Identify venomous and non-venomous snakes by their eyes, head shape, or belly scales…and you’ll be wrong

This post was inspired by a friend of mine who sent me a diagram proposing a simple rule to identify whether a snake is venomous or not. I’ve seen three of these simple rules shared on social media and all three are wrong.

The most popular rule normally consists of a diagram that looks like the one below, except that it claims that all venomous snakes have triangular heads and all non-venomous snakes have round heads.

Snake heads lie
This work by Eric Butler is licensed under a Creative Commons Attribution 4.0 International License.

Vipers (by which I mean any snake in the Viperidae, including rattlesnakes, copperheads, and water moccasins) do tend to have triangular heads, although the head shape changes in response to the amount of venom in the snake’s venom glands. But pythons and boas also have triangular heads because they eat large prey and have to have large heads. Meanwhile, round heads are found on many other species, including mambas, which are some of the most venomous snakes in the world, and coral snakes, which are also venomous.

The next most popular rule looks like this diagram, except it claims that venomous snakes all have slit-pupils and non-venomous snakes all have round pupils.

This work by Eric Butler is licensed under a Creative Commons Attribution 4.0 International License.

This is simply not a helpful characteristic. Slit-pupils and round pupils are both found in venomous and non-venomous snakes. In fact, while putting this article together I found a site claiming that all venomous snakes have slit-pupils and all non-venomous snakes have round pupils, no exceptions. It had a photo of a python (non-venomous) to illustrate slit-pupils. (Even if this characteristics worked more reliably it would require being quite close to the snake, and slit-pupil eyes with expanded pupils can look a lot like round-pupils.)

The last characteristic is whether the belly scales on the tail are in one or two rows. The claim is that venomous snakes have one row and non-venomous snakes have two rows.

Belly scales past anal plate in snakes
This work by Eric Butler is licensed under a Creative Commons Attribution 4.0 International License.

This characteristic (known as having a single anal plate or a divided one) is actually used by professional herpetologists and can be found in many field guides. However, while it can help identify what group of snakes a given snake belongs to it doesn’t tell you whether that group is venomous or not. Indeed, in a few snake species, individuals of the same species can vary in this characteristic.

What’s really going on here?

It turns out that all three of these characteristics are making the same mistake: they are taking characteristics that vary between the Viperidae and the Colubridae (or most colubrids) and attempting to use these to identify whether a snake is venomous or not.

In North America most venomous snakes (the rattlesnakes, cottonmouth/water moccasin, and copperhead) are vipers. Many other snakes are colubrids. This means that these shortcuts often seem to work in the United States. In South America there are some venomous colubrids and these rules wouldn’t work. In Africa, Asia, and Australia some of the most feared venomous snakes are elapids, including cobras, kraits, death adders, and mambas. Elapids and colubrids tend to match in the characteristics these shortcuts use.

However, even if you restrict your use of these rules to North America you run afoul of the coral snakes, venomous elapids found in the southern parts of the USA and into Central America.

The coral snake above has a small, non-triangular head, round pupils, and a divided anal plate. It also has distinctive, bright colors (although it has non-venomous mimics) and so you could learn to avoid those instead. But that won’t work in Central America, where some coral snake species are much duller in color.

What rules can you use?

Learn the venomous snakes in your area. Most snakes aren’t venomous (or aren’t venomous enough to harm humans) and so there’s normally a pretty short list of snakes to learn. For instance, where I live the venomous snake that people are most likely to run across is the copperhead. It has a distinctive pattern with only minor variations. Learning that pattern is far more effective than any of these rules, especially since the pattern will let you identify a snake that is visible only as a back half slithering away under cover.

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