Are Hognosed Snakes venomous?

As hognosed snakes grow in popularity, more and more amateur herpetoculturists and "backyard breeders" have begun to question their toxicity. Long known to possess enlarged rear teeth, hognosed snakes have long been suspected of using this adaptation for the deflation of toads during the ingestion process. However, the question of whether or not members of Heterodon possess the ability to deliver the contents of their Duvernoy's gland has taken on greater importance recently with the hognosed snake's increase in popularity in the pet trade.

While many hobbyists have kept Western hognosed (Heterodon nasicus) snakes without regard for the potential threat posed by possible envenomation, more and more people are keeping the other, less well-known members of the xenodontid group. This has forced a reexamination of the potential threat posed by so-called rear fanged non-venomous colubrids and an increase in their exposure through the popular herpetocultural media.

While less common in captivity, Eastern hognosed snakes (heterodon_platirhinos), and Southern hognosed snakes (Heterodon simus), have joined the ranks of the more commonly kept Western forms. Collectors are also adding Madagascan, Central and South American species (Leioheterodon sp., Lystrophis dorbignyi and Lystrophis sp. respectively) to their collections. All of these species are members of the Xenodontid group and therefore possess enlarged or modified rear teeth or "fangs." Snakes possessing these modified teeth which may or may not be slightly grooved, are known as opisthoglyhs. Other snakes that possess modified teeth and are more well known for delivering toxic bites include the boomslang and Kirtland's Tree Snake (Thelotornis).

While some believe the primary purpose of these modified teeth in Heterodon is to puncture the air sacs of inflated toads, this seems unlikely and has not been recorded in the scientific literature. Moreover, studies of the diet of Western Hognosed snakes and other Xenodontids have failed to yield an explanation for the enlarged teeth where toads make up less than 50% of the overall diet.

According to studies conducted by numerous researchers and reported by Platt, throughout its range, amphibians made up 57% of the total diet while the rest consisted of reptiles and their eggs, birds, and mammals. Of the 57% of the amphibians consumed, approximately 88% were toads, representing only 50% of the total diet.

In another study, amphibians made up 44% of the total diet (toads made up only 22% of the total) while reptiles and mammals accounted for 39% and 33% respectively. Curiously, Ruthven also reported instances where western hognosed snakes attempted to eat ornate box turtles.

Based on these and other studies, Western Hognosed Snakes in the wild feed on a variety of foods with toads making less than half of its total diet. Therefore, to say that the hognosed snake's rear teeth developed solely to deflate toads seems improbable.

Also, in contrast to Miller's assertion that the primary use of these fangs "...is to puncture the lungs of live prey (such as toads, their primary food item), thereby helping the snake to swallow large items," recent reports show that live toads have been regurgitated from the stomachs of Eastern hognosed snakes intact, without punctures or deflation of the air sacs.

If we look at the other xenodontids resembling American hognosed snakes, we see that members of both Leioheterodon and Lystrophis feed on a variety of foods, amphibians comprising only a small part. In fact, members of Leioheterodon do not have access to toads that inflate and instead feed largely lizards, their eggs, and small mammals. The Central and South American members of Lystrophis feed on amphibians, reptiles, and small mammals; thus the requirement to have a method of puncturing inflated toads is too limited to be useful.

The jaws of Heterodon are sufficiently strong enough to crush small prey while the enlarged rear teeth are large enough to cause extensive damage to the prey's internal organs and may result in massive hemorrhaging. Minton suggests the teeth are useful in holding struggling prey. From a size perspective, this seems logical. Where the prey is too large for the hognosed snake to subdue easily, large, rearward pointing teeth would facilitate holding the prey in place while the snake swallows it whole.

Anyone who has observed an American hognosed snake in the wild or in captivity has no doubt noted that these pudgy little snakes are slow moving and incapable of capturing most live prey through pursuit. Rather, they are, as Platt has noted, most likely ambush hunters as well as carrion eaters. Some Eastern and Southern Hognosed snakes have also been observed burrowing after toads in soft substrate to capture and eat toads. Of course, possessing the ability to deliver venom would of course add to the ability of the snake to capture and subdue prey.

Enlarged teeth may also facilitate the injection of a toxic substance into a prey item for the purpose of subduing it prior to ingestion. This is true for the large and powerful members of Crotalus as well as the more diminutive Micrurus Micruroides. So how toxic is Heterodon "venom?"

Human saliva, when injected into a hand, results in local pain, swelling, and discoloration. However, this does not mean that Humans should be considered venomous. And yet, this is precisely the approach taken by many in the herptile hobby when discussing the American hognosed snake.

According to Smith and Brodie: "...the fangs do not deliver a venom, although the saliva is believed to be toxic," (1982).

In several cases involving bites by Heterodon nasicus, victims reported a combination of symptoms ranging from no reaction to mild tingling, swelling and numbness. While some researchers have attempted to explain this as the result of bufotoxins in Eastern hognosed snakes eating toads, it cannot be explained in those cases where captive born Western hognosed snakes, fed exclusively mice caused similar reactions.

According to Kapus and Weaver , the parotid [sic] element of the superior labial gland is enlarged and differentiated in Western and Southern hognosed snakes (but not Easterns). The parotid gland, correctly known as the Duvernoy's gland, is that gland which possesses many of the toxic components found in the venom of more deadly colubrids (because the parotid gland found in mammals is not homologous to the gland found in snakes, future references to this gland will be referred to as the Duvernoy's gland). Therefore, it is thought that this contributes to toxic reactions from the bites of some of the more well known venomous species.

Numerous sources explain the origination of the contents of Duvernoy's glands and the potential lethality of some species, especially in snakes such as boomslangs (using bungarotoxins) and members of Boiga. The ability, however, of a snake to deliver the contents of its Duvernoy's gland may severely limit its ability to envenomate prey and predators. Apparently, the contents of the Duvernoy's glands in Heterodon are toxic, albeit on a lower level.

Weaver reports the toxicity of Duvernoy's secretions to be sufficient to subdue prey in both the Southern and Western hognosed snakes. As with crotalids and elapids, the expulsion of the Duvernoy's glands may be triggered during feeding responses. The severity of the secretion appears to be sufficient only to paralyze and not kill the prey. Injections of the macerated contents of the inferior and superior labial salivary glands of the Eastern Hognosed Snake had an effect only on frogs and toads but not on mice. The effects on the frogs did not take effect for several hours, much too late to be useful in subduing prey prior to ingestion. McAlister therefore concluded the maxillary teeth of Eastern hognosed snakes are not used for the introduction of the salivary gland contents

In keeping Leioheterodon, I have repeatedly observed both L. madagascariensisand L. geayi to employ both a chewing action as well as constriction to subdue and hold rodents. In some instances, I have observed my large female Giant Madagascan Hognosed Snake eating one rat while pinning another against the enclosure wall. Of course, the case for a rear-fanged snake using envenomation in conjunction with constriction is not without precedent - the rufous beaked snake ( Ramphiophis sp) of Africa is a good example of a snake that uses both methods.

In the case of American hognosed snakes, while they have been observed to hold prey down with their coils during ingestion, there have been no reports of these snakes employing constriction as part of their killing mechanism. So then, we are left with the question, how do Western, Southern and, to a lesser extent Eastern, Hognosed snakes capture, subdue and devour prey ? We can look to the Western garter snake (Thamnophis elegans) for our answer.

In the wild, Thamnophis elegans is known to feed on mice. How can a relatively small, inoffensive Colubrid possibly take down and devour a mouse in the wild ? According to studies, these snakes are apparently capable of delivering the contents of the Duvernoy's gland to the prey item and thus immobilize it prior to consumption. All that the Western garter snake lacks is enlarged teeth or grooved fangs to channel the Duvernoy's contents into the prey item. However, through a chewing action, Thamnophis elegans is able to envenomate its prey item and immobilize it prior to swallowing. The contents of the Duvernoy's gland seeps into open wounds produced by the maxillary teeth and temporarily immobilizes the prey. Species of Heterodon, Leioheterodon and Lystrophis have further improved on this design by incorporating enlarged teeth on the maxilla.

Personally, I have been bitten numerous times by Heterodon nasicus during feeding responses all without incident or toxic reaction. Therefore, reactions by individuals to bites from Heterodon may be directly related to individual anaphylactic or allergic reactions.

Taub reports the construction of the Duvernoy's gland to consist of both serous and mucous cells in Heterodon nasicus while in heterodon_platirhinos and Lystrophis dorbignyi, the gland is constructed of purely serous cells. These species have previously been classified together as members of subfamilies colubrinae and xenopeltinae based on hemipenal construction and dentition.

Further, based on toxicology studies and the behaviors of other, related snakes, it would appear that the secretions of the Duvernoy's glands of the genus Heterodon can be considered mildly venomous; Hognosed snakes possess both a slightly toxic saliva within their Duvernoy's glands, as well as enlarged rear fangs to facilitate its delivery. As far as the genus Leioheterodon, while the snakes in this genus mimic the far more dangerous elapid group of cobras found in Africa (in both coloration and in hooding), there is currently no evidence to suggest they are any more toxic than their cousins in the Heterodon group. According to Henkel,members of the Leioheterodon are not at all toxic to humans. Having been bitten by a subadult male L. madagascariensis during a feeding response, and a female adult, I can personally vouch for the fact that there were no toxic reactions to the bite.

And lastly, inasmuch as Lystrophis largely mimic members of the coral family and small viperids, due to their smaller size, they are also believed to be as non-lethal as Heterodon

Since the American hognosed snake group (all three species) are well known as bluffers and are loathe to bite their keepers, it is unlikely that they present any kind of threat to humans other than during feeding periods. Then, if keepers use safe handling procedures and use tongs to feed their charges, the likelihood of a stupid feeding error (SFE) and envenomation can be greatly reduced. Those with increased sensitivity and/or other immunological problems should take extra precautions and not tempt fate.

REFERENCES:

Bragg, A.N. 1960. Is Heterodon venomous? Herpetologica 16: 121-124

Bush, Brian 2000 .On snake fangs: grooved or hollow?

Conant, Roger. 1975. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. Houghton Mifflin Company, Boston. pp.168-171.

Desmond, Dennis. 1982. Envenomation by non poisonous colubrids. Unpublished report, Oregon State University.

Johnson, Tom R. 1997. The Amphibians and Reptiles of Missouri. Missouri Department of Conservation, Jefferson City. pp.248-252.

Kardong, Kenneth V. 1995. SNAKE TOXINS AND VENOMS: AN EVOLUTIONARY PERSPECTIVE Department of Zoology, Washington State University Pullman, WA 99164-4236, USA

Grogan, W.L. 1974. Effects of accidental envenomation from the saliva of the Eastern hognose snake, Heterodon platyrhinos. Herpetologica 30: 248-249.

Hoser, Raymond. On the question of the immunity of snakes. 1985 in Litteratura Serpentium, 5 (6), pp. 219-232.

Henkel, Friedrich-Wilhelm and Wolfgang Schmidt. 2000. Amphibians adn Reptiles of Madagascar and the Mascarene, Seychelles, and Comoro Islands.

Minton, S.A. 1998. Rear-fanged snakes and their venom. Abstract. 22nd Meeting, International Herpetological Symposium. Cincinnati, OH.

Morris, M.A. 1985. Envenomation from the bite of Heterodon nasicus (Serpentes: Colubridae). Herpetologica 41(3): 361-363.

Rodriguez-Robles, J.A. 1994. Are the Duvernoy's gland secretions of colubrid snakes venoms? Journal of Herpetology 28(3): 388-390.

SAKATE, M. and C. LUCAS DE OLIVEIRA . 2000 .TOAD ENVENOMING IN DOGS: EFFECTS AND TREATMENT. J.Venom.Anim.Toxins, Vol. 6, No. 1 pp.52-62, 2000. Review article - ISSN 0104-7930

Smith, Hobart M. and Edmund Brodie. 1982. A Guide to Field Identification of Reptiles of North America. Golden Press, New York. pp.164-166.

Taub, Aaron M. 1967. Comparitive Histological Studies on Duvernoy's Gland of Colubrid Snakes. Bulletin of the American Museum of Natural History. vol. 158, Article 1. New York.

Weaver, W.G., jr. 1965. The cranial anatomy of Hog-nosed snakes (Heterodon). Bulletin, FLA State Museum 9(7): 275-304.

WEINSTEIN, S. A., AND K. V. KARDONG. 1994. Properties of Duvernoy's secretions from opisthoglyphous and aglyphous colubrid snakes: A critical review. Toxicon 32:1161-1185.

NOTES:

1Xenodontidae: While current taxonomy denotes snakes with enlarged rear teeth on their maxillary bones as members of the Xenodontinae, sufficient evidence exists to suggest this group should be elevated to family level thus beign renamed Xenodontidae.

2Kardong, Kenneth V. 1995. SNAKE TOXINS AND VENOMS: AN EVOLUTIONARY PERSPECTIVE Department of Zoology, Washington State University Pullman, WA 99164-4236, USA

3 The Duvernoy's gland is homologous to the venom gland of elapids and viperids.

4 Parotid gland: The largest of the three major salivary glands, found in mammals it is located in front and below the ear and behind the jaw bone. Taub felt that this gland was dissimilar from the gland found in mammals and thus recommended it be renamed Duvernoy's gland.

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