Introduction Evolution
3
Rattlesnake
8 Hognose Snake
2 Diversification
4
Egg-eating snake
Green Anaconda
14
12 Long-nose Whip Snake 16
Belcher’s sea Snake
Inland Taipan
Flying Snake
20 Burrowing Asp
24 Tiger Kneelback
References and Acknowledgements
18 22
Tentacled Snake
26
Horned Viper
28 Glossary
10
30 32 34
Reptiles are fascinating. This is the same opening statement as the previous mini guide in this series however hopefully it will be proven once again. This mini guide focuses on the snakes,
which make up over a third of all known reptiles and are extremely diverse, found in almost every habitat in the world. They have developed ways of surviving in temperatures too low to incubate eggs and places that are so hot prolonged contact with the ground can lead to burns. They have climbed high into the trees, burrowed into
the ground and taken to the oceans. I have selected twelve examples that help demonstrate the different abilities and adaptations of the suborder Serpentes which contains all extant snake species. These twelve are a mixture of both single species, which show a remarkable behaviour or adaption, or families of snakes that evolved skills in specific areas that surpass their phylogenetic cousins. Each exceptional snake example will give you some background on the species as well as a description of how they use their unusual talent. The information in this guide has been taken from a variety of sources. In each section a reference will be noted like this (Ord and Stuart-Fox, 2006) following information that was taken from that particular study, in this case Terry Ord and Devi Stuart-Fox’s 2006 study on ornament evolution. All these references are in full on the back pages if you wish to read the full study to learn more. A glossary of a key words or terms is included on page 32.
The origin of snakes has long been debated. Older studies have suggested that they came from a lizard-like reptile ancestor. Over time the use of limbs became obsolete and they decreased in size until they no longer developed 25
during the embryonic stages. This new, almost legless, reptile would be the
start of the snake lineage. This was long since believed as lack of fossil evidence due to the soft bodied and thin boned anatomy of snakes meant fossilisation was unlikely. More recent studies have shown a different side. The first question that needed to be asked was what makes a snake a snake. There are many legless lizard species, most of which share very similar body shape and structure, therefore an elongated limbless body cannot be the determining factor. The skull of a snake is quite unique in its layout and structure. Each side of the lower jaw can move independently, they have recurved teeth to prevent any prey from escaping and a quadrate capable of pivoting to swallow larger prey items. Current studies now use the skull as a way of judging if a fossil specimen is part of the snake linage. 4 recently discovered specimens have been examined leading to dating of the earliest known snake species
Eophis underwoodi to approximately 167 million years ago (Caldwell et al 2015). The unusual feature of these species is that some had both front and hind limbs, in a much smaller form but still functional. In more modern snakes there is still hint of their past. Python and Boa species have small bone-like spikes called spurs either side of their vent. These spurs are no longer attached to the vertebrae but are used to aid breeding in the larger species. Further studies have shown even in those species lacking spurs specific vertebrae resemble those of four limbed animals even though no limbs are present.
This guide focuses on the unusual and intriguing adaptations shown by special species but there are several adaptations shown by a large proportion of snake families which cannot go without note. Snakes have taken venom to new levels with numerous concoctions causing sophisticated tissue damage as well as original delivery systems. They have formed several types of locomotion to adapt to their environment and navigate that environment with senses we cannot comprehend. Venom produced by snakes varies in its effect depending on the desired outcome, such as killing vs wounding and defence vs offence. There are approximately 20 different compounds which can make up the known snake venoms, mainly proteins and polypeptides. These are not all used by each species and vary in their quantity. These complex mixtures form toxins, most commonly neurotoxins and hemotoxins, used by elapids and viperids respectively. Therefore it matters greatly where and how deep the fangs penetrate to inject the venom. Venom mixtures will not only affect dissimilar species differently but also vary in the effect on that species system. Since all venom varies, comparing factors such as potency is difficult (Page 20). Venom potency is measured in the milligrams of toxin per kilogram of body mass (mg/kg) that would kill 50% of the test population. This is referred to as Median Lethal Dose or LD50.
Fig 1. One type of venom delivery system.
The test population is usually humans or mice. When the human LD 50 is estimated bovine serum album, a cow protein, is used as it is similar to human proteins. This provides the ‘facts’ about how many men a snake can kill in a single bite.
There are 4 main tooth conditions in snake species, 3 of which house venom delivery systems. The diagram (Fig 2.) demonstrates these. 1.
Aglyphous teeth have no prominent fangs, this is seen in non-venomous snakes such as pythons and boas. These snakes are mainly constrictors so need many small sharp slightly curved teeth for maximum hold on prey.
2. Opisthoglyphous teeth have fangs positioned at the rear to hold onto prey. They have a grooved channel so the venom runs direct to the tip. This can be seen in Hognose snakes (Page 12). 3.
Proteroglyphous teeth are curved posteriorly and have a hollow venom channel running through the centre. This means no venom is wasted and the prey gets the full dose. This can be seen in cobra species.
4.
Solenoglyphous teeth have the most advanced and largest fangs. They are hinged meaning they can be extended out to penetrate deep into prey with
Fig 2. Four Main tooth conditions seen in snake species.
a large volume of venom. A common feature of vipers (Page 30). Although there are 4 main tooth conditions some snakes have put their own spin on theirs such as the burrowing asp (Page 24). Venom is designed to be injected into prey, the solenglyphous fangs can penetrate the deepest, through the skin and into the underlying muscle. This means the venom has a quicker effect in immobilising the prey. Due to the hollow nature of two of the conditions venom can be forced out at high pressure giving species, like the spitting cobra, an extra defence by spraying the venom at a possible threat.
When it comes to sensing the environment no other group of animals show such a wide variety of different systems. Most of the time these systems coincide with the other standard senses, in a slightly diminished form, rather than replacing them. This gives them a much greater understanding of the world around them compared to humans. One of the key features of a snake is Brain
Nerve
the forked tongue. This tongue no Jacobson’s organ Point of contact
longer serves the purpose of taste but as an addition to the olfactory system. The Jacobson’s organ plays a key role in this. Located in the
Tongue retracted
Tongue extended
Fig 3. The Jacobson’s organ.
roof of the mouth it analyses the chemical composition of the air from a sample off the tip of the
tongue. The sample is being continuously updated through rapid tongue flicking. This is much more effective than drawing air through the nostrils. Pit vipers, pythons and some boa species have undergone parallel evolution by developing a further means of sensing the environment. The species in question either have a row of labial pits located on the upper lip or in the case of pit vipers a large loreal pit beside each eye. These pits are able to detect infrared thermal radiation. In a way they can ‘see’ heat. The pit viper’s loreal pits are the most advanced possessing an extra sensory membrane increasing its effectiveness. Inside these pits are rows of heat sensitive receptors connected to terminal nerve masses. This extraordinary ability gives the snakes a very clear ‘image’ of their environment. There are several documented cases of blind snakes that have these pits striking their prey perfectly on the most vulnerable body parts.
Recent studies have shown in some species they are also linked with other systems such as thermoregulation. The 27
pit vipers can sense their surrounding environment better that any other family. This means that they may sense a rise or fall in temperature even if it is
gradual. They can also locate the best place to shelter if caught out in intense heat preventing dehydration. Whatever the initial reason behind them, survival aid or prey detection, they are very effective. Like most aspects of snakes these are of great interest to science and robotics (Zhang, 2015). Snake movement varies with the species and environment. They don’t all move in an S-shape. Concertina locomotion is the slowest. Using the tail as an anchor the snake goes through a pattern of stretching out and bunching up. This method makes an arboreal lifestyle safer. Serpentine is the most common. Moving through lateral undulation the wave-like pattern propels the
Fig 4. Snake locomotion
snake forwards. Sidewinding is mainly seen in desert species but is for movement on any loose substrate. The pattern of small jumps and maximum contact with the ground gives as much friction as possible on sand dunes (Page 28). Finally rectilinear movement is slow but energy saving. If the snake has a large enough body size such as anacondas (Page 12) the muscles attached to either side of the ribs can be used to contract and relax the ventral belly scales, pulling the snake forward.
If you live in certain parts of the Americas, rattlesnakes are something you are 281
often warned about. Rattlesnakes are a group of venomous snakes in which 34 species exist from two genera. Their range extends from Southern Canada through to Argentina. Although being venomous and having a unique predator deterrent many immature rattlesnakes do fall prey to hawks, king snakes and weasels. They are often killed by humans both in passing and extermination campaigns especially as human populations extend into their habitats. In spite of this rattlesnakes rarely bite, only reacting in self-defence. The natural prey are generally birds and rodents which makes them a keystone species in many ecosystems. These snakes lie in wait for their prey, using their keen olfactory sense to locate nearby rodents. Once bitten, the hemotoxic venom takes affect relatively quickly in small prey. It is a mixture of up to 15 enzymes which both immobilize prey and aid digestion. The rattlesnake, a member of the pit viper family, has thermal detecting pits along the upper lip (Page 6). Like many other organs, these have multiple purposes, so not only can they act optically using thermal radiation but are used to judge when the prey item is finally incapacitated and will no longer try to defend itself.
Probably one of the world’s most recognisable snake features is located at the tip of a rattlesnake’s tail. This feature, which they are aptly named after, is a ridged keratinous mound. The ridged appearance is made from segments termed ‘rings’. This ‘rattle’ makes a specific sound which warns any potential predators that they t a distance.
are dangerous and signals their location if any larger animals come too close. Although its existence is common knowledge, the positioning, anatomy and development of this auditory structure is quite unusual in the animal kingdom. As mentioned previously, most predation of rattlesnakes occurs when they are hatchlings. When the snake hatches it only has a very small mound at the caudal tip known as a primary rattle. Although capable of producing sound it is limited and practically inaudible to larger animals. With each shed of their skin a new ring is formed. During
shedding the primary rattle, which is composed of fibrous connective tissue, secretes keratins that form the new rattle ring (Meik & Pires-daSilva, 2009). These rings encircle a small hollow chamber in each segment. This eventually reaches a point at which new rings are no longer formed but the existing rattle is built upon. It has been shown that the larger the species the more segments are developed. However, due to the position of the rattle, the rings can break off with general wear, therefore are not an accurate means of determining age. The rings are positioned around the tail, which has modified muscles that can contract and release rapidly. This means the snake can vibrate each segment. The segments are interlocked, but loosely, so when vibrating they collide creating the distinctive sound. The hollow chambers amplify the sound making it audible at a distance.
3
Like the other groups of reptiles the diet of snakes varies with the different families and even individual species. For most snakes rodents are the main source of food. Some have taken to eating aquatic animals like fish, birds and bats may form a large proportion of the diet of others and king snakes have taken to consuming other snakes. However one genera of snakes, Dasypeltis, has a different taste. These small non-venomous snakes rarely grow larger than 3ft (90cm) in length. They are found throughout Africa mainly inhabiting primarily forested areas as these are often home to numerous bird species.
Lacking in venom and being such a small species they do fall prey to many animals. It defends itself by inflating and hissing rapidly. Due to the elasticity of snake skin it can dramatically increase its perceived size. They normally show quite dark colours such as greens and browns, even black, but the patterns resemble some mildly venomous species, referred to as Batesian mimicry. Feeding mainly on bird eggs most of the species are arboreal, spending most of their time climbing through low tree lines. They travel large distances to locate food. When they have their own eggs they lay them in separate places. If this is to give the eggs their best chance of survival or due to their nomadic lifestyle is still unknown.
4
There are a few generalist snakes that may consume eggs, such as a few species in the rat snake genera Pantherophis which are known to raid nests for nestling birds but also eat unhatched eggs. However Dasypeltis sp. specialise solely in the consumption of eggs. These egg-eaters have developed two unique morphological adaptations that circumvent a couple of issues associated with this diet. Firstly the size of a bird’s egg is an issue, which is comparatively large for a snake of this size. Snakes’ skulls have evolved to take in large prey items and show an extraordinary elasticity in the skin. Due to the immobility of bird’s eggs they no longer require large teeth in order to keep hold of their prey. Therefore egg-eaters have a greatly reduced dental structure, to aid in swallowing of even larger eggs. The skull of egg-eaters are highly kinetic meaning they are surprisingly flexible. The second issue arises once the egg is firmly in the mouth. The shell is designed to be structurally tough and would normally take an extensive period to digest. This would also cause an egg-shaped protrusion from the snake, slowing escape time if required. The decreased dental structure is not the only anatomical change. They have a unique set of vertebral hypapophyses, which are spines that extend ventrally from a small portion of the vertebrae. These extend into the oesophagus and, as the egg is swallowed, pierce the shell until it concaves. The oesophagus has developed loose folds, to which these spines fit when not eating, preventing self-puncturing. Once the shell is cracked the yoke drains into the stomach and the snake regurgitates the crushed shell. This allows the snake to move onto the next egg in a short period of time (Gartner & Greene 2008).
5
There are 3 species of hognose snake found in North America, named by their locations: western, eastern and southern. These all belong to the genus Heterodon and are quite stout and small bodied snakes. The name hognose refers to the upturned rostral scale that makes the snake appear hog-like. This adaptation more than likely arose from their burrowing nature. They are also another snake that benefits from Batesian mimicry, with their pattern matching that of a rattlesnake (Page 6). This means many humans do mistake them for a more dangerous snake species often leading to death of the animal. Like quite a few other species the snakes can be highly variable in colouration and pattern. It is generally accepted that this species is venomous, however some do debate whether it has mildly toxic saliva rather than venom, similar to the Komodo dragon. Either way the impact of a bite shows only mild localised symptoms in mammals but affect ectotherms considerably more. Their diet consists of amphibians, mainly toads, referred to as bufophagy. Due to their small stature they cannot constrict as well as some other species therefore use the mild venom to slow down prey. This venom is delivered from rear maxillary fangs and they have to almost ‘chew’ in order to get it into the prey’s bloodstream. Much like their pattern they will often try to mimic more dangerous snakes when faced with predators. They will flatten their anterior ribs in order to replicate a cobra and will raise their head off the ground and false strike. If this fails they turn to more erratic behaviours.
Hognose snakes have a repertoire of behaviours they can exhibit if a predator is not fooled by their mimicry. The snakes can perform one or several behaviours in no particular order. It can be assumed that this complex range is to prevent predators learning which behaviours are false. They can show vigorous writhing, body snapping, head cocking, semi-rigid locomotion, head hiding, false aggression, tail coiling, cloacal sac discharge, defecation and death feigning to name a few. They are more likely to show the other erratic behaviours before they death feign as this is generally a last resort, triggered when the predator is in very close proximity. Death feigning happens almost instantaneously due to the behaviour being triggered by hormones (Durso 2011). The mix of hormones secreted by the adrenal gland in response to stress cause parasympathetic effects in hognose snakes, this results in death feigning. This can be linked to the diet of the eastern hognose which is mainly toads. The snakes have enlarged adrenal glands in order to cope with the toxicity of the toads. This resistance to the toxins also alters other behaviours such as climbing abilities and tongue flicking. During death feigning the body is twisted and contorted, they invert and display the ventral side then present a wide gape with tongue extended. Many add extra flare to their well-rehearsed display by defecating and regurgitating. Duration is generally 10 to 45 minutes before the snake corrects its posture and examines the situation. The snake may repeat this several times before assuming they are clear. If the snake is flipped over it immediately reorientates so its stays ventral size up.
8
Although the green anaconda does not necessarily display any unique behaviours or features this species gets an honourable mention in this guide due to its size. Recognised as the world’s largest snake the green anaconda is a member of the boa family, Boidae. The species is fairly widespread throughout South America. They inhabit marshes and swamp areas but are also found in rainforests like the Amazon basin. Generally, they are swift stealthy swimmers and are often found close to the water’s edge, spending more time in the water than any other boa species. They prefer to move underwater as due to their sheer size and weight they are cumbersome on land. Their colouration and pattern, which is olive green with dark spots and yellow underside, like most animals, helps with camouflage. Like other species, green anacondas do show some unusual adaptations when compared to the rest of the animal kingdom. This species is ovoviviparous, therefore appears to give birth to live young. This is not as it seems, the female does produce eggs but retains them inside the body. Like most snakes the offspring get no parental care. They have also known to be facultative parthenogenetic, so in areas where males are sparse the female can produce eggs without the need for fertilisation. The young have one of the animal kingdom’s most dramatic size changes, increasing up to 500x from hatchling to adult. They also show the biggest difference between the sexes, referred to as sexual dimorphism, of any terrestrial vertebrate with females being 5x heavier.
To be classed as the largest of any animal you need to factor in length, girth and mass. It is debated which is the longest snake species as variation amongst populations is high. With actual measurements of extant individuals the reticulated python, Python reticulatus, is currently the longest with a wild individual at 32ft (9.75m). Estimates place possible wild anaconda individuals at a similar length if living under optimal conditions although none over 30ft has been recorded. The anaconda, however, has a larger girth and can double the mass of a same sized python, therefore earning it the title of largest extant species. The largest in known history was Titanoboa
cerrejonensis reaching up to 49ft (15m). Titanoboa was from the same family as the green anaconda but lived some 58 million years ago. As mentioned, due to their size they spend most of their time underwater were their weight can be supported. This means they have adapted to eating more aquatic animals as well as those that come to the water’s edge to drink. Their nostrils and eyes have shifted to nearer the dorsal part of the head. This is so that less of the head is exposed when scanning the surface for prey and they can breathe almost completely submerged (O’Shea 2007). They kill their prey by constriction rather than venom, using their sheer body size to subdue their prey. Smaller and young individuals feed on fish, small birds and other reptiles. The larger snakes have been documented taking deer, tapirs, capybaras and even caimans. Large meals such as these can provide enough nourishment so the individual can last several months before needing to consume more food.
Forested habitats can hide some of the largest bodied snakes, using the thick ground brush they are almost undetectable. This is not unlike some of the smallest snakes which may inhabit similar regions. Long-nosed whip snakes, or green vine snakes as they are also named, are tree dwelling snakes that blend in remarkably with thick vines and leaves. Colouration is generally light to dark green throughout with no particular pattern. Due to the small body size it would normally fall prey to many animals however like most venomous species it has a way of advertising the danger of this meal. Between the green scales they hide an array of brighter colours so when threatened they expand their sides revealing the hidden warning signs. Camouflage is not the only adaptation to life in the canopy. Their body shape has also changed. A few tree dwelling snake species have developed a transversally flattened shape. Many similar snakes have the common name ribbon snake due to this appearance. This body shape would be useless on the ground but is quite the opposite when climbing through branches. The smooth elongated scales are constantly in contact with various surfaces providing a vastly superior grip, when compared to a rounded body plan. This shape helps when moving to cause as little motion in the surrounding foliage as possible. It also synchronises with the wind so even when lying in wait for prey it will match the small jerky movements of nearby leaves.
11
When it comes to eyes snakes have the same types of cones and rods as most birds and mammals. Different optical layouts mean although they do see colour it is not as broad of a range. They have also developed a yellow filter which protects the lens from high levels of ultraviolet light. However with most animal groups individual species vary in how well developed their vision is. Some species like the brahminy blind snake’s eyes are simple photoreceptive cells used for just detecting light. A large proportion of snakes have simple eyesight relying on other senses like thermal radiation detection those snakes living in certain and a keen olfactory sense. For environments, like thick forested habitats, the ambient thermal radiation and almost unlimited amount of escape routes for prey make hunting difficult. Many arboreal species have excellent vision allowing them to navigate their ever-changing and complex environment as well as to find food. The whip snake has one of the most well developed visual systems for this type of habitat. The eyes have rotated in their positioning to allow the relaxed line of sight to be straight forward. To prevent blocking this line the face has broad grooves in front of each eye. Just like primates, their field of vision overlaps slightly. The result of this is binocular vision and greatly improved depth perception, which is generally lacking in most snakes. They also have ‘keyhole’ shaped pupils which differs from the almost standard round or elliptical pupils of most snakes. Although the exact mechanism behind how this helps is still being researched it is hypothesised this further advances the depth perception.
12
One of a snake’s most recognisable features is the movement used in locomotion. The S-shaped rhythm helps snakes pull themselves forward on land without the need for limbs but has also helped them in water. Although most snake species are impressive swimmers one particular family, Hydrophiinae, have thrived in a completely aquatic lifestyle. The sea snake family consists of 62 recognised species from 17 genera. These are mainly found in warm coastal waters from the Indian to the Pacific Ocean but a few species can be found in Oceania. One species, the yellow bellied sea snake (Pelamis platura), has a geographical range greater than that of all other snakes and even rivals that of sea turtles. Almost all sea snake species are highly venomous. For many years it was believed that the Belchers sea snake was the most venomous species in the world, with an LD50 toxicity of 0.024mg/kg, however this is only when injected intramuscularly, when it comes to biting animals, other than fish, this cannot be achieved. It is now in popular agreement that the Inland Taipan (Page 18) should take that title. Most human fatalities from this family are fishermen accidentally picking them up in their catch however only 10% of those bitten are envenomed. Venom is quite expensive to produce, in energy terms. Therefore when threatened biting a larger animal, such as humans, is warning enough without wasting venom. The venom causes rapid breakdown of skeletal muscle and paralysis, which like most snakes, is to catch fast moving prey in this case fish.
The majority of sea snakes never actually leave the water apart from the sea kraits, 5 species in the genus Laticauda. Due to this lack of necessity to return to land the ventral scales, which acts as the ‘foot’ of terrestrial snakes, have shrunk in size to those of the adjoining scales. Therefore if they ever attempted to move on land they would essentially fall over. This shrinking could be because of the more laterally compressed body shape, making them more streamlined. The tail is flattened further to a paddle-like shape. Located on this tail are rudimentary photoreceptors, this is thought to be useful when ambushing prey from coral crevices. They will know if their tail is sticking out. Over their evolution certain organs have shifted to adapt themselves to this lifestyle. The nostrils are located more dorsally than terrestrial species. This is so they can breathe from the surface with minimal protrusion. The nostrils also contain a spongy tissue which acts as a valve taking in only air. Like with most air breathing vertebrates the maximum
dive time they can have the better. The sea snakes have developed a couple of means to extend this. The trachea has been modified to absorb oxygen alongside the lung. Like with the majority of snakes they have a primary lung however sea snakes have extended it to almost the full length of the body. The lung serves a dual purpose, providing buoyancy as well as oxygen. During periods of rest a single breath can sustain an individual for up to 7 hours. Through breathing and consuming prey the snake ingests salt water. This usually leads to more salts than the body needs therefore sub-lingual glands under the tongue expel excess salt every time the tongue extends.
13
Australia is known for its dangerous wildlife, there is very few places you are completely safe. In the ocean you have to look out for box jellyfish, your shoes at home may hide funnel web spiders and even a trip through the woods you could get caught by the giant stinging tree. There is no doubt that the ‘world’s most
dangerous’ snake would be found here. The Eastern brown snake is responsible for the most deaths per year than any other snake yet the argument for most dangerous is not that simple. The inland taipan is found mainly in central eastern Australia, like the Eastern brown, but in a much smaller range. This elusive snake was ‘missing’ for over 90 years until its rediscovery, by a single head, in 1972. Since it was first documented it has had at least 8 different scientific Latin names. Due to a lack of specimens and seasonal colour changes it was hard to study. The colour changes, from brown to olive-yellow, are to aid thermoregulation in winter months. The diet consists mainly of desert rodents such as the plains rat and long-haired rat. These particular rodents are known to be aggressive and can inflict severe wounds when defending themselves therefore the taipan’s venom has to act fast. When grabbing prey they also bite several times in quick succession to inflict multiple punctures.
Depending on where the information is sourced you may get several answers to .14
the popular question ‘Which is the most dangerous snake on earth?’ The term danger is ambiguous therefore narrowing the question down to the most venomous snake should surely be easier to answer. Unfortunately this is not the case either. Venom is a substance created by a particular species generally aimed to be used on another. Venom has a varied effect depending on the chosen species, the depth and method of injection, types of toxins present and the potency or LD50. All these factors combine to create the desired outcome for the snake. So narrowing to just a single factor, for example potency, we can form an argument for a single snake species. If a single species were to be selected to represent the most lethal snake bite the inland taipan would definitely be high ranking. The taipan delivers a much smaller dose than many of the other ‘dangerous’ snakes, approximately 10 times less than the king cobra, ~ 110mg, and is injected subcutaneously. Although a much smaller dose, the potency is 0.025mg/kg on mice and 0.01 mg/kg on bovine serum albumin (Page 4). Basically this means a single bite at maximum yield could show a potential mortality rate of 250 adult humans or over a million mice. This has of course not been tested on humans but laboratory mice have not been as fortunate. The venom itself is a potent presynaptic neurotoxin (Hodgson et al 2007), causing paralysis or severe muscle weakness. It also contains smaller amounts of myotoxin which triggers myolysis, essentially causing the muscles to dissolve. This would aid in the incapacitation of the prey but its probable main purpose would be to aid digestion. These effects would almost be instantaneous in rodents and death would follow quickly, in humans this could lead to death in approximately 45 minutes.
Flight, or to be more accurate gliding, has appeared in many types of animal that you wouldn’t normally expect it to be associated with. There is the gliding squirrel, the flying fish and even some lizards have developed rudimentary flight (See Draco lizards in ‘A lizards isn’t just a lizard’). Therefore it shouldn’t be surprising that a few snake species have developed a means to keep up with this evolutionary air race. To be precise five species, all from the genus Chrysopelea, have taken to the air. The species can be found throughout Southeast Asia. Arboreal in nature, the snakes occupy the canopies of tropical rainforests. They rarely descend to the forest floor since it is more energy efficient to glide across to new locations and they can avoid ground dwelling predators. This unusual ability means they have to stay small and light in stature, therefore not possessing the strength larger snakes have. Venom is used to subdue their prey which mainly consists of small lizards, amphibians, bats and birds. To reach their prey they have to be agile in the tree tops and have developed keeled ventral scales. This creates more friction in comparison to other species making them seem sluggish but gives them superior grip when scaling the bark of a tree.
Many animals seem to use the modification of limbs to develop a type of symmetrically paired ‘wings’ which can generate lift. Snakes seem to have the most unfortunate body plan to attempt such a feat as they generally lack any type of appendage that would assist them. The flying snakes have developed an 16
ingenious way of getting round this problem with very little morphological changes. However first they must take off. Most forested flyers leap from a branch before triggering the glide response. This again provides more difficulty for flying snakes. To gain enough momentum for a successful glide they hang, using their tail, from the branch in a J-shape. Once leaning at a suitable inclination for the desired location and location they spring forward from the branch.
Once in the air the body alters its shape. The snake ‘sucks’ in its abdomen and flattens out ribs (Fig. 4). This new position creates a concaved shape, with the cross section similar to that of a Fig 5. The body plan in flexed and normal
positions.
frisbee. This flattening can almost double the width of the snake’s body. This change may seem drastic but snakes in general have much more
flexibility regarding their ribs and organs with highly elastic skin due to consuming large prey.
The snake then moves as it would on the ground with the standard lateral undulation seen in most snakes. This movement when done parallel to the ground stabilises its direction whilst in mid-air. This movement also keeps constant air pressure underneath its body to keep the snake in the air. Although direction and altitude
change is limited the flying snake, with the concave shape and constant motion, can glide further than almost all other gliding animals even without wings (Socha 2002).
The burrowing asps are a small group of snakes consisting of 21 species. They have many other common names including stiletto snakes, burrowing vipers and mole vipers. Most of these common names are misleading as they are closer to the Colubrid and Elapid families than the Viperids. Almost all the species are found in sub-Saharan Africa including the small-scaled burrowing asp. As part of the name suggests these small snakes are fossorial in nature, spending most of their lives underground. The body plan is developed for this lifestyle. The body is uniformly cylindrical in shape with a short, blunt tail. The head is relatively small with a longer upper jaw creating an upturned shovel-like appearance, perfect for underground travel. Their fossorial nature limits their prey to several species of burrowing rodent. Like many snakes discussed venom plays a key role in their effectiveness as a predator. They are the only species of snake that will kill multiple prey items before consuming them. This would be in pre-empted self-defence as when
swallowing prey all snakes are extremely vulnerable. Rodents in enclosed spaces can cause serious damage regardless of the size of the snake.
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Fang and venom systems are highly varied between snake families but a few groups have developed similar systems. Front-fanged delivery systems are found in three clades: Elapids, Viperids and Atractasipids (burrowing asps). This system is well studied in two of the three but the asps are often overlooked. gooves Asp’s fangs inject venom through hollow grooves much like the other families. The relative fang size however is much larger therefore when attacking their standard prey they can inject venom much deeper into the body. However it’s how they bite their prey which is the key difference. Burrowing asps do not need to open their mouths in order to envenom prey. The retractable fangs can stick out even when the mouth is completely shut. This has been theorised to be linked to their fossorial lifestyle. When hunting prey underground the ‘normal’ strike action would cause the snake to ingest large
amounts of soil and other indigestible substrates. So when a burrowing asp gets alongside a prey item it slashes backward unilaterally with generally just one fang. With the high potency of the venom and the depth of the injection just a single side-stab can
subdue most prey items. The venom has to act fast. Due to the fangs location the asps have lost several dental structures that would normally be used to hold onto the prey. This evolutionary focus on the fangs, which has meant compromise in other areas, has lead researchers to believe this species is at an evolutionary endpoint, where any further development would hinder rather than benefit the survival of this species (Deufel & Cundall 2003).
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The tentacled snake, sometimes known as the fishing snake, can be found throughout Cambodia, Thailand and Vietnam inhabiting lakes and slow moving streams. They are known for their rather innovative take on the standard ambush technique used by most snakes. It is the only species of the genus
Erpeton and the only snake to have specialised tentacles to help catch prey. This uniqueness is why they are such successful hunters and why their prey have yet to adapt. The prey is exclusively fish so this has lead the tentacled snake to be completely aquatic. This lifestyle is noteworthy in itself as it has meant many physiological changes to cope, much like the sea snakes (Page 16). Unlike the sea snakes
who prefer vast, open, clear waters the tentacled snake much prefers enclosed lakes or ponds. Generally the murkier the better. The scales of a tentacled snake are keeled. This is common place with many species as it provides additional friction for those that need extra grip in their environment. When ambushing prey underwater it is difficult to remain in one
location even if there is very little water movement. Using a relatively prehensile tail the snake is able to hold onto aquatic plants or debris to anchor themselves. This lets the rest of the body move with the water to mimic its surroundings.
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Mimicking the surroundings is a very common practice for many ambush predators but the tentacled snake goes one step further. Each time the snake repeats the process in 3 stages. To set up the ambush the snake uses a pre-set position each time. When the tail is anchored the body is bent in a J-shape which is stage 1, this may be inverted depending on the local environment. After successfully
Twitch
avoiding detection by a fish that is in close proximity the snake moves to stage 2. It twitches the middle of the body as if by accident, which would seem like their cover is blown. However this is no accident. Fish have an inert danger reflex referred to as C-start. This acts in about 0.05 seconds. The body bends into a C shape powering the fish into a new direction. This new direction is stage 3, the snake has to react with minimal strike response as the fish seems to swim straight into its mouth.
Fig 3. 3 stages in 0.23 seconds.
This behaviour may seem effortless but timing for this specialised strike is key. Due to the location of this interaction the visibility can be limited, although this improves camouflage it affects vision. This is where the sensitive tentacles come into play. The tentacles are mechanoreceptors but are not directly linked to the brain. The signals are sent to the optic tectum first combining both mechanosensory and visual information together. The tentacles can pick up the faintest water movements much like the fish can. The visual information regarding the angle of approach and then the sensory information of the fish’s final movements is combined. The snake can then tilt its head ready to catch the prey before the fish notices any danger. This whole interaction can happen in less than 0.2s (Catania 2011).
The most common mistake, and that which irritates most herpetologists on a regular basis, is when people discuss snakes with the phrase ‘poisonous snake’. A poisonous animal would only do harm if ingested whilst a venomous animal would do harm if the venom is injected into the blood stream. In the case of snakes the correct term would be venomous as they inject venom through their fangs. However there is one species in which this is technically false, being both venomous and poisonous simultaneously. The tiger keelback, or yamakagashi, is found is various locations throughout Asia, with the highest populations found in Japan from Honshu to Kyushu. They inhabit open grassy plains and mountainous regions generally near a water source such as a river or lake. This close proximity to water is not purely for hydration but is where they will find most their prey. This particular snake is a specialist in bufophagy, they consume frogs and toads. There are a few populations which are located on off-shore islands, most of which are toad-free. These populations have adapted to eat rodents and lizards. Although never defenceless, due to their venom, the snakes on these islands have been found to be missing their poison. This alters their behaviour as they are more likely to flee than stand their ground.
For several invertebrate groups sequestering toxins from their food source is common place. In vertebrates it is very rare. The most understood 23
would be the poison dart frogs from tropical Central America. These small, seemingly defenceless frogs use the toxins from certain ants to make themselves harmful to ingest. However a vertebrate taking toxins from a vertebrate prey source was unheard of until recently. Even now the process used by the tiger Keelback is unique amongst all reptile species. Although its poisonous nature has been known for around eighty years only recently has the actual method of gaining these toxins been proven (Hutchinson et al, 2007). The toxins secreted by these snakes are bufadienolides, which have a strong effect on cardiac rhythm. As the name suggests the only other place these toxins can be found is in the glands of some frog and toad species. This strange choice of poison lead scientists to research further. The nuchal glands, which are located dorsally just behind the skull, are not found in any other snake species. On closer inspection the cells found in the glands do not contain the necessary organelles that would synthesise these toxins, such as the secretory epithelia. Instead they have a complex network of capillaries. So these glands are for storing not creating toxins. Like most poisonous vertebrates the toxins are secreted through the skin. The behaviour of the snake when threatened is unusual in itself. They arch their neck and rub against the threat, much like an affectionate pet to its owner. This coats the possible predator in the fast acting toxin. The behaviour is instinctual with hatchlings possessing the toxin if the mother had high levels in her system when laying her eggs.
The desert horned viper is a part of the Cerastes genus which consists of three species, two of which have supraorbital horns. Many of the other features of this species are shared among other desert viper genera. The Sahara is a harsh environment for any animals that may live there. Species have to adapt many behaviours and features to survive, such as locomotion. The horned viper quickly climbs the soft sand dunes through sidewinding, moving in arches with rigid contact to the ground. This happens when the substrate cannot provide the lateral resistance required to slither. It is very effective and has been replicated many times in robotics. The scales of the horned viper are keeled and in places have a serrated edge. This gives protection from both the elements and predators but is not the main purpose. Like most viper species this is an ambush predator, so when a suitable location is found a quick stationary movement causes the body to submerge in the sand. The nostrils and eyes are left exposed. When the prey approaches close enough a single venomous bite quickly incapacitates it. The species also has a slender tail, with a colour difference, that can be used as a caudal lure. It has been found that rather than olfactory or visual stimuli they use vibrations through the sand to trigger a strike response (Young & Morain 2002).
Although many species have unusual facial 24
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features, for various purposes, the horned viper may have the most impressive. The ‘horns’ are a single elongated and grooved scale. The horn itself is retractable and can lay flat against the head. This is their most recognisable feature but cannot be the sole
factor as many individuals are completely hornless. No one has explained why identifying entire populations are hornless and even why siblings may vary but it has no effect on survival rates (Wagner & Wilms 2010). Snakes are highly efficient predators and survivalists, with a vast array of adaptions to aid in ways too numerous to list. Each adaptation has been refined to be used in a specific way for a specific purpose. The horns of a horned viper have confused herpetologists since 1758. Many theories exist but none have been proven. Many facial features of other desert animals are to protect the eyes from sand irritation. However snakes have a transparent scale, called a spectacle, which would protect their eyes from any materials when lying in wait for prey underneath the sand. The movability of the horns could suggest communication but pheromones are still being used for that purpose. Luring prey is of high importance when in a very open environment however caudal lures have proven to be more effective. A final theory suggests since part of the face is exposed when buried in the sand it may break up their outline. No difference between horned and hornless individuals in survival or reproduction rates has ever been recorded. So what is their purpose? Although this is still an active debate we may never come to a solid conclusion. This is just one of many mysteries this unique and unusual group of animals still elude us with today.
This guide was written to be understood by a wide audience not just professional herpetologists however there are some terms which are not common knowledge. I have compiled a few words or terms which may need further explanation. Batesian mimicry – Form of mimicry which a relatively harmless species evolved to look similar to that of a harmful species. Bovine serum albumin – A protein which is derived from cows. This is often used in lab experiments as it shares many properties with human proteins. Ectotherm– The correct term for ‘cold-blooded’. An organism which cannot generate its required body temperature internally so relies on external sources. Fossorial – An organism which has adapted to digging and life underground. Hemotoxins – Toxins which aim to destroy red blood cells, prevents clotting and can cause deterioration of most tissues. Hypapophyses – Small bone outcroppings or spines which are connected to the underside of the vertebra.
Intramuscularly – Injection, in the case venom, straight into the muscle tissue. Keratinous – Composed of keratin. Keratin being a strong fibrous protein, also found in hair, nails, horns and hooves. Keeled Scales –These scales have a ridge running down the centre of each one. Increasing friction and subsequently grip on surfaces. Keystone Species – A species which affects the whole ecosystem in which it belongs. Its abundance directly effects the productivity of the other species found in the ecosystem. Labial Pits – A facial pit which is normally located on the upper lip. A simple heat sensing system. Lateral Undulation – A wave-like motion, in snakes, forms the standard ‘S’ shape movement.
Loreal Pits – A deep indentation generally located behind the nostril but before the eye on each side of the head. Located in which a very sensitive heat sensing membrane. Maxillary – Refers to the bones which connect to form the upper jaw. Mechanoreceptors – A sensory receptor in the skin which can detect changes in pressure which would indicate any local movement. Neurotoxins – A toxin which damages nerve tissue and/or alters function adversely affecting the nervous system. Nomadic – Animals that have no fixed base location or feeding grounds. Generally linked sparse food sources or to prey movements. Ovoviviparous – Egg laying animals that retain the fertilised eggs inside the mother’s body until hatching. Giving the appearance of live birth. Parasympathetic – Refers to the parasympathetic nervous system. Controls and regulated heart rate, intestinal activity and gland functions. Parthenogenetic – A natural form of asexual reproduction in which growth and development of embryos occur without the need for fertilization. Photoreceptors – Specialised receptors which convert light energy into electric signals which can be understood by the brain. Light detectors. Phylogenetic – Refers to the phylogenetic tree - a scientific diagram which shows the evolutionary relationships between species. Sequestering – To remove a substance from another organism and maintain it within the new organism.
Subcutaneously – An injection, in this case by fangs, into the layer of skin just under the epidermis (outer surface layer). Thermoregulation – A process in which an organism maintains its internal body temperature within a certain range.
All the information presented in this guide was a combination of personal knowledge, current scientific papers and reputable websites. To my knowledge the information is accurate and up to date with recent findings. Some information of certain features has been briefly mentioned whilst others were covered in depth. For the complete picture I have listed recommended papers and websites.
Snake Evolution: Caldwell MW, Nydam RL, Palci A, & Apesteguía S (2015) The oldest known snakes from the Middle Jurassic-Lower Cretaceous provide insights on snake evolution. Nature communications, 6. Diversification: Figure 1 – http://biology-forums.com Figure 2 – http://itg.author-e.eu/Generated/pubx/173/snakes/description.htm Figure 3 - http:// cherrycreekschools.org/studentprojects/PublishingImages.htm Figure 3 - http://animals.howstuffworks.com/snakes/snake3.htm Zhang Z, Zhang Y, Zhang Q, Cheng T, & Wu X (2015) Bionic research of pit vipers on infrared imaging. Optics Express, 23(15):19299-19317. Web link - http://snakesarelong.blogspot.co.uk/2012/09/snakes-that-can-see-withouteyes.html Rattlesnake: Meik JM, & Pires-daSilva A (2009) Evolutionary morphology of the rattlesnake style. BMC
evolutionary biology, 9(1):35. Egg Eating Snake: Gartner GEA, & Greene HW (2008) Adaptation in the African egg‐eating snake: a comparative approach to a classic study in evolutionary functional morphology. Journal
of Zoology, 275(4):368-374. Hognose Snake: Durso AM "Interactions of diet and behaviour in a death-feigning snake (Heterodon nasicus)" (2011) Masters Theses. Paper 47. Green Anaconda: O'Shea, M. (2007) Boas and Pythons of the World. New Holland Publishers, London. Long Nose Whip Snake: Van Doorn K, & Sivak JG (2013) Blood flow dynamics in the snake spectacle. The Journal of experimental biology, 216(22):4190-4195.
Belchers Sea Snake: Karleskint G, Turner R, & Small J (2012) Introduction to marine biology. Cengage Learning. 3: 298 Inland Taipan: Hodgson WC, Dal Belo CA, & Rowan EG (2007) The neuromuscular activity of paradoxin: a presynaptic neurotoxin from the venom of the inland taipan (Oxyuranus microlepidotus). Neuropharmacology, 52(5):1229-1236. Flying Snake: Figure 5 - http://www.barking-moonbat.com/index.php/weblog/2014/04/P14/ Socha JJ (2002) Kinematics: Gliding flight in the paradise tree snake. Nature. Burrowing Asp: Deufel A, & Cundall D (2003) Feeding in Atractaspis (Serpentes: Atractaspididae): a study in conflicting functional constraints. Zoology, 106(1):43-61. Web Link - http://people.whitman.edu/~jacksok/Atractaspis.pdf Tentacled Snake: Catania KC (2011) The brain and behavior of the tentacled snake. Annals of the New York Academy of Sciences, 1225(1):83-89. Tiger Kneelback: Hutchinson DA, Mori A, Savitzky AH, Burghardt GM, Wu X, Meinwald J, & Schroeder FC (2007) Dietary sequestration of defensive steroids in nuchal glands of the Asian snake Rhabdophis tigrinus. Proceedings of the National Academy of Sciences, 104(7):22652270. Web link 1 - http://link.springer.com/article/10.1007%2Fs00049-011-0086-2#page-2 Web Link 2 - http://www.japantimes.co.jp/life/2003/06/19/environment/tigerkeelback/#.Vfmob5dtf6d Horned Viper: Wagner P, & Wilms TM (2010) A crowned devil: new species of Cerastes Laurenti, 1768 (Ophidia, Viperidae) from Tunisia, with two nomenclatural comments. Bonn Zool. Bull, 57(2):297-306. Young BA, & Morain M (2002) The use of ground-borne vibrations for prey localization in the Saharan sand vipers (Cerastes). Journal of Experimental Biology, 205(5):661-665.
This guide was created as a follow on from ‘A lizard isn’t just a lizard’ mini guide. I hope the material I have discussed on these unusual creatures has provoked further research. 30
All statistics and figures including number of species, genera and families were checked with the Reptile-Database.org and affiliated pages. These were correct as of 30th October 2015. Photo Credits: Diversification – Quitoon -Tumblr & snakesarelong.blogspot.com Rattle Snake – Ratenslagen.nl & Rob Oliver Egg-Eating Snake – Warren Photographic Hognose – James van Dyke
Green Anaconda – M. Watson Log Nose Whip Snake – Jonathan Hakin & trimek.pl Sea Snake – Jurgen Freund, Richard Seaman & J Benthomien Inland Taipan – Anthony von Plettenburg Flying Snake – Jake Socha & Oscar Ocelotl Burrowing Asp – snakesarelong.blogspot.com Tentacled Snake – Nick Michalski & Dr Ken Catonia Tiger Kneelback – Cowyear - Flickr & Kim Hyun-Tae Horned Viper – Ajmal Hason, Dave Welling & Michael Fogden Acknowledgements – Authors image: Oscar the Burmese Python
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