Animals belonging to the class Reptilia are collectively known as Reptiles. This class consists of the amniotes, a group of vertebrates that are neither mammals nor birds. The amniotes are characterized by their eggs that feature an amnion or a double membrane allowing the embryo to respire on land.
Other features that help to distinguish reptiles from other tetrapod animals are their scute or scale covered skin and their cold-bloodedness. They are found all over the world from rainforests to tropical and temperate regions.
According to the taxonomy, they belong to the kingdom Animalia, phylum Chordata and the clade Amniota.
The Reptilia class is further divided into several extant sub-groups:
Testudines: There are around 330 species in this sub-group, including turtles, tortoises and terrapins.
Sphenodontia: This subgroup includes the two living tuatara species from New Zealand.
Squamata: This is the biggest sub-group of the Reptilia class, having over 9,000 species including lizards, worm lizards and snakes.
Crocodilia: This subgroup consists of 25 different species including crocodiles, alligators, caimans and gavials.
Reptile Defense Mechanism
The useful defenses of these animals help them to survive in their wild habitat. The defense mechanism can vary from one sub-group to another.
Camouflage
Birds and larger Reptiles are among the main predators of many smaller lizards and snakes. The brown, green and mottled green colorations of the small animals help them to remain camouflaged in their natural habitat and avoid detection of their predators, who have well-developed color vision.
Tail Shedding and Regenerating
Various lizards like skinks and geckos can shed their tail when captured by it. This process, called autotomy, enables these animals to run away by distracting their enemies. The detached tail continue to wiggle which makes the predator think that the lizard is still struggling, keeping their attention from the escaping prey.
The shed tail can be regenerated within a few weeks, but it is smaller than the original one and looks quite different from the rest of the body.
Defense in Snakes
The principal defense mechanism in various snake species is their ability to deliver poison through their fangs. Poison glands located inside their mouth produce deadly venom that helps the creatures to protect themselves from enemies.
Evolution of Reptiles
These animals originated around 320 million to 310 million years ago with the first reptiles evolving from the advanced reptiliomorph labyrinthodonts. Animals from the Casineria genus are the earliest animals suspected to be amniotes rather than advanced amphibians. The origin of reptiles occurred in steaming swamplands of late Carboniferous period.
Fossils found in Nova Scotia show footprints with imprints of scales and typical reptilian toes. The prints have been attributed to Hylonomus which is the earliest recognized reptile. Hylonomus was a lizard-like creature (8 to 12 in long) with sharp teeth that indicates an insectivorous diet.
Earlier, the bigger labyrinthodont amphibians like Cochleosaurus largely overshadowed the reptiles, who remained inconspicuous until the CRC or Carboniferous Rainforest Collapses. This significant extinction event affected the existence of many large animals with the Amphibians being among the worst affected populations. But, the reptiles coped better with the drier conditions post CRC.
One of the biggest problems the Amphibians faced was the lack of water bodies which prevented them from reproducing. This did not affect reptiles as they were able to lay eggs on dry lands. Gradually, the reptiles became a dominating class with new diet strategies including both carnivory and herbivory. This established the foundation for the Mesozoic stage, also referred to as the Age of Reptiles.
The genus Mesosaurus from the Permian period is counted among the most recognized early reptiles.
Reptile Anatomy
Eyes
Most Reptiles are unable to see properly during nighttime as their vision is mainly adapted to the daylight conditions. They have color vision with the visual depth perception being much more advanced than Amphibians and many Mammals.
The vision is reduced in species like the Blind Snake, while some snakes have extra visual or sensory organs that make them sensitive to heat and infrared radiation.
Skin
The horny epidermis layer makes their skin watertight, allowing these animals to inhabit dry land. Reptiles have thinner skin compared to mammals and it also lacks the dermal layer present in mammal skin. The exposed skin areas are covered in scutes or scales which may have a bony base, creating their armors. In turtles, a hard shell made up of fused scutes covers the entire body.
Respiratory System
Reptiles use their lungs for breathing. The skin of the aquatic turtles is more permeable for allowing them to respire while the cloaca is modified in various species to increase the gas exchange area. Despite these adaptations, lungs remain a very important part of their respiratory system.
The main Reptile groups accomplish lung ventilation in different procedures. Squamates are known to ventilate the lugs mainly by their axial musculature. Certain lizard species are capable of buccal pumping apart from the normal axial breathing. The proto-diaphragm in Tegu lizards separates their pulmonary cavity from visceral cavity, helping with their respiration by allowing greater lung inflation.
The muscular structure of the diaphragm in the Crocodilians species resembles that of various mammals. However, there are some differences in their diaphragmatic setup.
Circulatory System
Most of these animals have three chambers in their heart _ two distinctly divided atria and one ventricle, which may be variably partitioned in different species. They also have two aortas playing a major role in their systemic circulation. The oxygenated and deoxygenated blood may get mixed with each other in their three-chambered heart with the level of mixing depending on the species and the physiological state of the animal.
Their circulatory system is capable of shunting back the deoxygenated blood to the body and the oxygenated blood to the lungs if necessary.
Unlike other Reptiles, animals in the crocodilian subgroup have four-chambered hearts. But, their two systemic aortas are only capable of bypassing their pulmonary circulation. On the other hand, the three-chambered hearts in various lizard and snake species can function as the four-chambered ones during contraction.
Digestive System
Majority of these animals have short digestive tracts because their diet mainly consists of meat, which is very simple to digest. Their digestion process is slower than that in mammals due to their inability of mastication and their low metabolism rate while resting. The energy requirements for their poikilotherm metabolism are very low which allows large animals from this class such as various constrictors and crocodiles to survive for months from one large meal, digesting it slowly.
Herbivorous reptiles are also unable to masticate their food, which slows down the digestive process. Some species are known to swallow pebbles and rocks that help in grinding up plant matters within the stomach, assisting their digestion.
Nervous System
The basic nervous system in the Reptiles is similar to that in the Amphibians. But, Reptiles have slightly larger cerebrum and cerebellum. Most of the important sensory organs are properly developed in these creatures.
However, there are certain exceptions such as the absence of external ears in snakes (they have the inner and middle ears). Reptiles have twelve cranial nerve pairs. They have to use electrical tuning for expanding the range of their audible frequencies because they have short cochlea.
These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter. However, the brain development can be more complex in some larger Reptiles. Modern species also have pineal glands in their brains.
Skeletal System
Most of these animals are tetrapods, meaning they have four legs. Snakes are examples of legless Reptiles. Their skeletal system is similar to other tetrapods with a spinal column supporting their bodies.
Excretory System
Their excretory system consists of two small kidneys. The diapsid species excrete uric acid as the principal nitrogenous waste product. But, turtles excrete mainly urea. Some of these species use their colons for reabsorbing water, while some are able to absorb the water stored in their bladders. Certain Reptiles excrete the excess salts in their bodies through the lingual and nasal salt glands.
Characteristics of Reptiles
Reptiles have certain characteristic features that help in distinguishing them from Amphibians, Mammals and Aves:
Respiratory system: Unlike Amphibians who have gills during their juvenile stage, Reptiles breathe through lungs at all stages of their lives.
Circulatory System: Apart from crocodiles, all Reptiles have three-chambered hearts.
Excretory System: Unlike Birds and Mammals, their kidneys cannot produce liquid urine with higher concentration than the body fluids. This happens because these animals lack the special structure known as Henle’s loop, which is located within the nephrons of Mammals and Birds.
Body Temperature Control: Reptiles are cold blooded animals, meaning their body temperature is controlled by their surroundings. Due to this reason, they need to bask in the sun regularly for keeping themselves warm.
Skin: Their characteristic tough and scaly skin preserves the body heat while also preventing loss of moisture.
Sensory Organs: Many reptiles are blind or have primitive eyes. Some snake species are blind and use their tongues to understand their surroundings and to recognize the scent of potential prey. Certain Reptilian species have eardrums located close to their eyes to help them sense their prey.
Physical Development: Unlike Amphibians, Reptiles do not go through metamorphosis in order to develop into adults. The juveniles are born as miniature versions of adults with the same physical characteristics.
Reptile Habitat
They are capable of adapting to almost all kinds of habitats and environmental conditions, except for extremely cold regions. These animals can inhabit dry deserts, forests, grasslands, wet meadows, shrub lands and even marine habitats.
Desert Habitat
Reptiles are capable of adapting to extremely high temperatures because they are cold blooded. Various snakes including the Rattle Snakes and King Snakes as well as different lizards like the Gila Monsters live in desert habitats.
Grassland Habitat
Grassland is another common type of habitat for various snakes and lizards (e.g. Garter Snakes, Fox Snakes). The vegetation in this habitat attracts many insects and rodents, making it easier for the Reptiles to catch prey.
Wetland Habitat
Swamps and large water bodies are inhabited by different Reptiles such as crocodiles, alligators, certain turtles and snakes. Water-dwelling species include the American Alligators, Red-Eared Turtles and Mud Snakes.
Marine Habitat
Animals like the Saltwater Crocodile and Marine Iguana inhabit seaside, travelling in and out of ocean as necessary. Some species, such as the Sea Snakes and Sea Turtles, live in the ocean. They leave the waters only during the breeding season for laying eggs.
Reptile Reproduction
These animals typically practice sexual reproduction with some specific species using asexual reproduction. Majority of these animals are amniotes, laying eggs covered with calcareous or leathery shells. The eggs are generally laid in underground burrows dug by the females.
The viviparity and ovoviviparity modes of reproduction are used by many species such as all boas and many vipers. However, the level of viviparity may vary with some species retaining their eggs until shortly before hatching while others nourish the eggs for supplementing the yolks. In some Reptile species, the eggs do not have any yolk with the adults providing all the necessary nourishment through a structure resembling the mammalian placenta.
Six lizard families and one snake family from the Squamata sub-group are known to be capable of agamogenesis or asexual reproduction. In some squamate species, the females are capable of giving birth to unisexual diploid clones of themselves. This type of asexual reproduction is known as parthenogenesis, occurring in various teiid lizards, geckos and lacertid lizards. Komodo dragons have reproduced through parthenogeny in captivitiy.
Reptile Life Cycle
Like many mammals, birds and Amphibians, their embryonic life consists of an amnion, chorion, as well as an allantois. The incubation period may vary depending on the species and other factors like the temperature of the surroundings.
Usually, hatchlings are able to take care of themselves almost immediately after coming out of the eggs. But, the females of some species are known to protect their eggs and hatchlings. For example, female Pythons coil themselves around the eggs in order to protect them and regulate their temperature. Similarly, crocodiles are known to guard their young after the eggs hatch.
TDSD or temperature-dependent sex determination can be observed in many Reptiles. In TDSD, the incubation temperature determines the sex of the offspring. This characteristic is most commonly seen in crocodiles and turtles, but can also occur in tuataras and lizards.
Reptile Diet
Different food habits can be observed between the four sub-groups. Animals belonging to the Crocodilia, Squamata and Sphenodontia sub-groups are carnivores, feeding on a wide range of prey from vertebrates to fish and insects. The Testudines are herbivorous in nature with their diet comprising of fruits, shrubs, leaves, grass and marine plant materials like kelp and algae.
Reptile Conservation
The populations of many Reptilian species are facing rapid decline due to various factors like deforestation, loss of habitat, hunting for hide and to use them for edible purposes. Many Testudines have been categorized as Threatened or Critically Endangered by the IUCN. Various Reptile populations have faced extinction in some specific locations. Due to these reasons, many species are protected by law in most of the countries where they are found.
Reptiles as Pets
Many Reptile species, including various lizards and turtles, are very popular all over the world as exotic pets. There is a widespread popularity even for the venomous snakes, especially among keen animal lovers. However, there is a common misconception that these animals do not need as much care as required by mammal pets. The truth is that, Reptiles need to be taken care of properly; otherwise, they cannot survive in captivity. They should be kept in large tanks or cages where they can move freely.
It is not advisable to keep more than one to two animals in a single enclosure. Their tanks should have suitable substrate and the animals should be provided with special heating lamps to allow them to bask and maintain their body temperature. The temperature of the tanks needs to be monitored with the help of thermostats. Heating pads can also be used for this purpose.
Custom made cages can also be used for this purpose as these mimic the natural habitat of a specific species in the best manner possible. It is advisable to place stones, pieces of wood and small plants in their tanks to provide them with basking spots. Using a fogger to mist the enclosure may be advisable for certain species. When keeping snakes or poisonous lizards as pets, one should keep in mind not to handle them with bare hands.
One should do extensive research and read appropriate books regarding how to best take care of them before petting any Reptilian animal. These pets can survive as much as mammalian pets of similar size when properly taken care of.
