Lizards (Lacertilia) form a suborder of the scale reptiles (Squamata), which has more than 7100 species. This makes lizards the largest group of all modern reptiles.
Lizards usually remain small, most species do not exceed 50 centimeters including tail, and few species reach a length of more than one meter. Most lizards are green to brown in color and have a flattened body with a clearly distinguishable head and tail and four well-developed legs. However, a few groups have cylindrical bodies and degenerate legs, a few species have only front legs and a few groups are completely legless; such lizards can easily be mistaken for snakes.
The colors and various body modifications can be quite different. A number of lizards have body protrusions such as spines, crests, horns, collars or hover wings. Chameleons have highly laterally flattened bodies with several very distinctive adaptations.
Lizards are cold-blooded and like to take sunbaths to warm up so they can move faster and hunt and flee more efficiently. Lizards eat mostly insects and other arthropods; larger species sometimes eat vertebrates or switch to plants. They eat pests such as house crickets, grasshoppers and cockroaches.
There are 38 different families recognized today. The lifestyles of lizards are highly variable and are mostly related to the family to which a species belongs. For example, geckos are usually active at night and live in trees, while true lizards are active during the day and live on the ground. However, both groups again have exceptions, such as day-active geckos and climbing true lizards.
Distribution and habitat
Lizards are found almost worldwide, with only northern North America and far northern Asia lacking species. Lizards live mostly in humid tropical and subtropical regions, but there are also species that live in more temperate areas or have specialized in dry, arid environments such as mountainous regions, steppes and even deserts. Unlike crocodilians, turtles and snakes, no lizard lives permanently in the sea.
The range is related to the family, agamas for example are found in all continents, but anoles live only in South and Central America and deaf monitor lizards, represented by only one species, are found exclusively on the island of Borneo.
Lizards occur in a wide variety of habitats, both in terms of altitude and vegetation type as well as associated (air) humidity and temperature. Some lizards lead a burrowing existence underground and others live on the ground among rocks or climb trees and shrubs. In turn, these different lifestyles can be further subdivided; an example is lizards that live in trees. Many of these species have a more specific part of the tree as their habitat called the microhabitat. For example, there are species that live more in the lower branches, species that prefer to crawl against the trunk of the tree, and species that can be found higher up in the canopy.
All lizards have basically a similar build, but different species can differ greatly in length, color and body shape. The smallest species grow only a few centimeters long, while the largest species reach lengths of more than three meters. The smallest species are the bullet-fingered geckos; the largest species belong to the monitor lizards. The largest and heaviest lizard known is the komodo dragon (Varanus komodoensis), which can reach a length of more than three meters.
Despite the variation in length and the great variety of shapes within lizards, they generally follow roughly the same build plan. Notable are the elongated body consisting of a long trunk and tail, but a relatively small and flat head. The four curved legs face not downward but sideways, causing the abdomen to drag on the ground. The skull is distinctive: the shape and placement of the nostrils, auditory organs and eyes is unmistakable and easily distinguished from crocodilians and turtles. With bridge lizards, things are a bit more difficult. They look remarkably like lizards, but are a very ancient group of reptiles. Despite the name, they are not directly related to lizards.
Because of their flattened body shape and ditto head, lizards can easily crawl under and between rocks and other objects. However, there are exceptions, some burrowing species have a rolling round body and short tail, the chameleons, on the contrary, have a laterally flattened leaf-like body and a retractable prehensile tail. They also have a large broad head and do not resemble the other groups of lizards in build.
A lizard’s skin is always covered with scales, species from a number of groups additionally have small bony inclusions called osteoderms that provide additional reinforcement. Many lizards have a ossified underlayer; however, this is lacking in some groups such as the geckos. Osteoderms also occur in other reptiles such as the crocodilians, examples of lizards with osteoderms are the crusted lizards (Helodermatidae) and the knobbed lizards (Xenosauridae). Lizards with such armor are well protected, but have stiffer skin that is not very mobile. An unscaled skin fold is often present on the flanks to allow the skin to stretch for respiration and gestation.
The skin consists of two layers; from the inside to the outside respectively, the dermis or dermis and the epidermis or epidermis, which is the part of the skin on the surface. The dermis consists of connective tissue, any osteoderms lie within the dermis. The epidermis is further divided into three zones. In the lower layer, the stratum germinativum or germinal layer, the scales are formed by mitosis from cuboidal cells. Above this is an intermediate layer, in which the scales “mature,” so to speak: they become more compact and harder. Finally, the upper layer is called the stratum corneum or stratum corneum and consists of the fully developed scales formed from the hard beta-keratin, a horn-like substance. A cuticle is made up of many very thin layers of keratin; any spines and horns are also made up of keratin. The scales provide protection and have an insulating function. They are also water repellent and have no pores to prevent the lizard from drying out. A disadvantage is that the lizard cannot sweat to cool off.
Like other reptiles, lizards need to molt regularly, this is also called ecdysis. Molt is a result of mitosis of the germ layer, which moves the cells to the interlayer. The cells of the interlayer harden and grow into a new stratum corneum, after which the old skin comes off. Only during mitosis does wound healing take place, which is why lizards, like other reptiles, are slow to recover from injuries.
The shedding frequency depends on the life stage of a lizard, among other things, especially when they are still small they must shed often due to rapid growth, older lizards shed their skin less frequently. Unlike snakes that molt all at once and turtles and crocodiles, whose scales or horny plates come off one by one, lizards molt in patches. The old skin tears off further and further until it is completely lost. The new skin looks brighter in color, is smoother and is instantly dry. Lizards do not need to let the new skin harden after a molt as is the case with arthropods.
The skin has several conditions and diseases. Sharp objects or claws and bites from competitors and enemies cause slow-healing penetrations and lacerations, gram-negative bacteria cause abscesses. Ulcers are often caused by overly moist or polluted conditions. In animals such as mammals, minor infections of the skin are normal. This is due to the presence of a very large number of pores and hair follicles that inflame easily but heal quickly. Lizards do not have pores and hair follicles, therefore an infection of the skin is always a potential danger to the animal. Lizards can carry a wide range of parasites that usually manifest themselves on the skin. Examples of such parasites include fungi, worms, bacteria, mites and ticks.
Skin color is caused by melanin, an organic pigment produced by the melanocytes. These are cells positioned in the lower layer of the epidermis. They transport the pigments through dendrites (tentacle-like structures) to the keratocytes (the scale-forming cells) during the creation of new skin. Located in the upper layer of the dermis are the chromatophores, which are pigment-containing cells that contain different pigments. Xantophores cause a yellow color, erythrophores a red and leukophores and guanophores a white color. Finally, Iridophores are not color pigments but reflect or irradiate light, which causes the oily sheen of many reptiles. Some lizards, unlike their counterparts, have an all-white coloration. These specimens are often called albino but, unlike “true” albinos, usually do not have a complete lack of melanin but only a deficiency.
Lizards that can change color, such as chameleons, but also (although to a lesser extent) many anoles, geckos and true lizards, do so by redistributing the pigments in the chromatophores to create a different color. For a long time, it was thought that the lizards that can change color used this ability solely to be less conspicuous in the natural environment and adapt colors to a different environment. However, changing color is mainly related to environmental conditions such as light intensity, ambient temperature and humidity. In particular, the species that can change color quite a bit, such as anoles, change color primarily to show their expression. A stressed or irritated lizard will color darker, a lizard trying to attract a conspecific of the opposite sex will show very variegated colors. Well known are the pregnant females of several species of chameleons that not only change color very quickly but also show colors that are exceptional in nature such as pink and blue. Pregnant females thus indicate that they are already fertilized and have no desire for advances.
The head of a lizard is usually strongly flattened and generally distinct from the elongated and pointed tail. The scales on the lizard’s head are often large and shiny. The shape of the skull and the structure of the scales is basically about the same in every group of lizards, but varies somewhat among species. Some species of lizards are so similar that looking at the scales on the head, among other things, is the only way to identify them. The scales have different names depending on their position; for example, the scales on the head are called the frontal scales, which means front. The scales near the nostrils are called nasal (nasal: relating to the nose), those on the sides of the top of the head are called parietal, meaning relating to the wall. Some lizards have coarse spines, crests formed from deformities of the skull or even horn-like protrusions on the head.
The eyes are relatively small and are always clearly visible and positioned on the side of the head. Most lizards have movable eyelids, but some groups (such as geckos) do not have them. In these lizards, the lower eyelid is fused to the upper eyelid over the eyeball and forms a protective layer, the eyelid functioning as a window is regularly licked off with the tongue to keep it clean.
Some lizards, such as the snake-eyed lizard (Ophisops elegans), have movable eyelids that are transparent. The window, which acts like a pair of glasses, allows them to still see clearly with their eyelids closed. In species with movable eyelids, the eye is protected by a cutaneous membrane, which closes in a reflex; this so-called “third eyelid” is also found in other reptiles. However, it is missing in most geckos, which cannot close their eyes in any way because of this. The pupil of lizards can be round, oval or slit-shaped, diurnal species have a round pupil, nocturnal lizards often have a slit-shaped, vertical pupil. The iris can have different colors in lizards, ranging from green, brown, gray, orange, red or yellow.
Many lizards, such as iguanas and agamas have a third eye on the center of the top of the head. This “eye” is very primitive and anatomically unlike the other eyes; it consists of light-sensitive cells under the skin, which are in direct connection with the Epiphysis or pineal gland. Among other things, the third eye plays a role in determining the lizard’s day and night rhythm. It can also detect enemies approaching from above, such as birds of prey, because they cause a change in incident light that can be detected by the third eye. In some reptiles, such as bridge lizards, the third eye is much better developed.
The mouth of lizards contains teeth that are not placed in tooth cavities as in mammals (thecodont). The teeth are attached to the inside of the jawbone (pleurodont) or are positioned on top of the jawbone rim (acrodont). Not only the upper and lower jaws bear teeth, but teeth may also be present in the palate: the vomerine teeth.
The tongue of lizards does not serve to taste with, but mainly to smell. As in snakes, the tongue is regularly protruded from the mouth to capture odor particles that are carried to Jacobson’s organ, an organ located in the palate with odor receptors. The tongue is also used for drinking by licking up dewdrops. After a meal, the tongue is used to lick the mouth clean to remove food debris. The geckos, which as a rule do not have movable eyelids, lick clean the eyelid functioning as a kind of spectacles with the tongue.
Lizards have external ear openings, unlike snakes, and a well-developed inner ear. They can detect sounds but rely mainly on vibrations in the ground. This allows them to detect prey and evade enemies before they have noticed the lizard. The presence of external ear openings behind the eyes is one of the most obvious differences between snakes and legless lizards, as the presence or absence of eyelids is not conclusive in all species.
Most lizards have four legs, each with five relatively long toes that bear nails. The legs and especially the claws vary depending on their function in shape, size and strength, this is often related to the group to which the lizard belongs. Species that live in trees have large legs with curved claws and often long, curved nails. An example is chameleons, which have claws reminiscent of mittens, the fingers and toes are paired and face each other. This allows them to cling well to a branch but running over branches or walking quickly on the ground is not possible, and many chameleons move slowly here and are very vulnerable. Other examples are the fringe lizards of the genus Acanthodactylus, which live in deserts and run across hot sand. To protect their legs from being burned, they have a kind of “snowshoes” under their toes consisting of scaly flaps.
Geckos and anoles have lamellae, small grooves with a relatively large number of tiny hairs, each of which in turn has many spurs. This makes the absolute contact area very large, so they stick to everything, even upside down against smooth surfaces such as glass.
Lizards that live on the bottom also tend to have large, powerful legs and long claws and toes to get around quickly. Many bottom dwellers are very adept at digging burrows to crawl away quickly in the face of danger and to take shelter in poor conditions. Some species can even run away on their hind legs, such as basilisks. This lizard can run up to eight meters across the water thanks to a special running technique. Lizards that swim a lot have short but powerful legs; the toes are often webbed for more efficient swimming. Such species are mostly arboreal, jumping into the water to escape when in danger. Only a few species, such as the marine iguana, actively search for food underwater. A number of lizards have reduced limbs or no legs at all, examples are skinks and hazelworms. In a number of species, the front legs are completely missing and the hind legs are greatly degenerated. They are no longer used for walking but still play a role in mating.
All lizards have a tail that in many cases is as long as the body. Where the body ends and the tail begins is often difficult to see from above, especially in the legless species. On the underside, however, the boundary is clearly visible: the tail is that part of the body located behind the cloaca.
The tail is used as a balance when running; many climbing species have a more mobile tail that serves as an additional grasping organ. Chameleons are the best-known group of lizards with prehensile tails, but some other species such as some geckos, the emerald dragon and several species that live among grasses can anchor themselves with their tails, such as the long-tailed lizard (Takydromus sexlineatus). When swimming, the tail is used for propulsion by moving it rapidly back and forth. In many aquatic species, such as the Chinese water dragon (Physignathus cocincinus), the tail is flattened laterally to increase its propulsive effect.
The tail can be shed in many lizards if it is grabbed by an enemy. This is called autotomy and occurs in geckos and true lizards, among others, but not in chameleons, agamas and monitor lizards, among others. When the tail is shed, it always occurs at a specially shaped, weaker tail vertebra. The shedding of the tail can be controlled by the lizard by muscle contractions in the tail that cause the vertebra to snap. The muscles in the tail then squeeze together to prevent too much blood loss through the veins in the tail. After being shed, the tail continues to squirm because the body-independent nerves and muscles remain active for some time after the fracture. As a result, an enemy will be strongly attracted to the spastically moving tail so that the lizard can escape. The enemy is thus distracted and even has something to eat because the lizard uses its tail as a fat store. The tail grows back over time but has an abnormal, dark color and remains smaller because the tail vertebrae no longer grow. As a result, its body fat storage capacity is smaller.
Some lizards use abnormal shapes to cause confusion between head and tail. In some geckos and skinks, the head looks strikingly similar to the tail so that an enemy is deceived and attacks the wrong side. An example is the pine cone skink (Tiliqua rugosa) from Australia that has a short tail with a greatly thickened tail tip that resembles the head. However, the tail does not let go in this species. Especially the larger species use their tail as a striking weapon by reflexively moving it sideways.
Anatomy and physiology
Lizards have about the same organs as mammals, the digestive system occupies most of the body cavity space and consists of the esophagus, stomach, small and large intestines and the liver, which has a uniform reddish-purple color. The heart is surrounded by the pericardium or pericardium; the kidneys are lobulated. The respiratory system is located in the upper part of the body cavity, the lungs consist of a pinkish-red sac with a mesh-like structure. The sexual organs are located in the middle of the upper part of the body, the testes of the male are relatively large. The ovaries of the female can be recognized by the spherical, white eggs in development, their size depends on the stage of development. Fully developed eggs are considerably larger; they are sometimes located in the oviduct.
Snakes, due to their elongated bodies, have only one functional lung to use the body space more efficiently, in addition, the kidneys and testes are an extension of each other. A number of snakes sometimes lack the left lung completely. Lizards, on the other hand, always have two equivalent lungs, and the kidneys and testes are also next to each other.
Skeleton and muscles
From nose to tail tip, the skeleton of the lizard consists of the skull, cervical vertebrae, shoulder girdle, forelimbs, ribbed thoracic vertebrae, pelvic girdle, hind limbs and finally the tail vertebrae. Snakes differ skeletally in a number of areas, for example, snakes do not have a shoulder girdle and only a few snakes have remnants of a pelvic girdle. Forked ribs sometimes occur in snakes and worm lizards but not in lizards.
Because of lizards’ build, their flexible skeleton and muscular limbs, many species are quite fast. A few species reach speeds of 24 kilometers per hour, rivaling (warm-blooded) mammals of equal mass and length. However, because of their different energy balance, lizards maintain such speeds for only a short time.
Lizards are internally similar to other reptiles and have the same organs and roughly the same build as, for example, bridge lizards and crocodilians. Lizards, like most reptiles, have a three-chambered heart, unlike crocodilians, birds and mammals that possess four-chambered hearts. The lizard’s heart consists of a left and right atrium, connected by a single ventricle. However, the ventricle is divided by a muscular septum, which forms a septum except for an opening.
The left side of the ventricle runs full of oxygenated blood from the left ventricle and pumps it into the body. The right side of the ventricle is supplied with deoxygenated blood by the right atrium and pumps it to the lungs where it is reoxidized. During contraction or systole, blood is pumped out of the ventricle with both sides of the ventricle connecting. However, while the heart fills up during relaxation or diastole, the ventricular opening is completely closed. This ensures that the oxygenated and deoxygenated blood remains separated, giving the single ventricle the same function as a double ventricle. As a result, the three-chambered heart of lizards still functions like a four-chambered heart as in mammals. One of the advantages of this is that higher blood pressure can be maintained.
Lizards use the cloaca to release urine and feces. Feces consist of a watery white mass (uric acid) and a brown thicker substance composed of the digested food particles. They have a well-developed urinary system; all species have paired, symmetrical kidneys. The waste products from the kidneys are concentrated into uric acid and released by the cloaca. The uric acid is poorly soluble in water and precipitates, requiring only a relatively small amount of water to excrete the uric acid. The secretion has a high viscosity and is visible as the bright white substance in the lizard’s feces. Because uric acid precipitates and is not soluble, the glomerulus, a part of the kidney that filters soluble substances from the blood, becomes redundant, and in a number of reptiles the glomerulus is completely absent.
Lizards generally have a urinary bladder which is a marked difference from snakes evolved from lizards. Snakes and crocodilians lack a urinary bladder. The bladder, especially in desert-dwelling species, is used as a water reservoir. Water is returned to the body when dehydrated to replenish the deficiency. In some species, such as the gila monster (Heloderma suspectum), water can be stored for weeks to months. Among several other adaptations in appearance and behavior, such as inactivity during drought, the bladder contributes significantly to the survival of lizards in very dry environments.
Lizards are generally carnivorous or carnivorous, they eat live prey and are usually not very picky. They will grab anything that moves, does not look repulsive or dangerous, looks edible and fits in the mouth. Because many species do not grow very large, most lizards eat mostly insects, snails and other small invertebrates. Larger species also do eat the eggs of various animals or small vertebrates such as mice or other lizards. Small conspecifics are not spared either; many lizards are very cannibalistic. Very large species such as monitor lizards and crusted lizards actively hunt larger prey such as somewhat larger rodents or birds.
Among the species with a special menu is the green iguana that hunts prey as a juvenile but once adult eats only plants and is herbivorous. It does become more common in some larger iguanid species (Iguania) that as they grow older the animals adopt a more vegetarian diet and eventually eat only plant parts such as flowers or fruits. The marine iguana lives on algae and dives into the sea to scrape it off rocks underwater. Caimanteju specialize in snails, some species of lizards live on ants and termites and are formicivorous. Only fish-eaters (piscivorous) are lacking among the lizards, with the exception of certain extinct marine reptiles such as the mesh lizards. The komodo dragon has occasionally attacked humans and sometimes hunts large vertebrates such as ungulates.
A few species stay still and wait for the prey to suddenly strike, for example chameleons and some iguanids. Most lizards are true hunters that actively forage and are attracted to moving prey. Many lizards track prey by using the tongue, see also under the skull section. The teeth serve mainly to hold the prey; lizards often do make some kind of chewing motion but this serves to kill the prey or reduce it slightly so that protrusions such as legs and wings are lost. However, they swallow the prey in one go.
Enemies and defense
Lizards are hunted by several enemies, the main ones being birds of prey, snakes and predatory mammals. Birds of prey pluck the lizard from the ground while many mammals chase the lizard into the burrow. Many species of larger lizards and snakes also have lizards on their menu. However, the main enemy is humans, who are destroying the natural habitat of many species through landscape change and desiccation.
Many species of lizards have gone extinct or are threatened by the introduction of animals such as dogs, cats, rats and pigs into their habitat, which hitchhike with humans. Dogs and cats hunt the lizards; rats and pigs dig up the eggs. An example of a species that has become extinct due to the introduction of exotics is the Rodrigues day gecko (Phelsuma edwardnewtoni).
A number of species of birds and snakes rely heavily on lizards as a food source. An example is the cat snake, whose venom is only effective on lizards; other animals are not susceptible to the venom.
Lizards have limited forms of passive defense such as certain colors. Many species of lizards with bright colors are mistakenly considered poisonous by humans and killed by local people. Only the two species of crusted lizards from Mexico are poisonous to humans and have yellow or pink to red colors. In all other lizards, the variegated colors play a role only in distracting enemies and luring the opposite sex during the reproductive season. The colors have nothing to do with danger or toxicity, unlike some venomous and brightly colored snakes such as coral snakes. Some species that look strikingly like snakes are therefore unfairly considered dangerous. They suffer in urban areas, while in nature there are lizards that clearly imitate snakes. With its brown color and black head, the black-headed scaled lizard imitates the highly venomous Australian taipan. This imitation of dangerous species is called Bates’ mimicry.
In a number of species, the tail of juveniles is bright red or blue in color making it extra conspicuous. However, this serves not only to distract enemies but probably to indicate that they are not yet sexually mature. As a result, sexually mature males do not see them as competitors which prevents them from attacking their own young.
The main active defense of lizards consists of rapid flight. In doing so, almost all species seek out burrows or crevices between rocks or walls and expand the ribs by filling themselves with air, causing them to become trapped. Many species do not hesitate to dive into the water and swim away. Some agamas are found high up in the trees and, when in danger, can float bits through the air with the skin membranes, sometimes reinforced with the ribs, an example being the flying dragon (Draco volans).
Only a few lizards have specialized threatening behavior, an example being the ruffed lizard (Chlamydosaurus kingii). This agame opens its mouth, putting on its impressive collar. If the attacker does approach, the agame sets off running, using only its hind legs.
A lizard that is cornered further often makes hissing to growling sounds. Geckos in particular are known for this but chameleons and many large lizards can also be noisy. Larger species will attempt to strike with the tail, aiming for the face. If an enemy succeeds in grabbing a lizard despite the speed and thumping, the defense depends on where on the body the lizard is grabbed.
When an enemy grabs the lizard’s body, the lizard bites off with the mouth full of small but sharp teeth, many species have a firm bite because they have to crack relatively hard prey. Virtually all species give a quick, powerful bite that is intended to produce a startle effect and then quickly move off. Few species are known to be very tenacious but an exception are species such as the tokeh (Gecko gecko) and the gila monster (Heloderma suspectum).
When an enemy grabs the lizard’s tail, it breaks off functionally in some species, this is called autotomy. See also under the heading tail.
For a long time, only two species of venomous lizards were known; the crusted lizard and the very similar and closely related gila monster. It has recently been discovered that there are as many other species, such as monitor lizards and iguanas, that produce a venom. Like the previously mentioned species, the venom is not transmitted by direct injection through fangs as happens with venomous snakes. However, we do find nine different types of venom in venomous lizards that are also found in snakes, such as rattlesnakes. The venom runs in grooves along the teeth and enters the prey by making chewing movements. It causes changes in blood pressure and lowers blood clotting, but is typically harmless to humans.
Larger lizards, in addition to venom, have permanent putrefactive bacteria in their mouths that can be life-threatening if they enter the blood. The komodo dragon is thought to use its dangerous bite to kill its prey; although it not infrequently escapes, the prey often succumbs later. The corpse is detected by the monitor lizard using its well-developed sense of smell. However, recent research suggests that its large venom glands may play a more important role after all.
Some iguanas have an unusual way of scaring off enemies; they increase the pressure in veins under the eyes until they snap and aim the blood jet at the face.
Reproduction of lizards generally takes place in the spring, allowing eggs to be deposited in early summer when it is warm. They hatch in late summer when there is enough food for the young. Lizards that live in tropical regions reproduce all year, many northern and southern species are not active all year because of the temperate climate. Many of these species hibernate where metabolism is reduced, body temperature is lower and heart rate slows sharply.
Males and females are often sexually dimorphic, meaning the male and female have slightly different characteristics. This can often only be seen in adults because juveniles do not yet have all the adult characteristics. Males often have a bulkier build, a larger head and a thicker tail. On average, males grow longer than females. In many species like chameleons that have horns or other protrusions, these are underdeveloped or completely absent in females. Especially during the mating season, the males of many species become brightly colored, a well-known example being the sand lizard (Lacerta agilis). In some species, however, there is little difference between the male and female, and because lizards have no external genitalia, the sexes are difficult to tell apart. By probing, looking at how deep the cloaca is, the sex can be determined. Despite the internal presence of the testicles and penis in males, the cloaca in males can be probed deeper. Often seen from the outside, the cloaca is more curved due to the presence of the male sex organs.
Gender can also be determined by looking at the shape and size of the pori femorales. The pori femorales are located on the thighs and serve to release odorants that lure the opposite sex. The pores are always markedly enlarged in males and secrete a hard, oily, yellow substance that contains various lipophilic chemical compounds such as esters and alcohols as well as steroids including cholesterol. The secretion has an irritating effect on the opposite sex. Because they lie on the underside of the body, the substance is probably given off to the substrate, similar to an olfactory flag. It is suspected that the female “reads” the chemicals in the secretion, so to speak, and thus obtains information about the health of the male and bases her choice of mate on this.
Interestingly, in many species that live in groups, the least dominant males become more similar to females; they remain smaller and typical male characteristics such as a thicker tail, coarser spine crest or larger head are less well developed. This is due to the stress they are subjected to but they have less to fear from the dominant males as a result. The reverse is also true, dominant females are more similar to males.
Unlike some amphibians, no lizard can change sex. However, some species do have virgin reproduction, from which only females are produced, a phenomenon that also occurs in amphibians and insects, among others. An example is the European species Darevskia armeniaca, which reproduces almost exclusively in this way.
Once a pair has found each other, mating takes place; lizards have internal fertilization. The male must make contact with the female’s cloaca, which is complicated by the tail. This is why male lizards have a “double,” actually split or forked penis. The whole is called a hemipenis, hemi meaning “half. The paired penis allows contact on both sides which facilitates mating because the male can make contact on both the left and right. Incidentally, the hemipenis serves only for sperm transfer, and is not used as a urethra. During mating, the male often bites the neck of the female. Things can get pretty violent with the animals appearing to be fighting, injuries are not rare in many species.
Some lizards such as some monitor lizards and chameleons are known to store sperm for a while after mating, only to use it later for fertilization. Most lizards lay eggs; typical egg-laying groups are iguanas and geckos. A healthy female produces eggs every year, regardless of whether mating or fertilization has occurred. Eggs are often not eaten during pregnancy, this is because there is simply no room for food in the abdomen, which is greatly distended during gestation. Like other reptiles, lizards are able to draw on their reserves for a long time, so the females are not harmed by pregnancy. Lizards that lay eggs often bury them shallowly in the soil so they are not visible to enemies but are easily warmed by the sun. Most species produce several dozen eggs a year, deposited in a single clutch. Some anoles produce only a single egg per clutch; among geckos, two eggs per clutch are common.
Lizards, like all reptiles, have no sex chromosomes; sex is determined in the egg by ambient temperature. At a certain temperature, which varies by species, primarily males hatch. At higher or lower temperatures, females hatch. The eggs must not be turned after deposition because this can cause the embryo to die.
Most lizards lay eggs and are oviparous, a few are ovoviviparous or ovoviviparous. This means that the eggs do not have a hard shell but a thin membrane from which the young sometimes hatch inside the dam before they are born. This often serves to ensure survival in cooler regions, where it is too cold for the eggs to develop, by the female giving off her body heat to the eggs she carries with her until they are fully developed. Well-known examples of lizards whose juveniles are born alive are the hazelworm and the viviparous lizard. Both species occur as far as the Arctic Circle because of this. A number of chameleons and some skinks are also oviparous. Some lizards even have complex embryonic development, which has much in common with placental development as found in mammals. One disadvantage of egg-living barrenness is the relatively long gestation period, during which the females must carry the unborn young with them. This prevents them from eating for extended periods of time and makes fleeing from enemies more difficult. Because the development of offspring is slower in viviparous species, females regularly withdraw from reproduction. As a result, offspring are not produced every year, but the animals can regain strength. Egg-laying species, on the other hand, can often produce several clutches per year.
After depositing eggs, the mother animal generally leaves the nest; brood care is quite exceptional in lizards. When the juveniles crawl out of the egg they are on their own. Opening the leathery to parchment-like egg involves the use of the egg tooth, an ossified protrusion on the snout that serves only to open the egg and is released after some time. Juveniles often have to watch out for larger conspecifics; many lizards are cannibalistic. An exception are some skinks where the mother animal protects the juveniles for a while. In most species, the juveniles have distinctly different markings to prevent the adult males from seeing them as competitors and attacking them.
Smaller lizards from tropical regions often mature after a year; in larger species or those living in more temperate regions, it can take many years for the lizard to reach sexual maturity. The life expectancy of lizards depends greatly on the group. For example, many chameleons do not live beyond about five years, larger species such as monitor lizards can reach an age of more than 20 years. Living conditions and natural decay also have an impact. Specimens that have experienced food shortages or suffered from parasites, competitors and enemies do not live nearly as long as animals kept in captivity under ideal conditions.
Lizards are cold-blooded or ectothermic which means they cannot directly affect body temperature themselves. Once warmed up they are much faster, so they warm up in the sun in both the morning and afternoon so they can hunt prey faster and be more alert to enemies. During the hottest part of the day, lizards often shelter in a cooler place such as under a rock. Night-active species do not sunbathe and are found only in areas with relatively high nighttime temperatures.
Because they have scaly armor they are well insulated on one hand but a disadvantage is that they cannot sweat to cool off. When a lizard is hot it seeks shade, during prolonged very hot periods the lizard retreats to burrows, under rocks or tree trunks for several weeks to months and an aestivation or summer hibernation is held to avoid overheating or dehydration. During this time the lizard is not active and the same is true during prolonged cool periods such as the harsh winters in more temperate areas, this rest period is called hibernation.
In subtropical areas, where neither very warm nor very cold seasons occur, lizards do not have rigorous alternations of periods of inactivity and activity but have a less active period, during which they do not go into dormancy but require less food, become a duller color and move around less actively. The end of the less active period usually heralds the beginning of the reproductive period; an example of such a species is the bearded dragon (Pogona vitticeps).
Lizards in culture
Lizards, because of their shy and unexpected behavior and their “smooth” scaly skin, are not very pettable. Because of their shyness, lizards have often fled before one has a chance to observe them and because of the good camouflage of many species, they are overlooked even at close range. Humans generally have a negative impact on lizards although there are exceptions. The habitat of many species that live in forests is destroyed and the trade in exotic animals also causes many animals to be captured and removed from their natural habitat.
Lizards do not play a major role in human daily life. Ecologically, lizards play a role as cleaners of various pests such as crickets, cockroaches and caterpillars.
Virtually all lizards are harmless to humans, despite any spines and bright colors. All lizards will bite if picked up, however, most species remain small (15-25 cm total length) and a bite will at most result in a “finger-with-a-lizard’s-ear. Many species are tenacious but do not penetrate the skin with the small teeth and can be easily dislodged. Some larger species (25-100 cm) also tend to have more powerful jaws and can inflict superficial wounds or painful biting with the teeth, notoriously the common and widely domesticated tokeh. Very large species, longer than a meter, can inflict deeper flesh wounds, consisting of lacerations and penetrations. Tetanus infection must be considered in many species that can damage the skin.
Only monitor lizards and lichen lizards can be classified as truly dangerous, and in the case of the monitor lizards this is mainly due to the dangerous bacteria in the mouth. Lizards have a venom that acts on the nervous system and because they bite very vigorously and are tenacious, the venom has greater efficiency. For dangerous lizards, see also under the heading enemies and defense.
Lizards are sometimes considered bushmeat, the most famous example being the green iguana. This lizard is sometimes referred to in Suriname as tree chicken because the meat is said to taste like chicken. Especially the tail is considered a true delicacy. In Africa and India, mainly spiny-tailed iguanas (Uromastyx) are on the menu. Compared to other reptiles, such as snakes and turtles, lizards play a modest role in ancient cultures and symbolism. An exception are the monitor lizards, whose terrifying stature makes them one of the best-known groups. Monitor lizards, according to an Indian legend, were used to add motifs to the walls of the Sinhagad fortress.
Because lizards usually remain small and the skin of larger species is often penetrated by spines and osteoderms (hard bumps), the skin is not as suitable for making leather. As a result, they have suffered much less hunting for skin than many snakes (snakeskin) and crocodilians (crocodilian). An exception are the monitor lizards, but these species are protected without exception and skin hunting is prohibited.