How to Tell If a Snake Is Venomous — And Why the Usual Advice Is Wrong

Search for how to identify a venomous snake and you will be told to look at the head shape and the pupils. Both pieces of advice are repeated everywhere, both are genuinely unreliable, and following them gets harmless snakes killed and occasionally gets people bitten.

Here is what is actually going on, and what to use instead.

Why the triangular head test fails

The idea is that pit vipers have broad, arrow-shaped heads because of their venom glands, while harmless snakes have narrow heads. That much is true as a generalisation.

The problem is that harmless snakes know about this too.

When threatened, water snakes, ratsnakes, hognose snakes and gopher snakes all flatten their heads and necks to look bigger and more dangerous. A frightened northern water snake produces a head shape that is genuinely difficult to distinguish from a copperhead's, and it is doing so precisely because the impression works.

The test fails in the other direction as well. Coral snakes have narrow heads no wider than their necks, and they are among the most dangerously venomous snakes in North America.

Why the pupil test fails

US pit vipers — copperheads, cottonmouths and rattlesnakes — do have vertically elliptical pupils, and most harmless species have round ones. So far so good.

Two things ruin it.

First, coral snakes have round pupils. Applying the rule to a coral snake gives you exactly the wrong answer about the most neurotoxic snake on the continent.

Second, and more importantly: to see a snake's pupil clearly you need to be within a couple of feet of its head, in good light, looking at it straight on. That is a description of the exact position you should never be in. A test that requires you to be inside striking range is not a safety test.

Pupil shape also changes with light. A pit viper's pupil dilates in dim conditions and can look almost round at dusk — which is when most encounters happen.

What about the pit?

Pit vipers are named for the heat-sensing pit between the eye and the nostril, and it is genuinely diagnostic — no harmless North American snake has one. But it is a small hole on the side of the face. Seeing it means being close enough to count scales.

Same category as the pupil: real, but not usable.

What actually works

1. Start with your state

This is the single most effective filter available and almost nobody uses it.

There are only about twenty venomous snake species in the continental United States, and they are not evenly distributed:

• Maine, Rhode Island, Alaska and Hawaii have no venomous snakes at all.
• California, Nevada, Oregon, Washington, Idaho, Montana, Wyoming, Utah, North Dakota and South Dakota have only rattlesnakes — no copperheads, no cottonmouths, no coral snakes.
• Most eastern states have one to three species, usually the copperhead plus a rattlesnake.
• Only a handful of southeastern states have all four groups.

Before you have described a single marking, checking which snakes live in your state usually reduces the dangerous possibilities to one or two.

2. Learn the four groups, not the twenty species

Every dangerously venomous snake in the continental US belongs to one of four groups, and each has a look:

That is the entire list. If a snake does not fit one of those four descriptions and you are in the United States, it is very unlikely to be dangerous.

3. Use behaviour, which you can see from a distance

Behaviour is safer to observe than anatomy and often more diagnostic:

• A rattle means a rattlesnake. A buzzing sound with no rattle visible does not — ratsnakes, gopher snakes, milk snakes and racers all vibrate their tails in dry leaves, which sounds convincingly like a rattle.
• A white open mouth means cottonmouth. Nothing else does this.
• Playing dead means hognose snake, which is harmless. Nothing venomous in North America feigns death.
• A flattened cobra-like neck and loud hissing almost always means hognose or gopher snake — theatre, not threat.
• Fleeing at speed suggests a racer, coachwhip or ratsnake. Pit vipers rely on camouflage and tend to freeze.

4. Look at how it swims

For water snakes specifically, this is the best test there is and it works at fifty feet.

Cottonmouths float high, with much of the body visible on the surface, because they are buoyant. Harmless water snakes swim submerged, with only the head and a little of the neck above water.

The one rule that matters more than any of this

You do not need to identify a snake to be safe from it. You need to be somewhere else.

Roughly half to three-quarters of snakebites in the United States happen to people who were deliberately handling, catching, moving or killing a snake. The single most effective safety measure available is walking away — and that works perfectly whether or not you got the identification right.

Identify it afterwards, from a photograph, with your feet up. That is what the identification key is for.