Introduction
Reptiles are often referred to being “cold-blooded”, which can be misleading. More appropriately they should be considered poikilothermic or ectothermic, which means that unlike mammals and birds reptiles are unable to regulate their body temperatures internally and change their body temperature in adaptation to their environmental temperature. Because reptiles do not need to expend as much energy heating their bodies, they have a much lower metabolic rate than that of mammals. Each reptile species has what is referred to as its preferred optimal temperature zone which is a narrow temperature range at which they are active and undergo typical functions such as feeding, digestion and reproduction. Outside of this range these functions may be hindered or cease altogether. Some species will hibernate during colder months and during this time their metabolic rate will decrease.
Reptiles belong to the Class Reptilia, which can be broken further down into 4 Orders: Squamata, Chelonia, Crocodilia, and Tuatara.
Tuatara
The Tuatara is an ancient type of reptile only found in New Zealand and is a protected species only rarely seen in captivity in zoos or very special private collections.
Crocodilia
Crocodilians include crocodiles, alligators, and caiman. Members of Crocodilia are illegal to own as pets in Arizona.
Squamata
The order squamata can be broken down into 2 suborders: Serpentes (snakes) and Sauria (lizards).
There are over 2,700 species of snakes worldwide and snakes can be found on every continent besides Antarctica
. While the obvious answer to how snakes differ from lizards might appear to be that they lack limbs, it gets confusing when you consider legless lizards. Thus, to be more precise, snakes differ from lizards by lacking external ears and lacking remnants of forelimbs. That said, some snakes do have vestigial hind limbs, which can be seen externally as a pair of spurs on either side of their cloaca. Snakes also lack eyelids. Instead their eyes are protected by a clear scale known as a spectacle, which is shed at the same time as the snake sheds its skin.
All snakes are carnivorous, most feed on rodents, lizards, birds, and even eggs. Since snakes are unable to chew or take bites of their meals, they must swallow their food whole. To do this they have a very flexible skull and jawbones. Snakes can easily consume an animal as big as they are wide. The majority of snakes are non-venomous and kill their prey by constriction. Those that are venomous produce venom in glands located behind the eyes and inject it via fangs by biting. Venomous snakes should be discouraged as pets due to the risk of injury to humans.
Most snakes lay eggs (oviparous), with the exceptions of nearly all boas and some vipers such as rattlesnakes who give live birth (viviparous). Snake eggs are oval shaped and have flexible, leathery shells.
Snakes are largely non-social and live solitary lives, rarely seeking out company outside of the breeding season.
Lizards far outnumber snakes with nearly 5,000 different species worldwide. And like snakes, lizards can be found on every continent except Antarctica. Lizards have well developed color vision and communicate via body language and pheromones. Most lizards are solitary animals and keep a loose territory. Most lay eggs (oviparous) but some do give live birth (viviparous). There are two species of venomous lizard, the Gila monster and the beaded lizard.
Some lizards have unusual adaptations. Many species of gecko have adhesive lamina on their toes to enable them to climb on sheer surfaces. Many geckos also lack eyelids and, similar to snakes, have a spectacle.
Many species of lizard have the ability to drop their tails, an adaptation referred to as autonomy. Most lizards will regrow their tails with the exception of Rhacodactylus geckos. Lizards that do not practice autonomy will not regrow a lost tail, however.
Both snakes and lizards shed their skin as they grow. Snakes shed their skin in one large piece, while lizards often do so in multiple smaller pieces. Proper shedding is very important to their health.
Chelonians
This suborder includes turtles, terrapins and tortoises. Turtles most accurately should be used to describe sea turtles, which are not kept as pets due to their size and care requirements. Terrapins are the freshwater and land dwelling species that are what we commonly refer to as turtles. Terrapins are omnivorous to carnivorous. Tortoises are strictly land dwelling and predominantly herbivorous. Often, the confusion arises when trying to distinguish a box turtle from a tortoise. While they are primarily land animals, box turtles are terrapins. They are omnivores and have limbs more similar to that of a terrapin.
All Chelonians have a protective shell, however some may be more protective than others. Soft shelled turtles, for instance, have a shell covered with leathery skin that does very little for protection. Other than soft shells, a chelonian’s shell is comprised of living bone covered with plates of keratin, similar to scales, called scutes. It is normal for terrapins to shed their scutes as they grow, but not for tortoises.
One of the most well known characteristics of chelonians is that they are very long lived; even the shortest-lived aquatic terrapins routinely live 20-30 years. Tortoises can live 60-100 years, depending on species. Chelonians are very social animals and establish loose territories. Males can be very territorial and will fight over resources and females. All chelonians lay eggs (oviparous); they produce eggs with a hard shell similar to a bird’s egg and are usually spherical.
Basic Anatomy
Most reptiles have a 3-chambered heart as well as a renal portal system where blood passes through the kidneys both as it leaves the heart and as it returns to the heart after traveling through the back end of the body.
Reptile lungs are simple and sac-like and often have a vascular epithelium towards the front and non-respiratory epithelium towards the back. They do not have aleoli like mammals, but instead their pulmonary functional unit is called faveoli, small air spaces in the lungs that resemble honeycomb cells.
Reptile gonads are internal and the right and left gonads are located at different levels of the snake between the pancreas and the kidneys. Male snakes and lizards have paired copulatory organs called hemipenes while male chelonians and crocodilians have a single phallus. These are not associated with the urination and only function for mating, thus if diseased these may be amputated.
Snakes
Snakes possess very elastic to allow for ingestion of prey. Periodically shed whole, usually between 2 to 4 times per year depending on growth rate. They also are very delicately built compared to other reptiles. They have two rows of teeth on the top (maxillary and pterygoids) and one on the bottom (mandibular). Their teeth are recurved and are periodically shed throughout life. Their tongue is long, deeply forked and lies in sheath under the glottis and is used in olfaction (smell) together with the Jacobson’s organ found on the palate. They have no external ears and no tympanic membrane or middle ear, yet they are able to hear low frequency sound using the inner ear. Their vision geared for movement and they have no eyelids, instead their eyes are covered with a spectacle. Heat-pits are present on the heads of boas, pythons, and pit vipers allowing them to find prey in complete darkness.
The right lung is larger than the left in snakes, with the left lung completely non-functional (vestigial) in colubrids like corn snakes and kingsnakes. Like all reptiles, snakes lack of diaphragm.
Venom glands in snakes are modified salivary glands and located behind the eyes on the head.
Like all reptiles, the snake ureter empties into part of the cloaca called the urodeum. Snakes lack a urinary bladder. Male snakes have a sexual portion of the kidney that enlarges during the breeding season and produces seminal fluid.
Lizards
Unlike snakes, most lizards shed their skin in pieces. Their skin is usually fairly thick and scaled, with the exception of geckos who possess very thin skin. Many practice tail autonomy, meaning they can drop their tails as a defence mechanism. While most species will regrow their tails, not all can do this, i.e. crested geckos. Bearded dragons do not drop their tails and their tails cannot regenerate if lost.
Their ear drum, or tympanic membrane, is visible and there is no external or ear lobe.
Most lizards have eyelids, with the exception of some species of geckos, and have well developed color vision. Many also possess a degenerate 3rd eye or parietal eye located on the top of the head. It contains a retina and lens and is used to stimulate hormone production as well as moderate behavior associated with thermoregulation and basking times.
The heart is 3-chambered and located between the shoulders in most lizards, with the exception of monitor lizards where it is located low in the chest.
Lizard tongue morphology varies based on function.
Chelonians
Probably the most promanent part of chelonian anatomy is their shell witha dorsal carapace and ventral plastron. Their pectoral and pelvic girdles are uniquely located within their rib cage. Comprised of living bone covered with protectie keratin, their shells are well innervated and can feel touch and pain.
Their eyes are similar to birds and they possess scleral ossicles, small circular bones embedded in the whites of the eyes.
The chelonian ear is hidden behind a large scale or several smaller scales that overlies their tympanic membrane (ear drum).
Chelonians have large lungs that extend the length of the shell. Like all reptiles, they lack a diaphragm. Their shell prevents movement of the rib cage so respiration requires movement of the head and forelimbs.
While chelonians have a relatively simple stomach, the large intestines of many tortoises is extensive and used for microbial digestion of fiber.