Desert reptiles

Deserts are some of the most extreme habitats on the planet. The Sahara, the Gobi and the Sonora are some examples of warm deserts where the high temperatures and the lack of water pose a great challenge to animals that live in them. Reptiles are one of the animal groups that present the most incredible adaptations for life in deserts. In this entry we’ll explain the difficulties that desert reptiles must face in order to survive, and we’ll introduce you to different species of snakes and lizards that in the deserts have found their home.

REPTILES IN THE DESERT

The characteristic which unites all deserts is the scarce precipitation as, unlike most people think, not all deserts present high temperatures (there are also cold deserts, like the Arctic and the Antarctic, both in danger because of the climate change). Reptiles thrive better in warm deserts than in cold deserts, because the low temperatures would not allow them to develop their life activity.

Map by Vzb83 of the warm deserts, both arid and semiarid, of the world.

Warm deserts not always have extremely high temperatures. While during the day temperatures may rise up to 45°C, when the sun goes down temperatures fall below freezing point, creating daily oscillations of up to 22°C. The different desert reptiles, being poikilotherms and ectotherms, use different behavioural strategies in order to avoid overheating during the day and to keep their body heat during the night (for example, climbing to elevated areas or sleeping in burrows).

The Namaqua chameleon (Chamaleo namaquensis) regulates its body heat changing its colouration. During sunrise it is black in colour in order to absorb as much radiation of the sun and activate its metabolism. When temperatures become higher, it turns white to reflect solar radiation. Video from BBC.

As we have already stated, the main characteristic of any desert is the lack of water. Generally, in a desert, it rains less than 250 mm of water a year. The scaly and impervious skin of reptiles prevents the loss of water, and their faeces contain uric acid which, compared to urea, is much less soluble in water, allowing them to retain more liquids. Most desert reptiles extract the water they need from their food and some drink water from the dew.

Both the extreme temperatures and the shortage of precipitations make the desert a place with very few living beings. Vegetation is scarce and animals are usually small and secretive. This lack of resources causes desert reptiles to be usually smaller than their cousins from more benevolent environments. Also, these saurians usually exploit any available food resource, although they think twice before wasting their precious energy to get their next meal.

SAND SNAKES

In many sandy deserts we can find various species of snakes (and legless lizards) that have adapted to a life among the dunes. Many of these ophidians share a locomotion method called “sidewinding”, in which they raise their head and neck from the ground and move them laterally while the rest of the body stays on the ground. When they place their head on the ground again they raise their body, making these snakes move laterally in a 45° angle. This method of locomotion makes these snakes move more efficiently in an unstable terrain. It also reduces the contact of their body with an extremely hot substrate, as the body of these ophidians only touches the ground in two points at a time.

As we can see in this video from RoyalPanthera, sidewinding allows desert snakes to move minimizing the contact with the hot terrain.

Many desert ophidians bury themselves in the sand both to avoid sun exposure and to blend in and catch their prey unaware. This has made many desert-dwelling snakes very sensitive to vibrations generated by their prey as it moves through the sand. In addition some species present an overly developed rostral scale (the scale at the tip of their snout), being much thicker in order to aid during excavation in sandy soils.

An example of this are the North American snakes of the Heterodon genus, also known as hog-nosed snakes, as they present an elevated rostral scale giving their snout a characteristic shape. Photo of Heterodon nasicus by Dawson.

The horned vipers of the Cerastes genus also present various characteristics that facilitate life in the deserts. These vipers evade high temperatures becoming active at night and they spend the day buried in the sand. Their hunting method consists in burying themselves waiting for a prey to pass by, this way saving most of their energy. It is believed that their horn-shaped supraocular scales cover their eyes when they are buried in order to protect them from the sand.

Photo by Tambako The Jaguar of a Sahara sand viper (Cerastes vipera), a species from North Africa and the Sinai Peninsula.

SPINY CRITTERS

In different deserts of the world we find reptiles with their bodies covered in spines. This not only provides them with certain protection against predators, but is also helps them blend in in a habitat with plenty of thorny plants. Two of these animals are members of the Iguania suborder: the thorny devil and the horned lizards.

Photo of a thorny devil (Moloch horridus) by Christopher Watson.

The thorny devil (Moloch horridus) is an agamid that lives in the Australian sandy deserts. This lizard presents spines all over its body, making it difficult for its predators to swallow. It also has a protuberance behind its head that acts as a fat storage.  When it feels threatened, it hides its real head between its legs and it exposes its neck protuberance as a decoy head. Probably, the most interesting adaptation of this animal is the system of small grooves among its scales, which collect any water that contacts its skin and conducts it directly to its mouth.

Horned lizards (Phrynosoma genus, affectionately called “horny toads”) are iguanids which are found in different arid habitats of North America. Similarly to the thorny devil, their body is covered in spines making them hard to eat for their predators. Also, when they are caught, they inflate their bodies to make the task even more difficult. Finally, some species like the Texas horned lizard (Phrynosoma cornutum) are known for their autohaemorrhagic abilities: when they feel cornered they squirt a stream of stinky blood from their eyes which scares away most predators.

Photo from the U.S. Fish & Wildlife Service of a Texan horned lizard (Phrynosoma cornutum).

As you have seen, in the deserts we can find reptiles with some of the most inventive (and disturbing) adaptations of the world. These are only a few examples of the astonishing diversity of squamates that are found in the deserts of the world, which only seek to survive the harsh conditions of these extreme environments. Sometimes, it’s just a matter to avoid burning your feet with the hot sand.

Video from BBCWorldwide of a shovel snouted lizard (Zeros anchietae) making the “thermal dance” in order to diminish the contact with the hot sand.

REFERENCES

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

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Digital-Desert. Desert Reptiles.
• Arizona-Sonora Desert Museum. Adaptations of Desert Amphibians & Reptiles.
• In the desert. A comprehensive list of the venomous snakes found in the desert.
• H.E.R.P. Herpetological Education & Research Project.
• Christopher J. Bell, Jim I. Mead & Sandra L. Swift (2009). Cranial osteology of Moloch horridus (Reptilia: Squamata: Agamidae). Records of the Western Australian Museum. Vol 25. Pp: 201-237.
• Horned Lizard Conservation Society.
• Cover image by Yathin S. Krishnappa.

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.

Reptiles and mammals: same origin, different stories

Did mammals evolve from reptiles? The truth is they didn’t. Reptiles and mammals both have independent evolutionary histories that separated soon after the apparition of the so-called amniotic egg, which allowed the babies of these animals to be born outside of water. Previously, we talked about the origin of vertebrates and about how they managed to get out of the sea to start walking on land for the first time. In this entry we’ll explain how the ancestors of reptiles and mammals, the AMNIOTES, became independent of the aquatic medium and became the dominant land animals.

THE AMNIOTIC EGG

The characteristic that unites reptiles and mammals in the same group is the amniotic egg. While amphibian eggs are relatively small and only have one inner membrane, the eggs of amniotes are much bigger and present various membranes protecting the embryo and keeping it in an aqueous medium. The outer layer is the eggshell which, apart from offering physical protection to the embryo, prevents water loss and its porosity allows gas interchange. Beneath the eggshell we can find the next membranes:

Diagram of a crocodile egg: 1. eggshell 2. yolk sac 3. yolk (nutrients) 4. vessels 5. amnion 6. chorion 7. air 8. alantois 9. albumin (white of the egg) 10. amniotic sac 11. embryo 12. amniotic fluid. Image by Amelia P.

• Chorion: The first inner membrane, which offers protection and, together with the amnion, forms the amniotic sac. Also, being in contact with the eggshell, it participates in gas interchange, bringing oxygen from the outside to the embryo and carbon dioxide from the embryo to the outside.
• Amnion: Membrane that surrounds the embryo and constitutes a part of the amniotic sac. It offers an aqueous medium for the embryo and connects it with the yolk sac (a structure that brings food and that is also found in fish and amphibians).
• Allantois: The third layer, it is used as a storage for nitrogen waste products, and together with the chorion, helps in gas interchange.

Diagram of an amphibian egg: 1. jelly capsule 2. vitelline membrane 3. perivitelline fluid 4. yolk 5. embryo. Image by Separe3g.

All these different kinds of membranes eliminate the need amphibians had of laying their eggs in water. Also, unlike amphibians, amniotes don’t go through a gilled larval stage, but are instead born as miniature adults, with lungs and legs (at least those that have them). All these made the first amniotes completely independent of the aquatic medium.

AMNIOTE ORIGINS

The first amniotes evolved around 312 million years ago from reptiliomorph tetrapods. At the end of the Carboniferous period lots of tropical forests where the great primitive amphibians lived disappeared, leaving a colder and drier climate. This ended with many of the big amphibians of that time, allowing the amniotes to occupy new habitats.

Reconstruction of Solenodonsaurus janenschi, one of the candidates in being the first amniote, which lived around 320-305 million years ago in what is now the Czech Republic. Reconstruction by Dmitry Bogdanov.

CHARACTERISTICS

These early amniotes had a series of characteristics that set them apart from their semiaquatic ancestors:

• Horny claws (amphibians don’t have claws) and keratinized skin that prevents water loss.
• Bigger large intestine and higher density of renal tubules to increase water reabsorption.
• Specialized lacrimal glands and a third membrane in the eye (nictitating membrane) which keep the eye wet.
• Larger lungs.
• Loss of the lateral line (sensory organ present in fish and amphibians).

The skeleton and musculature also evolved offering better mobility and agility on a terrestrial medium. The first amniotes presented ribs that encircled their body converging at the sternum, making their inner organs more secure, and a series of muscular receptors offered them better agility and coordination during locomotion.

AMNIOTE SKULLS

Traditionally, the different amniotes were classified based on the structure of their cranium. The characteristic used to classify them was the presence of temporal openings (fenestrae), by which we have three groups:

• Anapsids (“no arches”): No temporal openings (turtles).

Diagram of an anapsid skull, by Preto(m).

• Synapsids (“fused arches”): With only one temporal opening (mammals).

Diagram of a synapsid skull, by Preto(m).

• Diapsids (“two arches”): With two temporal openings (reptiles, including birds).

Diagram of a diapsid skull, by Preto(m).

Previously it was believed that the first amniotes presented an anapsid skull (without openings, like turtles) and that subsequently they separated into synapsids and diapsids (the temporal openings formed “arches” that offered new anchor points for the jaw’s musculature). Yet, it has been discovered that this three-group classification is not valid.

Even though we still believe that the first amniotes were anapsid, it is currently known that these, soon after their apparition, separated into two different lineages: the synapsids (clade Synapsida) and the sauropsids (clade Sauropsida).

SYNAPSIDA

This lineage includes mammals and their amniote ancestors. Even though the first synapsids like Archaeothyris looked externally like lizards, they were more closely related to mammals, as they shared one temporal fenestrae where the jaw muscles passed through.

Drawing of the skull of Archaeothyris, which is thougth to be one of the first synapsids that lived around 306 million years ago in Nova Scotia. Drawing by Gretarsson.

The ancestors of mammals were previously known as “mammal-like reptiles”, as it was thought that mammals had evolved from primitive reptiles. Currently it’s accepted that synapsids form a different lineage independent of reptiles, and that they share a series of evolutionary trends that makes them closer to modern mammals: the apparition of different kinds of teeth, a mandible made of one single bone, the vertical posture of their limbs, etc…

Reconstruction of Dimetrodon grandis, one of the better known synapsids, from about 280 million years ago. Reconstruction by Dmitry Bogdanov.

Even though most modern mammals don’t lay eggs and give birth to live offspring, all groups maintain the amniote’s three characteristic membranes (amnion, chorion and allantois) during embryonic development.

SAUROPSIDA

Sauropsids include current reptiles and their amniote ancestors. Currently, in many scientific papers the word “sauropsid” is used instead of “reptile” when discussing phylogenies, as the sauropsids also includes birds. The first sauropsids were probably anapsids, and soon after their appearance they separated into two groups: the Parareptilia which conserved anapsid skull, and the Eureptilia which include the diapsids (current reptiles and birds).

Evolutionary tree of current vertebrates, in which green color marks the groups previously included inside reptiles. As you can see, the traditional conception of "reptile" includes the ancestors of mammals and excludes birds. Image by Petter Bøckman.

Diapsids are currently the most diversified group of land vertebrates. They diversified greatly in the late Permian period (about 254 million years ago), just before the Mesozoic (the Age of Reptiles). These can be divided into two main groups: the Lepidsaurs and the Archosaurs, both with representatives in our days.

LEPIDOSAURIA: SMALL AND PLENTIFUL

Lepidosaurs (literally “reptiles with scales”) appeared in the early Triassic (around 247 million years ago) and, even if most of them didn’t grow to big sizes, they are currently the largest group of non-avian reptiles. These are characterized by presenting a transversal cloacal slit, by having overlapping scales and shedding their skin whole or in patches and by other skeletal characters.

Shed skin of a rat snake. Photo by Mylittlefinger.

The current lepidosaurs belong to one of two different orders:

• Order Rhynchocephalia: That includes the two species of tuatara. Currently endangered, they are considered living fossils because they present skulls and characteristics similar to the Mesozoic diapsids.

Photo of a tuatara (Sphenodon punctatus), by Tim Vickers.

• Order Squamata: Current squamates include iguanas, chameleons, geckoes, skinks, snakes and other legless lizards. With more than 9000 living species, squamates are a large group with a wide array of adaptations and survival strategies.

Photos of some squamates, from left to right and from top to bottom: Green iguana (Iguana iguana, by Cary Bass), king cobra (Ophiophaga Hannah, by Michael Allen Smith), Mexican mole lizard (Bipes biporus, by Marlin Harms) and Indian chameleon (Chamaeleo zeylanicus, by Shantanu Kuveskar).

ARCHOSAURIA: ANCIENT KINGS

Archosaurs (literally “ruling reptiles”) were the dominant group of land animals during the Mesozoic. These conquered all possible habitats until the extinction of most groups at the end of the Cretaceous period. Some of the extinct groups were the pseudosuchians (relatives of modern crocodiles, order Crocodylia), the pterosaurs (large flying reptiles) and the dinosaurs (excepting birds, clade Aves).

Drawing of the skull of the dinosaur Massospondylus in which we can see the different characteristic openings of diapsid archosaurs. Image by Steveoc 86.

As you see, both groups of modern archosaurs couldn’t be more different. Yet, crocodiles and birds share a common ancestor, and they are both more closely related with each other than with the rest of reptiles.

Photo of two species of modern arcosaurs: a Nile crocodile (Crocodylus niloticus) and a yellow-billed stork (Mycteria ibis). Photo by Tom Tarrant.

AND WHAT ABOUT TURTLES?

Turtles (order Testudines) have always been a group difficult to classify. Turtles are the only living amniotes with an anapsid skull, without any post-ocular opening. That’s why previously they had been classified as descendants of primitive amniotes (clade Anapsida, currently disused) or as primitive anapsid sauropsids (inside the Parareptilia clade)

Skeleton of the extinct tortoise Meiolania platyceps which lived in New Caledonia until 3000 years ago. In this photo it can be seen the compact cranium without openings. Photo by Fanny Schertzer.

Recent molecular studies have revealed that turtles are actually diapsids that lost their temporal openings secondarily. What still divides the scientific community is if testudines are more closely related to Lepidosauromorphs (lepidosaurs and their ancestors) or to Archosauromorphs (archosaurs and their ancestors).

Individual leopard tortoise (Stigmochelys pardalis) from Tanzania. Photo by Charles J. Sharp.

As you have seen, the evolution of amniotes is an extremely complex matter. We hope that with this entry some concepts have been clarified:

1. Mammals (synapsids) come from an evolutionary lineage different from that of reptiles (sauropsids).
2. Sauropsids include traditional reptiles (lepidosaurs, archosaurs and turtes) and birds (inside archosaurs).
3. There’s still so much to investigate about the placement of turtles (testudines) in the evolutionary tree of sauropsids.

Modified diagram about the evolutionary relationships of the different amniote groups.

REFERENCES

During the elaboration of this entry the following sources have been consulted:

• Cover image by Henri Pidoux.
• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Hickman, Roberts, Keen, Larson, l’Anson & Eisenhour (2009). Principios integrales de Zoología (decimocuarta edición). Mc Graw Hill.
• http://www.biologyreference.com/A-Ar/Amniote-Egg.html
• http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amniota.html
• http://tolweb.org/Amniota
• http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.1979.tb00027.x/abstract
• https://courses.candelalearning.com/osbiosec/chapter/29-4-reptiles/

Herpetological parachuting: gliding amphibians and reptiles

Currently, the only flying reptiles are birds, direct descendants of theropod dinosaurs. Although the age of the great flying reptiles has passed, nowadays, various species of reptiles and amphibians have acquired the ability of gliding to escape their predators. Gliding is defined as falling at an angle less than 45^o from the horizontal with the help of membranes that create resistance to the air. In this entry I’ll show you some gliding herp species which currently exist.

Gliding frogs

Gliding frogs (also called “flying frogs”) include species from the Polypedates, Rhacophorus (Rhacophoridae family) and Ecnomiohyla (Hylidae family) genera. These have gained similar characteristics through a process of convergent evolution.

Photo of Ecnomiohyla rabborum by Brian Gratwicke.

Both hylids and rhacophorids are popularly known as tree frogs. Their limbs are specialized for an arboreal lifestyle, with long legs and fingers with sucker-like structures for a better grip.

 Male and female false Malabar's gliding frogs (Rhacophorus pseudomalabaricus) mating. Video by Sandesh Kadur.

The gliding genera have also acquired big membranes on its limbs and between their toes to help them glide and therefore, being able to escape predators more efficiently.

Wallace's flying frog (Rhacophorus nigropalmatus) gliding.

Gliding geckoes

Between the members of the Gekkonidae family there are two Southeast Asian genera which have acquired adaptations for gliding: the Ptychozoon genus and the Luperosaurus genus.

Photo of a Kuhl's flying gecko (Ptychozoon kuhli) by Bernard Dupont.

Geckoes are a group of lizards which have evolved for an arboreal lifestyle which allows them to adhere practically on any surface. Their feet present tiny filaments which allow them to move even upside down.

Detail of the underside of a Kuhl's flying gecko (Ptychozoon kuhli) in which the skin flaps can be appreciated. Photo by Fenchurch.

The Ptychozoon and Luperosaurus genera also present membranes on their neck, body, limbs and tail that help them to blend in the surface of trees and also to glide at some extent from tree to tree to escape possible predators.

Flying snakes

Speaking of predators, the snakes from the Chrysopelea genus also have developed an efficient method to move through the rainforest’s canopy. The snakes from this genus are diurnal, feed on lizards, frogs, birds and bats, and are found throughout Southeast Asia.

Couple of Paradise flying snakes (Chrysopelea paradisi) in the Singapore Zoo, by Alan Couch.

Unlike the former gliding herps, flying snakes have no membranes to slow down their descent, instead they have a more complex method. When arriving at the tip of a tree branch, these snakes drop themselves. After a brief fall, they retract their inner organs, compressing them against their thoracic cavity and flaring out their ribs laterally, taking a semi-concave shape, similar to that of a plane’s wing.

Explicative image of the gliding mechanism of the flying snakes. Image from Biomechanics.

With this method and with the help of serpentine movements, the snakes of the Chrysopelea genus can control with great precision the direction of their descent. These snakes have a more controlled glide than many gliding mammals such as the gliding squirrel and are able to glide through a horizontal length of up to 100 metres.

 Group of scientists testing a Paradise flying snake’s (Chrysopelea paradisi) ability to glide. Video by All of These Videos.

Flying dragons

And we finally get to the most spectacular of all flying herps, the reptiles known as flying dragons. These agamids (Agamidae family) of the Draco genus are found in the tropical forests of Asia, where they survive hunting insects on the forest canopy.

Photo of a five-lined flying dragon (Draco quinquefasciatus) from Sarawak, Malaysia. Image by Bernard Dupont.

The main characteristic of flying dragons is their ribs, some of which are extremely elongated and present dermal membranes between them acquiring the function of wings. These “wings” are usually retracted against the body and can be erected for both gliding and sending visual signals to other members of their species (wings are usually brightly coloured).

Drawing from the book On The Genesis of Species of the skeleton of a Draco volans.

Flying dragons use their wings to move from tree to tree, to hunt, to escape predators, to chase their own kind during both territorial disputes and courtship. Aside from their brightly coloured wings, many species also present colourful dewlaps (especially males) to indicate their reproductive state to other members of their species.

Photo of a Sulawesi's lined flying dragon (Draco spilonotus) by A. S. Kono.

The flight record of these agamids is of 60 metres of distance with a vertical descent of only 10 metres. Flying dragons are small, fast and active animals, so few predators are able to hunt them. In addition, they are totally arboreal with only females descending to the ground to lay their eggs underground.

 Flying snake chasing a flying dragon. Video found in Venomous Animals.

As we have seen, most species of gliding amphibians and reptiles live in tropical climates. This is due to the fact that these are habitats with a dense vegetation cover and trees grow very close to one another, allowing these animals to glide from one tree to the other easily. The main threats to these creatures are deforestation and habitat loss since, without an optimal vegetation cover, these animals may be preyed easily by terrestrial predators.

References

During the elaboration of this entry the following sources have been used:

• Cover photo: Premaphotos.
• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• http://australianmuseum.net.au/blogpost/science/aerodynamic-amphibians
• http://www.naturia.per.sg/buloh/verts/flying_tree_snake.htm
• http://www.siam-info.com/english/snakes_chrysopelea.html
• http://phys.org/news/2005-05-reveal-secrets-snake-flight.html
• http://www.uchospitals.edu/news/2005/20050513-snake.html

Having no legs doesn’t make you a snake

With the arrival of good weather it becomes more probable that we go out to the forest to enjoy nature, and the possibilities of finding snakes and other reptiles sunbathing on a stone or running among the grass increase. Snakes are the best known legless squamates, even though there are many other species of lizards which have also lost their extremities during their evolution. In this entry I’ll explain some distinctive characteristics of the three species of legless lizards that we can find in the Iberian Peninsula, the slow worm and the Iberian worm lizards.

LIMBLESS LIZARDS

The loss of legs is an evolutionary phenomenon that has happened more than once in the Squamata order. In fact, currently there are at least nine different lineages of legless lizards (not counting snakes).  In most groups this happens as an adaptation to a subterranean lifestyle (these usually present a short, round tail) or to a life among grass and vegetation (which usually show a long, slender tail).

Scheltopusik or European legless lizard (Ophisaurus apodus) a limbless lizard of the Anguidae family, photo by Tim Vickers.

Even though technically snakes are also legless lizards, unlike other groups, some ophidian species may pose a potential threat to human beings. This is why it’s important to know how to tell a snake from a legless lizard. There are some characteristics which can help us to differentiate a snake from a non-venomous lizard:

• Snakes haven’t got movable eyelids, while the rest of lizards do have.
• Ophidians have no external ear, while in most lizards the auditory channel can be appreciated.
• Snakes present specialized ventral scales for locomotion, while most limbless lizards have to move with the aid of the irregularities of the substrate.
• Many legless lizards can shed their tail as a defense mechanism (caudal autotomy) while snakes can’t.

Picture of a western green mamba (Dendroaspis viridis), a typical ophidian, by Jon Sullivan.

In a previous entry we already explained the different snake species that can be found on the Iberian Peninsula. Below, I’ll present you the three different species of legless lizards that we can find when we go out to visit natural landscapes of our country.

SLOW WORM (Anguis fragilis)

The slow worm is a legless squamate within the anguid family (Anguidae), in which we find the Anguinae subfamily, in which many species have lost their limbs or have them extremely reduced in size. The slow worm’s scientific name, Anguis fragilis, means literally “fragile snake”, referring to its ability to shed its tail to escape predators.

Photo of a slow worm close to Nismes, by © Hans Hillewaert.

Description

The slow worm is a small lizard with no visible legs, which can grow to 40 centimetres in length. It presents shiny, smooth scales and a small head with a poorly differentiated neck. Unlike snakes, it has movable eyelids, a forked tong and a small tympanic aperture.

Juvenile individuals usually have a golden or silver brown colouration with their sides and belly of a black coloration. Females and juveniles are similarly colored, being ochre with a dark brown or black belly and a black dorsal band, even though their coloration varies a lot.

Female slow worm, photografied at Hertfordshire by Peter O'Connor.

Males are more uniformly colored, with its back and sides of brown or grey coloration, while some older individuals show dark brown spots on their sides which may become of a bluish coloration with age.

Male slow worm, with distinctive blue spots, by Maria Haanpää.

Habitat and distribution

It’s a widely distributed reptile throughout most Europe, all being found from the Iberian Peninsula, England and Scotland up to Iran and west Siberia, passing through Greece and Turkey.

Map showing the slow worm's distribution, by Osado.

In the Iberian Peninsula it is found mainly in the northern half, occupying most Galicia, Asturias, Basque Country and Castile and León and the north of Aragon and Catalonia. The slow worm is a common species that goes unnoticed thanks to its inconspicuous customs. We can find slow worms in a wide variety of open habitats, such as grasslands, scrublands and open forests.

Distribution of the slow worm in Spain, by Lameiro.

Unlike most reptiles, which look for sunny places to warm up, the slow worm has a strong preference for wet and shadowy places, with plenty of low growing vegetation. It usually shelters under stones, tree logs, plastic wastes or small mammal’s burrows.

A male slow worm on its habitat, on the Netherlands, by Viridiflavus.

Biology and ecology

In the Iberian Peninsula the slow worm is active from the end of February to November, when hibernation starts, during which groups of up to 100 individuals can be found. Mating lasts from middle March up to July, during which males can be found fighting. Their gestation period lasts about three months, they are ovoviviparous species (females produce eggs but babies hatch inside their mothers) and females give birth from 2 to 22 young.

Many different species of reptiles, birds and mammals prey upon this species. As other lizards, the slow worm can shed its tail as a defence mechanism, which continues moving while the rest of the animal flees. The tail starts to regenerate after a few weeks.

Picture of a slow worm after shedding its tail, by SuperMarker.

Slow worms feed on snails, earthworms, insect larvae and many other small invertebrates, because, unlike snakes, they can’t unhinge their jaws to swallow big prey. This animal has been unfairly persecuted even though it is a helpful species for fields and gardens, as it feeds on many species considered pests for many cultivated plants.

Photo of a slow worm feeding on a slug, by Biosphoto/Thiebaud Gontard.

WORM LIZARDS (Blanus cinereus and Blanus mariae)

Amphisbaenians (clade Amphisbaenia) are a group of highly specialized subterranean squamates known as worm lizards. Even though externally they resemble some primitive snakes, they are different in that, while snakes first lost their front limbs and their left lung, worm lizards first lost their hind limbs and their right lung. Currently about 180 species of amphisbaenians are known, two of which are found on the Iberian Peninsula: the Iberian worm lizard (Blanus cinereus) and the Maria’s worm lizard (Blanus mariae), both differentiated by distribution and genomic studies.

Iberian worm lizard in Andalusia, photo by Antonio.

Description

Worm lizards are reptiles strictly adapted to a subterranean lifestyle, with bodies externally resembling that of earthworms. At first it’s difficult to tell the head and the tail apart, which is useful for worm lizards when it comes to escape predators (just as the slow worm, worm lizards can shed their tail, which doesn’t regenerate completely).

Iberian worm lizard next to Murcia. Note the similarity between head and tail. Photo by Jorozko.

Adults may measure more than 15 centimetres in length, with some individuals reaching 30 centimetres. The head is blunt and short, with a wide frontal scale to aid them while digging. Their eyes are vestigial (they can only detect changes of light intensity) and are covered by scales, while they have very acute hearing and smell.

Photo of the head of an Iberian worm lizard, where you can see the scale-covered eyes, by J. Gállego.

Scales are rectangular and are distributed evenly forming rings around their body. Coloration goes from pale pink, to dark purple and brown, and there is no sexual dimorphism between males and females. Like all amphisbaenians, worm lizards can move both forwards and backwards.

Adult worm lizard next to Cáceres, in which we can see the rectangular and evenly distributed scales. Photo by Mario Modesto.

Habitat and distribution

The two peninsular species of worm lizard are found exclusively in the Iberian Peninsula, except in the north and northeast, from sea level up to 1800 metres of altitude (in Sierra Nevada). The Iberian worm lizard (Blanus cinereus) is more widely distributed, while the Maria’s worm lizard (Blanus mariae) occupies the southwest of the peninsula.

Distribution map including both Blanus cinereus and Blanus mariae, by Carlosblh.

Worm lizards are found in a wide variety of habitats, from forests of holm oaks, pine trees and oaks to crops, gardens and sandy areas. They have subterranean habits, and usually take shelter under rocks and logs. Like the slow worm, worm lizards prefer humid zones and with soft soil, easy to dig into.

Biology and ecology

Worm lizards are active all year round, even though their activity specially intensifies during spring, summer and after rainy weather. During the day they usually shelter in underground galleries or under logs and rocks. In winter they maintain their body heat, moving through galleries at different depths or staying under sun warmed stones.

Photo of an Iberian worm lizard next to Cádiz, photo by Jorge López.

Their diet is composed of insects, arachnids and other arthropods found between leaves or underground. Worm lizards are eaten by a great number of terrestrial vertebrates, and their defense mechanisms include: tail scission, escaping to some of their galleries or curling up to form a ball.

Video of an Iberian worm lizard from Albacete, by Encarna Buendia.

The reproduction season goes from February to June, while mating usually occurs between April and May. Females lay a single relatively large egg, which is abandoned buried underground. Incubation period lasts for 69 to 82 days, and the newborn measure between 78 to 86 millimetres.

Photo of a pair of Iberian worm lizards in a garden near Seville, by Richard Avery.

OTHER LEGLESS LIZARDS

As I’ve already said, apart from the species described above, there are many other groups of limbless lizards over the world. Some of these other groups are:

Scincidae family: A family of chubby, short legged lizards, many of which have no functional limbs. In the Iberian Peninsula we can find two species: the Bedriaga’s skink (Chalcides bedriagai) and the western three-toed skink (Chalcides striatus).

Western three-toed skink, photo by Benny Trapp.

Pygopodidae family: A group of lizards with absent or reduced limbs, related to geckos.

Photo of a Burton's legless lizard (Lialis burtoni) from southern Australia, by Matt.

Dibamidae family: Legless tropical lizards of subterranean habits.

Photo of a dibamid called Anelytropsis papillosus, taken from Tod W. Reeder et al.

Anniellidae family: American legless lizards.

A legless lizard from the Anniella genus, form California, by Marlin Harms.

Even if most legless lizards are harmless, it doesn’t mean we can touch them and handle them in any form we want when we find them in nature. Legless lizards, as most wild animals, are easly stressed by human handling and shouldn’t be handled except for scientific purposes. The best way to enjoy nature is by observing it without disturbing it.

REFERENCES

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

• Cover photo: David López Bosch.
• http://animals.howstuffworks.com/snakes/legless-lizard-vs-snake1.htm
• http://www.mapoflife.org/topics/topic_378_Limblessness-in-lizards/
• http://www.vertebradosibericos.org/reptiles/listareptiles.html
• http://www.magrama.gob.es/es/biodiversidad/temas/inventarios-nacionales/0904712280003d45_tcm7-21472.pdf
• http://www.asturnatura.com/especie/anguis-fragilis.html
• http://www.vertebradosibericos.org/reptiles.html
• http://reptile-database.reptarium.cz/species?genus=Anguis&species=fragilis

Iberian ophidians: nice snakes and venomous vipers

In my first blog entry I talked about the different kinds of snake that exist based on their dentition. In this entry, I’ll explain what species of ophidian we can find in the Iberian Peninsula, which species are venomous and which aren’t, and how we can identify the different species we can find when we are on the field. As we will see in this entry, snakes have been unfairly demonized, as the species in the Iberian Peninsula pose no threat to us.

INTRODUCTION

In the Iberian Peninsula we can find 13 different species of snakes, with representatives of three of the four types of dentition I talked about in my last post. There aren’t any proteroglyphous snake because the members of the Elapidae family are restricted to tropical and subtropical habitats. Most of the iberian species are snakes of the Colubridae family (aglyphous or opisthoglyphous) or vipers and adders of the Viperidae family (solenoglyphous).

Viperine water snake (Natrix maura), aglyphous

Montpellier snake (Malpolon monspessulanus), opisthoglyphous

Snub-nosed viper (Vipera latastei), solenoglyphous

COLUBRIDS vs. VIPERS

When we find a snake in the wild it’s important to know if that animal is a colubrid or a viper. Bites from Iberian colubrids are mostly harmless because they have either an unspecialized non-venomous dentition (aglyphous) or posterior venomous fangs (opisthoglyphous) which usually doesn’t inject venom and even if they do, normally they don’t inject enough venom for it to be dangerous. On the other hand, as Iberian vipers are solenoglyphous, they inject large quantities of venom, being vipers responsible for most of the snake bite accidents in Spain. Yet, bites are extremely rare, and most happen after a too prolonged manipulation of the animal.

To identify a snake as a colubrid or a viper there are some anatomical characteristics which tell them apart. These characteristics are usually only applicable to iberian ophidians; species from outside the Iberian Peninsula may present different combinations of characters.

The most cited character is the pupil. Normally vipers show an elliptic, slit-like pupil, while colubrids present a round pupil. However, this character is variable, because with low-light conditions a viper’s pupil may look round as the eyes of these animals can adapt to darkness.

Colubrid with round pupil (ringed snake, Natrix natrix) and viperid with elliptic pupil (snub-nosed viper, Vipera latastei). Photos by Honorio Iglesias.

The second character refers to the shape of the body. While colubrids are mostly thin, have an undifferentiated neck and a long slim tail, vipers have a triangular-shaped head with a neck differentiated from the body, and a short and conic tail.

Aesculapian snake (Zamenis longissimus) and Baskian viper (Vipera seoanei, photo by Daniel Gómez)

Although it may be difficult to look at, scales can be useful to tell colubrids and vipers apart. Vipers always present keeled scales, which have a little keel-like protuberance longitudinally on it. On the other side, even though they can have some keeled scales, most colubrids present smooth scales.

Smooth scales of a horseshoe whip snake (Hemorrhois hippocrepis, photo by Saúl Yubero) and keeled scales of an asp viper (Vipera aspis, photo by Grégoire Meier)

Finally, while colubrids are active animals and usually flee before we can get close to them, vipers rely on their camouflage to avoid predation. Therefore, they stay still so we can’t see them, and may bite if they feel cornered.

IBERIAN OPHIDIANS

Colubridae family:

Coronella genus: Known as smooth snakes. In the Iberian Peninsula we can find the northern smooth snake (Coronella austriaca) which presents a dark mask-like spot covering from the nasal openings up to the neck and dark irregular markings on its back, and the southern smooth snake (Coronella girondica) which presents a pair of parietal marks and dark transversal spots on its back.

Northern smooth snake (Coronella austriaca, left, photo by Christian Fischer) and southern smooth snake (Coronella girondica, right, photo by Evaristo Corral)

Hierophis genus: The green whip snake or western whip snake (Hierophis viridiflavus) is a brightly-coloured snake with a pattern of black, yellow and green spots over its body. Even though they can grow up to 170 cm of length they are not venomous. It can be usually found from temperate forests to crop fields, and even in abandoned buildings.

Green whip snake (Hierophis viridiflavus), juvenile (left, by Polypterus) and adult (right)

Natrix genus: Commonly known as water snakes due to their affinity for aquatic habitats. In the Iberian Peninsula we can find two species, the viperine water snake (Natrix maura) named after its zigzag marking and its keeled scales similar to a viper, and the grass or iberian ringed snake (Natrix astreptophora) which presents reddish pupils, an extremely variable coloration and a black “ring” in juvenile individuals.

Viperine water snake (Natrix maura, left, photo by Honorio Iglesias) and iberian ringed snake (Natrix astreptophora, right photo of Fafner).

Zamenis genus: The Aesculapian snake (Zamenis longissimus) is a slim, long and harmless colubrid with a characteristically narrow and elongated skull. It is normally found on forested areas, with different microclimatic variations to aid it on its thermoregulation. This species is the one represented coiled around the rod of Aesculapius and the Bowl of Hygieia, symbols of medicine and pharmacy respectively.

Aesculapian snake (Zamenis longissimus) (left by Amiralles).

Hemorrhois genus: The horseshoe whip snake (Hemorrhois hippocrepis) is an aglyphous colubrid that, even if it may bite if touched or grabbed, it’s not considered a venomous species. It presents a transversal mark on its head from one eye to the other, and another mark in the shape of a horseshoe on its neck, which gives this species its common name. It’s a species typical of rocky habitats.

Horseshoe whip snake (Hemorrhois hippocrepis). Photos by Accipiter and Raúl León.

Rhinechis genus: The ladder snake (Rhinechis scalaris) receives its common name due to the stripes that juvenile specimens present on their back, similar to a ladder, even though adult individuals may present only longitudinal stripes on their body without any transversal marks connecting them. Despite being an apparently aggressive snake, it rarely bites and is harmless to human beings.

Ladder snake (Rhinechis scalaris). Photos by Matt Wilson (left) and by Fernando Fañanás (right).

Macroprotodon genus: This is one of the few venomous species in the Peninsula. The western false smooth snake (Macroprotodon brevis) is an animal common on many different Mediterranean habitats. Even if it’s venomous, its small opisthogyphous mouth and its calm behavior make it totally harmless. It is characterised by a dark mark on the back of its head, and its short and flattened skull.

Western false smooth snake (Macroprotodon brevis). Photos by Saúl Yubero and Amiralles, respectively.

Malpolon genus: With specimens growing up to two and a half meters of length, the Montpellier snake (Malpolon monspessulanus) is the largest ophidian of the peninsula. Due to its opisthoglyphous dentition it normally doesn’t inject venom when biting (which is extremely rare), but larger individuals with much wider mouths may inject venom, but to cause symptoms it should hold its bite for a long period of time (most bites, even if rare, are dry warning bites). It is easily recognisable for its prominent eyebrows which give it a ferocious look.

Montpellier snake (Malpolon monspessulanus). Photos by Herpetofauna and RuizAraFoto respectively.

RuizAraFoto

Viperidae family:

There’s only one genus of vipers on the Iberian Peninsula with three representative species. Vipers and adders usually have a wide and triangular head, a lightly elevated snout and usually present a zigzag pattern on their back which help them camouflage. The three Iberian species are venomous, but thanks to modern medicine, their ocasional bites aren’t harmful to human beings. The asp viper (Vipera aspis), the most venomous snake in the peninsula, presents grey, golden or yellow scales, with black or green spots. The snub-nosed viper (Vipera latastei) is the most common viper in the peninsula and its coloration varies from brown to grey. Finally the Baskian or Portuguese viper (Vipera seoanei) is a middle-sized viper and with a highly polymorphic pattern.

Asp viper (Vipera aspis, top left, photo by Felix Reimann), snub-nosed viper (Vipera latastei, top right, photo by Honorio Iglesias) and Baskian viper (Vipera seoanei, bottom, photo by Andre Schmid).

As we have seen, snakes and vipers aren’t as bad as they are portrayed to be. Most species flee from human beings, and accidents and bites happen when we force them to interact with us too much. Also, ophidians help farmers and agriculturers by hunting and eating species traditionally seen as vermin. If we leave snakes and vipers alone, we will be able to enjoy the beauty of this animals in peace.

REFERENCES

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

• Cover photo: Aecuvis.
• http://www.trebago.com/revistas/01/08serpientes.asp
• http://anfibiosyreptileslarioja.blogspot.com.es/
• http://www.moroccoherps.com/Inicio/
• http://www.faunaiberica.org/?page=categoria&tipo=ofidios
• http://www.vertebradosibericos.org/reptiles/listareptiles.html
• http://manual-ofidico.blogspot.com.es/2012/08/diferencias-entre-serpientes-venenosas.html

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Snakes: Show me your teeth and I’ll tell you who you are

This week’s entry focuses on snakes, creatures that have caused an intense hate from human beings since ancient times. One of the main reasons why snakes are so deeply rooted in the human mind is the posed by the fact that some species are venomous, possessing venomous glands which open through ducts into grooved or hollow teeth. Although most species are harmless to human being, these animals still give the shivers to more than one.

Venomous fangs appeared as a modification of maxillary teeth. Depending on the level of specialization in both the jaw and these fangs, each species of snake may be classified into one of four different groups.

AGLYPH (lacking grooves)

Reticulated python’s skull (Python reticulatus)

Aglyphous snakes have the most primitive condition in which teeth are solid, without grooves or specialized venom-injecting fangs. This is the less specialized dentition, which is found in many snake families, from the great boas and pythons to the primitive blind snakes from the infraorder Scolecophidia, and even in some members of the great Colubridae family. Teeth usually have the same size and morphology. This type of dentition is usually linked with non-venomous species, although a few aglyphous snakes do have venom yet most are nonlethal to human beings.

OPISTHOGLYPH (rearward grooves)

These snakes posses venom which is injected with specialized fangs found at the posterior end of the maxilla, which are backward-oriented and grooved so that toxins are canalized to the tip of the tooth. To correctly inject venom, these snakes must hold their prey and move it to the rear of the mouth, a pretty arduous task if the prey is of a considerable size.

Black-headed cat snake’s skull (Boiga nigriceps), a colubrid from Southeast Asia

This type of dentition is found in various species into the large Colubridae family, in which it has evolved twice independently.

Common kingsnake (Lampropeltis getula), colubrid from the USA

Even though most opisthoglyphous snakes are harmless to humans (because fangs are found at the back of their mouths, and these snakes aren’t usually very big) some species are lethal to humans, like the “boomslang” (Dispholidus typus) and the bird snakes (Thelotornis sp.) which bite with the mouth wide open (up to 170 degrees to insert their venomous fangs firmly) and generate powerful haemotoxins against which no efficient antitoxin has been developed yet. Haemotoxins are toxins which destroy red blood cells collapsing the circulatory system and provoking severe necrosis to the other tissues.

PROTEROGLYPH (forward grooved)

Proteroglyphous species have venomous fangs at the front of their mouth and these aren’t usually very long. That’s why these snakes must apply pressure on their bite long enough to inject the necessary venom into their prey.

Death adder’s skull (Acanthophis sp.), and Australian elapid

This kind of teeth is characteristic of the Elapidae family, which includes cobras and sea snakes. The members of this family have venoms most of which consist in neurotoxin (toxins that destroy the nervous system), and are amongst the most venomous of all vertebrates.

Mozambique spitting cobra (Naja mossambica)

Also, some elapids of the Naja genre are known as spitting cobras, because their anterior fangs are modified and present orifices which allow them to spray their venom with the contraction of muscles of their venomous gland.

SOLENOGLYPH (pipe grooved)

This is the most evolved form and it is exclusively found in the members of the Viperidae family. In these species the maxilla is extremely reduced and serves as the support to a pair of hollow fangs which can make up to half the skull’s length.

Rattlesnake’s skull (Crotalus sp.)

These fangs are usually folded against the roof of the mouth but can articulate with the rest of the cranium when the mouth is opened up to 180 degrees to bite. This allows vipers to penetrate their fangs deeper into their prey and inject large quantities of venom, which being usually less powerful than the one of proteroglyphous snakes, in large quantities can be lethal.

Puff adder (Bitis arietans), showing how fangs articulate with the cranium

Intern taxonomy of the different snake groups is based on many different anatomic characteristics. The classification presented here only refers to the dentition and jaw structure, and that may not be directly correlated to the evolutionary relationships between different families. For example, inside the Colubridae family (family which includes two-thirds of the extant snake species), we can find species with aglyphous, opisthoglyphous and proteroglyphous dentitions, even though the proteroglyph type is characteristic of the Elapidae family, where it has evolved independently.

REFERENCES

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

• Cover photo: Siddhesh Chafekar.
• http://hydrodictyon.eeb.uconn.edu/eebedia/images/b/b7/Snakedentition.pdf
• http://bugsinthenews.info/?p=1285
• http://biostor.org/reference/107209
• http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2205449/pdf/procrsmed00690-0099.pdf

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