Albeit extremely mild, garter snakes are venomous. The venom is produced in the Duvernoy's gland and is not harmful to humans. How did I not know this before?
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excerpt from: What snakes are venomous and which are poisonous?
Well, although the answer to that question can be relatively straightforward, there are a few special cases that must be discussed first. For instance, Spitting Cobras (Genus Naja) are capable of launching their venom into the eyes of a potential predator, thus using their venom as a defensive poison instead, permitting the toxins to be absorbed through the eye's mucous membranes and causing blindness (among other envenomation symptoms). This is not to say that other snake venoms applied to your eye will do no damage, as many of them will, it's just that Spitting Cobra venom (along with their fangs) has specifically evolved to be used for that defensive purpose (in addition to being used in the procurement of food via a venomous bite). Also, certain species (and populations) of Garter Snakes (Genus Thamnophis, species sirtalis, couchii, and atratus) and one of their cousins (Tiger Keelback Snake, Rhabdophis tigrinus) are both venomous AND poisonous. How can this happen, you ask? Simple: they produce their own venom in a specialized gland inside their upper lip (called the Duvernoy's gland) for use on prey, while sequestering prey toxins (tetrodotoxin from newts or bufotoxins from toads) in their body (Garter Snakes) or poison glands (Tiger Keelback Snake only) for use against predators. So, basically these animals possess two different types of toxins in different places in their bodies, which come from different sources, and serve different purposes. The "Venomous AND Poisonous Animal" photo below illustrates an example from one of the species of Garter Snakes (Thamnophis sirtalis) known to be poisonous (tetrodotoxin) and venomous (although Garter Snake venom is not dangerous to humans), while also pointing out where the poison glands (containing bufotoxins) of the Garter Snake cousin, the Tiger Keelback Snake, are located. So, if anyone ever asks you if there are any poisonous snakes in the world, you can give them an answer of at least four species (Thamnophis sirtalis/couchii/atratus and Rhabdophis tigrinus), with the caveat that they are both poisonous and venomous. Aside from those species, the remaining snakes in the world are either nonvenomous/nonpoisonous or venomous, with constrictors belonging to either of those categories. The >1,200 species of venomous snakes may be further categorized into front- or rear-fanged species and are introduced in the next hub on snake fangs, which you may feel free to explore after taking the quiz below to test your knowledge about how to distinguish venoms and poisons. You can also check out the video below, which shows how a nonpoisonous/nonvenomous constrictor kills prey. If you would like to learn more about venomous and/or poisonous creatures, please see the Amazon links below for some useful book resources.
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What snakes are venomous/poisonous?