Watertight Skin

Reptiles are covered by a thick, dry, scaly skin that prevents water loss.  This scaly covering develops as surface cells fill with keratin, the same protein that forms your fingernails and hair.  Lipids and proteins in the skin help make the skin watertight.  The tough skin of a reptile not only helps conserve body water but also protects the animal against infections, injuries, and the wear and tear associated with living in terrestrial environments.

Respiration & Excretion

All reptiles have lungs for gas exchange.  Thus all the tissues involved in gas exchange are located inside the body, where they can be kept moist in even the driest environments.  The excretory system of reptiles also helps them conserve body water.  Snakes, lizards, and other land-dwelling reptiles excrete nitrogenous wastes in the form of uric acid.  Uric acid is much less toxic than ammonia or urea.  Thus, it requires little water for dilution, and reptiles lose only small amounts of water in their urine.

Circulatory System The circulatory system of a reptile, like those of all terrestrial vertebrates, is composed of two loops.  The pulmonary loop carries deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart.  The systemic loop transports oxygenated blood to the tissues of the body, where oxygen and nutrients are unloaded and carbon dioxide is picked up, and returns deoxygenated blood to the heart.  Click here to learn more about the reptilian heart.

Respiration

The lungs of reptiles are large, and they are often divided internally into several chambers.  The lining of the lungs may be folded into numerous small sacs called alveoli.  Alveoli greatly increase the internal surface area of the lungs, thus increasing the amount of oxygen that can be absorbed.  In most snakes, only the right lung actively functions.  It is elongated and may be half as long as the body.  The left lung is either reduced to a small nonfunctional sac or absent entirely.

     A reptile fills its lungs be expanding its rib cage.  This expansion reduces the pressure within the thorax and draws air into the lungs.  When the ribs return to their resting position, pressure within the thorax increases and air is forced out of the lungs.

Nervous System The brain of a reptile is about the same size as that of an amphibian of the same size.  However, the reptilian cerebrum is much larger.  This region of the brain is involved in controlling and integrating behavior.  Because vision is an important sense for most reptiles, the optic lobes, which receive input from the eyes, are large.      Most reptiles rely on their sense of sight to detect predators and prey.  The eyes of reptiles are usually large, and many species have keen vision.  Hearing is also an important sense.  Sound waves first strike the tympanum, or eardrum, and are transmitted to the inner ear through the movements of a small bone called the columella.  The inner ear contains the receptors for sound.  Snakes lack a tympanum and are sensitive only to low-frequency sounds.  They are able to detect ground vibrations, which are transmitted to the columella by the bones of the jaw.      Jacobson's organ is a specialized sense organ located in the roof of the mouth of reptiles.  Jacobson's organ is sensitive to odors.  Reptiles use their tongue to gather chemicals from the environment.  These chemicals are transferred to the Jacobson's organ when the tongue is drawn back into the mouth.  Jacobson's organ is found in all reptiles except crocodiles and most turtles, but it is highly developed in lizards and snakes.      Pit vipers, such as rattlesnakes, copperheads, and water moccasins, are able to detect the heat given off by warm-blooded prey, such as mammals and birds.  These snakes have one heat-sensitive pit below each eye.  Input from these pits allows a snake to determine the direction of and distance to a warm object. Thermoregulation The control of body temperature is known as thermoregulation.  An ectotherm warms its body by absorbing heat from its surroundings.  It has a slow metabolism that produces little heat.  Reptiles are ectotherms.  Reptiles regulate their body temperature by controlling how much heat they absorb.  For example, when a lizard emerges from its nest after a cool night, its body temperature is low and must be raised before it can become active.  The lizard warms itself by basking in the sun.  The lizards warm blood is diverted from the skin to the interior of the body.  The lizard continues to bask until it is warm enough to become active.  A lizard can maintain its body temperature within a narrow range by using a variety of behaviors and body positions, despite variations in air temperature.      Ectothermy requires very little energy because their metabolism is very slow, and they also need only about one-tenth as much food as an endotherm of the same size.  Ectotherms cannot live in very cold climates, and they can survive temperate climates only by becoming dormant during the coldest months.  Furthermore, ectotherms can run or swim at maximum speed only for short periods of time.  Ectothermic metabolism cannot provide enough energy for sustained exertion.

Reproduction and Parental Care

There are three patterns of reproduction among reptiles.  The differences between these three patterns lie in how long the eggs remain within the female and in how she provides them with nutrition. 

     In oviparity, the female's reproductive tract encloses each egg in a tough protective shell.  The female then deposits the eggs in a favorable place in the environment.  Oviparity is characteristic of most reptiles.

     Once released from the female's body, an egg is particularly vulnerable to predators and to the environmental hazards.  One way to reduce exposure to these hazards is to retain the eggs within the female's body for a time.  This strategy is called ovoviviparity.  The eggs may be laid shortly before hatching, or they may hatch within the female's body.  The eggs absorb water and oxygen from the female, but they receive no nutrition other than the yolk.

     In viviparity, a shell does not form around the egg, and the young are retained within the female's body until they are mature enough to be born.  Nutrients and oxygen are transferred from mother to embryo through a structure called the placenta.  The placenta forms from the membranes within the egg, and it brings blood vessels from the embryo near the vessels of the mother.  Viviparity is the reproductive pattern shown in a few species of lizards and snakes.

     Many reptiles provide no care for their eggs or young.  However, some species of lizards and snakes guard and warm the eggs until they hatch.  Crocodiles and alligators provide the greatest amount of parental care.  A female crocodilian, for example, builds a nest for her eggs.  She remains nearby while the eggs incubate, guarding against nest robbing predators.  After the young hatch, they produce a high-pitched yelp, which summons the mother.  She then breaks open the nest and carries the hatchlings to the water in her mouth.  The mother crocodile may protect her young for a year or more.

For any questions email Mr. Reilly at biosnake@aol.com