I recently participated in a Zoom event for Museum members focused on toxins and venom in the natural world. Mason Heberling, Assistant Curator of Botany, and Ainsley Seago, Associate Curator of Invertebrate Zoology, spoke about how the organisms they study produce toxins, and how these chemicals impact their environments and other organisms around them. As an ecologist focused on impacts of climate and land use change, I don’t consider myself an expert on toxins and venoms in amphibians and reptiles, but researching my portion of the joint presentation reminded me once again why herps are the best.
First, a refresher: toxins are poisons, and they have to be consumed or encountered (touched) by an organism to do harm. For example, many frogs produce toxins in their skin, but you would have to either consume that frog or touch its skin for the toxin to do you any harm. Most frogs don’t produce toxins strong enough to hurt humans, though a few notable exceptions exist. Some species of poison dart frogs have skin toxins strong enough that if you touched them and then touched your eyes, nose, or mouth, or if you had a cut on your hand, you could indeed become very ill and perhaps die. Venom, on the other hand, is a toxin that one organism can inject into another. Typically, we think of snakes when we think of injectable toxins. Many snake species have venom glands that produce toxins, and they can forcibly inject that toxin into their prey. The action, which can occur in a flash, involves the use of fangs to puncture the skin, and muscles surrounding the venom gland to force the toxin out along the fang and into the other organism.
Most people tend to think of amphibians as toxic and snakes as venomous. This is true, but it turns out that snakes aren’t the only venomous reptiles, and amazingly two frogs are known to be venomous. Gila monsters (Helodermatidae) and water monitors (Varanidae) produce venom, but their venom glands are in their lower jaws (unlike snakes whose venom glands are in their upper jaws), and they lack the muscles to forcibly inject that toxin the way snakes do. Instead, the act of chewing on their prey causes their jaw motion to work the venom toward their grooved teeth, which then enables the venom to be injected through the bite wound. Using a very different delivery system, two frogs in the family Hylidae (tree frogs from the Americas) have very spiny skulls. Their skin produces toxins, and by “head-butting” another organism, they can effectively inject that toxin into another organism. This unusual delivery system technically makes them both toxic (the toxin can be transferred to you if you touch their skin) and venomous (they can inject that toxin into you).
One of the most surprising things I learned is that there are snakes that are both toxic and venomous, and these are snakes I see frequently in the field. The genus Rhabdophis is common across South and Southeast Asia, and have long been known to be venomous. What I didn’t know is that in addition to making their own venom, they sequester toxins from their prey, and store it in a gland on the back of their neck called a nuchal crest. Rhabdophis feed on toads, which are toxic, and the snakes are able to sequester that toxin, rather than being adversely affected by it. Interestingly, scientists have shown that Rhabdophis tigrinus are toxic only where their range overlaps with Bufo japonicus, a highly toxic toad—so on some islands of Japan the snakes are toxic, while on other islands they are not.
There are numerous other interesting adaptations involving toxin and venom in the herp world—tweet me (@JenASheridan) if you want to learn more!
Jennifer Sheridan is Assistant Curator in the Section of Amphibians and Reptiles at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.