Monsters and dragons: Venomous lizards

When we think about venomous animals most people think about the same ones. Usually, we think about spiders, scorpions and snakes, despite knowing there are also venomous amphibians, fishes and mammals. Even if snakes are the best known venomous reptiles, in time we have learned that they are not the only group that present venomous glands and that many other reptiles also have the capacity of injecting venom. In this entry we’ll get to know the least known venomous saurians and we’ll try to explain their relationship with snakes.

EVOLUTION OF VENOM IN REPTILES

Everybody is familiar with the toxic abilities of snakes. Traditionally it was believed that venom evolved independently in the different groups of venomous snakes (colubrids, elapids and viperids) and in a lizard family (the helodermatids). Yet this vision has changed over the years and with the discovery of other species of venomous squamates.

The venom of many animals is used for both antivenom development and pharmacological research of analgesics and other medicines. Photo of the extraction of venom from a saw-scaled viper (Echis carinatus), by Kalyan Varma (Image under a GNU license).

Currently, it’s been shown that there are different species of saurian which present glands and organs capable of injecting venom, along with many other species with genetic material related to venom production (even if most aren’t venomous). This occurs, for example, in many apparently non-venomous snakes and lizards that retain genetic material related to the synthesis of venom. This has caused many scientists to group these reptiles under a common clade called Toxicofera, “those who bear toxins”.

This new clade includes the different squamosal taxa, which are believed to have had a venomous common ancestor. These groups are:

• Ophidia: Ophidians, snakes.

Indian wolf snake (Lycodon aulicus), example of an ophidian. Photo by Shantanu Kuveskar.

• Iguania: Iguanas, agamas and chameleons.

Brown basilisk (Basiliscus vittatus), example of an iguanian. Photo by Steve Harbula.

• Anguimorpha: Monitors, slow worms and others.

Earless monitor lizard (Lanthanotus borneensis), example of an anguimorph. Photo by Kulbelbolka.

Even though most current iguanians and anguimorphs don’t present venom, the Toxicofera theory proposes that many species would have lost their capacity to inject venom secondarily. Below we’ll present some of the lesser known venomous saurians.

MONSTERS OF THE NEW WORLD

The most famous venomous lizards are the anguimorphs of the Helodermatidae family. From their discovery it was known that these lizards where venomous, as they present a pair of venomous glands in their lower jaws and various pairs of grooved teeth similar to those of venomous snakes with which they inject venom.

Helodermatid skull, in which we can see the sharp teeth with which they inject their venom. Image from Heloderma.net.

The helodermatis are carnivorous animals which feed on small mammals, birds, wall lizards, amphibians, invertebrates, eggs and carrion. Considering its generalist diet and that their prey are pretty defenceless, it is thought that venom evolved in these reptiles as a predator deterrent method, not as a hunting strategy.

Photo by Walknboston of a Gila monster (Heloderma suspectum), in which we can see its black and yellow coloration, with which it warns its predators about its toxicity (aposematic coloration).

The Gila monster and the beaded lizard (Heloderma horridum) are slow animals which aren’t really dangerous to human beings. Yet their raising popularity as exotic pets has ended with some bite cases. The bite of a Gila monster causes some serious and burning pain, local edema, weakness, dizziness and nausea. Even if heavy bleeding is usually associated with bites, this isn’t due to some sort of anticoagulant substance but to the helodermatid’s sharp teeth and to the fact that to inject the venom they must chew their aggressor strongly , causing deep lacerations.

THE BEARDED DRAGON

The saurians of the genus Pogona are iguanians of the Agamidae family. These Australian reptiles are known as bearded dragons for the spines that they present on their throats. Even though they are adapted to live in arid places, the environmental temperature can affect the sex of their offspring.

Photo of an eastern bearded dragon in which we can see its yellow coloured mouth. Could it be that this coloration is indicating anything? Photo by Matt.

Bearded dragons are inoffensive animals, but there’s one species with a secret weapon. The eastern bearded dragon (Pogona barbata) is a venomous lizard but, while the rest of venomous reptiles only have one pair of venomous glands, the eastern bearded dragon has two pairs: two in its upper jaw and two in its lower jaw.

Transversal section of the mouth of an eastern bearded dragon, in which we can see the incipient venomous glands both in its upper jaw (mxivg) and its lower jaw (mnivg). Image extracted from Fry, Vidal et al.

The venom they produce isn’t really strong (in human beings it only causes a minor swelling) and the glands are considered vestigial. Yet, the Toxicofera theory argues that the glands of the bearded dragon show us the primitive form which the first toxicoferan reptile would have presented, with two pairs of venom glands instead of a single pair like most current venomous reptiles.

THE BIG MONITORS

Everyone has heard about monitor lizards (anguimorphs of the Varanidae family). There are hundreds of documentaries about the Komodo dragon in which we are told that these animals have so many bacteria in their mouths that their bites inflict an infection, deadly enough to kill an adult bull. Yet recent studies have shown that the monitor’s poor buccal hygiene is not what causes the death of their victims.

Perente or perentie (Varanus giganteus) a typical varanid, with long neck, strong legs, active metabolism and developed senses. Photo by Bernard Dupont.

Even if there are three frugivorous species, the rest are obligate carnivores. It has always been said that the mouth’s bacteria of the monitors is what causes the death of their prey, even if there isn’t any studies which prove it. In fact, in many studies it has been seen that the monitor’s saliva isn’t very different from that of other herbivorous reptiles.

Photo in which we see the feared monitor’s saliva, specifically from an Asian water  monitor (Varanus salvator). Image by Lip Kee.

In a study, it was demonstrated that various species of monitor lizards present venom glands in their lower jaws. These glands are among the most complex venomous glands known of all reptiles. In the case of the Komodo dragon, these are compound glands with a larger posterior compartment and five smaller anterior compartments. These compartments have ducts that carry the venom between the teeth.

Even if varanids are closely related to snakes (they share, for example, a bifid tongue), these don’t present the snakes’ characteristic grooves in their teeth. This is due to the fact that instead of injecting the venom directly, monitor lizards use their serrated teeth to open a deep wound in their prey, through which the venom will enter the organism.

Skull of megalania (Varanus priscus) in which we can see the teeth without gooves. This extinct monitor with more than 5 metres long, was the largest venomous animal known. Photo by Steven G. Johnson.

The utility of the venom for the predatory monitors is also supported by the large quantities of venom that they produce. In constrictor snakes that don’t utilise venom, the genes which codify the synthesis of venom are atrophied because of the great amount of energy required to produce it. Monitors, instead, secrete lots of venom with the slightest stimulation of their glands. This venom contains anticoagulant compounds which prevent the wound to close and also produces a cardiovascular shock in the animal by lowering the blood pressure.

A group of Komodo dragons (Varanus komodoensis) feeding on a recently killed pig. Image extracted from Bull, Jessop et al.

Even if we still don’t know for sure if the common ancestor of all these animals was venomous, nor if venom appeared independently in the different families, the relationship between the different members of the clade Toxicofera has been supported by posterior phylogenetic analyses. What we know is that venom is an extremely powerful weapon in the struggle for survival and that, even if snakes are the most numerous venomous reptiles, many other squamate species have been benefiting from the use of toxins, both for self-defence and to subjugate their prey.

REFERENCES

The following sources have been used during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• http://www.nature.com/nature/journal/v439/n7076/full/nature04328.html
• http://www.sciencedirect.com/science/article/pii/S0041010114003353
• http://www.leidenuniv.nl/en/researcharchive/index.php3-c=134.htm
• http://www.heloderma.net/en/heloderma.html
• http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0011097
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2690028/
• Cover image by Rocío Corzo.

The world from the eyes of a snake

Imagine you are a snake. You’re crawling along the path, with a long slithering body behind you. You have no ears and, even if your eyes are large and well-developed, you cannot blink. You’re constantly flicking your tongue, which informs you about everything that has happened around you, especially about the smell of that tasty mouse you’ve been looking for for days. Ophidians have suffered so many bodily modifications that their senses have had to adapt to their lifestyle. With more than 3,000 current snake species it’s difficult to generalize, but in this entry we’ll explain some of the most curious sensorial adaptations of current ophidians, trying to shed some light over the world of these fascinating and unfairly treated animals.

SMELL: TASTING THE AIR

One of the most developed ophidian senses is smell. It’s common knowledge that snakes use their tongue to smell the air and detect chemical substances. It was once thought that snakes used only their tongue to smell and that the nasal epithelium was only used to activate this mechanism. Now it’s known that snakes smell using both their nose and their tongue, even if the latter is more useful in certain situations.

Microscope image of a transversal slice of a snake skull, where we can see the olfactory epithelium of both the nasal cavity and the vomeronasal organ. Image by Elliott Jacobson.

Snakes taste the air using their tongue and the vomeronasal or Jacobson’s organ. This organ isn’t found only in snakes, as it is also found in other lizards, some salamanders and many mammals. The vomeronasal organ is used to detect non-volatile chemical substances (which need direct contact with the epithelium to be detected) such as pheromones or the scent of a prey.

Scheme of the position of the vomeronasal organ. This forms during the embryonic development from the nasal cavity and it has an opening to the palate. Image by Fred the Oyster.

The snakes’ distinctive bifid tongue is very specialized into particle transport to the vomeronasal organ. It has a set of microscopic papillae or depressions (depending on the species) that help to catch and retain odorous particles. Then it brings this information to the palate, where it gets in contact with the vomeronasal organ.

Monitor lizards (relatives of snakes) also present bifid tongues which allows them to smell the air. Photo of an Asian water monitor (Varanus salvator) from India, by Dibyendu Ash.

Snakes flick their tongue in the air or against some surface to collect “chemical samples” from the environment. Also, the fact that the tongue is bifid is thought to be useful in detecting the direction from where the stimulus comes, as the information obtained from each tip of the tongue goes to one of the two cavities of the vomeronasal organ and goes to the brain by separate ways.

Photo of a European grass snake (Natrix natrix) flicking its tongue to taste the air. Image from WikiImages.

Snakes use this chemical information to follow the trail of a prey, to find a mate and to detect the reproductive state of another individual. Also, a recent study shows that snakes (thanks to their keen sense of smell) are able to recognize their siblings and relatives, choosing them before a stranger to share their hibernation grounds.

Hearing: listening without ears

Hearing is one of the least developed ophidian senses. The absence of an external ear caused that for a long time it was believed that snakes were deaf. Yet recently, it has been demonstrated that snakes do have different methods to detect different types of vibrations.

Portrait of an Indian python (Python molurus) in which the absence of external ears can be seen. Photo by Holger Krisp.

As we explained on an earlier entry, snakes do not have neither external ears nor eardrums. Yet, they do present all the elements of the inner ear characteristic of tetrapods. What changes is the way the vibrational stimulus is transmitted, which in ophidians is accomplished via a bone called columella.

Scheme of the auditory apparatus of a common snake. Image by Dan Dourson.

The columella is a small, long and thin bone attached by ligaments and cartilaginous tissues to the posterior end of the upper jaw and that articulates with the lower jaw. Snakes have one on each side of their skull, which have an equivalent function to the stapes (bones of the mammalian middle ear). The columellas are completely surrounded by tissues, so aerial, terrestrial and aquatic vibrations, are transmitted to these bones which are in contact with the fluids of the inner ear.

Yet, the snakes’ sensitivity to aerial waves is pretty much limited. For example, while human beings are able to hear aerial vibrations between 20 and 20,000 Hz, snakes can only detect vibrations between 50 and 1,000 Hz. Even though they have such limited hearing range, in some species it has been observed that they are able to receive vibrational stimuli with any body part, as these are transmitted through the bodily tissues to the columellas.

Aquatic snakes like the anaconda (Eunectes murinus) can detect with all their body the sounds of an animal moving through the water. Photo by Ddouk.

Even with their limitations to hear aerial waves, what snakes do best is to detect vibrations coming from the ground or the water. Most snakes can detect with great precision vibrations generated by the steps of a prey by keeping their lower jaw (which is in contact with the columellas) in contact with the ground.

The Arabian horned viper (Cerastes gasperettii) is a snake that lives in sand deserts, where the terrain allows a great transmission of terrestrial vibrations. Image by Zuhair Amr.

SIGHT: LIGHT AND HEAT

The eyes of snakes are not very different from the eyes of most terrestrial vertebrates. Yet they have some special characteristics, probably due to their subterranean or subaquatic origins. Most scientists think that snakes had to somehow “reinvent their eyes”.

Some primitive ophidians, like this European blind snake (Typhlops vermicularis), have small and poorly-developed eyes. Image by Kiril Kapustin.

The structure of their eye is mostly identical to that of the rest of tetrapods. A difference is the focusing method: while most tetrapods focus by changing the curvature of the crystalline lens, snakes focus moving the crystalline lens forward and backward. Also, while most terrestrial vertebrates have eyelids to protect the eye, snakes have an ocular scale called the spectacle which is renewed each time they shed their skin.

Western rat snake (Pantherophis obsoletus) about to shed its skin, moment when the spectacle turns opaque. Photo by Bob Warrick.

Depending on the snake’s lifestyle, its sight will have different adaptations, even if in most species the retinas present both rods (sensitive to low light conditions) and cones (allow to see details and colours). Subterranean, more primitive snakes present quite simple eyes, with only rods which allow them to distinguish light and darkness. On the other hand most diurnal snakes have round pupils and both cones and rods.

Many arboreal snakes like this green vine snake (Ahaetulla nasuta) present horizontal pupils which allow them to have a wider range of vision, making it easier to calculate the distance between one branch and another. Photo by Shyamal.

Aside from visible light, some snake are able to see other wavelengths. Pit vipers and some pythonomorphs (pythons and boas) can detect infrared radiation, being able to see the thermic signature around them. This is extremely useful to detect prey in low light conditions, as they can perceive their body heat.

Photos of a python and a pit viper where both the nostrils (black arrows) and the pit organs (red arrows) are highlighted. Image by Serpent nirvana.

They can do this using the pit organs, cavities that appeared independently in pit vipers (from which they got their name) and pythonomorphs. While pit vipers only have a pair of facial pits on both sides of their snout, pythonomorphs have various labial pits on the upper or the lower lip. Despite having fewer pits, the pit vipers’ ones are more sensitive that the ones of the pythons.

Scheme of the structure of a pit organ of a pit viper. This presents a membrane sensible to temperature variations, behind which there’s a chamber with air and nerves sensible to heat. This air dilates when the temperature rises and it activates the trigeminal nerve. Image by Serpent nirvana.

These pits are extremely sensitive and can detect temperature changes of up to 0.001°C. The trigeminal nerve reaches the brain via de optic tectum, making the image detected by the eyes superpose with the infrared image from the pits. Therefore snakes detect both the visible light (as we do) and the infrared radiation in a way that is impossible for us to imagine.

Video from BBCWorldwide in which they explain how a timber rattlesnake (Crotalus horridus) uses infrared detection to hunt a rat in the dark.

As you have seen, snakes perceive the world very differently than we do. Snakes do not leave anyone indifferent and, in the same way that different people see snakes in different ways, different ophidian species present different and diverse adaptations to perceive the world that surrounds them. We hope that with this entry, you’ve been able to understand a little better the incredible world in which snakes live.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Harvey B. Lillywhite (2014). How snakes work. Structure, function and behaviour of the world’s snakes. Oxford University Press.
• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
• http://news.nationalgeographic.com/news/2004/02/0223_040223_rattlesnakes.htmlutm_content=buffer44d50&utm_medium=social&utm_source=facebook.com&utm_campaign=buffer
• Cover image by Bernard Dupont.