Evolutionary Pressures in Reptiles


The reptiles have striking adaptations for terrestrial life.

Adaptations in Chuckwalla (Sauromalus obesus)

It lives in the deserts of  Southwestern United  States. It has following adaptations:

(i) Adaptation in summer: It can survive in late summer at temperate above 40 degree centigrade. Chukwallas browse on plants. These plants wither. Therefore, chuckwallas aestivate to withstand these hot and dry conditions. Chukwallas disappear below ground during aestivation.

(ii) Adaptation for winter:

Temperature becomes model ate during the winter. But little rain falls. Thus lift in the desert is still not possible. Therefore,  summer sleep of chuckwalla enters into winter sleep. Rain fall started in March. Therefore, greenery and appears in the desert. The chukwalla comes out from sleep. The chuckwalla browses and drinks water. It stores large amount of water under its skin.


(iii) Defense from predators: Predators cannot prey chukwallas easily. If threatened, a chukwalla enters into rock crevice. It inflates lungs with air. It increases its forms a wedge entrance or the rock walls. There is friction of its the rocks.  Therefore, chukwalla cannot be removed from rock.



Skin of reptiles has no respiration functions. Reptilian skin is thick, dry  or keratinized. Scales are modified for various functions.  For example, the snakes have large belly scales. The scales provide contact with the substrate during locomotion. Reptilian skin is less glandular than that of amphibians. Skin glands secrete phermones. These pheromones functions in sex recognition and defense.


The chromatophores of reptiles are dermal in origin. Cryptic coloration,  mimicry and aposematic coloration occur in reptiles. Colors also function in sex recongnition and thermoregulation.


The process in which reptiles periodically shed their outer epidermal layers of then is called ecdysis. All reptiles undergo ecdysis. The blood supply to the skin does not move in the epidermis. The outer epidermal cells lose contact with the blood supply and die. The lymph moves between moves  the inner and outer epidermal layers. It loosens the epidermis. Ecdysis begins in the head region. The epidermal layers come off in one piece in many lizards and snakes . In other lizards, epidermal layers broken into  small pieces. The frequency of ecdysis is different in different species . It occurs more in young than in adults.

Suppport and Movement


The reptailians are inherited skeleton from ancient amphibians. The skeletons of reptiles show many notifications. The skeleton is highly ossified. Thus it provides greater support.

1. Skull: Their skull is longer than that of amphibians. They have                    secondary palate. Secondary palate partially separates the nasal passages from the mouth cavity. Palate was evolved in archosaurs. It was an adaptation for breathing when the mouth is full of water or blood. It is also present in other reptiles. They also have longer snouts. It increases the sense of olfaction.


2. Vertebrae: Reptiles have more cervical vertebrae than amphibians. The first two cervical vertebrae are atlas and axis. They provide greater freedom of movement to head. An atlas articulates with a single condyle on the skull. It helps in nodding. Axis is modified for rotational mo ements. They have different number of cervical vertebrae. It provides additional neck flexibility.

3. Ribs: The ribs of reptiles may be highly modified. The ribs of snakes have muscular connections to large belly scales. It helps in locomotion. The cervical vertebrae of cobras are attached with some special ribs. Cobra flares these ribs in aggressive displays.

4. Pelvic girdle: The pelvic girdle is attached to the vertebral column by two or more sacral vertebrae.

5. Autotomy: The caudal yertebrae ot man !um ds possess a vertical fracture plane. If a lizard is grasped by the tail, caudal vertebrae are broken. Therefore a portion of the tail is lost. The loss of tail is called autotomy. Autotomy is an adaptation that allows a lizard to escape from a predator. Sometimes, the predator runs away from lizard after seeing its broken moving tail. The lizard later regenerates the lost portion of the tail.


There are three types of locomotion in reptiles:

1. Locomotion in primitive reptiles is similar to salamanders. The body move low between paired appendages. The appendages extend laterally and move in the horizontal plane.

2. The limbs of other reptiles are elongated and slender. They remain closer to the body. The knee and elbow .joints rotate posteriorly. Thus, the body  moves higher from the ground. Thus the legs support the both vertically.

3. Many prehistoric reptiles were bipedal. The walking on the hind limbs is called bipedalism. They had a narrow pelvis. Ihey have a heavy outstretched tail for balance. Bipedal locomotion treed the front appendages. Thus these appendages are used capturing of prey or flight in some animals.


Tongue: Most reptiles are carnivores. But turtles eat almost all the things. The tongues of the turtles and crocodilians are nonprotrusible. It helps in swallowing. Some lizards and the tuatara late sticky tongues. It is used or capturing the prey. The extended tongue of chameleons exceeds their body length.


Modification in snakes for swallowing

The skulls of snakes are greatly modified for feeding. The bones of the skull and jaws are attached loosely. These bones move away from for ingestion of prey. In this way, snakes can ingest larger than a snake’s normal head size. The bones of the upper jaw on the skull. The halves of both of the upper and lower jaws are attached loosely by ligments at anterior side. Therefore, each half of the upper and lower jaws can move independently. Opposite sides of the upper and lower jaws are moved forward and retracted alternately alter the capturing of prey. Their teeth are posteriorly pointed. These teeth prevent the prey from escaping. They also force the food into the esophagus.The glottis is much forward in snake. Thus they can breathe during swallowing of prey.

Evolutionary Pressures in Reptiles

Biting apparatus and biting mechanism

1. Vipers

Vipers possess hollow fangs. These fangs are present on the maxillary bone at the anterior margin of the upper jaw. These fangs are connected to venom glands. The Maxillary Bone of the vipers is hinged. It can moved backwards. Thus when the snake mouth is closed the fangs fold back and it lie along the upper jaw. When the mouth opens, the maxillary bone rotates. It swings down the fangs. Thus the fangs project outward from the mouth. Now vipers may strike at objects.

2. Rear fanged snakes

Rear-fanged snakes have groove in rear teeth. Venom is passed through grooves and injected into the prey during swallowing.These snakes usually do not bite.Therefore,  they are harmless to humans. However, the African boomslang (Dispholidus typus) have killed men.

3. Coral snake, sea snake and cobra

The fangs of coral snakes,  sea snakes. and cobras are attached to the upper jaw. It remains in an erect position in opened mouth. The fangs lit into a pocket in the outer gum of the lower jaw when the mouth is closed. Fangs have a groove or it is hollow. The muscles of the venom glands contract and inject venom into the fangs. Some cobras can spit venom at its prey. This venom may cause blindness.


Venom  glands are modified salivary glands. The venoms of most snakes are mixtures of neurotoxins and hemotoxins.

1. Neurotoxin: Neurotoxin attacks on nerve centers. It causes respiratory paralysis. The venoms of coral snakes, cobras, and sea snakes are neurotoxins.

2. Hemotoxins: Hemotoxins break blood cells. It attacks blood vessel linings. The venoms of vipers are primarily hemotoxins.




Th circulatorysystem of reptiles is based on amphibians. The blood of reptiles must travel under high pressures.

1. The reptiles possess two atria. These atria are completely separated in the adult. Veins from the body and lungs open into them. The sinus venosus is absent in reptiles except in turtles. It has become a patch of cells and act as a pacemaker.

2. The ventricle of most reptiles is incompletely divided. The ventricular septum is complete only in crocodilians.

3. The ventral aorta and the conus arteriosus divide during development. They form three major arteries that leave the heart.

 (a) A pulmonary artery: It leaves the ventral side of the ventricle. It takes blood to the lungs.                                   •

(b) Two systemic arteries: One systemic artery arises from the ventral side of the heart. Second systemic arterv arises from the dorsal side of the heart. It takes blood to the lower body and the head.


Circulation of blood

The deoxygenated blood enters into the ventricle from the right atrium. It leaves the heart through the pulmonary artery and moves to the lungs. Pulmonary veins bring oxygenated blood from lungs and transfer it into left atrium. Blood then enters into the ventricle from. It leaves the heart through left and right systemic arteries.

Mixing of blood: An adaptation

There is incomplete separation of the ventricle in most reptiles. The pulmonary artery contracts and some blood moves from pulmonary circuit to the systemic circuit. All reptiles do not breathe constantly. Therefore, the movement of blood from pulmonary circuit to systemic circuit has advantage for reptiles. The breathing by lung stops when turtles withdraw into their shells. They also stop breathing during diving. During periods of apnea (“no breathing), blood flow to the lungs is limited. It conserves energy. It allows more efficient use of the pulmonary oxygen supply .


Reptiles exchange respiratory gases through internal respiratory surfaces. Thus they do not lose large quantities of water. Larynx is present in them. However, vocal cords are absent in them. Cartilages support the respiratory passages of reptiles. Their lungs are partitioned into sponge like interconnected chambers. Lung chambers provide the large surface area or gas exchange.

Mechanism of respiration in most reptiles

Negative-pressure mechanism is responsible for lung ventilation. A posterior movement of the ribs and the body wall expands the body cavity. It decreases pressure in the lungs. Thus lung draws air into the lungs. Elastic recoil of the lungs and forward movements of the ribs and body wall compress the lungs. Thus air is expelled out of it.

Mechanism of respiration in turtles

The ribs of turtles are a part of their shell. Thus movments of the body wall and ribs arc impossible. Therefore, turtles exhale by contracting muscles. These contractions force the viscera (Internal organs) upward and compress the lungs. They inhale bv contracting muscles that increase the volume of the visceral cavity. It creates negative pressure lung. This pressure draws air into the lungs.




The terrestrial animals face high temperature (65 to 70°C). This temperature is not suitable for life. Thus temperature regulation is important in terrestrial animals. Reptiles inn be:

1. Ectotherms: The animals which use external heat sources for thermoregulation arc called ectotherms. Most reptiles are ectotherms.

2. Endotherms: The animals which generate internal heat during metabolism are called endotherms. Some reptiles like monitor lizards and brooding Indian pythons arc endotherms. Female pythons coil around their eggs. It raises its body temperature by 7.3oC above the air temperature. It uses metabolic heat to raise this temperature.

Reptiles regulate their body temperature by following methods.

1. Heat regulation by hibernation and aestivation: Some reptiles can survive in wide temperature fluctuations (2 to 41o C or some turtles). However, body temperatures are regulated within a narrow range between 25 and 37oC. If they are unable to maintain this range, they remain within the range in this retreat.

2. Behavioral methods of heat regulation: Most thermoregulatory activities of reptiles are behavioral. A lizard orients itself at right angles to the sun’s rays to warm itself. It presses its body tightly on a warm surface to absorb heat by conduction. A lizard orients its body parallel to the sun’s rays to cool itsels. It seeks shade or burrows. It take its body erect prostrate (legs extended and tail arched) to reduce conduction from warm surfaces. Many reptiles are nocturnal in hot climates.

3. Physiological methods of heat regulation: Various physiological mechanisms also regulate body temperature. Some reptiles use panting  for releasing neat. Panting releases heat through evaporative cooling. Marine iguanas absorb heat by basking in the sun. It divert blood to the skin and arm up quickly. Marine iguanas reduce heart rate and blood flow to the skin during ing into the ocean. It slows down heat loss. Chromatophores also help in emperature regulation. Dispersed chromatophores increase the rate of heat bsorption.

4. Heat regulation by torpor: Many temperate reptiles withstand cold winter temperatures by entering into torpor. Torpor is an inactive stage. The body temperatures and metabolic rates decrease during torpor. The body temperatures of reptiles in torpor are not regulated. It is a difference from the true hibernators.

5. Heat regulation by hibernacula: The solitary reptiles migrate to a common site and spend winter there. These animals clumped together. Heat loss from these groups is called hibernacula. Exposed surface area reduces hibernacula. Sometimes, the animals can freeze and die in cooler winter. Death from freezing is an important cause of mortality for temperate reptiles.


The brain of reptiles is similar to the brains of other vertebrates. The cerebral hemspheres are larger than amphibians. This increase of size of brains has improved the  smell. The optic lobes and the cerebellum are also enlarged. It shows that reptiles much depend on vision. They have better coordination of muscle functions.

Sense organs

The reptiles have complex sensory systems. It is evidenced by a chameleon’s method of feeding. It has protruding eyes. Its eyes move independently. Each eye has different field of view.  Initially, the brain keeps both images separate. But when they an insect, both eyes converge on the prey. As a result binocular vision is formed. It helps chameleon to determine whether the insect is within range of the chameleon s tongue.

1. Eyes

(a) Focusing mechanism: Vision is the dominant sense in most reptiles. Their eves are similar to amphibians. Snakes moves the lens forward for focusing the nearby objects. Iris contract and places pressure on itrcous body. Vitreous both is gel-like in the posterior region of the eye. The displacement of this gel pushes the lens forward. All other reptiles have different method to locus on nearby objects. Their ciliary muscles press the ciliary body against the lens. It changes the shape of lens from elliptical to more spherical. The spherical lens is used for focusing on nearby object. Reptiles have a greater number of cones than amphibians. Thus they have well-developed color vision.

(b) Protection: The eyes of reptiles have upper and lower eyelids, a nictitating membrane and a blood sinus. These structures protect and cleanse the surface of are eye. In snakes and some lizards, the upper and lower eyelids fuse in the embryo. It forms a protective window of clear skin called the spectacle. The blood sinus is present at the base of the nictitating membrane. It swells with blood and force debris to the corner of the eye. It is rubbed out from this corner. Horned lizards rupture this sinus and blood come out from it. It is a defensive act to confuse the predators.

(c) Median eye: Some reptiles possess a median (parietal) eye. This eye is developed from outgrowths of the roof of the forebrain. In the tuatara, this eye has a lens,  nerve and a retina. The parietal eye is less developed in other reptiles. Parietal eyes are covered by skin. Thus it cannot form images. However, parietaleye can differentiate light and dark periods. Thus it is used to locate the position of the sun.

2. Ear

The structure of reptilian ears varies. The ears of snakes detect substrate vibrations.They lack a middle ear cavity, an auditory tube and a tvmpanic membrane. A bone of the jaw articulates with the stapes. The jaws and stapes receive substrate vibrations. Snakes can also detect airborne vibrations. In other reptiles, a tympanic membrane is present on the surface. Or it may be in a small depression in the head. The inner ear of reptiles is similar to amphibians.

3. Olfactory senses

Olfactory senses are better developed in reptiles than amphibians. They have partial secondary palate. It provides more surfaces for olfactory epithelium. Many reptiles possess blind- ending pouches. This pouch opens into the mouth cavity through the secondary palate. These pouches are called Jacobson’s (vomeronasal) organs. These organs are present in diapsid reptiles. However, they are best developed in the squamates. Jacobson’s organs develop in embnonie crocodilians. But it degenerate in adults. Anapsids (turtles) lack these olfactory organs. The protrusible,  forked tongues of snakes and lizards are accessory olfactory organs. It detects chemicals present in air. A snake’s tongue come out and then moves to the Jacobson’s organ. Jacobson detect odor molecules. Tuataras use Jacobson’s organs to taste objects present in its mouth.


4. Pit organs

Rattlesnakes and other pit vipers have pit organs. Pit organs are present on each side of the face between the eye and nostril. It is a heat-sensitive organ. Pit organs form depressions. These depressions are lined with sensory epithellium. These are used to protect objects with temperatures different from the snake’s surroundings. Pit vipers are nocturnal. Their pit organs help them to locate small, warm-blooded prey.



Excretory organs: The kidneys of embryonic reptiles are similar fishes and amphibians. Terrestrial animals have larger body size. They have higher metabolic rates. However, kidneys are capable of processing wastes with little water loss. Their kidneys have in many nephrons. The functional unit reptiles are called metanephric kidneys. Their function depends on a circulatory system. It delivers more blood to kidney at greater pressures. Thus kidney filters large quantities of blood.

Mechanism of excretion: Most reptiles excrete uric acid. It is nontoxic and insoluble in w tier. It precipitates in the excretory system. The urinary bladder or the cloacal absorb water. The uric acid is stored in them in a paste like form. Nontoxic uric acid be stored in egg membranes. Thus it has made possible the development of embryos in terrestrial environments.


There are many adaptations in reptiles to reduce water loss by evaporation. These are:

1. Their excretory system reabsorbs water.

2. They have internal respiratory surfaces.

3. They have impermeable exposed surfaces.

4. The behaviors that help regulate temperature also help conserve water.

5. Most reptiles are nocturnal. They do not come out hot day time. The burrowing at du% time reduces water loss.

6. When water is available, many reptiles store large quantities of water in lymphatic spaces. Lymphatic spaces are present under the skin or in the urinary bladder.

7. Many lizards possess salt glands below the eyes. These glands remove excess salt from the body.


Vertebrates have internal fertilization and the amniotic egg. It has adapted them completely on land. The amniotic egg is not completely independent of water. Pores are present in the eggshell. They allow the gas exchange. But it allows water to evaporate. Amniotic eggs require a large amount of energy expenditures. This energy is provided by parents in the form of stored food. Parental care occurs in present in some reptiles. They maintain high humidity around the eggs.


Reptiles have internal fertilization. Fertilization occurs in the reproductive tract of the female. Then protective egg membranes are formed around the eggs. All male reptiles possess an intermittent organ. It transfers sperm into the female reproductive tract. Intermittent organs are absent in tuataras. Lizards and snakes possess paired hemipenes at the base of the tail. Hemipenes are erected by turning inside out, like a linger of a glove.

Gonads lie in the abdominal cavity. A pair of ducts transfers sperms into the cloaca in males. The female may store sperms in seminal receptacle after copulation. Secretions of the seminal receptacle nourish the sperm. Sperm may be stored for up to four years in some turtles, and up to six years in some snakes. Sperm can be stored for winter in temperate latitudes. The individuals grouped in hibemacula in the fall and copulation take place. Female stores sperms. Fertilization and development occur in thu spring. Fertilization occurs in the upper regions of the oviduct. Oviduct opens into cloaca. Glandular regions of the oviduct secrete albumen and the eggshell. The shell is tough and flexible. The egg shell is calcareous and rigid in some crocodilians.


Parthenogenesis occurs in six families of lizards and one species of snakes. In these species, no males are present. Populations of parthenogenetic females have higher reproductive rate than bisexual populations. A large population of reptiles died in the cold winter. The surviving reptiles can repopulate rapidly in winter.

Reproductive behaviour

Reptiles have complex reproductive behaviour. Males actively seek females. Courtship behaviour helps in sexual recognition. It is involved in physiological preparation for it production.

(a) Some males display head bobbing. These males have bright patches of color on the throat and enlarged folds of skin.

(b)  Courtship in snakes is based on tactile stimulation. The male displays tail-waving activity. It brings it chin along the female. Then it entwines his body around her. Then male produces wavelike contractions that pass from posterior to verior side of the body.

(c) Recent research indicates that lizards and snakes also use sex pheromones.

(d) Voclizations are important only in crocodilians. During the breeding season, males bark or cough. It is a territorial warning to other males. Roaring vocalizations also tract females and mating occurs in the water.

Parental care

Most reptiles freely lay eggs. They do not care about them. Turtles bury their eggs on the ground or in plant debris. Other reptiles lay their eggs under rocks, in debris, or in burrows. About one hundred species of reptiles show parental care of eggs.

One example is the American alligator, Alligator mississippiensis. The  female builds a nest or mud and vegetation. It is about 1 m high and 2 m in diameter. She hollows out the cell of the mound. She partially fills it with mud and debris. She deposits her eggs in the cavity and then covers the eggs. Temperature within the nest influences the sex of the hatchlings. Temperature at or below 31.50 C produce females offspring. Temperatures between 32.5 and 33° C produce male offspring. Temperatures around 32°C result in both male and female offspring. Similar temperature effects on sex determination are found in some lizards and many turtles. The female remains near the nest throughout the development and protect the eggs from predation. She frees young from the egg shell. Then she picks them up in her mouth, and transfers them into water. She forms shallow pools for the young and remain with them for up to two years. The female feeds on small vertebrates and invertebrates and drops the food for young. The young scraps this food.

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