What Is a Reptile? 3 Reptile Evolution Thorny Devil
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Marine Iguana
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Chameleons
Draco Lizards
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10 Whiptail Lizards Frilled Dragon Komodo dragon
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Geckos
16 Basilisks
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Alligator Lizards
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Italian Wall Lizards
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Horny Toads References and Acknowledgements
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Reptiles are fascinating. They have conquered almost every type of environment with an array of amazing adaptations that allow them to catch food, avoid predators, cross natural barriers and
even communicate with one another. Some of these adaptations are not seen anywhere else in the natural world. This mini guide focuses on the lizards, which make up nearly two thirds of all known reptiles and are extremely diverse not just in location but in size, shape, diet and colouration. This guide has just a select few to
show how unique each lizard species is. There are twelve lizard sections that help demonstrate the different abilities that can be seen in the reptile kingdom. These twelve are split into two categories, the first of which is ‘Unique Species’, focusing on just one species of lizard that has a unique talent. ‘Unusual Adaptations’ is the other, looking at a select group of lizards, generally a genus but not always, which collectively uses a remarkable skill or feature. The information in this guide has been taken from a variety of sources, most of which are scientific research papers or literature summaries. This, to a certain degree, insures the information on each of the species is current and up-todate on the latest scientific findings. In each section references 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 in dragon lizards. All the references used in this guide are in full on the back pages if you wish to read the full study.
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Reptiles are a paraphyletic group of vertebrates, meaning the group contains the same ancestor but not all of its descendants. In
traditional classification Tetrapoda (four limbed vertebrate) is split into Aves (Birds), Mammalia (Mammals), Reptilia (Reptiles), Amphibia (Amphibians) and Synapsida (Extinct group).This has been the layout for many years as
morphologically birds and reptiles are very different. If the taxa are split into monophyletic groups they would be like the diagram above, Sauropsida would include all modern ‘reptiles’ and birds with their fossil ancestors. This is arguably the correct way to classify them as crocodiles are more closely related to birds than lizards, turtles diverged so early on they almost could be a separate class. The term
‘reptile’ commonly still just covers lizards, snakes, crocodiles, turtles and sphenodonts (Tuatara). The reptiles and amphibians are also commonly grouped together as they are both known as ‘cold-blooded’. This term, although widely used, is not accurate. Reptiles are ectothermic, meaning they cannot generate body heat so they absorb sunlight instead, since they still require ‘warm’ blood to be active. Some lizards, like the marine iguana (Page 10), at peak temperatures have warmer blood than those of most mammals.
Like all vertebrates reptiles started life on the evolutionary tree as simple fishlike creatures. The first steps onto dry land were taken by the amphibians, which later became reptiles, which would split into birds and mammals. This is an oversimplification but it shows the order in which things came. The first
amphibians are thought to have arisen in the Devonian period around 370 million years ago (mya) from lobe-finned fish similar to that of today’s lungfish. Around 320 mya in the Carboniferous period the first basic reptiles are thought to have evolved. The switch from amphibian to reptile is characterised by two key elements, firstly the development of watertight skin. Reptile scales form a tight skin which holds moisture in so they do not have to stay by the water’s edge like amphibians. Secondly the amniote egg, which is hard-shelled compared with soft
water-bound amphibian eggs, meant reptiles could expand into drier climates. These two key elements meant they eventually became the dominant forms on earth from 280 to 65mya, a period known as the ‘Age of Reptiles’. From the amphibian
ancestor the reptiles branched out into a wide variety of forms seen here. Some went back to the water like the plesiosaurs, ichthyosaurs and later the crocodiles and turtles. The
archaeopteryx lineage at the top became the birds and therapsids became the mammal-like reptiles.
Lizards and snakes are a monophyletic clade known as Squamata. The earliest known possible ancestor to lizards is Paliguana. Fossils found in South Africa and 27
Australia date back to 250mya, suggesting the lizards evolved on Gondwana just after the breakup of the supercontinent Pangaea. To understand the morphology
and physiology of the early lizards scientists look to the tuatara. The tuatara is the only member of the sister group of the lizards, Sphenodontida, and looking at the fossil record they have remained almost unaltered for nearly 250 million years. They look very similar in appearance to modern lizards however they have a few physiological differences. They have a very large fleshy tongue for feeding on small prey
and do not have a moveable quadrate bone in the jaw like other lizards. The male copulatory organ is not like the paired hemipenes of lizards but two saclike structures which could have been how lizards first came to be. Early in the evolutionary history of lizards two distinct groups arose separating the Squamata in Iguania and Scleroglossa. Iguania contains the iguanas, chameleons and agamids, these groups kept the primitive fleshy tongue used in feeding. The Scleroglossa switched to using their jaws when feeding rather than the tongue. This freed up the tongue for other uses like chemoreception, which led to the development of the Jacobson’s organ found in a variety of species and the snakes. An early split from Scleroglossa produced the geckos (Gekkota) and Autarchoglossa, which later led to many different groups including Varanoidea. The latter group split further producing monitor lizards and later the snakes around 110mya (Pianka & Vitt 2003).
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One of Australia’s most unusual and fascinating animals, the thorny devil (Moloch horridus) lives in the arid outback where very little food or water is available and predation can be quite high. This desert dweller has come up with some certainly
unique adaptations to survive in such a hostile environment. Within the vast barren landscape there is limited food for an insectivorous reptile to consume. The thorny devil has a specialised diet consisting of just a select few species of black ant. Their stomachs are designed to tolerate the increased acid levels from this type of prey, and it has been known for an adult to consume up to 3000 ants in one sitting.
Various traits such as minimal lunge and fast tongue protrusion make the thorny devil, in a single feeding event, the fastest lizard studied to date (Meyers and Herrel 2004). Although covered in rather sharp spines it is still preyed upon, mainly by birds such as bustards. As they are slow moving and rigid, with a maximum burst speed of 1.2 m sec-1 (Clemente et al 2004), they cannot flee like most other similar sized lizards. Instead they take up a defensive stance by putting their head down and pushing the
tail into the ground to avoid being flipped over. This makes a fleshy mound on the neck stick into the air, this ‘false head’ distracts the predator from its real head.
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The conical spines are great for defence but they serve another purpose which is to help retain as much water as possible, much like the spines of cacti. They have to do this due to the lack of water available in this arid environment. The little water that may be found, either as shallow puddles or as dew on plants
and the lizard itself, is not wasted. A unique adaptation for utilising every drop of this precious fluid is a complex system of grooves all over the body, in which any drop of water is moved round the body to the lizard’s mouth. There are only a handful of lizards which possess the ability to move water around the outside of their body, each with a unique system like the thorny devil. These systems where looked into by Commans et al (2011) who found that these grooves between the spines had honeycomb-like micro structures at certain angles to get a specific surface tension. This made the surface superhydrophilic, meaning the water does not ‘wet’ the surface but is repelled like a lotus leaf. The collected water is effectively transported through capillary forces acting on this network of grooves. The combination of surface tension (similar to that which makes it possible for a Basilisk to run on water – page 21) and capillary action makes this a truly remarkable adaptation. To read further into this check out the companion website or find references of the scientific studies and articles at the back.
Chameleons are a highly specialised group of lizards with almost 200 described 3
species inhabiting rain forests to deserts in Africa, Asia and southern Europe, with the highest concentration of individuals and species being found in Madagascar. The oldest known chameleon, found in China, is approximately 60
million years old but chameleons are thought to have arisen 100mya. Brooksia
micra is a tiny chameleon which is currently the smallest known reptile in the world with females reaching a maximum of 2.8cm snout to tail. Chameleon species have a remarkable ability to change colour using specialised chromatophores in their skin cells. This is not to improve camouflage as one may think
but to thermoregulate or to signal to other individuals to defend territories and attract mates. Other unique features include specialised pincer-like feet (zygodactyl), independent stereoscopic eyes, a swaying gait, extended facial horns and noses, cranial crests, prehensile tails and modified rapid ballistic projecting tongues. All these highly adaptive features help these slow-moving, short-lived reptiles to be highly accurate predators.
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One of the best known prey-capturing mechanisms seen in lizards is the chameleon’s tongue. When an insect has been targeted the chameleon fires a missile-like tongue, which is up to two times the body length, gripping the prey before reeling it back in. The anatomy of the tongue is rather unusual. Like
humans, chameleons have a horseshoe-shaped hyoid bone which attaches the tongue to the bottom of the mouth. Unlike many other animals they have an extra section called the hyoid horn. This is covered by a hollow tongue and attaches to the hyoid bone at the back of the mouth. This whole apparatus, sometimes referred to as the entoglossal process, sits in the base of the mouth (Anderson et al 2012). When a chameleon has spotted its prey it moves the cartilaginous hyoid horn and tongue above the lower jaw. At the base of the tongue are rings of accelerator muscles. These muscles squeeze against the hyoid horn which is smooth and tapered to a point, much like squeezing an apple seed, which fires the tongue off the end. The tongue can reach an acceleration of 500 ms-2, much more power than the accelerator muscles generate. Surrounding the accelerator muscles are sheaths of connective tissue which were originally thought to act as lubrication. Prior to tongue projection they are contracted, building up elastic potential. The accelerator muscles are tightened, starting the motion causing the tissues to relax, rapidly releasing the energy (Groot & Leeuwen 2004). It has been shown that only around 10% of the firing power
of the tongue is from the accelerator muscles and the rest is elastic energy. This process has further benefits from elastic energy as temperature drops, that would normally slow muscle movements in lizards, do not have a serious effect on tongue projection in chameleons (Anderson & Deban 2009).
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On his trip to the Galápagos archipelago in 1835, Charles Darwin first saw these “large most disgusting, clumsy lizards” and referred to them various times in his field notes as the “imps of darkness”. One could argue that these are not the most attractive of lizards but they are uniquely specialised. Out of the nearly 10,000
species of reptile only around a hundred have gone back to the sea. The majority of these are sea snakes and a few sea turtle species, but only one lizard has taken to the ocean. Being scattered across many islands has meant several subspecies have evolved due to these isolated populations however, they all can be found basking in the morning sun on the rocky shores surrounding each island. The ocean surrounding the islands is much cooler than the land temperature due to cold currents. This can be an issue to a lizard species which feeds exclusively on marine plants, however this has been overcome by the ability to slow down their heart rate. This is referred to as heart rate hysteresis. This feature allows them to have heart rate of ~43bpm on land, to speed up the time taken to warm up to optimum a temperature, but then drop to ~7bpm whilst diving to slow down the cooling effects of the sea (Rasmussen et al 2011).
Like most marine animals the body of the marine iguana has morphological adaptations which aid them in their aquatic lifestyle such as a flattened tail and webbed feet. With its marine lifestyle however, comes a downside in the form of a considerable increase in salt intake. All marine reptiles have developed salt glands in order to expel the excess salt. There are several locations glands can develop, the diagram shows the location of these glands in red in the different animal groups (Babonis and Brischoux 2012). Just like the marine dinosaurs the marine iguana has nasal salt glands, which gives them the appearance of having a ‘white wig’. Many other species of lizard also possess nasal glands for specific dietary purposes to excrete excess salt or other ions which they gain from their food (Hazard et al 2010). Those lizards that have to excrete ions without salts glands have to excrete nitrogenous waste as insoluble uric acid. Like birds, most reptiles are unable to produce liquid urine more concentrate than their body fluids,
therefore getting rid of excess salt would mean losing large amounts of water, which is very important for both desert and marine reptiles. Marine iguanas have to excrete much more salt than most other lizards, yet surprisingly have not developed hyperosmotic urine. They rarely
expel waste ions like other lizards through uric acid or urate salts, meaning they have the highest salt gland excretion rate of any reptile (Rasmussen et al 2011).
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Deep in the jungles of Southeast Asia nimble draco lizards scamper up trees picking off ants and termites. These little insectivorous lizards are unfortunately prefect prey for many of the other inhabitants of these tropical forests but have developed a few unusual morphological adaptations to keep them out of sight or to evade predators that get to close. Draco lizards predators that get too close. Draco lizards refer to a group of lizards in the genus Draco, with currently 42 described species all capable of gliding.
Draco lizards vary in both size and colouration depending on the cover, and ease of evading predators, in different microhabitats. Sexual ornaments also differ greatly, with some species having quite pronounced colourful ornaments and others completely lacking them (Ord and Stuart-Fox 2006). Some stick to large rocks and shrubs close to the ground whilst others are completely arboreal staying against tree trunks. The arboreal dracos were found by Huang and Liu (2005) to favour a certain height on a tree trunk. Around a
third up a tree trunk is the optimum position to avoid ground-based predators like snakes and aerial predators such as birds.
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Many of the draco lizards are also referred to as flying dragons due to their ‘wings’. This gliding membrane called a patagium is a flat parachutelike expansion of the skin that allows gliding flight by catching the air. Although the draco lizards cannot flap their wings like a bat or a bird they do have control over them. Many other mammals, insects, a few reptiles and even fish have a gliding ability, many through a patagium system but the draco is unique among all living animals. The ‘wings’ are an actively controlled structure which is supported by a special muscle structure and elongated ribs (McGuire and Dudley 2001). This means they have complete control over contraction and retraction of the patagium. To open the membranes, muscle contraction pulls the first two thoracic ribs forward, while ligaments between the ribs pull the other 4 to 5 ribs out in series much like a collapsible fan. The diagram taken from Colbert (1967) shows this connection between the ribs. The muscles that do this are the intercostal muscles which are normally used for pulling air into the lungs, therefore draco lizards had to reorganize their respiratory apparatus (McGuire and Dudley 2001). The pectoral muscles, which normally power flight in birds, have been reassigned to control breathing.
Whiptail lizards, or racerunners, aptly named as they are almost constantly in motion, are found in two genera, Cnemidophorus and Aspidocelis with currently 18 and 40 described species respectively. These small active lizards are common throughout the United States and on first glance don’t seem to be different from many other lizard species around the world in appearance, size and diet however they do have hidden talents. Most species of whiptail are crepuscular, being most active during the early mornings and late afternoons, spending the middle of the day under cover of shrubs and bushes to escape from the midday heat. Whiptails have many adaptations, some shared with other lizard groups such as the Jacobson’s organ in the roof of the mouth in order to ‘taste’ the air with their slender forked tongues just like monitor lizards. The whiptail’s agility and speed help it to evade predators quite effectively, however if it does come into contact with one it can sacrifice its tail, just like alligator lizards (Page 24), which will cause a distraction allowing them to make a quick getaway. The pointed snout will not only help in decreasing drag at high speeds but is very useful for probing the earth for ground-dwelling insects.
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Certain whiptail species have been studied intensively for many years with new information coming to light even now. The reason for such scientific interest in these little lizards is that some of these species are entirely female. The New Mexico Whiptail (B), Aspidoscelis neomexicana, is a clear case study as this all
female species produces offspring via parthenogenesis, which means the young are clones of the mother. The species itself is a hybrid of two closely related species, a tiger whiptail (A) and a little striped whiptail (C) (Manning et al 2005). This hybridisation is not uncommon in whiptails with many new species arising through the pairing of two similar species. Although parthenogenetic whiptails can produce eggs without coming into contact with another individual those that demonstrate pseudocopulation have a higher percentage of healthier offspring. This process comes into effect when a female in oestrus, in a sexually reproducing species, would normally mate with a male, then is mounted by another female. This male-like female has just laid her own eggs so progesterone is now her dominant hormone. This false mating increases the hormones needed to produce a higher number of healthy offspring (O’Connell et al 2011). In the natural world producing hybrids is not unheard of but generally the outcome is a mule, an infertile animal. All the hybrid species of whiptail are parthenogenetic, producing fertile eggs without the need for a male. What has peaked interest is that these hybrids can then further reproduce with males from a different species creating a second generation hybrid cross that can
produce triploid parthenogenetic clones which are offspring with three sets of genes rather than the normal two (Cole et al 2010). These double hybrids can produce offspring that are as genetically diverse as any normally reproducing species.
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Geckos are a group of lizards which are found on both islands and mainlands across the world generally in warm or tropical climates. There are 7 gecko families one of which is Pygopoda, the legless lizards, which are closely related but are not generally classed in the term ‘geckos’. In the other 6 families there are around 1470 recognised species in 118 genera, which makes up around a quarter of all lizard species on the planet. They are extremely diverse and seem to have conquered most environments and habitats. They have been known to glide through the air, scale the tallest trees, vocally warn each other, lose and regrow tails, lick their own eyes clean and even mimic venomous invertebrates. Between the different species the most varied body part is the tail. An active tail, which is the case for many arboreal geckos, can be seen aiding in multiple capacities such as rapid climbing, gliding and aerial descent. A feature of the tail is revealed when a gecko loses grip on a particular surface. The tail has a scansorial pad at the end which acts like a prop or ‘fifth’ leg to support the gecko and prevent it falling backwards (Pianka & Vitt 2003). Within the flying gecko species the tail acts like a rudder when gliding through the air.
Adhesive toepads are seen in many lizard and frog species but possibly most noticeably in the geckos. The adhesive properties of the toepads allow geckos to scale vertical as well as inverted surfaces, both smooth and coarse, with ease. This ability has attracted scientists for many years with some trying to recreate the ability. These adhesive properties are achieved through millions of keratinous setae (B) located in the lamella (A) on the toepads. The setae ends (C) are split furthermore into fine structures known as spatula (D). These microscopic hairlike outgrowths manage to adhere to virtually any surface through complex frictional interactions such as van der Waals forces (Rogerson 2014). This complex system has been so well refined over millions of years it has developed a process of self-cleaning with every step. The oldest known gecko to have adhesive toepads was Cretaceogekko burmae over 110 million years old. Out of the 6 ‘gecko’ families approximately 60% of gecko species possess adhesive toepads. Over a dozen gecko genera are named after the characteristics of their digits such as Pachydactylus meaning ‘thick toe’ and Gymnodactylus meaning ‘naked toe’. These adhesive properties are absent in certain lineages but have re-emerged again in others, which is referred to as convergent evolution. On multiple occasions repeated evolution of adhesive toepads has appeared in unrelated lineages 11 times and been lost in 9 (Gamble et al 2012).
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One of the most well-known of the Australian lizards is the frilled dragon. Being the only species in the genus Chlamydosaurus means it is definitely unique with one clear feature which sets it apart from other similar agamid lizards. When escaping from a predator, the frilled dragon is capable of bipedal locomotion, sprinting on its hind legs, across open terrain. Despite this, it spends the majority of its time in trees. The grey brown colouration means it blends in well with the branches. It is in the branches that it catches most of its insect prey but it has also been known to take small mammals if given the chance. Compared to its body size the frill of a frilled dragon is the largest, and possibly one of the most spectacular display structures in the animal kingdom. It was originally not known what the primary purpose of the frill was, many ideas from gliding, food storage and auditory enhancement were discussed but no evidence to support these theories was ever found. After many studies on the behaviour of these lizards were conducted the main uses were discovered to be for communication and predator defence. Another use for such a large fold of skin is to enhance camouflage when resting on a tree branch by breaking up their outline. This particular adaptation was perhaps not the reason for developing such an oversized frill but a useful consequence of it (Shine 1990).
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The structure of the frill is supported by long spines made of cartilage which extend from the lower jaw bone, giving complete control over the extension of the frill. This is beneficial when they come into contact with predators such as birds of prey, as a quick pulse of the frill would scare off predators approaching quickly. The lizard itself looks larger as the diameter of the frill can be up to four times larger than the actual body. This increase in size is useful against predators like snakes, monitor lizards and dingoes (Shine 1990).
Male frilled dragons are territorial so general displays are common to communicate such as slapping tails onto tree trucks, which is audible to individuals up to 30 meters away. If another male is closer, push-ups, head bobs and partial frill erections will be the display of choice (Wilson 2012). If the opposing male gets even closer, full frill displays leading to combat can occur, this is more common in the breeding seasons. Many male frilled dragons have scars from bites during combat over territories however the majority of the time the display is enough to settle disputes before combat begins. It varies geographically but the colouration of the frill is usually a mix of reds, oranges and yellows. These colours are caused by carotenoid pigments in the skin which are directly linked to
diet. A healthy and older male will have mostly red whilst a juvenile will have a very yellow frill. In frill displays between similar sized males the victor has the most colourful frill, particularly red colouration (Hamilton et al 2013).
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Basiliscus is a genus comprised of four species of large basilisk lizard – common, brown, western and plumed which are found throughout the Americas. These tree-dwelling lizards are normally found in patches of forest around or near large expanses of water such as rivers or lakes. They can be seen clasping the
end of branches as their colourations makes impressive camouflage but if spotted by a predator the position means they can descend rapidly to the floor or into water to escape. Juvenile basilisks tend to hide amongst the leaf litter and in shrubs whilst feeding on a diet solely comprising of insects. When adults they expand their diet to include vegetation, small mammals, small lizards and birds’ eggs. Generally the ranges of juvenile and adults do not overlap as it is possible hatchlings may be preyed upon by adults (Lattanzio & LaDuke 2012). Juveniles position themselves closer to the water’s edge than adults do, which could be in response to the higher number of predators at that size. If approached by a predator the basilisk of any size will show off their unique adaptation which has earned them the nickname ‘Jesus lizard’. They can sprint on their hind legs across the water surface at speeds of up to 1.5ms-1. Land based predators are unable to follow and for those aerial predators they can
sprint to a deep section and sink as they are also exceptional swimmers.
Throughout the stages in life, juvenile to adult, all basilisks possess the ability to run on the water surface. This ability is shared with a couple of other 16
physiologically similar species. Unlike solid ground, water offers little resistance so a specially adapted limb and an altered form of locomotion are required. The large hind feet have scaly fringes on the sides of the third, fourth and fifth toes. When walking on land these are compressed but these fringes open
against the water surface, increasing the surface area of the foot and creating a pocket of air underneath when the basilisk takes a step. The strides that basilisks take are divided into three phases referred to as the slap, stroke and recovery (Hsieh 2003). During the slap the foot moves vertically downward with sufficient force to keep the lizard above the water surface. This is different to a step on land, with a normal step it follows the spring-mass model which states that during contact with a surface energy is stored to propel the limb forward, so most animals walk with a slight spring in their step. The basilisk ignores this rule and only produces the energy for the step then stops. During the stroke they focus the energy backward, forcing their body forward, shifting the centre of mass. The recovery step lifts the leg high off the water and resets ready for the next step. In amongst all this the lizards also create forces off the sides called lateral reaction forces enabling them to stay upright (Hsieh and Lauder 2004).
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Reaching lengths of up to 3m long and weighing approximately 70kg, the Komodo dragon is the largest living lizard. Found on a select few Indonesian islands they feed on large mammals such as deer through a combination of stealth and force, as well as large amounts of carrion. The Komodo dragon is the only large carnivore on these islands. Like most monitor lizards they track their prey using a long forked tongue and a specialised scent organ called the Jacobson’s organ. Although they have no natural predators, apart from other Komodo dragons, the skin is reinforced with specialised structures called osteoderms which are small bony deposits creating almost armoured scales much like that of crocodiles. The Varanus genus is thought to have originated from Asia and later moved to
Australia where the Komodo dragon evolved. When the landmasses of Australia and Southeast Asia collided, the varanids’ range expanded. They are thought to be a relic from the time of megafauna although, along with many other giant lizards and mammals, they became extinct in Australia and across Asia probably due to climate change and modern humans. The Komodo dragon managed to survive unchanged in the select few islands. One reason could be these islands were not only home to the dwarf elephants (Stegodon) but also hobbits (Homo
floresiensis). They may have adapted to live with and hunt humans, before modern humans were a threat to their existence (Hocknull et al 2009).
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The mechanisms surrounding how Komodo dragons kill their prey have been debated for many years. Until recently it was believed that they did not have venom like snakes but instead harboured a
vile mix of bacteria within their mouths, so upon biting prey the wound quickly got infected. Recent analysis of Komodo specimens showed they have in fact developed venom glands with ducts that run through serrated teeth. Some of the larger more ancient lizards have been shown through fossil records to also have these venom
ducts therefore the presence of simple venom is an ancestral state. The two known lizards with well-developed venom systems, the Gila monster and the beaded lizard, are thought to share the same ancient lineage (Fry et al 2009). Analysis of the compounds found in the mouth of a Komodo dragon showed that they do not possess anything extra compared with other monitor lizards regarding toxic bacteria. The venom has toxic properties such as acting as an anti-coagulant, causing hypertension and inducing shock. Compared to other similar sized carnivorous crocodiles, Komodo dragons have a relatively weak bite force about a sixth of the strength. The skull itself is comparatively light weight being strongest along its length due to the difference in catching its prey. Crocodiles hold onto their prey until dead, sometimes drowning them. Komodo dragons have a different approach, they inflict deep wounds caused by the prey pulling away from the thick serrated teeth, pump in the toxic venom and wait until the prey succumbs to its injuries from blood loss and shock (Wroe et al 2011).
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Alligator lizards are from the genus Elgaria and the eight species are found throughout North America. In cooler weather they are diurnal but become crepuscular during the summer months. They have large bony scales, large thick heads and quite powerful jaws for their body size hence their common name.
Much like skink species, alligator lizards have a long elongated body and short limbs. When traveling at speed they flatten their legs against their bodies and move with an undulation motion much like that of a snake, this also means they are powerful swimmers. The larger bone-reinforced scales on the back are separated from the much smoother belly scales with small granular scales creating a slightly indented fold on each side. This fold can stretch and expand to hold extra food or eggs then contract when not needed so the skin is always tight and streamlined. The ear of the alligator lizard is highly developed in comparison to other lizards. Most lizards have a short frequency range, so iguanas can only detect low pitch sounds whilst for geckos it is only high pitched sounds. Alligator lizards are unusual as they can detect a very high frequency and a comparatively high range.
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When trying to escape a predator many lizards perform caudal autotomy which is the act of releasing the tail. This can happen through elevated stress levels as well as physical contact on the actual tail. Dropping the tail creates the deception of a continued struggle, as the autotomized tail continues to wriggle for up to thirty minutes after detachment due to anaerobic metabolism. This is a useful escape tool with two thirds of all lizard families having species that perform caudal autotomy (Bateman & Fleming 2009). Caudal autotomy takes place at pre-weakened areas of the tail. Autotomy has two main patterns. The first is intra-vertebral where the tail has diagonal fracture planes along the length of the tail at which point the caudofemoralis tail muscle is responsible for contracting to ‘break’ the tail between certain vertebrae. The second is inter-vertebral where the tail breaks at a vertebrae but doesn’t include any special adaptations that allow this to take place. Sphincter muscles then close off the opening to stop bleeding quickly. It is believed that intervertebral autotomy is the ancestral state. Bateman and Fleming (2009) hypothesized that populations that had very little predation pressures quickly lose the intravertebral function with ease. Since this is such an important body part most arboreal lizards have more fracture planes within the tail than terrestrial species as even the removal of a small portion can deteriorate the performance. Alligator lizards have long tails with multiple fracture planes so the smallest amount of tail is lost. The less tail is needed to regenerate the more beneficial to the lizard (Pianka & Vitt 2003).
Horned lizards, or horny toads as they are better known, are found throughout North America in dry arid landscapes. There are currently 17 described species all with similar bodies but very distinct head shapes. The name horny toad comes from their physical size, the largest species is 10cm long but most are much smaller, and have spikey rough appearance. Horny toads fill much the same ecological niches that thorny devils (Page 6) fill in Australia. Both are small armoured lizards, found in dry barren environments feeding on large numbers of ants as a primary food source. This is referred to as convergent evolution. Horny toads are most vulnerable when moving around so they position themselves near several ant colonies and unlike the thorny devil they harvest up to a maximum of 25% of the forager ant numbers over ten days, keeping the colony at a healthy population (Eifler et al 2012). Horny toads are particularly remarkable at both camouflage and mimicry since they remain
stationary between feeding sessions. If laid flat their colouration blends in well with the sand and dirt but if balled up they look just like rocks.
When it comes to dealing with predators the horny 23
toads have an assortment of manoeuvres. If the mimicry and camouflage do not fool the predator, the lizards try to run but they do so in short bursts and stop abruptly in order to confuse. If they are still being hunted by a snake after the chase they puff out their bodies to make themselves look harder to swallow. If being pursued by a canine or feline predator they have one final defence if all else fails. If a predator gets into close proximity several horny
toad species can squirt blood from the orbital sinuses up to 5 feet. They can control the movement of the jet of blood to aim for three sensory target areas, eyes, mouth and nose (Sherbrooke & Mason 2005). This unusual action causes a startle response in the predator. If the projected blood gets into the mouth of a predator, there is a foul taste from the chemicals of the harvester ant diet (Sherbrooke et al 2004). They restrict blood flow to the rest of the body causing blood pressure in the head to increase rapidly. The blood vessels in the orbit are abnormally thin therefore with the increased pressure they rupture from the corner of the eye socket. This stream can then be directed with the eye muscles. With all the blood flow going to the head this jet of blood can be repeated multiple times in a matter of minutes (Sherbrooke et al 2004).
Italian wall lizards, also known as ruin lizards, are native to not just Italy but are widespread throughout Croatia, France and Switzerland as well as being introduced to many other countries. They are commonly seen close to human inhabitation darting across buildings and basking on rubble. Italian wall lizards were part of a unique long term experiment. In 1971 scientists took five breeding pairs from a small island, Pod Kopište in Croatia and put them on a neighbouring uninhabited island, Pod Mrčaru that didn’t have any lizard species at all. In 2007 a team went back to Pod Mrčaru to find a thriving population of lizards. This population however was quite different to the original. They had much bigger heads which delivered a much higher bite force as well as being larger on average with shorter limbs. This was down to two factors, diet and predators. The new population had very different selection pressures, less bird predation meant they could increase size and in their diet they ate much more vegetation. This new diet has even meant they have started to develop cecal valves in order to digest the cellulose in plant cells much more effectively. This part of the stomach is completely absent in the original population. This dramatic change in just thirty six years is seen as evolution in action (Vervust et al 2007).
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The parietal eye or ‘third eye’ is located on top of the head. Unlike its name suggests it is generally quite rudimentary being just a photoreceptive scale, distinguishing between light and dark. This is particular useful for circadian rhythms and responding to predation threats from above. Early tetrapods are thought to have had a fully functional third eye, therefore it is considered to be the ancestral state. The tuatara, a lizard-like reptile, is from a very ancient lineage and possesses a well-developed parietal eye with a simple lens and retina. The parietal eye in lizard species isn’t quite as developed. The parietal eye works closely with the pineal gland which mammals and birds still possess under the skull (Foà et al 2009). In mammals the pineal gland helps control the circadian rhythm, in the lizards the combination of the parietal eye and the pineal gland serve a similar function. Using this parietal eye the lizard can almost ‘read’ the sky. The amount of time a wall lizard stays out foraging will be determine by the time of day which will be calculated by the lizards internal clock set by the strength of sunlight. As the amount of sunlight changes over seasons, it combines with temperature change to prompt hormone production. Italian wall lizards have also used their parietal eye is another way. Scientists using a Morris water-maze found that they can actually use their internal body clock along with the sun’s position in order to navigate almost like a basic compass. Even more incredible is that even on a cloudy day without knowing the sun’s location they can orientate themselves just using polarised light (Beltrami et al 2010).
Lizard Evolution: Pianka ER, Vitt LJ (2003) Lizards: Windows to the evolution of diversity. University of California, Los Angeles. pp 13-18 29
Thorny Devil: Comanns P, Effertz C, Hischen F, Staudt K, Böhme W, Baumgartner W (2011) Moisture harvesting and water transport through specialized micro-structures on the integument of lizards. Beilstein Journal of Nanotechnology 2:204-214 Clemente CJ, Thompson GG, Withers PC, Lloyd D (2004) Kinematics, maximal metabolic rate, sprint and endurance for a slow-moving lizard, the thorny devil (Moloch horridus). Australian Journal of Zoology 52:487-503 Meyers JJ, Herrel A (2005) Prey capture kinematics of ant-eating lizards. Journal of Experimental Biology 208:113-127 Chameleons: Anderson CV, Deban SM (2010) Ballistic tongue projection in chameleons maintains high performance at low temperature. Proceedings of the National Academy of Sciences 107:5495-5499 Anderson CV, Sheridan T, Deban SM (2012) Scaling of the ballistic tongue apparatus in chameleons. Journal of Morphology 273:1214-1226 Groot JH, Leeuwen JL (2004) Evidence for an elastic projection mechanism in the chameleon tongue. Proceedings of the Royal Society of London-B 271: 761 Marine Iguana: Babonis LS, Brischoux F (2012) Perspectives on the Convergent Evolution of Tetrapod Salt Glands. Integrative and Comparative Biology 52: 245-256 Hazard LC, Lechuga C, Zilinskis S (2010) Secretion by the nasal salt glands of two insectivorous lizard species is initiated by an ecologically relevant dietary ion, chloride. Journal of Experimental Zoology: Ecological Genetics and Physiology 313:442-451 Rasmussen AR, Murphy JC, Ompi M, Gibbons JW, Uetz P (2011) Marine reptiles. PLOS one 6: e27373 Draco Lizards: Colbert EH (1967) Adaptations for gliding in the lizard Draco. American Museum novitiates 2283 Huang SC, Liu JX (2005) Microhabitat selection by gliding lizards (genus Draco). International Field Biology Course 2005 McGuire JA, Dudley R (2011) The biology of gliding in flying lizards (Genus Draco) and their fossil and extant analogs. Integrative and Comparative Biology 51:983-990 Ord TJ, Stuart‐Fox D (2006) Ornament evolution in dragon lizards: multiple gains and widespread losses reveal a complex history of evolutionary change. Journal of Evolutionary Biology 19:797-808 Whiptail Lizards: Cole CJ, Hardy LM, Dessauer HC, Taylor HL, Townsend CR (2010) Laboratory hybridization among North American whiptail lizards, including Aspidoscelis inornata arizonae× A. tigris marmorata, ancestors of unisexual clones in nature. American Museum Novitates 3698:1-43 Manning GJ, Cole CJ, Dessauer HC, Walker JM (2005) Hybridization between parthenogenetic lizards (Aspidoscelis neomexicana) and gonochoristic lizards (Aspidoscelis sexlineata viridis) in New Mexico. American Museum Novitates 1-56 O’Connell LA, Matthews BJ, Crews D (2011) Neuronal nitric oxide synthase as a substrate for the evolution of pseudosexual behaviour in a parthenogenetic whiptail lizard. Journal of Neuroendocrinology 23:244-253 Geckos: Gamble T, Greenbaum E, Jackman TR, Russell AP, Bauer AM (2012) Repeated origin and loss of adhesive toepads in geckos. PloS one 7: e39429
Pianka ER, Vitt LJ (2003) Lizards: Windows to the evolution of diversity. University of California, Los Angeles. pp 172-186 Rogerson TC (2014) A review of the major gecko adaptations with comparison between the families and noteworthy species. Journal of the International Herpetology Society 39:24-30 Frilled Dragon: Hamilton DG, Whiting MJ, Pryke SR (2013) Fiery frills: carotenoid-based coloration predicts contest success in frillneck lizards. Behavioral Ecology 24:1138-1149 Shine R (1990) Function and evolution of the frill of the frillneck lizard, Chlamydosaurus kingii (Sauria: Agamidae). Biological Journal of the Linnean Society 40:11-20 Wilson SK (2012) Australian lizards: a natural history. Csiro Publishing. pp 140-141 Basilisks: Hsieh ST (2003) Three-dimensional hindlimb kinematics of water running in the plumed basilisk lizard (Basiliscus plumifrons). The Journal of Experimental Biology, 206:4363-4377 Hsieh ST, Lauder GV (2004) Running on water: Three-dimensional force generation by basilisk lizards. Proceedings of the National Academy of Sciences of the United States of America 101:16784-16788 Lattanzio MS, LaDuke TC (2012) Habitat Use and Activity Budgets of Emerald Basilisks (Basiliscus plumifrons) in Northeast Costa Rica. Copeia 2012:465-471 Komodo Dragon: Fry BG, Wroe S, Teeuwisse W, van Osch MJ, Moreno K, Ingle J, Norman JA (2009) A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus. Proceedings of the National Academy of Sciences, 106:8969-8974 Hocknull SA, Piper PJ, van den Bergh GD, Due RA, Morwood MJ, Kurniawan I (2009) Dragon's paradise lost: palaeobiogeography, evolution and extinction of the largest-ever terrestrial lizards (Varanidae). PloS one 4: e7241 Wroe S, D'Amore DC, Moreno K, McHenry CR (2011) The effects of biting and pulling on the forces generated during feeding in the Komodo dragon (Varanus komodoensis). PloS one 6:e26226 Alligator Lizards: Bateman PW, Fleming PA (2009) To cut a long tail short: a review of lizard caudal autotomy studies carried out over the last 20 years. Journal of Zoology 277:1-14 Pianka ER, Vitt LJ (2003) Lizards: Windows to the evolution of diversity. University of California, Los Angeles Horny Toads: Eifler DA, Eifler MA, Brown TK (2012) Habitat selection by foraging Texas horned lizards, Phrynosoma cornutum. The Southwestern Naturalist 57:39-43 Sherbrooke WC, Mason JR (2005) Sensory modality used by coyotes in responding to antipredator compounds in the blood of Texas horned lizards. The Southwestern Naturalist 50:16-222 Sherbrooke WC, Middendorf GA. Jones CA (2004) Responses of kit foxes (Vulpes macrotis) to antipredator blood-squirting and blood of Texas horned lizards (Phrynosoma cornutum). Journal Information 2004 Italian Wall Lizards: Beltrami G, Bertolucci C, Parretta A, Petrucci F, FoĂ A (2010) A sky polarization compass in lizards: the central role of the parietal eye. The Journal of Experimental Biology 213:2048-2054 FoĂ A, Basaglia F, Beltrami G, Carnacina M, Moretto E, Bertolucci C (2009) Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye. Journal of Experimental Biology 212:2918-2924 Vervust B, Grbac I, Van Damme R (2007) Differences in morphology, performance and behaviour between recently diverged populations of Podarcis sicula mirror differences in predation pressure. Oikos 116:1343-1352
This mini guide was created for a final year information project. All the information was personal knowledge combined and backed up from scientific papers all of which are referenced. All number figures including number of species, genera and families were checked with the ReptileDatabase.org. 30
The author would like to thank the following for direct help in the project: Peter Simmons for supervising the project. Robert and Laura Brooke for proof-reading and checking how interesting the information was. Printkiosk for printing the final booklets. Everyone else who took the time to read through and fill out feedback forms. Photo Credits The author would like to thank the following for the use of their images in this booklet: Maine Iguana Tongue Image – Arthur Morris / Birds As Art Thorny Devil Profile Image – Jason Edwards / National Geographic Creative Thorny Devil Walking Image – Brooke Whatnall / National Geographic Creative Jacksons Chameleon Profile Image – Lizard Types / lizardtypes.com Chameleon Computer Generated Image – Science.howstuffworks.com Chameleon Hunting Mantis Image – Scott Cromwell / Solent News and Photo Agency Marine Iguana Profile Image - Rod Stewart / animals.nationalgeographic.co.uk Salt Gland Diagram – Leslie Babonis and François Brischoux Marine Iguana Thermal Image – Glen Tattersall / panoramio.com Draco Lizard Profile Image – Alamy / Premaphotos / animals.nationalgeographic.co.uk Whiptail Blue Lizard – G. Peterson Whiptail Hybrid Image – Dr William Neaves / Stowers.org Texas Whiptail lizard image – Eric Siegmund / The Fire Ant Gazette Mossy Leaf-Tailed Gecko Profile Image – Mark Scherz / The Travelling Taxonomist Gecko Feet Image – Kellar Autumn / National Geographic Gecko Lamella Images – Paul Rincon / BBC Science and Environment Frilled Dragon Display Image – Belinda Wright / National Geographic Creative Alligator Lizard Profile/Autotomy Images – Gary Nafis / Californian Herps Horned Lizard Blood Squirt – National Geographic Wild Horned Lizard in Blood – Randomtruth /Flickr Italian Wall Lizard Profile / Gape Image - Gary Nafis / Californian Herps
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