Tetrapod Zoology

Why do some snakes have horns?

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Dave Hone (of Archosaur Musings and Ask A Biologist) kindly provided the photo you see here of two captive West African gaboon vipers Bitis rhinoceros (NOT East African gaboon vipers B. gabonica: see comments), and initially I was going to use it in a ‘picture of the day’ post. One thing led to another and what you’re reading is the result. Gaboon vipers, the largest of the African viperids, are among the best known members of the genus Bitis (in which there are about 11 species). Like several other species in the genus, they’re stocky, broad-headed snakes of forest floors, but other species inhabit deserts and even rocky, montane habitats. Anyway, one feature of gaboon vipers interests me in particular: its nasal horn(s). Why does these snakes have nasal horns?

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They’re far from the only snakes to have such structures. A few Eurasian viperids have nose horns, including Lataste’s viper Vipera latasti and the Nose-horned viper V. ammodytes [shown here, from wikipedia]. The Rhinoceros viper or River jack B. nasicornis also has nasal horns. Horns over the eyes are present in the Eyelash boa Trachyboa boulengeri, Mexican horned pitviper Ophyracus undulatus, Montane pitviper Porthidium melanurum, Desert horned viper Cerastes cerastes, Persian horned viper Pseudocerastes persicus, Horned adder Bitis caudalis, Many-horned adder B. cornuta, Eyelash viper Bothriechis schlegelii and Horned rattlesnake or Sidewinder Crotalus cerastes. Small horn-like tubercles have been reported in Pseudocerastes fieldi from the Middle East. Finally, really weird horn-like structures are present in the Fishing snake Erpeton tentaculatum and in the Madagascan Langaha species.

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In all of these snakes, the horns are soft structures make of spikes or scales. The nose horns of most species are single, but gaboon vipers possess two, and there is a cluster of little horns in the Rhinoceros viper. Supra-orbital horns are sometimes single (as in the Desert horned viper) and sometimes consist of a row of small spikes (as in the Eyelash boa) or a cluster of small upright spikes (as in the Many-horned adder). Interestingly, the horns are variable in some species: not all gaboon viper or Lataste’s viper individuals possess horns, for example (Phelps 1981). In the colubrid Langaha, strong sexual dimorphism is present: in L. nasuta the nasal ‘horn’ is rod-like and pointed in males, but elaborately serrated in females. In L. alluaudi, only females are adorned. As usual with reptile photos on the internet, the only images I can find are fiercely copyrighted*, so I hope I’m ok in using this excellent image: it’s from here.

* Someone really needs to embark on some massive creative commons project involving photos of the world’s reptiles and amphibians. ALL the good images on the internet are copyrighted (which makes you wonder why they’re on the internet at all).

Returning to the question in hand, I don’t think there’s a single answer. While it’s interesting that the majority of horned snakes belong to the same group (Viperidae), the diversity of horn-like structures, and the ecologies and lifestyles of the snakes in question, suggest that horns evolved for diverse reasons. In some species – like the gaboon and Rhinoceros vipers – they might help break up the animal’s outline, as these are species that rely on crypsis while remaining motionless on cluttered forest floors.

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Desert-dwelling species with horns might, it has been suggested, somehow gain protection from sand grains thanks to their horns [Desert horned viper shown here, from wikipedia]. One idea is that, when the snake is submerged in the sand, the horns are bent down and therefore protect the eyes (Cohen & Myres 1970). I find this doubtful as the horns are rarely long enough for this, and are they really flexible enough to allow this? And since when do snake eyes need protection from sand grains anyway? (snakes lack eyelids but most possess a specialised transparent scale, the brille or spectacle, over the eye. This is shed with the skin). I wonder if supra-orbital horns might somehow help keep sand grains clear of the eyes when the snake is lying at the sand surface, slightly submerged, but then this doesn’t quite follow, as wind-blow sand doesn’t move around an obstacle; rather, it gathers up around it. Someone should look into this. In the same vein, it has also been suggested that the horns help deflect vegetation from the eyes when the snake is moving through undergrowth. Again, I am sceptical given that the vast majority of snakes do fine scooting through the undergrowth without horns.

Finally, could the horns play a role in display or, at least, visual signalling of some sort? The strong sexual dimorphism present in Langaha suggests that this might be the case here, but I don’t know if anyone has yet worked out whether it is. As for visual signalling in other horned snakes, I’m not aware of any studies on this and it seems that we’re all still guessing. Does anyone know any better?

Refs – -

Cohen A. C. & Meyers, B. C. 1970. A function of the horn in the sidewinder rattlesnake Crotalus cerastes, with comments on other horned snakes. Copeia 3, 574-5.

Phelps, T. 1981. Poisonous Snakes. Blandford Press, Poole, Dorset.

Comments

  1. #1 Neil
    February 24, 2009

    Ive always assumed they are used for display but its interesting to read the other theories. I guess in some species they could aid camouflage?

  2. #2 Mokele
    February 24, 2009

    I’ve always suspected that snake horns were either disruptive, to obscure the organism’s outline, or just non-selective artifacts of drift or founder effect.

  3. #3 Darren Naish
    February 24, 2009

    Hmm – I will admit that I have fallen into the usual trap of assuming that a structure must have ‘a function’. But, yes, there may well be neutral selection on such things as soft-tissue horns and spikes.

  4. #4 Dr Vector
    February 24, 2009

    They’re obviously for combat. RAAAAHHHHHH!! Take that! Charge, gore, spurting blood!

    You might argue that the horns, being made out of soft tissue, would be useless for poking holes in other organisms. But you’re forgetting about jellyfish.

  5. #5 gostonethecrows
    February 24, 2009

    Protection from frost? Desert nights can be cold, and snakes need to hunt at night. A seed for ice-crystals to form prevents ice damage to the eyes, c.f. ear-tufts on lynxes and owls.

  6. #6 Andy
    February 24, 2009

    I like Dr. Vector’s idea. Now, I just need to get some plastic snakes, and see if I can get them to fight each other. . .

  7. #7 Darren Naish
    February 24, 2009

    Dammit, if only toy horned snakes weren’t so thin on the ground [read: non-existent] :)

  8. #8 Mokele
    February 24, 2009

    Another possibility that’s selective, but not on the horns – what if selection for particular scale morphology or pattern (namely for camoflage) results in ‘horns’ or ‘spines’ at points where the developmental system underlying the scales hits a ‘glitch’ due to the shape of the body? That could be especially powerful for nose-horns, since it’s at the tip.

    Colubrids like Erpeton, Rynchophis, and Langha probably wouldn’t fall under this, given their large, fleshy protrubances, but given the species involved, I think a reasonable argument can be made for crypsis in colubrids, especially if you analagize the long noses of Rynchophis and Langha to the elongate, pointed snouts of Oxybelis, Ahaetulla, and Thelotornis. Given Erpeton’s habit of mimicing a twig in the water, I think that argument can be reasonably made too.

    Of course, we can speculate all we like – what’s needed are actual *experiments*

  9. #9 wolfwalker
    February 24, 2009

    Regarding the nose-horns: does anyone know if those particular species have an egg-tooth? Could the nose horn be a a way of breaking out of the egg that simply never goes away?

    The other possibility that comes to my mind is (as with several other commenters) display, or species recognition. How good is a snake’s eyesight?

    Regarding “a massive creative commons project involving photos of the world’s reptiles and amphibians” — have you looked at Wikimedia Commons? Go to Wikipedia, enter “Gaboon viper” (or any other species you want) and hit return. Then scroll to the very bottom of the article, and look on the right for a link to “Wikimedia Commons.” It’s usually in a gray box. Wikimedia includes pictures that are public domain and pictures for which it has an explicit release on file. For example, this excellent shot of a Gaboon Viper is released for use by the photographer under the terms specified in the GNU Free Documentation License, which is just a formalized version of the traditional GNU copyleft.

  10. #10 Art
    February 24, 2009

    Just brainstorming here but if the animal spends time buried in soft sand the horns might act as sensory periscopes to check out surface conditions. Camping I have been known to stick a finger out of my thick and well insulated bivy bag to determine the temperature.

    I note that temperature swings can be quite wide and fast in the desert and the snakes would want to preserve their warmth and, perhaps, avoid the worse of the heat. Being able to sense the temperature at the surface using these horns could prevent inadvertently leaving the comfort under the sand and the resulting need to expend energy reburying themselves to get comfortable again.

    Another though would be that the horn in some way can detect light. Buried in sand day and night are hard to detect accurately. When daytime predators are present knowing when it is dark, without having to expose the bulk of your body, could save your life.

    If able to detect light, or the heat of sunshine, could also give a snake clues as to the presence of predators as they block the sunlight.

  11. #11 John Scanlon FCD
    February 24, 2009

    Uhh, all of the above?
    The developmental mechanism or ‘potential’ to form horns may only have arisen a few times (Trachyboa, Viperinae, one or more colubrid clades…).
    Once expressed in a mutant, it necessarily and immediately alters the profile of the snake’s head, making it less likely to match the search image of predators or prey trained on hornless ancestors (that much is trivial) and probably resemble some class of common harmless objects (sticks, curled leaves). Hence positive selection through crypsis.
    So why don’t all snakes have horns? The advantage depends on horned individuals of a particular type being rare in the snake community; once nose-horns are common, you’re better off with supraocular horns or none.
    For desert snakes that shuffle into sand until nearly buried, wind-blown sand is not the only thing to consider. Typically, the snake ends up with a pile of sand on top of its flat head, which the slightest breeze or twitch will cause to start sliding off. Brow-horns can’t protect the eye from grains flying in horizontally, but neatly part the curtain of sand trickling off the top of the head and thus provide a less-obstructed view.
    Why don’t pitvipers have horns? (or do they ever? – any examples? – some have very pointed snouts, some steps in the direction of Langaha, but no raised horns at all?) – that hole in the face must have its own problems with sand, mud, fungus and stuff, so it could be to do with keeping the face out of the muck.
    Interestingly, no Australian snakes have horns except supraocular ones in some Death Adders (some mainland desert species, and some New Guinea forest ones, while closer relatives of both have smooth brows). Would be interesting to compare the distribution and details of horns among Viperinae, Acanthophis and a few other such clades. And don’t leave out the sea-snakes – different kinds of horns in Emydocephalus, Acalyptophis, and Aipysurus at least.

  12. #12 Owlmirror
    February 24, 2009

    Thinking about nasal horns reminded me of the electrosensory organs of star-nosed moles, paddlefish, platypuses, sharks, etc. Maybe?

    The eye-horns are different, of course. Would they be useful in thermoregulation? Has anyone take IR pictures of these snakes?

    Another idea for eye-horns is simply as natural eye-shades; just to keep the amount of sunlight from supersaturating the retina. How do they hold their heads when heading away from or towards the sun?

  13. #13 Lilian Nattel
    February 24, 2009

    Maybe there’s no reason at all and it’s a byproduct of something else, that might someday evolve into something useful?

  14. #14 Mokele
    February 24, 2009

    I’m skeptical of the use of horns as egg-teeth, in part because they occur in live-bearing species as well as egg-layers. As for species recognition, most snakes have so-so vision at best (with a few notable exceptions like Chrysopelea), and chemosensation is likely to be far more informative anyway.

    Sensory probes is an interesting idea, at least for the desert species, though not so much for Gaboons, though there’s also desert species which lack horns, including congenerics – Cerastes cerastes has horns (see photo above), while C. vipera (also a desert snake) lacks them. And AFAIK, Bitis vipers with horns are all jungle snakes, while the desert ones lack horns.

    Oh, and John, several pit-vipers have horns or eye-horns, notably the eyelash viper, the sidewinder rattlesnake, and the hognose viper.

  15. #15 DDeden
    February 24, 2009

    Vibrational “vibrissae” with closed mouth? Thinking of large animals walking by while the snake sleeps invisibly under otherwise exposed sandy surface. Eyebrow horn seems about the same width as the slit eye, shading in sunlight?

  16. #16 DunkTheBiscuit
    February 25, 2009

    In the desert viper illustrated above, at least, those horns look exactly placed to prevent dazzle from overhead sunlight. But I speculate, as I have no idea if that would be a problem for snakes.

    Incidentally I’d love to release some of my images under collective commons license, but don’t know how to go about it. My only snake images so far are of a gaboon viper behind glass, though.

  17. #17 Jonathan Warner
    February 25, 2009

    The photo of the Gaboon is of Bitis rhinoceros (West African Gaboon Viper), not Bitis gabonica (East African Gaboon Viper). The latter has two distinct ‘tear drops’ running from the eye to the labial scales. The subspecies were recently raised to specific status. If anybody is really into Gaboons, I just finished radiotracking the species for two years in South Africa…am happy to pass along a copy of my thesis to anybody interested.

  18. #18 Darren Naish
    February 25, 2009

    Hi Jonathan – I hadn’t realised this, thanks for the correction. The paper that splits B. rhinoceros and B. gabonica is…

    Lenk, P., Herrmann, H.-W., Joger, U. & Wink, M. 1999. Phylogeny and taxonomic subdivision of Bitis (Reptilia: Viperidae) based on molecular evidence. Kaupia 8, 31-38.

    Does anyone have a pdf? Would also be interested in your thesis Jonathan, I’ll email you privately.

  19. #19 Jason Adams
    February 25, 2009

    “Someone really needs to embark on some massive creative commons project involving photos of the world’s reptiles and amphibians.”

    Have you checked out Edward O. Wilson’s Encyclopedia of Life project? It’s a collaborative effort to catalog every species of organism on the planet. There is a Flickr group set up for people who want to contribute images, and the rules require that they be CC licensed or copyright-free. http://www.eol.org

  20. #20 D'oh!
    February 26, 2009

    In the picture of the Desert Horned Viper, the middle of the snake is noticeably thicker than the head. Could the horns let the snake know that the hole it is about to go down is too small? How easily can snakes back up out of a hole? Seems to me that they’d need some wiggle room.

  21. #21 Graham King
    February 26, 2009

    In the picture of the Desert Horned Viper, the middle of the snake is noticeably thicker than the head. Could the horns let the snake know that the hole it is about to go down is too small? How easily can snakes back up out of a hole? Seems to me that they’d need some wiggle room.

    Posted by: D’oh! | February 26, 2009 2:37 PM

    D’oh! beat me to it… I was just going to say that!

    This needs measurements to be made of maximum dorsoventral height of body, and height at horns, to see if the two are related.

    Problem I see with that idea is that the snake’s body is also wider than its head. So it would need a separate way to judge lateral width of hole as adequate, to avoid getting stuck. Maybe it can do that by sight in light, but not in dark… but maybe its natural snaky motion (head moving side to side?) gives it a sense of lateral space even in the dark?

    I know nothing about this really… just imagining.

  22. #22 Jerzy
    March 3, 2009

    I thought that nasal horns act as bull bar, moving vegetation aside when the viper crawls forward.

  23. #23 Jerzy
    March 3, 2009

    BTW, horn of male Langaha can be cryptic. These snakes in captivity have curious habit of hanging vertically head-down, looking like plant stems.

    About female I don’t know – sex-specific foraging strategy and crypsis?

  24. #24 Jerzy
    March 3, 2009

    Wow – snake mimicking fruit! Got to read the end of this:

    http://www.herpbreeder.com/pdf/pdfs/2003%20Krysko-Langaha%20reproduction.pdf

  25. #25 DDeden
    March 17, 2009

    DD: Eyebrow horn seems about the same width as the slit eye, shading in sunlight?

    Dunk: In the desert viper illustrated above, at least, those horns look exactly placed to prevent dazzle from overhead sunlight.

    See Between a rock and a Hyrax, Natural History Mar. 2009, paraphrase: they spend much of their time basking, with eyes to the sky watching for predators (black eagle), staring at the sun watching for raptors which often attack from that direction… they possess a operculum in the eye that shields vision, they often bask as mixed species, the yellow rock hyrax and the rock hyrax, but don’t cross mate.

  26. #26 Dave S.
    January 9, 2010
  27. #27 Sangey Dorji
    July 7, 2010

    Just recently one of my friend shows me a pair of small horn saying that it is a snak’s horn which would fetch up good price in the market. Yes I have seen in the photo provided above that some snak have a horn but it is true it can fetch a good price.

  28. #28 mohamed
    August 7, 2010

    this last picture is an egyptian snake called “toriesha”. im egyptian and i dont know y is it called like this.
    this type of snake burys itself under the sand, so i think these horns are like sensors left unburied. these snakes are extremely dangerous!

  29. #29 pingu
    August 16, 2010

    is the horn of snake costable.

  30. #30 ZIA ULLAH TAHIR
    April 9, 2011

    I saw Ist time double horns snake, it is just strange to me. But I could not understand about its home land.
    ZIA ULLAH TAHIR, ISLAMABAD, PAKISTAN.+92 344 5878687, 331 5878687.

  31. #31 Sean
    July 20, 2011

    I guess for digging porpouses, to look scary to predators, and for mating pourposes. these are my guesses

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