Spitting cobra

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Ring-necked cobra ( Hemachatus haemachatus )

As spitting cobras a number of man called poison snakes (Elapidae), which can spray their venom to potential enemies. A muscle contraction pumps poison from the poison gland into specially adapted poison teeth, from which it then emerges as a jet. The snakes aim at the attacker's face and can thus bridge distances of several meters, depending on the species. The poison has no effect on the skin; however, if it gets into the eyes, it causes severe pain and impaired vision. If the affected eyes are left untreated, long-term damage and even blindness are possible.

The spitting cobras include about a quarter of the real cobras ( Naja ) as well as the ring-necked cobra , which, however, belongs to the independent genus Hemachatus and is the only species. The spitting cobras are therefore not a related taxonomic group , the ability to inject poison is an adaptation that has been acquired several times independently of one another in evolution ( convergence ). Spitting cobras are found in Africa and southern Asia.

"Spew poison" as a defense against the enemy

With all spitting cobras, “spitting” their poison is an important part of defense against enemies . If the poison gets into the eyes of a potential aggressor, the pain and visual impairment are usually sufficient to drive it away. It is a beneficial, low-risk defensive strategy because it allows the spitting cobra to defend from a safe distance. Defending yourself with a poison bite requires physical contact with the aggressor, which is a significantly higher risk for a snake. This is also shown by the fact that spitting cobras tend to confront possible enemies more often than non-spitting cobras; the latter have a higher tendency to flee. In addition, only a fraction of the venom needed for a defense bite is consumed when spitting. Splashing poison is an innate behavior that can be observed even in hatchlings that are still partially in the egg.

To kill prey, spitting cobras use the poison bite, because under normal circumstances the death of a prey animal cannot be brought about by poison in the eyes alone. Likewise, spitting does not serve to fight intra-species, since the eyes of spitting cobras, as with all snakes, are protected by the transparent, so-called eyepiece scale.

Construction of the fangs

Schematic comparison of the venomous teeth of spitting cobras (right) and other cobras. The tooth is gray, the venom canal in green. First line cross section, second line horizontal section at the level of the outlet opening, third line front view of the outlet opening.

What all spitting cobras have in common is a special structure of their fangs, which enables them to spray poison in a targeted manner. The poison fangs of cobras have a closed venom canal that is connected to the poison gland and serves to secrete the poison. The poison from the poison gland enters the tooth through an opening in the venom canal at the top of the tooth, while it leaves the tooth through an opening at the bottom. Both openings are on the front of the tooth. The adaptations to the special defense method of the spitting cobras can be found mainly in the course of the poison canal and in the shape of the exit hole for the poison. In contrast to other cobras, the venom canal in spitting cobras does not run vertically throughout the tooth, but is strongly kinked shortly before its end, so that it leads roughly horizontally to the outlet of the poison. This causes the poison to leave the tooth in a horizontal, forward-facing jet rather than downward. The exit hole modified in Speikobras also contributes to this, which is relatively small and rounded to teardrop-shaped and can therefore function as a kind of nozzle . In spitting cobras it points forward and lies in the middle, while in non-spitting cobras it points downwards and is slightly shifted to the side. In the case of cobras that do not spit, this opening is also significantly larger and more slit-shaped, and in addition a groove runs from the exit hole to the tip of the poisonous tooth, which also favors the escape of the poison downwards.

A further adaptation of the venom canal is only available in spitting cobras, typically paired ridges on the inner surface of the venom canal. These usually begin in the lower third of the venom canal and end shortly before the outlet opening. They influence the flow behavior of the poison in such a way that a higher pressure is achieved for the poison flow in the tooth in front of the outlet opening and the cobra can thus continue to spew its poison.

Speiverbehavior

Course of the Speiact

When spitting cobras spew poison, they are usually already in the typical cobra threatening posture with their upper bodies erect and the neck shield spread. The mouth is opened slightly, then a jet of poison leaves the teeth, which initially points downwards, but then rises into its typical horizontal trajectory within about 10 milliseconds. At the end of the feeding process, the flow of poison bends back down. The entire feeding process takes about 50 milliseconds on average. Depending on the species, the poison can either be sprayed in the form of two defined jets (in the case of African spitting cobras) or as a mist-like spray cloud (in the case of Asian spitting cobras and the ring-necked cobra). With African spitting cobras, eating distances of up to three meters are possible, with Asian species and Hemachatus up to one and a half meters. The amount of poison released when spitting varies greatly - for the red spitting cobra ( Naja pallida ), for example, 3.2 milligrams dry weight is given.

Because the sprayed poison only has a specific effect on the eyes, spitting cobras have to be very targeted. However, the animals face a number of challenges: the target area is comparatively small, the animal perceived as an enemy is in motion, and the trajectory of the poison cannot be changed afterwards. All of these factors must be taken into account by the snakes in the short time it takes to feed. First, spitting cobras do not specifically target the eyes, but rather the center of the attacker's face. The cobras recognize the face by its shape, its short distance (since it is mostly facing the cobra) and its movements. This is an advantageous behavior, because the attacker's eyes cannot always be precisely recognized. This can be the case if the snake's vision is restricted by its clouded eye flakes at night or shortly before molting . In order to still ensure a hit in the eye, spitting cobras make quick movements of their head while the venom is released, which spreads the venom over a larger area. How far they swing with these circular movements, spitting cobras adapt to the perceived size of the target - for more distant targets that appear smaller on the retina, for example, the angle of the poison distribution is reduced and thus more accurate. Moreover cobras are able to spray poison in rapid succession - are experimentally, for example 45 consecutive Speiakte in the Red spitting cobra ( Naja pallida ) and the 57 African spitting cobra ( Naja nigricollis detected). Even if the first attempt fails, a spitting cobra will very likely hit at least one eye of the aggressor during an enemy encounter.

Spitting cobras also track the movements of their target and thus increase their accuracy. In experiments by Guido Westhoff and colleagues (2010), a test person protected with a mask carried out jerky movements with their head in order to provoke the snakes to attack with sprayed venom. The cobras tracked the movements of their target and adjusted their heads accordingly; when the target changed direction, poison was sprayed an average of 200 milliseconds later. These 200 milliseconds are the reaction time that elapses from the visual perception of the trigger (the change of direction) to the activation of the poison apparatus. Due to this reaction time from optical stimulus to muscle activation, the cobras cannot perfectly mirror the movements of the target, but rather “lag” 200 milliseconds behind, so to speak. When the trigger is perceived, the cobras accelerate their head disproportionately in the direction that the target takes immediately after the change of direction. In this way they “hurry” ahead of the goal, so to speak, and compensate for their own reaction time. Otherwise, the cobras would inject their venom where the target was 200 milliseconds earlier. Choosing the point in time directly after a change of direction makes sense, since a further change of direction is unlikely to occur shortly after a change of direction. In addition to these persecution movements, there are also the turning movements to disperse the poison; this requires complex motor skills and the neural processing of sensory impressions, which are unusual among snakes.

Individual spitting cobras often have very different tendencies to spray their venom. Experiments in captivity range from very nervous and aggressive specimens to snakes that can hardly be moved to spit.

Mechanism of toxin secretion

When spitting venom, the functioning of the venom apparatus is initially similar to the processes involved in a normal bite. At the beginning there is the contraction of the so-called adductor mandibulae externus superficialis muscle ; This is a two-part muscle of the cobra skull in which the upper half attaches to the parietal bone ( os parietale ) and posterior bones ( postorbital ) and the lower half to the non-tooth-bearing bone of the lower jaw ( compound bone ). Both partial muscles are connected to the poison gland located in the temporal region and exert pressure on it through their contraction. This forces the poison through the venom canal and the poison tooth. In order to prevent an unnecessary secretion of poison when the lower jaw moves, the poisonous teeth of poisonous snakes are additionally covered by the connective tissue , muscle-free dental sheath, which represents a physical barrier for the flow of poison. When prey or an enemy is bitten, it is pushed back from the surface of the victim's body, clearing the way for the flow of poison. Spitting cobras, unlike most venomous snakes, secrete their venom without physical contact with another animal, so they need a special mechanism to move their dental sheath. In spitting cobras , the protractor pterygoideus muscle is used for this purpose, which attaches to the parietal bone (parietal) and sphenoid bone (base phenoid) and ends on the posterior half of the palatine bone (pterygoid). Its contraction causes rotations and displacements of the bones and joints of the upper jaw and palate, which ultimately lead to an upward retraction of the sheath, thus removing a physical barrier for the poison. This mechanism is externally recognizable as deformations and vertical displacement of the snout complex of the cobras. The synchronous contraction of these two muscles then leads to the expulsion of the venom.

Systematics and evolution

Taxonomy

Well katiensis
Filipino Cobra ( Naja philippinensis )

Currently, 15 of the 28 species of real cobras ( Naja ) as well as the ring-necked cobra from the monotypic (only one species containing) genus Hemachatus are among the spitting cobras . All African species of spitting cobras are grouped together in the subgenus Afronaja , and all non-spitting cobras in Africa are assigned either to the subgenus Uraeus or Boulengerina . All Asiatic cobras, whether spitting or not, are placed in the subgenus Naja .

  • Genus Hemachatus

Evolution of the ability to speak



Hemachatus


 Well 

Well (subgenus)


   

Afronaja


   

Boulengerina


   

Uraeus






Cladogram of the Naja / Hemachatus group

The tribal history of the genera of poisonous snakes, often summarized as “ cobras ”, was researched in a study by the British herpetologist Wolfgang Wüster and colleagues (2007). The snakes commonly referred to as "cobras" do not represent a related ( monophyletic ) group - an example of this is the king cobra ( Ophiophagus hannah ), which is not closely related to the real cobras ( Naja ). Wüster and colleagues showed through the cladistic analysis of mitochondrial DNA, however, that Hemachatus and Naja are sister groups (descended from a direct common ancestor) and thus together form a natural (monophyletic) family group. It was also shown that within Naja all African species represent a monophyletic subgroup of three subgenus ( Afronaja , Boulengerina and Uraeus ) and thus form a sister group to the Asian cobras of the subgenus Naja .

The insights gained in this way on the relationships between Naja species and Hemachatus show that the ability to spray poison does not have a common origin, but was acquired evolutionarily in different groups independently of one another ( convergent evolution ). Hemachatus and Afronaja contain exclusively spitting cobras , and within Naja all Asian spitting cobras also seem to form a monophyletic kin. So there are three independent evolutionary lines of spitting cobras, so that the necessary adaptations of the body structure as well as the behavior have developed three times independently of each other in the cobra group. Therefore, the spitting cobras are not a taxon in the strict sense, but are grouped together due to a convergent adaptation.

toxicology

Symptoms

When a spitting cobra hits a person in the eye with its snake venom , the affected person experiences severe pain in the eyes. Swellings ( edema ) of the conjunctiva ( chemosis ) and eyelids form, epiphora occurs, and whitish deposits appear on the eye. The cornea becomes cloudy and an inflammation of the cornea (keratitis) forms. On the following day, iris inflammation ( uveitis ) and hypersensitivity of the eyes to light ( photophobia ) often occur. A collection of pus may form in the anterior chamber of the eye ( hypopyon ) and the iris may be cloudy. The eye is reddened.

In the days after the poisoning, the pain persists, but the damaged tissue also begins to regenerate. The eyesight remains restricted; In severe cases, visual perception can be limited to the distinction between light and darkness for up to 8 days. Vision is usually restored after about two weeks if treatment has been performed. However, cloudiness in the cornea can remain for longer. If, on the other hand, no treatment or treatment is delayed, infections , a perforation of the cornea and necrosis can occur in the eye . This can also lead to long-term impairment of vision and even blindness.

The exact course and severity of the symptoms vary from case to case, with the amount of poison received and, in particular, the species belonging to the cobra playing a decisive role. Some species are less toxic than others, and poisoning by African spitting cobras tends to be more severe than that of Asian species.

Composition and mode of action of the poison

The poison of spitting cobras is similar in composition to the poisons of other venomous snakes, and contains neurotoxins , cytotoxins , cardiotoxins and enzymes such as phospholipase A 2 (PLA 2 ). The cardiotoxic components in particular seem to be responsible for the harmful effects in the eyes. The exact mechanism of action has not yet been conclusively established, but a series of tests with the various fractions of cobra poisons identified the cardiotoxins in spitting cobras as being decisive for the toxic activity in the eyes. It is interesting that similar cardiotoxic components are also present in similar quantities in poisons from non-spitting cobras; the non-fractionated venom from such cobras is much less damaging to the eyes than the venom from spitting cobras. The toxicologist Mohammad Ismail and colleagues (1993) suspect that the poisons of non-spitting cobras contain more acidic proteins (for example acidic PLA 2 ) than spitting cobras . It is known that acidic proteins can bind with cardiotoxins ( dimerization ) - such bindings could limit the eye-damaging activities of cardiotoxins in non-spitting cobras.

Epidemiology

The epidemiology of Speikobra poisoning is still insufficiently researched. In a study carried out in Northern Nigeria ( Local Government Area Malumfashi ) in 1980, the incidence of eye poisoning from the African spitting cobra ( Naja nigricollis ) was six to eight cases per year among 100,000 people. In rural areas of Africa, attacks of this kind usually take place in the home of those affected, near their home or while working in the fields. There are no statistically relevant data on the frequency of incidents with Asian spitting cobras. In Africa, as in Asia, dogs and other pets also often suffer eye injuries from spitting cobras.

In addition to the poisoning of the eye, spitting cobras are also medically relevant due to their poisonous bite - in 1980, an average of 48 out of 100,000 people were bitten by the African spitting cobra per year in Malumfashi. Spitting cobra bites are also common in Asia, for example the Philippine spitting cobra is responsible for the majority of venomous snake bites in the Philippines .

treatment

If the venom of a spitting cobra gets into a person's eyes, they should be washed out very liberally with water or any other mild liquid as soon as possible. Alternatively, for example, milk or, if there are no other options, such as in dry areas, even urine can be used. This first aid measure has proven to be very effective in preventing further complications. In practice, however, this treatment is not infrequently made more difficult by the fact that the patient's eyes close convulsively due to the intense pain ( blepharospasm ). In such cases, the application of a vasoconstrictor ( vasoconstrictor ) to the eyes, such as 0.5% adrenaline and local anesthetic drops (for example 0.4% oxybuprocaine ), is recommended to relieve the spasms. At the same time, the patient is relieved of pain ( analgesia ). However, the use of anesthetic drops should only be limited, as the anesthetics themselves often have a toxic effect on cells of the cornea and hinder the production of tear fluid, thus promoting the development of bacterial infections in the damaged eye, for example. Another measure after eye contact with the poison is the administration of cycloplegics (e.g. homatropin ) and mydriatics (e.g. atropine or scopolamine ), which prevent cramping of the inner eye muscle , which is unpleasant for the patient , and an iris inflammation ( Uveitis) and adhesions in the eye ( synechiae ). However, a side effect of these drugs is a possible glaucoma attack in people with a shallow anterior chamber . The topical application or intravenous administration of an antivenin (antidote) is contraindicated in the case of spitting cobra poisoning in the eye : The use of an antivenin is not necessary in the eyes because it is washed out and can instead lead to irritation , and because the poison does not get out of the eyes spreads the bloodstream, intravenous antivenin has no effect and can lead to a harmful overreaction of the immune system ( anaphylaxis ). An eye patch can be worn until symptoms have subsided.

Following the first relief efforts after an incident with a spitting cobra eyes should with a slit lamp in the process of fluorescein angiography to examine a possible erosion to recognize the cornea. The dye fluorescein is put into the eye, which shows dead cells of the cornea by means of fluorescence . If such erosion is present, topical treatment with antibiotics ( e.g. tetracycline ) is indicated to prevent secondary infection of the eye.

Similar to humans, pets can be treated after being attacked by a spitting cobra by intensive washing of the affected eyes and then topical administration of antibiotics. Epinephrine or atropine can also be given to the eyes for analgesia.

Research history and culture

In the regions of origin of spitting cobras, people have always known how to defend them. For example, according to the Egyptologist Margaret Alice Murray, the ancient Egyptian symbol of the venomous and fire-breathing uraeus snake was derived from spitting cobras . In ancient Egypt it was a protective symbol, especially for the pharaoh. In the first century, Pliny the Elder (23 / 24-79) reported in his Naturalis historia of a snake called "ptya" that can spray its venom into the eyes of its victims; he was probably referring to one of the North African spitting cobras. Pliny also describes the basilisk , a snake supposedly also found in North Africa, which kills with its breath alone. According to Alexander (1963), spitting cobras or the symbolic uraeus could be the cause of the basilisk myth of antiquity and the Middle Ages. Travelers to Africa in the 18th and 19th centuries such as Prospero Alpini brought further reports of poison-spitting snakes to Europe.

The first formally described spitting cobra is the Indochinese spitting cobra ( Naja siamensis ); it was first described in 1768 by Josephus Nicolaus Laurenti (1735-1805). Laurenti was probably unaware that this species can defend itself by spraying poison. The first mention of poison-spitting cobras in modern scientific literature is from Friedrich Boie (1780–1870); in 1827 he described the species Naja sputatrix (literally "spitting cobra"). As recently as the early 20th century, it was a widespread belief that cobras spit poison by expelling air or shaking it off their teeth; only a work by the American herpetologist Charles Mitchill Bogert (1908–1992) from 1943 finally proved that the poison emerged as a ray from the fangs and explained its connection with the fine structure of the fangs.

Web links

Commons : Speikobras  - album with pictures, videos and audio files

swell

literature

  • R. McN. Alexander (1963): The Evolution of the Basilisk . Greece & Rome (Second Series) 10 (2), pp. 170-181.
  • Ruben A. Berthé (2011): Spitting behavior and fang morphology of spitting cobras . Dissertation Rheinische Friedrich-Wilhelms University Bonn.
  • Ruben Andres Berthé, Stéphanie de Pury, Horst Bleckmann & Guido Westhoff (2009): Spitting cobras adjust their venom distribution to target distance . In: Journal of Comparative Physiology A 195, pp. 753-757.
  • Ruben Andres Berthé, Guido Westhoff & Horst Bleckmann (2013): Potential targets aimed at by spitting cobras when deterring predators from attacking . In: Journal of Comparative Physiology A 199, pp. 335-340.
  • Edward R. Chu, Scott A. Weinstein, Julian White & David A. Warrell (2010): Venom ophthalmia caused by venoms of spitting elapid and other snakes: Report of ten cases with review of epidemiology, clinical features, pathophysiology and management . In: Toxicon 56, pp. 259-272.
  • Alexandra Deufel & David Cundall (200): Prey Transport in “Palatine-Erecting” Elapid Snakes . In: Journal of Morphology 258, pp. 358-375.
  • P. Gopalakrishnakone & LM Chou (1990, eds.): Snakes of Medical Importance (Asia-Pacific Region) . Venom and Toxin Research Group, National University of Singapore.
  • William K. Hayes, Shelton S. Herbert, James R. Harrison & Kristen L. Wiley (2008): Spitting versus Biting: Differential Venom Gland Contraction Regulates Venom Expenditure in the Black-Necked Spitting Cobra, Naja nigricollis nigricollis . In: Journal of Herpetology 42 (3), pp. 453-460.
  • Mohammad Ismail, Abdullah M. Al-Bekairi, Ayman M. El-Bedaiwy & Mohammad A. Abd-El Salam (1993): The ocular effects of spitting cobras: II. Evidence that cardiotoxins are responsible for the corneal opacification syndrome . In: Journal of Toxicology Clinical Toxicology 31 (1), pp. 45-62, PMID 8433415 .
  • Margaret A. Murray (1948): The serpent hieroglyph . In: The Journal of Egyptian Archeology 34, pp. 117-118.
  • RN Pugh, RDG Theakston (1980): Incidence and Mortality of Snake Bite in Savanna Nigeria . In: The Lancet 316 (8205), pp. 1181-1183.
  • Michael Triep, David Hess, Humberto Chaves, Christoph Brücker, Alexander Balmert, Guido Westhoff & Horst Bleckmann (2013): 3D Flow in the Venom Channel of a Spitting Cobra: Do the Ridges in the Fangs Act as Fluid Guide Vanes? . In: PLoS ONE 8 (5): e61548. doi : 10.1371 / journal.pone.0061548 .
  • Van Wallach, Wolfgang Wüster & Donald G. Broadley (2009): In praise of subgenera: taxonomic status of cobras of the genus Naja Laurenti (Serpentes: Elapidae) . In: Zootaxa 2236, pp. 26-36.
  • Guido Westhoff, Melissa Boetig, Horst Bleckmann & Bruce A. Young (2010): Target tracking during venom 'spitting' by cobras . In: The Journal of Experimental Biology 213, pp. 1797-1802.
  • Guido Westhoff, K. Tzschätzsch & Horst Bleckmann (2005): The spitting behavior of two species of spitting cobras . In: Journal of Comparative Physiology A 191, pp. 873-881.
  • Wolfgang Wüster, Steven Crookes, Ivan Ineich, Youssouph Mané, Catharine E. Pook, Jean-Francois Trape, Donald G. Broadley (2007): The phylogeny of cobras inferred from mitochondrial DNA sequences: Evolution of venom spitting and the phylogeography of the African spitting cobras (Serpentes: Elapidae: Naja nigricollis complex) . In: Molecular Phylogenetics and Evolution 45, pp. 437-445.
  • Wolfgang Wüster & Roger S. Thorpe (1992): Dentitional phenomena in cobras revisited: Spitting and fang structure in the asiatic species of Naja (Serpentes: Elapidae) . In: Herpetologica 48 (4), pp. 424-434.
  • Bruce A. Young, Melissa Boetig & Guido Westhoff (2009a): Functional Bases of the Spatial Dispersal of Venom during Cobra “Spitting” . In: Physiological and Biochemical Zoology 82 (1), pp. 80-89.
  • Bruce A. Young, Melissa Boetig & Guido Westhoff (2009b): Spitting behavior of hatchling red spitting cobras (Naja pallida) . In: The Herpetological Journal 19 (4), pp. 185-191.
  • Bruce A. Young, Karen Dunlap, Kristen Koenig & Meredith Singer (2004): The buccal buckle: the functional morphology of venom spitting in cobras . In: The Journal of Experimental Biology 207, 3483-3494.

supporting documents

  1. ↑ Wüster & Thorpe (1992), p. 431
  2. ↑ Hayes et al. (2008), p. 453
  3. ↑ Young et al. (2009b), p. 185
  4. ↑ Berthé (2011), p. 12
  5. ^ Wüster & Thorpe (1992), p. 432
  6. ↑ Berthé (2011), pp. 86-87
  7. ↑ Berthé (2011), pp. 13-14 & 86-87
  8. ↑ Triep et al. (2013), pp. 1 & 10
  9. ↑ Young et al. (2004), p. 3486
  10. ↑ a b Westhoff et al. (2010) p. 1797
  11. ↑ Young et al. (2009a), p. 82
  12. ↑ Berthé et al. (2013), p. 335
  13. ↑ Chu et al. (2010), p. 261
  14. ↑ a b Berthé et al. (2013), p. 339
  15. ↑ Berthé et al. (2009), pp. 753 & 755-757
  16. ↑ Westhoff et al. (2005), pp. 873 & 879
  17. ↑ Westhoff et al. (2010), pp. 1797 & 1800-1801
  18. ↑ Westhoff et al. (2005), pp. 879–880, see also Berthé (2011)
  19. ↑ Young et al. (2004), p. 3490
  20. ↑ Deuffel & Cundall (2003), p. 366
  21. ↑ Young et al. (2004), pp. 3488 & 3490-3492
  22. ↑ Berthe (2011), pp. 9-11
  23. ↑ see also Wallach et al. (2009)
  24. ↑ Wüster et al. (2007), pp. 437 & 443-445
  25. ↑ Wüster et al. (2007), pp. 444-445
  26. ↑ a b Chu et al. (2010), pp. 265-266
  27. ↑ Chu et al. (2010), pp. 265-267
  28. ↑ Chu et al. (2010), p. 262
  29. ↑ Ismail et al. (1993), pp. 45-46 & 54-58
  30. ↑ Chu et al. (2010), pp. 262 & 269
  31. ^ Pugh & Theakston (1980), p. 1181
  32. ↑ Toriba in Gopalakrishnakone & Chou (1990), pp. 463-469
  33. ↑ Chu et al. (2010), pp. 268-269
  34. ↑ a b Chu et al. (2010), p. 269
  35. ↑ a b Berthé (2011), p. 9
  36. ↑ Murray (1948), pp. 117-118
  37. ↑ a b Chu et al. (2010), p. 260
  38. ↑ Alexander (1963), pp. 171-174
  39. ↑ Josephus Nicolaus Laurenti (1768): Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena . Vienna.
  40. ↑ Friedrich Boie (1827): Comments on Merrem's attempt at a system of amphibians . Isis von Oken 20, pp. 508-566.
  41. ^ Charles Mitchill Bogert (1943): Dentitional phenomena in cobras and other elapids with notes on adaptive modifications of fangs . Bulletin of the American Museum of Natural History 81, pp. 285-363.
  42. ↑ Young et al. (2009a), p. 80