator adaptations in the salamander Pleurodeles waltl (Salamandridae). Copeia 1978:424-429.