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If you’ve ever called snakes “poisonous” around a herpetologist or, you know, that friend of yours, you know which one I mean, there’s a good chance you’ll receive a lecture about the difference between venoms and poisons. And it’s true that most snakes aren’t considered poisonous.

But nature loves an exception. Meet the tiger keelback. They’ve got the best of both worlds: poison for their predators and venom for their prey.

It can be easy to confuse the terms venom and poison, but the difference does actually matter. Both are made up of toxins, biological chemicals that mess with body functions in small amounts. The technical difference has to do with how they get into a victim’s body.

Poisons enter passively, by being eaten, breathed in, or absorbed through the skin. And because of that, they’re usually defensive, an organism’s way of saying “Hey! Don’t touch me!

You will regret that!” Venoms, on the other hand, are actively injected into the body, like, via a pair of pointy snake fangs, for example. Venom can certainly be used defensively, but often, venomous animals are predators that use their toxins offensively, so they can enjoy a meal without all that troublesome struggle. Since venoms and poisons tend to be used differently, they usually contain different kinds of toxins.

So a doctor would want to treat a snake poisoning differently than a snake bite, or what’s known as envenoming. And the distinction is particularly important if the snake in question is a tiger keelback. These snakes can be found across Southeastern Asia, and they belong to a huge and diverse family of snakes called the Colubridae.

These are sometimes referred to as “rear-fanged” snakes because unlike rattlesnakes or cobras, which pump venom forcefully through hollow fangs in the front of their mouths, colubrid fangs sit further back in the jaw and deliver venom along open grooves. And they have to kind of chew on their prey a bit to get the venom moving. Most of the time, tiger keelbacks use their venom to take down tasty fish, tadpoles, and their favorite: frogs and toads.

They rarely bite humans, but when they do, it’s not a fantastic experience for the human. The venom can cause hemorrhaging and mess with blood clotting, leading to excessive internal and external bleeding, and in several known cases, death. Thankfully, antivenom is usually an effective treatment, if delivered in time.

Now, all this talk of killer snake bites might sound scary, but it’s important to remember that snakes don’t want to bite you! Really, they don’t! They’d much rather save their venom for their prey.

They don’t want to waste it on something they can’t eat. And that may be partly why the tiger keelback usually fends off threats another way. On the back of these snakes’ necks are several pairs of nuchal glands.

If the right pressure is applied, these glands crack open and spew out a stinky, toxic yellow-ish pus. The poisonous nature of this stuff was first noted in 1935, when a scientist who was dissecting a keelback cut into its nuchal glands, accidentally spraying the poison right into his eye! Which is just one of the many stories that science teachers have heard, and why they get so fussy about you wearing your safety goggles.

The poison contains bufadienolides, which can cause acute pain and temporary damage to the cornea if they get in your eyes. When ingested or inhaled, they can irritate any tissue they touch and even cause heart problems. And the snakes know how to use their poison.

When confronted with danger, they assume an arched posture and aim their neck towards the threat. Sometimes, they’ll even swing their neck at an attacker, poison glands first, in a maneuver that scientists have delightfully named a “neck butt.” Maybe the most interesting thing about this poison is that they don’t make it. They pick it up from the toads they eat.

Research has shown that tiger keelbacks living on toad-free islands don’t have these poisons, but they can become poisonous if given toxic toads. And you’d think that a lack of poison would be a problem for a newborn snake, but it turns out mama keelbacks can pass toxins along to their young. It’s unclear if she adds it to the yolks or just bathes her eggs in the stuff.

Experiments have shown that keelback babies absorb toxins through their eggshells. Either way, by the time baby keelbacks hatch, they’re ready to defend themselves. And here’s something super weird: the snakes are more likely to flee from threats than face them neck-on if they’ve been deprived of toads, suggesting the animals somehow know when they’re poisonous.

Also, the snakes don’t just take the toad toxins as-is and stick them in their glands, they actually modify them chemically to make them even more potent. The tiger keelback is definitely the best-known example of a poisonous snake, but it’s likely not the only one. Other related species have nuchal glands, so they might have poisons of their own that simply haven’t been studied.

Research has found that other snakes, like garter snakes, can eat so much poisonous prey their organs become toxic to predators. Since most people don’t usually eat snakes, there could be a number of species that would make us sick that we just don’t know about. So future studies may reveal that the tiger keelback isn’t that much of an exception after all.

In the meantime, you can tell that pedantic friend of yours that there definitely are poisonous snakes. So Mleh! Thanks for watching this episode of SciShow!

And a special thank you especially to our patrons on Patreon. We wouldn’t be able to have all this fun making awesome educational science videos like this if it weren’t for the support of our patron community. If you’d like to learn more about joining that community, or how you can help us do what we do, you can head on over to Patreon.com/SciShow [♪ OUTRO].