is the process by which a reptile reduces its metabolism and becomes dormant during the winter. As opposed to hibernation in mammals, brumating reptiles may retain a low level of activity, wake up to drink water and then go back into dormancy. The stimulus for and emergence from brumation is primarily influenced by temperature, but other factors, including reproduction and food availability, may also play a role. There are four stages of brumation:
1 A decrease in temperature leads to inhibition of appetite.
2 Reptiles seek a hibernaculum that provides insulation against freezing and enough moisture. Oxygen availability is less important.
3 Fat stores (liver, fat bodies, tail) are used as energy source, especially when emergence occurs in the spring.
4 Emergence from brumation is triggered by an increase in temperature with photoperiod having little to no influence since most brumate underground. During this period a reptile should not lose more than 10% of their body weight.
Captive reptiles undergoing brumation should be weighed on a bi‐monthly schedule to monitor their weight. It is critical that water be available during brumation in captivity to avoid dehydration. Brumating reptiles can be soaked weekly to every other week.
Estivation is a period of inactivity during the dry season, mostly used by desert species in an effort to conserve water. Some turtles may leave the water during periods of drought and bury themselves on land until the conditions are favorable again. This is less likely to occur in captivity since a constant water source is available.
INTEGUMENT
The epidermis consists of the stratum corneum with an outer keratin layer, an intermediate lipid‐rich layer and the stratum germinatum with cuboidal cells. Two types of keratin—alpha and beta—are present. Alpha keratin is flexible and is found in hinge regions of the skin, while the more rigid beta keratin provides strength and hardness, as in the shell of chelonians. The wide ventral scales of snakes are called gastropeges. The dermis is composed of connective tissue, blood vessels, lymphatics, and nerves.
Chromatophores are pigment‐containing cells that lie between the epidermis and dermis. They may also occur in the peritoneum. Chromatophores function in camouflage, sexual display, thermoregulation, and behavior. Melanophores are located deep in the sub‐epidermal layer and produce melanins, black, brown, yellow, or gray in color. Carotenoids can be found beneath the epidermis but above melanophores and produce yellow, red, and orange pigments. Iridophores located within the dermis contain guanine (a breakdown product of uric acid), which reflects light in the blue wavelength (Tyndall scattering). The combination of the iridophores’ blue reflection and carotenoid yellow pigmentation create the green coloration common in many reptiles.
Ecdysis is the process by which reptiles shed their skin. Ecdysis is under pituitary and thyroid control. Continuous ecdysis occurs in chelonians and crocodilians while discontinuous ecdysis occurs in snakes, lizards, and tuataras. For simplicity, the ecdysis cycle can be split into a resting phase and a renewal phase, which has up to five stages. During the beginning of the renewal phase, the skin begins to appear duller, as regeneration of cells and enzymatic separation of the skin layers becomes activated. This period lasts approximately 5 ± 2 days. Afterwards, the skin becomes even duller and the spectacles turn opaque in what is commonly described as snakes being in “blue”. This appearance is in large part due to lymphatic fluid accumulation that helps separate the old and new skin layers. This second period lasts 5 ± 2 days, followed by the last stage, during which lymphatic fluid resorption occurs leading to a clearing of the skin and spectacle. At this time, the skin regains some of its sheen and shedding typically occurs 5 ± 2 days after that. Hydration is critical for proper ecdysis and dehydration is a common cause of dysecdysis. The frequency of shedding will vary with environmental temperature, food intake, and growth of the animal.
SKELETAL SYSTEM
Unlike mammals, the bone epiphyses do not close in chelonians, snakes, and crocodilians. Epiphyses close later in life in lizards. Reptiles have intervertebral bodies rather than disks. These intervertebral bodies are softer and form convexities in the anterior or posterior aspect to allow for additional flexibility and decreased pressure on the spinal cord. This arrangement allows reptiles the ability to tolerate and recover from spinal trauma better than mammals.
Reptile orders are classified based on the presence or absence of fenestrae in the temporal region of the skull. Anapsids lack temporal openings (chelonians). Diapsids have two openings in the temporal region behind the orbit, one superior (dorsal) and one inferior (tuataras and crocodilians). Squamates are modified diapsids, with lizards having only a dorsal opening while snakes have lost the upper temporal arch between the two openings. The quadrate bone articulates between the maxilla and mandible allowing increased movement of the skull, including the maxilla, a feature critical for some reptile species. Streptostyly is the ability of the quadrate bone to move back and forth in snakes due to their modified diapsid skull. Lizards and crocodilians have powerful snapping jaws due to the adductor muscles that arise from the temporal fossae and insert at right angles to open the jaw.
The vertebral column of reptiles can be divided into three sections: presacral (24 in lizards, 18 in chelonians, over 200 in snakes), sacral, and caudal. The atlas and axis are more rigidly connected than in mammals, so primary neck movement is between the single occipital condyle and the vertebral column. The number of cervical vertebrae varies across species, with crocodilians and varanids having nine, chelonians, tuataras, and most lizards eight, and chameleons three to five. There is no subarachnoid space but a subdural space between the leptomeninges and the dura mater is present.
CARDIOVASCULAR
The heart of crocodilians has four chambers (two atria and two ventricles) while that of all other reptile species has three chambers (two atria and one ventricle). The ventricle of non‐crocodilian reptile hearts is divided into three chambers separated by incomplete muscular ridges. The cavum venosum receives blood from the right atrium dorsally and the paired aortas ventrally. The cavum arteriosum receives blood from the left atrium. The cavum pulmonale opens into the pulmonary artery and is equivalent to right ventricle in mammals. The admixture of oxygenated and deoxygenated blood varies across species and is influenced by the presence of a muscular ridge separating the three areas of the ventricle. The right and left cranial vena cava and the left hepatic vein supply blood to the sinus venosus. Reptiles have two aortas, with the left aorta giving rise to the celiac, cranial mesenteric, and left gastric arteries before joining the right aorta caudal to the heart. The blood flow through a non‐crocodilian heart is outlined in Figure 1 (see web image supplementary content for section I).
RESPIRATORY SYSTEM
Nasal conchae are absent in chelonians but are present in other reptiles. Crocodilians and chelonians have complete tracheal rings, while they are incomplete in snakes and lizards. The lung anatomy is one of three basic types. Unicameral lungs are simple, single‐chambered structures found in most snakes and some lizards, including tuataras. Paucicameral or transitional lungs have some internal divisions, but these are mostly incomplete and do not split the lung into separate chambers. They also lack intrapulmonary bronchus. Most iguanids, chameleons, and agamids have paucicameral lungs. Multicameral lungs are more complex with internal divisions and multiple bronchi. These lungs can be found in varanids, helodermatids, chelonians, and crocodilians. Chelonians have an exception as they possess a single, unbranched bronchus running the entire length of the lungs, with all lung chambers opening into this single bronchus.
In snakes, the lungs transform into an air sac structure caudally near the liver. In terrestrial species, the lung air sac terminates near the gallbladder, while in some aquatic species it terminates near the cloaca. In
