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This image was recently shared by an Australian Facebook friend:

Diagram of snakes: A little education never hurts... Poisonous: Elliptical pupil nostril. pit. ]Scales on underside of tail in single row. Non-Poisonous: Round Pupil. Nostril. No pit. Scales on underside of tail in double-row.

The source appears to be a Facebook post of a man from India with a staggering 380,000 shares from around the world!

There are many different species of venomous snakes (and I assume that they mean venomous, not poisonous) and I struggle to see how this could be true.

Is it true? Is it true just for snakes in India?

  • Comments are not for extended discussion; this conversation has been moved to chat. – Sklivvz Dec 2 '17 at 22:01
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No, this is not true. Not even for just India, as Indian cobra (Naja naja) has round pupils and subcaudal (tail) scales are divided. There is also no pit visible. It is venomous species of snake.

This answer assumes, that author means venomous snakes instead of poisonous, as this is common mistake. Also, Wikipedia should have enough credibility for this answer:

https://en.wikipedia.org/wiki/Indian_cobra

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    There are actual poisonous snakes such as the Rhabophis. However, it shouldn't be poisonous according to the claim. – Jordy Nov 29 '17 at 13:56
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    I suspect the diagram was intended for use in North America, where the common venomous snakes (the pit vipers) match the description quite well. – Mark Nov 29 '17 at 18:37
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    IMHO the venomous vs. poisonous "mistake" is so common in regular English usage that it is the people who insist the English language has a hard difference in meaning between the two who are in error. – T.E.D. Nov 30 '17 at 2:07
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    @T.E.D. The problem with that opinion now becomes how to distinguish poisonous snakes from venomous snakes? – slebetman Nov 30 '17 at 8:08
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    @slebetman - My guess would be we have never really required that distinction because not a lot of English speakers are in the habit of eating snakes. – T.E.D. Nov 30 '17 at 14:28
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... maybe a bit more education hurts even less... but the answer is still no.

  • So to me the description with pits and slit pupils (and also the head drawing) looks like indicating pit vipers. I don't knot about their tail scales, though (and it doesn't seem very practial to me to check...). Pit vipers are venomous.
  • But there are lots of venomous snakes without pits and with round pupils. E.g. the cobras were mentioned already in another answer.

  • There are about 3500 species of snakes.
  • About 1/4 of them are venomous. Venomous species of snakes are not concentrated in a particular branch of the snake taxonomy. [Wiki venomous snake]
  • But there are several families of snakes where all species are venomous: Elapidae, Viperidae and Atractaspidinae. There are other Families with venomous and non-venomous species.

  • One family of venomous snakes are the vipers.

  • scales [Wiki Snake scale]:

    Scales do not play an important role in distinguishing between the families but are important at generic and specific level.
    [...]

    There is no simple way of differentiating a venomous snake from a non-venomous one merely by using a scale character.

    • It is also not so very practical:

      Distinguishing by using scale diagrams whether a snake is venomous or not in the field cannot be done in the case of uncaught specimens

    • In certain regions, distinguishing by scales may work:

      In certain regions, presence or absence of certain scales may be a quick way to distinguish non-venomous and venomous snakes, but used with care and knowledge of exceptions. For example, in Myanmar, the presence or absence of loreal scales can be used to distinguish between relatively harmless Colubrids and lethally venomous Elapids.

    Note though that this distinction tries to use scale patterns on the head, not below the tail.

    • Back to the vipers, they typically have keeled scales (i.e. with a ridge)

    • In my region (Germany), there are only 2 species of venomous snakes, both vipers (aspis viper and common European viper) but not pit vipers, and according to the respective Wiki pages they have paired scales on the lower side of the tail... => 2:1 failure of suggested rule.
      But then, the most common "snaky" animal we have isn't even a snake (neither venomous): slowworm(https://en.wikipedia.org/wiki/Anguis_fragilis)

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    Now you had me look it up and I found out that the English term is not "blind-sneaky" ;) – Hagen von Eitzen Nov 29 '17 at 20:39
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    After dozens enconters with snakes, scorpions, spiders and venous/poisonous fish I found the little criters actually knows they are dangerous (sure it's a importat part in they behaviour) so I follow a rule of thumb: 1. If it run away from you just be cautions to not step in it. 2. if it seems to ignore you or just is no moving be extremely cautious. 3. If it attacks you run like hell – jean Nov 30 '17 at 9:43
  • @HagenvonEitzen: I had to look up the "slowworm" as well ;-) – anonymized Dec 1 '17 at 16:10
  • @jean: Most wild animals actually follow more or less the same rule of thumb when they encounter an unfamiliar creature. This can sometimes lead to funny results like, say, a naïve housecat being scared of an overconfident mouse. It also makes running away from an animal yourself a bad idea in most cases, since it tells the creature you're facing that you're safe to chase (and since you're neither fast enough nor agile enough to actually run away from most things that could hurt you). Usually, option #2 (back off slowly, stand your ground if approached) is what you should aim for. – Ilmari Karonen Dec 2 '17 at 9:11
  • @IlmariKaronen Not quite really. First I refering to snakes and venous things and can extend it to nasty critters like wolverines and if it attacks you must run or fight anyway. Note I cannot give my back to big felines, they really have this thing to jump in anything giving the back to them. I growth in a place overrun by snakes and learned how to handle them if necessary, even killed dozens (venous) in my backyard (fear of accidents with my family and pets). This was a generic advice. – jean Dec 2 '17 at 10:04
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That is a diagram for identifying pit vipers. There are many, many venomous snakes in the world, including I believe all of the most deadly ones, that are not pit vipers.

The advice is particularly unfortunate for Australia, as not a single one of Australia's 10 most dangerous snakes are pit vipers. All 10 are instead elapids.

The origin of this graphic is almost certainly English-speaking North America. For much, if not all of that area, the only venomous wild snakes are pit vipers. As a kid in Oklahoma I was given these exact instructions verbally for identifying poisonous snakes*. This simplification is possible in the USA because the only venomous snake in the continental USA that isn't a pit viper is the coral snake, whose range is limited to the Southeast, is rarely seen even there, and is generally not aggressive.

enter image description here


* - I am repeating the term that was used at the time for these snakes when said people were talking about them. I know its fashionable these days to maintain that "poisonous" doesn't really mean that. For that reason when I'm trying to be technical I'll instead use "venomous". But consider that language is arrived at by consensus, not dictate. If you have to explain to everyone you meet what a word means, you should consider the possibility that it isn't they who are wrong.

  • Re: terminology - we've just been having that discussion on Meta. – Oddthinking Nov 30 '17 at 3:14
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    No, just you took a descriptionist approach to the meaning of "poisonous", and my answer on meta largely supports that approach. – Oddthinking Nov 30 '17 at 6:57
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    @Oddthinking - Yes. I believe there are two types of people: Lingustic descriptionists, and people who think Canute's only error was that he didn't wave a book of tidal tables at the tide to show it that it shouldn't be up right now. – T.E.D. Nov 30 '17 at 14:41
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As answers state, these are signs for some snakes in some parts of the world.

The only sign I know that is said to be generally applicable, is a broadening / widening of the head around the side of the skull (as if seen from above) - because poison glands and (for those snakes which have them) additional muscles and anchor points to control fang movement and compress the glands to send venom into the duct/groove in the tooth and thereby into the target can tend to shape the head.

In other words try looking at venomous vs. nonvenomous snake heads from above, rather than from the side, and see if that helps.

I don't know how universal that is, but it might stand a chance of being helpful.

Or perhaps it's unreliable as well.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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Another counterexample is the Eastern Brown Snake of Australia (see notes). It has the double caudal scale row pattern of the "nonvenomous" picture.

But the counterexample is not my main point. I'd like to raise the following line of reasoning as an indication of why you should treat any assertion such as this with extreme skepticism. And that reasoning is simply to ask yourself how any creature gets to be venomous. It gets that way because of evolutionary drivers: initially through random mutations, its saliva or other bodily fluids exchanged with a foe - either during defense against an antagonist or during bringing down prey - contains proteins or other substances that subdue that foe. This might be, for example, mouth chemistry that is friendly to symbiotic bacteria that produce tetrodotoxin, as in the blue ringed octopus. This effect conveys an evolutionary advantage that is then re-inforced by natural selection.

Now, ask yourself how plausible it is that the same evolutionary drivers might affect the skin patterns on the tail at the other end of the body. It's not plausible at all. If there were a relationship such as this picture alleges, there would be a very strong evolutionary explanation for the link (albeit it may not be causal). For example, venom use requires a means of delivery, so it is vaguely plausible that venom carriers might have general facial shape traits. However, none have ever been observed: effective fangs fold up so neatly as to have no effect on the animal's facial shape.


Eastern Brown Snake (Pseudonaja Textilis): This one is particularly and extremely dangerous; owing to its venom's action on the blood, you've only got a short time to get the antivenom or at least make sure a compression bandage / limb immobilization is properly in place to stave off a likely fatal outcome. Basically its venom wreaks the same havoc as a massive intravenous injection of factor X would bring about.

The central argument of this answer is theoretical in nature. We do not allow answers based uniquely on common sense or pure logic. Answers which are wholly based on a theoretical model are generally downvoted and may be deleted. See FAQ: What are theoretical answers?

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