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Tuesday, June 30, 2015

The Linnaean Snakes: Part II

This post will soon be available in Spanish!

Click here to read Part I.


Last month I wrote about "serpentine royalty": the first species of snakes to be formally described using the Linnaean system—those described by Linnaeus himself in the 1758 10th edition of Systema Naturae. Out of 100 snake species in that tome, let's take a closer look at the four that still bear their original names.

Infographic showing the "tenure" of 807 snake genera used by more than one taxonomist.
An additional 387 genera used only once are not shown, for a total of 1,194. Of these, about 500 are currently in use.
The three longest lines at the top are the original three genera coined by Linnaeus in 1758 and still in use today.
Data span 1758-2010, from The Reptile Database.
Click for full version.

Coluber constrictor

In the immortal words of Jeffrey C. Beane: "Linnaeus first gave me my scientific name, but reflecting upon it, I think: “What’s his game?” Perhaps he was drunk on that day (or smoked pot), for a snake I am, yes, but constrictor I’m not."

The Racer (Coluber constrictor) is one of only four snakes
that have gone by the same scientific name since 1758.
Today Coluber contains only 14 species, 11 of which
were recently reallocated to it from Masticophis
.
In 1758 Linnaeus placed 61 species into Coluber,
of which only C. constrictor remains.
Many contemporary naturalists have been puzzled by the scientific name of the North American Racer, which normally crams its live prey into its mouth rather than constricting it. Linnaeus's descriptions ("Inhabits Canada. Triangular head1. It approaches men, twisting itself around their feet, but it is harmless."[10th ed.]/"Inhabits North America. Runs swiftly and bites very hard, but is not poisonous. Twists itself around the legs of such as approach it. Very smooth and slender. Black, pale blue beneath, white throat."[12th ed.]) glibly paraphrase that of his student Pehr Kalm2, who gave a several-page account of racers in his 1753 book Travels in North America. Among other myths, Kalm recounts tales told by "numbers of credible people" that racers, especially males interrupted during mating in the spring, will chase and trip people, but he was unable to reproduce the behavior despite his best efforts, saying "I know not for what reason they shunned me, unless they took me for an artful seducer". He was also doubtful of the claim that racers and other snakes enchant or hypnotize their prey, but he was reluctant to discount the possibility entirely because "many of the worthiest and most reputable people have related [the story], and...it is so universally believed here that to doubt it would be to expose one's self to general laughter." Given their willingness to accept these and other myths, it's not unlikely that Kalm and some of his informants, including the naturalists John Bartram and Cadwallader Colden, also confused racers with ratsnakes, both species being black along the east coast, which accounts for Kalm's descriptions of racers constricting and climbing large trees to eat birds' eggs, and might explain their perpetual misnomer.

Racer plate from Catesby's Natural History.
Catesby also described cornsnakes but not ratsnakes,
suggesting that perhaps he too confused ratsnakes and racers.
The English naturalist Mark Catesby3 had previously described racers, which he called Anguis niger, and other snakes in his 1731 Natural History of Carolina, Florida and the Bahama Islands, the first published account of the flora and fauna of North America. Apparently Linnaeus didn't think much of this book, because he dismissed it in his introduction to the reptiles by writing "Catesby sketched a few serpents more beautifully than he made notes about them". He must have had some respect for Catesby, though, for he named both the lily-thorn Catesbaea and the bullfrog Rana catesbeiana after him. It is a bit of a mystery where the racer specimen that Linnaeus saw originated, or if he even saw one. If he did, it must have been collected and sent to him by Kalm, but the whereabouts of Kalm's specimens of North American snakes, if they exist, are unknown. There is a specimen of a racer in the Royal Museum in Stockholm which is marked with a Linnaean label. An 1802 catalogue states that the specimen came from the King, but of all the snakes with these labels it is the only one Linnaeus does not describe in his 1764 manuscript on the King's collection, Museum Adolphi Friderici (where the binomial system is used for the first time), so this information is probably erroneous. Also described by Linnaeus but subsequently lost are Kalm's specimens of Northern Watersnakes (today, Nerodia sipedon) and Common Gartersnakes (today, Thamnophis sirtalis), as well as two more mysterious specimens which Linnaeus named Coluber leberis and Coluber ovivorus4.

Boa constrictor

Top: Boa constrictor
Bottom: Boa constrictor plate from Linnaeus & Sundius's
1748 Surinamensa Grilliana, drawn by P. A. Petersson
and engraved by C. Bergquist
Unlike racers, this snake is the eponymous constrictor. The name boa comes from the Latin boa for ‘large snake,’ after an animal mentioned in the Natural History of Pliny the Elder, which ate cows ('bos' in Latin). Linnaeus, whose descriptions were written in a kind of telegram style, without verbs, in a deliberate effort to be as brief as possible and save space, was particularly laconic if descriptions had already been published by himself or others. Of this species he said only that it "inhabits the [West] Indies and warm parts of the Americas", because boa constrictors had already been described by two of his primary sources on snakes, the Dutch naturalists Albertus Seba and Laurens Gronovius. Additionally, a specimen collected in Surinam by Claes Grill reached Linnaeus in the 1740s, and is described and illustrated in a dissertation defended by Peter Sundius, one of Linnaeus's early students. The catalogue of the King of Sweden's natural history collection also contains a description of one. However, Linnaeus could have been even more succinct had he recognized that a dark-colored specimen from the collection of Charles de Geer, a Swedish entomologist, was also a Boa constrictor. This collection was also the source of his anaconda, Burmese Python, and a handful of other snakes he had seen nowhere else, all of which are now in the Royal Museum in Stockholm, but he did not recognize that the boa in de Geer's collection was the same species that he had already called Boa constrictor.

Image of an African Python (Python sebae) from Charles Challié Long's
1876 book Central Africa: Naked Truths of Naked People
The caption reads "Capture of a Boa-Constrictor"
The confusion may have arisen because de Geer's specimen had many more ventral scales than other boas Linnaeus had examined. Linnaeus preferred to use the number of ventral and subcaudal scales to distinguish species of snakes over their color or pattern (like his quantitative sexual system for classifying plants, Linnaeus's methods were a predecessor to modern ones), but he recognized that even these scale counts varied considerably within species. Kalm stated that his teacher thought "it was better to make use of an imperfect character than none at all" and he was hopeful that "time, and greater acquaintance with this class of animals may perhaps clear up their natural characters". Linnaeus named Boa constrictor earlier on the page than Boa orophias, which is why we use the former name rather than the latter, which is now used for a subspecies from St. Lucia. In his defense, Linnaeus did write of B. orophias: "Face of the constrictor, but dark", suggesting that he thought they might be the same snake. To Linnaeus's terse description, Gmelin, writing in the 13th edition of Systema Naturae, added: "beautifully variegated with rhombic spots, belly whitish" and noted that it is "of vast strength and size, measuring sometimes 12 yards long, and by twisting itself round the bodies of deer, leopards, and other larger quadrupeds, breaks the bones, and after covering them over with a slimy mucus gradually swallows them". Certainly these descriptions helped popularize these large and impressive snakes, specimens of which were curios of the highest value. This popularization led to many explorers and travel writers calling any large snake a boa constrictor (including pythons) for centuries to come.

Crotalus durissus


Global distribution of 35 species of the genus Crotalus
Data from IUCN; click for a larger version
Rattlesnakes have captured the attention of Europeans ever since they first started settling the New World. Using their Nahuatl (Aztec) name, Teuhtlacocauhqui, Francisco Hernández described them in his 1615 Quatro Libros de la Naturaleza"When they strike, the bite is fatal unless treated promptly...It has a tail with rattles, one for each year of its life...It has two curved fangs in its upper jaw to inject its venom...It moves in a slithering fashion. Indians hunt and capture them and hang them around their necks...Those who raise them at home say they can live for up to a year without eating anything...When wounded and angry, it whips around, shaking its rattles, and raises its neck to frighten those nearby. However, it does not bite unless provoked." Hernández's book also contains the earliest illustration of a rattlesnake, which is certainly Crotalus durissus. Many other 16th and 17th century authors also wrote about rattlesnakes, dating back to their earliest mention in print, by Cieça de Leon in 1554. Many of these writings contain both accurate information and the first printed iterations of several still-current myths. Clearly, native Americans had known of rattlesnakes since ancient times; the Aztecs and Mayans had a rattlesnake constellation which may have been part of their zodiac.

Top: Neotropical Rattlesnake (Crotalus durissus)
Bottom: The earliest illustration of a rattlesnake
in a book, from Hernández 1628
Because of their tails, Linnaeus thought that rattlesnakes were so unusual that he placed them in their own genus, Crotalus, separating them from other vipers (which he placed in the genus Coluber despite their solenoglyphous fangs). Linnaeus described three species of rattlesnakes: Crotalus horridus (see below), C. durissus, and C. dryinas. Like Boa constrictor and B. orophias, we now consider the latter two species to be the same, but unlike the boa names we use the name durissus for the species even though dryinas precedes it on the page5. Although most rattlesnakes are North American, Linnaeus's C. durissus specimen was collected by Claes Grill in Surinam and originally described in detail in 1748 in the same dissertation that contained the boa constrictor description. In contrast, the "C. dryinas" specimen was from the king of Sweden's collection and lacked geographic data. It is clear that King Adolf Frederick was not the most attentive curator—his curatorial record is incomplete, and over time many of his specimens have been lost and their labels mixed up or deliberately modified. He wasn't the most assertive head of state either, described as "little more than a state decoration"—although apparently he did like snakes. Both rattlesnake specimens from his collection that Linnaeus examined have apparently been lost for over 100 years, and the Grill specimen is lost as well. As a result, and because Linnaeus's descriptions are so terse, the names of the three rattlesnakes (horridus, durissus, and dryinas) were for many years confusingly and inconsistently applied. For example, both Holbrook and Duméril referred to the timber rattlesnake as C. durissus in their respective classic works, Duméril additionally called the neotropical rattlesnake C. horridus, and Boulenger refers to the eastern diamondback as C. durissus in his catalogue of snakes in the British Museum.

Crotalus horridus

Crotalus horridus is the type species of the genus Crotalus,
which today contains 39 species.
Of this snake which would come to symbolize America, Linnaeus wrote (appropriately) "Lives in America. Very venomous; its antidote is Senega (snakeroot). It is eaten by pigs, and calls down birds and squirrels from the trees into its jaws." This is a lengthy description for him, especially considering that timber rattlesnakes had already been described in 1683 (by Edward Tyson, who dissected one), 1721 (by Richard Bradley), 1734 (by Albertus Seba), and in 1745 and 1754 by Linnaeus himself (the first time in a dissertation defended by Barthold Rudolph Hast describing specimens from Count Carl Gyllenborg's collection of rare herps, insects, corals, and minerals, and the second time in his description of the collection of the Swedish king, Museum Adolphi Friderici). C. horridus is the only North American snake Linnaeus described that was not sent to him by Kalm or by his other primary North American informant, Alexander Garden. The specimen that he described had 7 rattle segments, and he was able to give a count of the ventral scales (167), which indicates that he examined a complete specimen, but the specimen that is now in the Royal Museum in Stockholm is represented only by a severed tail, which has 12 rattle segments, and a head, which is actually from a bushmaster (Lachesis muta). Like the specimens of the neotropical rattlesnakes, apparently the original specimen has been missing since at least 1899, and possibly much earlier.6

Catesby's Timber Rattlesnake (which he called Vipera
caudisona
, but which Linnaeus and we call Crotalus horridus).
Kalm considered this "an incomparable illustration".
Although Kalm did not collect a rattlesnake for Linnaeus, he gave a lengthy, detailed, and incredibly accurate description of rattlesnakes and their relationship with humans, which is as much an account of snake biology as it is of the cultural history of colonial America. His words suggest that rattlesnakes were already on their way out in eastern North America in the mid-1700s: "In all my travels, I saw only 3 living specimens. I frequently heard them in the nearby thicket, but it seemed inadvisable to pursue them." It is a testament to Kalm's scientific training that he surpassed many modern observers in accurately stating that "The snake is usually 3 to 4 feet long. The largest one I saw was 6 feet long and as thick as the calf of a man's leg. Usually they are as thick as the wrist" and "They travel slowly, thus one need not fear being overtaken" and "The rattler is peculiar in that he usually does not injure a person unless forced to defend himself."  Despite these honest observations, Kalm had no special love of snakes, including them in a list of reasons that he preferred Sweden to America: "The rattlesnakes, horned-snakes, red-bellied, green, and other poisonous snakes, against whose bite there is frequently no remedy, are in great plenty here".

Timber Rattlesnake from Bradley (1721)
Catesby also described the Timber Rattlesnake, which he called Vipera caudisona, at length. Gmelin, writing in the 13th edition of Systema Naturae, expanded Linnaeus's description in both inaccurate ("The most venomous of the serpent tribe") and accurate ("They seldom bite unless when irritated, or for the purpose of securing their prey, and the fascinating power which has been attributed to them is probably nothing more than that they first bite the animal and patiently watch till it dies to devour it") ways. In 1754 in Museum Adolfi Frederici Linnaeus wrote: "...nor can he escape with life who is bitten by the Rattle-snake (Crotalus horridus) in any part near a great vein. But the merciful God has distinguished these pests by peculiar signs, and has created them most inveterate enemies; for as he has appointed cats to destroy mice, so has he provided the Ichneumon [mongoose] (Viverra Ichneumon) against the [cobra], and the Hog to persecute the latter. He has moreover given the Crotalus a very slow motion, and has annexed a kind of rattle to its tail, by the motion of which it gives notice of its approach; but, lest this slowness should be too great a disadvantage to the animal itself, he has favoured it with a certain power of fascinating squirrels from high trees, and birds from the air into its throat, in the same manner as flies are precipitated into the jaws of the lazy toad."

Other notable snake species described by Linnaeus

Linnaeus described 83 other snake species between his 10th and 12th editions that are still considered valid today, plus 31 that are not (including Crotalus dryinas and Boa orophias). These include many familiar, widespread, and notable species, including 2 scolecophidians, Anilius scytale, and an Asian pipesnake from Sri Lanka (all of which he placed in the genus Anguis, which we today use for legless lizards), several huge constrictors including the Indian Python, Boa Constrictor, and Green Anaconda (but also three smaller tree boas and two sand boas, the latter also in Angius), 13 vipers including the fer-de-lance, copperhead, European adder, bushmaster, and pygmy rattlesnake, a pair of homalopsids, 46 colubrids (including many familiar European and American species but also an African egg-eater and an Asian flying snake), 5 lamprophiids, and 9 elapids (including 3 cobras, 2 coralsnakes, and 2 sea snakes). He also made a few brief comments about snake anatomy and biology as footnotes or in his introductory material, including his method for counting ventral and subcaudal scales (first used in Amphibia Gyllenborgiana and still in use today) as well as the correct observations that "Serpents of our country hibernate and in the early spring shed their skin, that is to say, their old age" and "Serpentes often swallow down prey twice as thick as their neck, on account of their expandable, unarticulated jaws". In other works, he presents a great deal of information on snakebite and, the consummate botanist, its treatment using various medicinal plants. Although Linnaeus bore no special love for snakes, he treated them as he did other biodiversity, and I encourage all modern biologists to do the same—to view snakes as wildlife rather than pests, as a beautiful and diverse part of our natural heritage, to see them as what they are rather than what we imagine them to be.

It is tempting to imagine Linnaeus as a brilliant solitary taxonomist, aided and sent specimens by his correspondents, colleagues, and students but intellectually working alone. But, as today, Linnaeus relied heavily on his network both to obtain specimens and to describe them with reference to those who had gone before. Of the 74 species in the 10th edition, only four were brand new original descriptions (these were Vipera aspis from southern Europe, Epicrates cenchria from South America, Erythrolamprus triscalis from Curaçao, and Duberria lutrix from Africa), and the 12th contained scarcely more, mostly southeastern North American species sent to Linnaeus by Alexander Garden. Almost ten times that many new snakes were described last year alone.

Coronella austriaca from Laurenti 1768
It's probably safe to assume that Linnaeus described every snake he ever saw, since this is what he did with everything else. And, considering he lived in Sweden most of his life and never traveled further south than Germany, he did pretty well, nailing numerous tropical species of snake collected by others and sent either to him, or that he examined in the collections of zoologists in Germany, England, and Holland. Systema Naturae contains snakes from every continent except for Australia, which was only just becoming known in Europe at the time of Linnaeus's death (his correspondant Joseph Banks and two of his apostles, Daniel Solander and Anders Sparrman, sailed around the world with James Cook and visited Australia and Oceania in the 1760s and 1770s; Linnaeus's health was poor throughout the 1770s and he died in 1778). But, there is one glaring oversight in Linnaeus's snake work: he described only two of the three native Swedish snakes (Natrix natrix and Vipera berus). Both of these he initially described in his 1746 Fauna Svecica, an account of the animals of Sweden containing 1,357 species in its original edition (upated 1761 with 2,266 species), in which he used cumbersome pre-binomial names such as Coluber natrix scutis abdominalibus CLXX squamis caudae LX ("Water snake with 170 ventral scales and 60 subcaudal scales"), which later became the much simpler yet no less unequivocal Coluber natrix in Systema Naturae. But he missed one: the smooth snake, Coronella austriaca, which was described by J.N. Laurenti7 and named for his native Austria (where it is also found) ten years after the 10th edition of Systema Naturae. Did Linnaeus ever see a Coronella in all the years he lived, worked, and botanized in Sweden? Smooth snakes are active during the day in dry, sunny clearings where they bask in bushes, and although they are not found as far north as Uppsala, they do occur in Småland, where Linnaeus grew up. It seems likely that Linnaeus would have seen them—did he think they were the same species as another kind of snake? If not, why did he leave them out of Fauna Svecica and Systema Naturae, which were intended to be as comprehensive as possible?



1 In the 10th edition Linnaeus confused specimens of racers with those of the black form of the Eastern Hognose Snake (Heterodon platirhinos), but by the 12th edition these had been separated and the phrase "triangular head" removed from the description of the racer.



2 After whom the mountain laurel genus Kalmia is named.



3 Like Maria Sibylla Merian before him, Catesby was among the first naturalists to draw his plants and animals interacting in their natural habitats, a style of representation that would later be used by Alexander Wilson and John James Audubon. He was also the first to abandon the Native American names for his subjects, instead establishing scientific binomials based on relationships a la Linnaeus. Had his work been published three decades later, he might have been immortalized as the father of North American herp taxonomy, and many of the scientific names that we use today could have been very different. Catesby's book, richly illustrated, was much more popular than Linnaeus's.



4 The specimen named Coluber leberis was likely a Storeria, the only genus found in the area traversed by Kalm (Pennsylvania, New York, New Jersey, and southern Ontario) with matching scale counts. Although the scale counts and pattern description match S. occipitomaculata better and this species is more common than S. dekayi in northeastern North America, the specimen could have been either, and since we cannot examine it, the name is not used. Coluber ovivorus is even more enigmatic, because the description does not match any northeastern snake well.



5 This is because, by the time it was all sorted out, the name C. durissus had ended up being in more widespread use, so the "proper" name dryinas was suppressed by the International Commission on Zoological Nomenclature.



6 There is a reasonable chance that the specimen that Linnaeus first named C. horridus was actually from South America, and thus was really C. durissus as well, but since we cannot prove this beyond a shadow of a doubt, in 1926 the International Commission on Zoological Nomenclature decided to continue to use it for the timber rattlesnake.



7 Little is known about Laurenti. No picture of him exists, and his 1768 thesis, 
Specimen medicum, was his only publication. In it, he elevated Linnaeus's order Reptilia to a class, distinguishing it from class Amphibia, into which Linnaeus lumped both amphibians and reptiles. Laurenti also tripled the number of reptile genera, coining some of today's most familiar genus names, including Vipera, Natrix, Laticauda, Dipsas, and Naja.


ACKNOWLEDGMENTS

Thanks to Todd PiersonPatrick Jean, and JD Willson for the use of their photos, and to my mom for getting me William Blunt's Linnaeus for Christmas this year, which inspired this article.

REFERENCES

Andersson, L.G. 1899. Catalogue of the Linnaean type-specimens of snakes in The Royal Museum in Stockholm. Bihang till Kongl. Svenska Vetenskaps-Akademiens Handlingar 24:1-35 <link>

Blunt, W. 2002. Linnaeus: The Compleat Naturalist. Princeton University Press, Princeton, New Jersey, USA <link>

Bradley, R. 1721. A philosophical account of the works of nature, London <link>

Campbell, J.A. and W.W. Lamar. 2004. The Venomous Reptiles of the Western Hemisphere (2 Vol.). Cornell University Press, Ithaca, New York <link>

Catesby, M. 1731. The Natural History of Carolina, Florida, and the Bahama Islands, London.<link>

Cieça de Leon, P. 1554. La Crónica del Perú, Seville <link>

Gronovius, L.T. 1756. Museum Ichthyologicum. Theodorum Haak, Lugduni-Batavorum <link>

Hernández, F. 1615. Quatro libros. De la naturaleza, y virtudes de las plantas, y animales que estan receuidos en el vso de medicina en la Nueua España, y la methodo, y correccion, y preparacion, que para administrallas se requiere con lo que el doctor Francisco Hernandez escriuio en lengua latina <link>

Kalm, P. 1771. Travels Into North America. T. Lowndes, London <link>

Kitchell, K. and H.A. Dundee. 1994. A trilogy on the herpetology of Linnaeus's Systema Naturae X. Smithsonian Herpetological Information Service 100 <link>

Klauber, L.M. 1941. The rattlesnakes listed by Linnaeus in 1758. Bulletin of the Zoological Survey of San Diego 17:81-95 <link>


Klauber, L.M. 1948. Some Misapplications of the Linnaean Names Applied to American Snakes. Copeia 1948:1-14 <link>

Klauber, L.M. 1956. Rattlesnakes: their habits, life histories, and influence on mankind. University of California Press <vol 1/vol 2>

Krecsák, L. and R. Wahlgren. 2008. A survey of the Linnaean type material of Coluber berus, Coluber chersea and Coluber prester (Serpentes, Viperidae). Journal of Natural History 42:2343-2377 <link>

Larsen, E.L. 1957. Pehr Kalm's Account of the North American Rattlesnake and the Medicines Used in the Treatment of its Sting. American Midland Naturalist 57:502-511 <link>

Laurenti, J.N. 1768. Specimen medicum: exhibens synopsin reptilium emendatam cum experimentis circa venena et antidota reptilium austriacorum. Typ. Joan. Thomae nob. de Trattnern, Vienna, Austria <link>

Linnaeus, C. 1745. Amphibia Gyllenborgiana. Uppsala University, Uppsala <link>

Linnaeus C. 1746. Fauna Svecica Sistens Animalia Sveciæ Regni: Quadrupedia, Aves, Amphibia, Pisces, Insecta, Vermes, Distributa per Classes & Ordines, Genera & Species. Differentiis Specierum, Synonymis Autorum, Nominibus Incolarum, Locis Habitationum, Descriptionibus Insectorum. Stockholmiæ [Stockholm] (Sweden): Sumtu & literis Laurentii Salvii <link>

Linnaeus, C. 1746. Museum Adolpho Feidericianum. Uppsala University, Uppsala <link>

Linnaeus, C. 1748. Surinamensia Grilliana. Uppsala University, Uppsala <link>

Linnaeus, C. 1764. Museum S:ae R:ae M:tis Adolphi Friderici Regis Svecorum, Gothorum, Vandalorumque &c. &c. &c. in quo Animalia rariora imprimis & exotica: Aves, Amphibia, Piscis describuntur. Tomi secundi Prodromus. Holmiae. Pp 110. <link/translated>


Linnaeus, C. 1758. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Holmiae, 823 pp. <link>


Linnaeus, C. 1762. Morsura Serpentum. Uppsala <link>

Linnaeus, C. 1766. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio duodecima, reformata. Holmiae, 532 pp <link>

Lönnberg, E. 1896. Linnean type-specimens of birds, reptiles, batrachians and fishes. Bihang till Kongl. Svenska Vetenskaps-Akademiens Handlingar 22:1-45 <link>

McIvor, R.S. 2010. Aztec Constellations Preserved by Duran. Journal of the Royal Society of Canada 104:46-50 <link>

Seba, A. 1734-1765. Locupletissimi rerum naturalium thesauri accurata descriptio, et iconibus artificiosissimis expressio, per universam physices historiam :opus, cui, in hoc rerum genere, nullum par exstitit. Apud Janssonio-Waesbergios & J. Wetstenium & Gul. Smith, Amstelaedami <link>

Smit, P. 1979. The zoological dissertations of Linnaeus. Svenska Linnesallskapets Arsskrift 1978:118-136 <link>

Tyson, E. 1683. Vipera Caudi-Sona Americana, Or the Anatomy of a Rattle-Snake, Dissected at the Repository of the Royal Society in January 1682/3 by Edw. Tyson MD Coll. Med. Lond. Cand. & RS Soc. Philosophical Transactions (1683-1775) 13:25-46 <link>

Creative Commons License

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Monday, May 25, 2015

The Linnaean Snakes: Part I


Although recent findings have shed new light on the (so far) oldest-known fossil snakes, extending the fossil record of snakes back in time an incredible 70 million years, this article is about a more anthropocentric definition of "the first snakes". It's about the first snakes to be named and described using the modern system of classification: those described and classified by Linnaeus in the 10th edition of his Systema Naturae, using consistently together for the first time a binomial naming system for genera and species and a hierarchical category system for higher taxa (i.e., families, orders, classes, phyla, and kingdoms). Although Darwin's theory of evolution has ultimately refocused modern taxonomy on cladistics and phylogenetic trees, the Linnaean system is not wholly incompatible with our new understanding of the common ancestry of all life, and has and will continue to be used.

Carl Linnaeus (left) and Peter Artedi (right)
Carl Linnaeus was primarily a botanist, coining Latin names for and describing over 7,700 species of plants in his lifetime. However, he did a pretty good job of naming and describing species of animals as well, with over 4,400 to his name. His interest in describing animals derived partly from an agreement he made with his friend and one-time rival, Peter Artedi, when the two men were students: that if either of them should die, the other would complete their life's work. Artedi, an ichthyologist, drowned at age 30 (wrote Linnaeus, "too early...did the most distinguished of ichthyologists perish in the waters, having devoted his life to the discovery of their inhabitants!"), so Linnaeus took it upon himself to organize, complete, and publish Artedi's work on the classification of fishes. In truth, the two men developed the basics of zoological nomenclature together, and if Artedi had lived he probably would have shared equally in the renown which has come to Linnaeus today.

Tantilla melanocephala from the King of Sweden's collection
The snakes that Linnaeus described came primarily from a few sources. Several small collections ('curiosity cabinets') made by European aristocrats and businessmen formed the basis of a handful of his zoological dissertations, short papers written primarily by Linnaeus and defended by his students at the University of Uppsala, as was the custom at the time. One such dissertation, Amphibia Gyllenborgiana (defended by B. R. Hast in 1745), describes a collection donated by the university chancellor, Count Carl Gyllenborg, which contained the first attempt to classify snakes according to their numbers of scales, rather than their colors or patterns. Another, Surinamensa Grilliana (defended by Peter Sundius in 1748), describes a collection acquired with the help of Claes Grill, a wealthy merchant with an interest in natural history who used his directorship of the Swedish East India Company to obtain plants and animals from Surinam. Some of these specimens are still in the museum in Uppsala, including a caecilian, two Red Pipesnakes, a false coralsnake, and a parrotsnake. These dissertations do not use the binomial nomenclature for which Linnaeus is now famous. A few years later, Linnaeus was asked by the King and Queen of Sweden to organize, describe, and publish accounts of their personal natural history collections. In those days, it was as fashionable to collect objects of natural history, such as shells, insects, and preserved specimens, as it is to collect art today. The king in particular had amassed a large collection of snakes, many of which are still in the Swedish Museum of Natural History today (and looking remarkably well for being almost 300 years old), and these are described in Linnaeus's 1754 Museum Adolphi Friderici. During the 1750s and 60s, many of Linnaeus's students (which he called his "apostles") traveled the world collecting and sending him specimens, but in accordance with his interests they mostly sent him plants. A few students, including Pehr Kalm, who explored and collected in North America, and Fredrik Hasselqvist, who explored the Middle East, sent Linnaeus a few reptiles. Almost half of the snakes in Systema Naturae are from the king's collection, and most of the others are from the collections and works of two Dutch naturalists whose collections Linnaeus had seen as a young traveler: Albertus Seba, who wrote a Thesaurus of animals with many engravings (including hundreds of snakes), and Laurens Theodorus Gronovius, who worked mostly on fish (the distinction between fish and reptiles was still a bit hazy at the time). Although Linnaeus no doubt could have read about other snakes, he was skeptical of anything he had not examined himself1, and limited his published descriptions to specimens he could examine personally.

Title page of the 10th edition
In the 10th edition of his Systema Naturae, Linnaeus listed a total of 110 species in the order Serpentes, in six genera: Crotalus, Boa, Coluber, Anguis, Amphisbaena, and Caecilia. The first three will be familiar to any snake enthusiast, but the latter three, while legless, have since been reclassified as lizards or amphibians2. Of the 100 species that are actually snakes, 74 are still considered valid today. Linnaeus added 18 more snake species in his 1766 12th edition3, 13 of which are still valid, for a grand total of 87 snake species currently bearing his name, over 2% of modern species; only the authors of Erpétologie Générale can claim more. For reptiles as a whole he still ranks as the 9th most prolific taxonomist4. Pretty good for a botanist. To be fair, Linnaeus had the distinct advantage of Systema Naturae's 10th edition being later declared the starting point of zoological nomenclature, so he has benefited from having any names which preceded his automatically invalidated, whatever their notoriety. His cavalier attitude towards the work of those who came before him rankled many of his contemporaries, although he cited their descriptions wherever he could verify them. This also means that it was technically impossible for him to have "redescribed" any taxa, as many later authors often did, even though in reality of course many kinds of snakes were already recognized and some had names dating back to antiquity (many of which he used). Only 14 of the 100 snake species in SN10 were described therein for the first time. All these advantages didn't stop him from naming invalid species though—26 of the 100 species in the 10th edition (and 5 in the 12th) he described twice, under two different names; that is, later herpetologists decided that the specimens in his descriptions were members of the same species and synonymized (or "lumped") them, which accounts for the reduction in his total number of snake species from 118 to 87.

The travels of Linnaeus's students (click for larger version)
Linnaeus worked on classifying many different groups of organisms, and he always worked in great haste, because there was so much to do. As a result, he could be fairly careless, particularly when it came to the geography of his specimens (i.e., his type localities). Because he had not actually been to many of the places where his specimens came from, he had to rely on the word of others for this information. When specimens came from his apostles or from other contemporaries, they usually had pretty accurate, if general, locations (e.g., 'America', 'Africa'). If they were older, such as those in the collections of Gronovius, Seba, and the king, they were often accompanied by unverifiable locations, many of which were incorrect. In fact, only 33 of the 74 snake species in Linnaeus's SNX have unambiguously correct location information. A further 21 are unambiguously wrong, and 20 bear the label 'Indiis', which might refer either to India or to the West Indies (and, in either case, is still incorrect for certain specimens). In certain cases, it almost seems that labels were switched, such as a South American Xenodon from 'Asia' and an Asian Amphiesma from 'America'. Overall his snakes are fairly diverse, with good geographic representation, except for Australia, which was first botanized in 1770, close to Linnaeus's death, by Linnaean apostle Daniel Solander, sailing onboard James Cook's Endeavour along with Joseph Banks (and resulting in the name of Botany Bay).

Many other later taxonomists reorganized Linnaeus's snake genera, breaking up his combinations by placing the vast majority of the snakes Linnaeus described into new genera. However, 4 of his snake species retain their original genus and species names today. That three of them would was inevitable because of the principle of priority and the "type" concept5, but the fourth is a bit of a bonus. Next month, in Part II, we'll take a closer look at these four species, named by Linnaeus when George Washington was in his twenties, 257 years ago.



1 Seba's Thesaurus contained a now-famous image of a hydra, which Linnaeus inspected in Hamburg in 1735 and exposed as a hoax made from weasels and snake skins. This and other mythical creatures he listed as "animalia paradoxa" in early editions of Systema Naturae, although some (like the paradoxical frog) turned out to be real! For instance, he was correct in stating that "All the other dragons listed by authors are fictitious, like the hydra, which I saw at Hamburg, but which was an outstanding work, not of nature, but of art.", but erred in thinking that "The horned viper is a coluber fabricated by the craft of the Arabs, who pierced its head with the claws of a small bird and then inserted them there".



2 Originally, two scolecophidians (Amerotyphlops reticulatus and Typhlops lumbricalis), the monotypic Anilius scytale, a pipesnake (Cylindrophis maculatus), and two sand boas (Eryx colubrinus and E. jaculus) were placed in Linnaeus's genus Anguis, but were later reclassified (correctly) as snakes.



3 Nothing new was added to the 11th edition, which was simply a reprint of the 10th. In 1789, 13 years after Linnaeus's death, Johann Friedrich Gmelin added three more species of snakes to the 13th and last edition, by which time other zoologists such as Laurenti (who also split reptiles from amphibians and tripled the number of reptile genera) had already contributed a great deal to snake taxonomy.



4 It's fair to say that Linnaeus didn't like snakes or other reptiles. In the first edition of Systema Naturae he wrote: "The Creator in his benignity has not wanted to continue any further the Class of Amphibia for, if it should enjoy itself in as many Genera as the other Classes of Animals, or if those things were true that the Tetralogists have fabricated about Dragons, Basilisks, and such monsters, the human genus would hardly be able to inhabit the earth." He continues in Museum Adolphi Friderici: "Truly formidable are the arms which the Lord of nature has given to some animals. Though he has left serpents destitute of feet, wings, and fins, like naked fishes, and has ordered them to crawl on the ground exposed to all kinds of injuries, yet he has armed them with dreadful envenomed weapons: but, that they may not do immoderate mischief, he has only given these arms to about a tenth part of the various species; at the same time arraying them in such habits that they are not easily distinguishable from one another, as the rest of animals are; so that men and other creatures, while they cannot well distinguish the noxious ones from those which are innocent, shun them all with equal care. We shudder with horror when we think of these cruel weapons. Whoever is wounded by the Hooded Serpent (Coluber Naja) expires in a few minutes; nor can he escape with life who is bitten by the Rattle-snake (Crotalus horridus) in any part near a great vein. But the merciful God has distinguished these pests by peculiar signs, and has created them most inveterate enemies; for as he has appointed cats to destroy mice, so has he provided the Ichneumon [mongoose] (Viverra Ichneumon) against the former serpent, and the Hog to persecute the latter. He has moreover given the Crotalus a very slow motion, and has annexed a kind of rattle to its tail, by the motion of which it gives notice of its approach...On account of these and various other poisonous serpents and worms of India, which crawl upon the ground, swim in the waters, or twine among the branches of trees, we prefer our barren and craggy woods to the everblooming meadows and fruitful groves of Indian climes; and we had rather suffer the inconveniences of our northern snows, than enjoy their enviable luxuries."



5 The principle of priority states that the first name given to a plant or animal is the correct one, and all subsequent uses of other names for that species or of that name for other species are invalid. Some formal exceptions are allowed on a case-by-case basis. The type concept permanently associates a species with a genus, which helps biologists decide which genus name to use for which species when genera are split or lumped.

ACKNOWLEDGMENTS

Thanks to my mom for getting me William Blunt's Linnaeus for Christmas this year, which inspired this article.

REFERENCES

Andersson, L.G. 1899. Catalogue of the Linnaean type-specimens of snakes in The Royal Museum in Stockholm. Bihang till Kongl. Svenska Vetenskaps-Akademiens Handlingar 24:1-35 <link>

Blunt, W. 2002. Linnaeus: The Compleat Naturalist. Princeton University Press, Princeton, New Jersey, USA <link>

Gronovius, L.T. 1756. Museum Ichthyologicum. Theodorum Haak, Lugduni-Batavorum <link>


Linnaeus, C. 1745. Amphibia Gyllenborgiana. Uppsala University, Uppsala. Dissertation (B. R. Hast, respondent)
 <link>

Linnaeus, C. 1748. Surinamensia Grilliana. Uppsala University, Uppsala. Dissertation (P. Sundius, respondent) <link>

Linnaeus, C. 1758. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Stockholm. <link>

Linnaeus, C. 1764. Museum S. R. M. Adolphi Friderici. Stockholm <link/translated>

Linnaeus, C. 1766. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio duodecima, reformata. Stockholm <link>

Kitchell, K. and H.A. Dundee. 1994. A trilogy on the herpetology of Linnaeus's Systema Naturae X. Smithsonian Herpetological Information Service 100 <link>

Seba, A. 1734-1765. Locupletissimi rerum naturalium thesauri accurata descriptio, et iconibus artificiosissimis expressio, per universam physices historiam :opus, cui, in hoc rerum genere, nullum par exstitit. Apud Janssonio-Waesbergios & J. Wetstenium & Gul. Smith, Amstelaedami <link>


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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Tuesday, April 21, 2015

Spitting cobras

Click here to read this post in Spanish!
Haga clic aquí para leer este artículo en español!

Spitting cobras have been known for centuries,
as you can see from this report published in the
Journal of the Bombay Natural History Society in 19001

A clever comic from birdandmoon
highlighting the fact that king cobras
are not true cobras
Cobras are some of the most iconic snakes in the world, instantly recognizable by their hoods even to those who have never seen one. They are also among the most dangerous snakes—fast-moving, with potent neurotoxic venom, cobra bites cause injury or death to many people in Asia and Africa. Cobras are elapids, together with coralsnakes, mambas, kraits, seasnakes, and numerous terrestrial Australian snakes both well-known and obscure. What unites these ~350 species of snakes is their short, immovable, and hollow ("proteroglyphous") fangs. Elapids probably evolved in Asia between 25 and 30 million years ago. By 16 million years ago, cobras were found in Europe, where they no longer live, and in Asia and Africa, where they are still found today. The core cobra clade consists of three small genera (Hemachatus, Aspidelaps, and Walterinnesia) and one large one, Naja. Other hooded snakes that are usually called "cobras" include tree cobras (genus Pseudohaje), whose placement remains uncertain, and the king cobra (Ophiophagus hannah), which is probably more closely related to mambas than it is to true cobras. Ironically, most people, if asked for a species of cobra, would almost certainly come up with the king first. But, probably they would think of a spitting cobra second, and with good reason from an evolutionary perspective, as we shall see.

Mozambique Spitting Cobra (Naja mossambica)
Almost all spitting cobras belong to the genus Naja, a large genus that comes from the Sanskrit word for snake, nāga. Literature buffs will recognize the name of the cobras in Kipling's Rikki Tikki Tavi, which led to the name of the snake Nagini in the Harry Potter books. Over the past 50 years, the number of species within the genus Naja has risen from six to 292, and more will probably become recognized in the future. At least 15 of these species can spit their venom through the air. The best of them are capable of aiming at targets the size of a human face with >90% accuracy up to 8 feet away. This adaptation represents the only purely defensive use of venom by any snake. Vipers and other venomous snakes occasionally eject venom from their fangs into the air, particularly when being handled, but these snakes are not aiming at anything, so they are not really using their venom defensively. Spitting in cobras is an adaptation that involves changes to the morphology of the fangs, their head musculature, and the chemistry of their venom.

Fangs of  cobras progressively adapted for spitting.
Dotted lines show the venom canal, dark arrows indicate
the flow of water injected into the top of the fang.
Left: "normal" non-spitting cobra fang (Naja kaouthia)
Right: spitting cobra fang (Naja pallida)
The sutures are visible above the exit orifices.

From Young et al. 2004
All snake fangs are modified teeth provisioned with grooves that vary in depth and degree of closure. In vipers and elapids, the grooves are completely closed, forming hollow tubes, along the front edge of which a narrow suture can still be seen where the ridges forming the tube have come together in the developing embryo. In spitting cobras, the inside of this tube contains ridges, which act like rifling in a gun barrel to impart spin on the venom. The discharge orifice, located near but not at the point of the tooth (like a hypodermic needle), is large and elliptical in non-spitting cobras but small and round in spitting cobras, which has the same velocity-increasing effect as putting your thumb most of the way over the end of a garden hose. A sharp 90° bend at the distal end directs the jet of venom forward or slightly upward, instead of downward as in most snakes, and venom stream spins towards the exit orifice, which prevents the flow from slowing down as it goes through the sharp bend at the exit (similar strategies are used in pressure washers). These adaptations of the fang enable a cobra to spit venom in defense but do not prevent venom injection when biting, which is used both defensively and for killing prey. In fact, spitting cobras can meter the duration of their venom pulse, which is normally about five times longer during biting (1/4th of a second) than during spitting (1/20th of a second). This affects the quantity of venom ejected, which varies considerably from bite to bite and may consist of up to 100 times more venom than the fairly consistent 1.9-3.7 milligrams (~1/10th of a milliliter) of venom per spit. Most estimates suggest that a single cobra has enough venom to spit about 40-50 times consecutively. The fluid dynamics of such tiny volumes over relatively long distances are complex, and spitting cobra venom has shear-reducing properties, such as high surface tension and viscosity, which hold the droplets together as they fly through the air. Some species of spitting cobra eject their venom as a spray, whereas others eject two pressurized parallel streams. Reports of the maximum distance achievable by a spitting cobra vary from surely exaggerated distances of 12 feet or more to more believable (though still impressive) distances of five to eight feet.

Venom spray patterns of Red Spitting Cobras (Naja pallida)
From Westhoff et al. 2005
Middle: Examples of head movement patterns of  Black-necked
Spitting Cobras (Naja nigricollis). Black dots represent the
positions of the upper and lower jaws,  red dots indicate the
period of venom spitting.
From Westhoff et al. 2005
Bottom: Congruence between target (back; blue)
and cobra’s head (red; front plot) motion during spitting.
Data are offset 180 ms to reflect the cobra's reaction time.
From Westhoff et al 2010
Unlike vipers, cobras cannot move their fangs, so in order to accurately hit their targets, they move their heads instead. When a spitting cobra spits, it opens its mouth slightly and contracts the muscles around the venom glands so that a small amount of venom is forced out of the glands and down the venom canal of the fangs. At the same time, the upper lip scales and the fang sheaths are levered up out of the way and the maxilla levered down, removing soft tissue barriers between the venom glands and the fangs as well as between the exit orifices of the fangs and the air around them3. Most often, the spit is accompanied by slight movements of the head in response to change in direction of the target, which disperse the venom over an area about the size of a human face. Measurements indicate that more head rotation corresponds to a larger area covered by the venom stream, allowing cobras to adjust for target size and distance. Splattering of the venom when it hits the target and partial disintegration of the venom stream as it travels through the air increase the chance that at least some of the venom will hit the target's eye. Consequently, cobras only need to aim at the center of the face, rather than precisely at the eyes, in order to hit the eyes 90-100% of the time. They adjust for target movement by using a strategy familiar to any Space Invaders or Galaga player: firing not at where you are but at where you're going to beChameleonsarcher fish and spitting spiders do the same kind of thing. In some species venom spitting is often accompanied by an audible hiss as the cobra exhales, but in contrast to early reports that spitting cobras propelled their venom with their breath, this is not an essential part of the spitting process. In one experiment, spitting cobras restrained in tubes did not seem to suffer from reduced spitting ability or range. How do they choose their targets? Cobras have good vision and moving human faces are the stimuli that normally elicit spitting, although in lab experiments they will also spit at masks, photos of human faces, and even plain ovals without eyes, as long as they are moving, but not at moving triangles. Adult cobras will not spit at stationary human faces or moving human hands, although newly hatched cobras will spit at nearly anything, even if it is beyond their maximum target distance, including human hands, unhatched eggs, other baby cobras, and even their own reflection. Hatchling cobras also spit more of their venom, proportionally, and rotate their heads in a more pronounced fashion; their spitting performance improves following their first shed. Like many stereotypical snake defensive behaviors, most spitting cobras apparently habituate to humans when in captivity and are disinclined to spit after a while, although some spit without hesitation and willingness to express defensive behavior is very variable from individual to individual.

Sumatran Spitting Cobra (Naja sumatrana)
Although the color and consistency of spat venom does not change noticeably with repeated spitting, the venom chemistry of at least one species, Red Spitting Cobras (Naja pallida), changed over 10 minutes of repeated spitting. The quantity of venom remained the same and the toxin concentration rose over the first 20 spits, but both decreased afterward. The first five spits contained a protein that was not found in later spits, which might be involved in venom storage. Although this protein is non-toxic, most of the other molecules in spitting cobra venom are not. African spitting cobra venom is rich in cytotoxins and PLA2s, which cause tissue damage; spitting cobra cytotoxins lack certain acidic proteins, which frees them to damage tissues in the eyes. If even a small quantity of venom contacts the eye it causes instant, intense pain and damage to the cornea and mucous membranes. If left untreated, it can lead to blindness. Treating spitting cobra venom in your eyes involves flushing it out with water for 15-20 minutes. Anti-inflammatory eye drops are sometimes prescribed.

Rinkhals (Hemachatus haemachatus)
The 29 living species of Naja fall into four groups: a basal Asian clade of eleven species (subgenus Naja, including six accomplished spitting members, two non-spitters, and three species of intermediate spitting ability), an African spitting group of eight species (subgenus Afronaja), and two African non-spitting groups of six and four species, respectively (subgenus Uraeus, found mostly in open areas, and subgenus Boulengerina, found mostly in forests). This pattern of species relationships suggests that spitting evolved more than once! In Asia, the six spitting cobras (Naja siamensis, N. sumatrana, N. sputatrix, N. mandalayensis, N. samarensis, and N. philippinensis4) are probably one another's closest relatives, and their closest cousins are a group of three cobra species (Naja atra, N. kaouthia, and N. sagittifera) with somewhat modified fangs and intermediate spitting ability. They can spit their venom, but they do so rarely and with less accuracy than the "true" spitters. The remaining Asian cobras, Naja naja and Naja oxiana, do not spit their venom but nevertheless are more closely related to Asian spitting cobras than to other cobras. This means that venom spitting arose independently in the common ancestor of the seven species of African spitting cobras (N. pallida, N. nubiae, N. katiensis, N. nigricollis, N. ashei, N. mossambica, and N. nigricincta), which form a monophyletic group sometimes referred to as Afronaja. Their cousins, the other African Naja (i.e., subgenera Uraeus and Boulengerina), do not spit. Finally, a member of one of those small genera, a very interesting cobra known as the rinkhals (Hemachatus haemachatus) also spits its venom, indicating that venom spitting has evolved three times in cobras (or, alternatively, been lost twice, in Naja naja/N. oxiana and in the common ancestor of Uraeus and Boulengerina, with a third partial loss in N. atra & kin). Because the details of spitting behavior and morphology differ slightly among the three groups of spitting cobras, the former hypothesis is more likely.

The largest Giant Spitting Cobras (Naja ashei) can top 9 feet.
This species was described in 2007.
From Wüster & Broadley 2007
Why do some cobras spit their venom? Herpetologist Thomas Barbour, who published one of the first studies on spitting cobras, thought that spitting cobras evolved venom spitting for much the same reason that rattlesnakes were thought to have evolved their rattles—to alert large ungulates to their presence and avoid getting stepped on. He was speculating in the absence of any direct evidence when he wrote in 1922 that "The African veldt is the only other region in the world where snakes abound and where hoofed animals grazed in numbers comparable with those of the western American plains. Snakes probably found the heavy antelopes equally dangerous though unwitting foes and many antelopes probably suffered from snake bite. No rattle was evolved, however but some of the common veldt-ranging snakes secured protection in another way. Several common cobras and cobra-allies learned to expel their poison in a fine spray for very considerable distances, and with a fairly shrewd aim at the eye."

A rinkhals (Hemachatus haemachatus) spits its venom
Nearly 100 years after Barbour, we have just as little direct evidence—published field observations of spitting cobras interacting with their non-human predators are non-existent. The main reason we now think that the evolutionary cause of these adaptations isn't so simple is that spitting is too old. Molecular dating methods suggest that African spitting cobras evolved about 15 million years ago, whereas the spread of open grasslands and their characteristic megafauna (elephants, etc.) didn't happen until about 5 million years ago. Asian spitting cobras don't inhabit open grasslands, so this hypothesis seems unlikely to explain their evolution either. African spitting cobras are eaten by birds and other snakes, against which spitting venom would be a relatively ineffective weapon, and in captive experiments cobras do not spit at mounted bird specimens. Given what we know about face targeting, it's possible that spitting may represent a defense that is specifically adapted for use against primates [Edit: Harry Greene hinted at this idea in his recent book, Tracks and Shadows]. Barbour's comment that "...[venom spitting] must antedate man's coming, for contact between man and the snakes can hardly be conceived as sufficiently frequent to account for the modification" may be technically correct, but the evolution of spitting cobras coincides roughly with the evolution of apes in Asia and Africa, which (as we all know) are diurnal primates with forward-facing eyes, some of which are omnivorous and many of which (ourselves included) habitually kill snakes either for food or in defense. Could it be that spitting cobras evolved their venom spitting capacity to deal with threats from our own ancestors? Only further research into the co-evolution of apes and snakes can tell us. Perhaps this is why, although certain toads, salamanders, insects, and scorpions can also eject their toxin defensively, spitting cobras are by far the longest- and best-known organisms to do so. Clearly, much remains to learn about them and their fascinating habits.



1 The cobra in this account was undoubtedly Naja mandalayensis, which was described by Joe Slowinski & Wolfgang W
üster 100 years later. Before 2000, no spitting cobras were known from Burma. Cobra specimens with fangs highly modified for spitting from northeastern India may represent a seventh species of undescribed Asian spitting cobra.




2 This number includes species of cobras formerly placed in the genera Boulengerina and Paranaja, both of which have been synonymized with Naja in the last 15 years. In part, the reason for this change is that, when scientists realized that some species of Naja were more closely related to Boulengerina and Paranaja than they were to other Naja (i.e., that Naja was paraphyletic), they were reluctant to split up the genus Naja because they didn't want to change the name of medically-important snakes and create potential confusion. However, a few sources use Afronaja and other other subgenera as full genera anyway.




3 The fang sheath is soft tissue that completely surrounds the fang at rest, including at the top, which keeps the venom from dribbling out. In other venomous snakes, physical contact with a target is required for displacement of the fang sheath and release of venom, but spitting cobras have co-opted the movements normally used for jaw-walking over a prey item (the ‘pterygoid walk’) to free their fangs for spitting in the absence of any external physical contact. This has been termed the "buccal buckle" (pronounced "buckle buckle") by the research group of Bruce Young, of Kirksville College, which has studied several aspects of the functional morphology of spitting in cobras.






4 Naja philippinensis is 
the only spitting cobra species with pronounced sexual dimorphism in discharge orifice size—females have longer orifices less well-adapted for spitting, whereas males have small round orifices. The evolutionary causes and consequences of this dimorphism are not understood.


This post is part of a Reptile and Amphibian Blogging Network (RAmBlN) online event called #CrawliesConverge. We are writing about convergent evolution in reptiles and amphibians. Find our event schedule here, or follow on Twitter or Facebook.

ACKNOWLEDGMENTS

Thanks to Stu PorterDan Rosenberg, and Ray Hamilton for allowing me to use their photos.

REFERENCES

Barbour, T. 1922. Rattlesnakes and spitting snakes. Copeia 105:36-38 <link>

Berthé, R., S. de Pury, H. Bleckmann, and G. Westhoff. 2009. Spitting cobras adjust their venom distribution to target distance. Journal of Comparative Physiology A 195:753–757 <link>

Berthé, R.A., G. Westhoff, and H. Bleckmann. 2013. Potential targets aimed at by spitting cobras when deterring predators from attacking. Journal of Comparative Physiology A 199:335-340 <link>

Bogert, C.M. 1943. Dentitional phenomena in cobras and other elapids, with notes on adaptive modifications of fangs. Bulletin of the American Museum of Natural History 81:285-360 <link>

Cascardi, J., B.A. Young, H.D. Husic, and J. Sherma. 1999. Protein variation in the venom spat by the red spitting cobra, Naja pallida (Reptilia: Serpentes). Toxicon 37:1271-1279 <link>

Chu ER, Weinstein SA, White J, Warrell DA (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. Toxicon 56:259-272 <link>

Goring Jones, M.D. 1900. Can a cobra eject its poison? Journal of the Bombay Natural History Society 8:376 <link>

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Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.