Clinical aspects of reptile anatomy and physiology (Proceedings)

The Class Reptilia consists of over 6,500 species, but only a few dozen species are likely to be encountered with any regularity in general practice. The diversity within the Class Reptilia necessitates the description of generalities.

Anatomy & Physiology

Ectothermia—Reliance on environmental heat and behavior to maintain a preferred body temperature (PBT). PBT is the core temperature at which a metabolic process is optimal. PBT varies with species, age, season and even time of day. The PBT for specific metabolic processes is variable such that the PBT for gametogenesis and reproduction is likely to be different from the PBT for immunocompetence. The preferred optimum temperature zone (POTZ) is the temperature range that permits the reptile to achieve the PBT, and should therefore be provided by the thermal gradient provided in the captive scenario. The metabolic rate of reptiles is lower than for mammals and birds, and consequently the k constant in determining energy expenditure, nutritional requirements and even calculating allometric drug doses are related to the equation: Basal Metabolic Rate (BMR) = K(Wkg0.75) = Kcal/day, where K is the energy constant = 10 in reptiles and Wkg is the animal's weight in kilograms.

Circadian Rhythms—In tropical species, constant temperatures are the norm, with 12-hr cycles. Large deviations from this pattern are not well tolerated. Desert spp, require high daytime temperatures, followed by cool nighttime periods. Temperate animals are more tolerant of fluctuations.

Hibernation—This is a period of dormancy, marked by a decrease in metabolic rate. This follows an adequate period of recovery after the most taxing activities, such as reproduction. It precedes a period when food resources are scarce, so the GI tract is typically empty at the time of hibernation.

Blood and Circulation—All snakes, lizards and chelonia possess a three-chambered heart (2 atria and 1 ventricle) while crocodilians have a four-chambered heart. All reptiles have both pulmonary and systemic circulations (i.e., similar to mammals). In non-crocodilian reptiles some mixing of blood occurs in the single ventricle, however, functional separation of venous and arterial blood is largely maintained thanks to a muscular ridge in the ventricle (termed the intraventricular septum or vertical septum). Heart rate is dependent on body temperature, body size, metabolic rate, respiratory rate and sensory stimulation. Heart rates elevate during active respiration and decrease during apnea.

Renal Portal System—Renal portal system means blood from tissues caudal to the kidneys can be shunted directly to the kidneys. In chelonian, blood from the hind limbs does not seem to enter the portal system, while that is the case in lizards. There is a valve at the junction of the abdominal and femoral vein. When the valve is closed→blood shunted through iliac vein→kidneys. When valve open→ blood flows into systemic circulation through abdominal vein. Blood entering kidneys through renal portal system only supplies the tubules, NOT the glomeruli. Drugs cleared by tubular secretion (ie: penicillins) are affected by this anatomic peculiarity. In addition, research suggests that in the red-eared slider, the "valve" is only closed during periods of dehydration.

Integument—Reptilian skin is usually heavily keratinized and protected by scales, with the chelonian shell composed of both dermal bone plates and epithelium. Reptiles do not have extensive skin glands and their skin is essentially dry. However, in many species of lizards the mature male will possess a series of pre-femoral pores located on the craniomedial aspect of the hindlimbs (e.g. Eublepharis macularius). Crocodilians have perianal musk and mandibular musk glands which are thought to produce pheromones. Some tortoises (Gopherus sp) have mental glands, which are lobular-alveolar glands similar to sebaceous glands that secrete pheromones. Chromatophores are a common component of the skin and enable many species, most notably the Chamaeleonidae, to change color. Some lizards have salt glands (oral or nasal) which excrete excess salt (ie: iguana-nasal glands, crocs-ventral tongue gland). Bony skin structures, osteoderms, are also encountered in the crocodilians and lizards (e.g. Corytophanes spp). Certain species of snakes (vipers and boids) have heat sensitive receptors located around the maxilla (pits) which are used in prey location. The parietal eye is an invagination of the thalamus and is connected to the pineal gland via the parietal nerve. It aids in regulation of circadian rhythms and annual rhythms. Skin characteristics (e.g. crests, spines, dewlaps etc) are often used for species identification or gender identification in those species that exhibit dimorphic variation.

Epidermis—The complex integument is typically vertebrate and consists of an outer epidermis and inner dermis. The outer epidermis prevents excessive dehydration and has reduced the reptile's dependence on water. The epidermis gives rise to the characteristic scales and interscalar skin. The most complex integumental design belongs to the squamates.

Ecdysis (Shedding or Sloughing) Cycle—All reptiles shed their skin. Snakes and lizards periodically, crocodilians and chelonia continuously in small pieces. The frequency of ecdysis is dependent upon species, age, nutritional status, environmental temperature and humidity, reproductive status, parasite load, hormonal balance, bacterial/fungal skin disease and skin damage. During the resting phase of skin shedding, i.e. immediately after a successful shed, three regions of the epidermis can be distinguished;

     1. Stratum corneum (outer, keratinized, acellular zone)

     2. Stratum germinativum (single layer of cuboidal cells immediately above the dermis)

     3. Intermediate zone (layers of cells produced by the Stratum germinativum in various stages of proliferation and differentiation)

Dermis—The epidermal-dermal division is marked by a basement membrane. The dermis tends to be thicker than the epidermis and contains connective tissue with abundant collagen fibers, blood and lymphatic vessels, smooth muscle fibers, sensory nerve endings, and chromatophores.

Regeneration—Most lizards can regenerate tails to some degree. There is an unossified membrane through the center of each tail vertebrae and valves in the blood vessels supplying the tail. The tail can be shed, and the valves constrict, avoiding significant hemorrhage. Cartilage, which mineralizes slightly over time, develops in place of vertebral bodies, surrounded by soft tissue.

Gastrointestinal System—Choanal slits→paired perimedial clefts that allow passage of air from nares into oral cavity and trachea. Nasolacrimal ducts connect to the eye beneath the spectacle. Venom glands are modified upper labial salivary glands in snakes. Squamates have no taste buds, but present particles to vomeronasal organ (highly innervated area on roof of mouth located in vomer groove, used in sense of smel). Crocs have taste buds and no vomeronasal organs. Snakes have open mandibular symphysis and flexible rami to swallow large prey intact. Fangs are hollow teeth. If in front and retractable→vipers, if rigid→cobras, sea snakes; rear in some sp; croc teeth are conical and open rooted. Lizards are either acrodonts (have teeth ankylosed to the surface of jaw bones) or pleurodonts (have teeth attached to the medial side of the jaw bone). Some chelonians have cornified papillae on dorsal pharynx and length of esophagus to deal with eating abrasive food. In crocs, the stomach is two-chambered, and stones may be found in the larger section that aid in mechanical grinding of food. The cecum or distal colons are highly developed in herbivorous reptiles. No cecum in crocs, or snakes. The liver of most reptiles has melanin aggregates. Most reptiles have gallbladders. The gall bladder produces a variety of bile acids, commercial bile acid tests only test for one kind, which may not be useful for all reptiles. Contrary to generalizations, some reptiles do have biliverdin reductase (ie: some snakes) and thus can convert biliverdin to bilirubin. Cloaca receives products from kidneys, gastrointestinal tract, reproductive tract, is involved in respiration, water resorption and reproduction. Pancreas located between ascending/descending portion of duodenum in chelonia and lizards. In snakes, located caudal to pylorus near gall bladder and spleen, in some snake spp, it is fused with spleen=splenopancreas.

Respiration—Breathing is accomplished by expansion of coelomic cavity by intercostals muscles, pectoral limb movements and smooth muscle contraction of the lungs. Crocs use movement of liver to create some negative pressure. Chelonians use movement of front limbs and viscera. Some reptiles can extract oxygen from water through the skin. Some aquatic turtles can exchange dissolved oxygen through the mucosa of the pharynx and cloaca while being submerged. Crocs posses a gular flap (pharyngeal membrane) which allows breathing while the mouth is full of water or food. Gular movements in chelonians is actually associated with olfaction. The glottis is located at the base of the tongue. Tracheal rings are complete in chelonian and crocodilians; incomplete in lizards and snakes. The trachea bifurcates more cranially in chelonians. The reptile lung is made up of open-ended sacs lined with alveoli-like structures which are supported by smooth muscle. Each sac connects directly to a bronchus or to secondary bronchi. Lungs end in thin walled air sacs, which can be used as a defense mechanism or buoyancy control. Vipers have a tracheal lung (collection of alveolar tissue on dorsal portion of trachea) which allows for gas exchange when prey items compress lungs. In most snakes, the right lung is larger than left.

Renal System—Terrestrial reptiles excrete uric acid (uricotelic). Aquatic reptiles either excrete ammonia (ammonotelic), urea (ureotelic), or uric acid. Some examples: sea turtles excrete mostly ammonia or urea; semi-aquatic turtles excrete 50% urea, 50% uric acid; desert tortoises excrete >90% uric acid. No loop of Henle→only isosthenuric urine possible. Uric acid is extremely insoluble and thus in extreme dehydration, urates can precipate in the nephron leading to obstruction. Right kidney is cranial to left in snakes. Some reptiles (geckos, skinks, iguanas, snakes) have a sexual segment of their kidneys, which means that cells lining the distal convoluted tubule hypertrophy during breeding season and excrete eosinophilic granules into the tubules. These granules can be found in the vas deferens. Since there are no accessory glands, the sexual segment provides seminal fluid. Some reptiles have bladders-mostly terrestrial chelonians and some lizards (iguanas do, monitors do not). Water is resorbed in the bladder. Isosthenuric urine flows from kidney→ureter→cloaca→bladder→cloaca→outside. Those without bladders retroflux urine from cloaca into distal colon for water resorption. Salt is also resorbed in this way.

Reproduction—Reproductive organs are paired, even in parthenogenic females, except only one phallus in turtles and crocs. Right ovary is cranial to left. Yolk is very high in calcium, which the embryo needs for skeletal formation. In chelonians and crocs, this calcium is acquired from the inside of the shell. Fertilization occurs in upper portion of oviduct. Crypt-like pits can store sperm for up to 6 yrs! The development of testes is seasonal. Chelonians (vascular) and crocs (fibrocartilagenous)→single penis; squamates→paired hemipenes (highly vascular and fibrocartilagenous).

Many species are sexually dimorphic:

     1. Lizards. Males are larger, possess pre-anal or pre femoral pores, hemipenal bulges at tail base, and often more brightly colored.

     2. Snakes. Snakes are sexed by probing the hemipenes with a blunt lubricated probe. In males the probe will enter to a depth of 6-14 subcaudal scales, females only 3-6 scales.

     3. Chelonia. Sexual dimorphism is usually obvious in adults; Male desert tortoises and box turtles tend to have concave plastron, males have longer tails such that the cloacal opening is caudal to the caudal rim of the carapace.

Temperate species require hibernation at 5-10° C for at least 2 months, and usually 3-5 months.

Subtropical species are often induced to breed after a period of seasonal reduction in temperature and photoperiod, often coupled by a reduction of food intake. Tropical species are more challenging but breeding can be stimulated by a reduction in nighttime temperature (but maintain daytime temperatures), increasing humidity, decreasing food availability and changing social grouping. Reptiles are not considered to be highly social creatures and multiple male groups can lead to intraspecies aggression. Harem groupings can work well for certain species.

Reproductive behavior:

     1. Lizards. Males are often highly territorial and will fight to the death.

     2. Chelonia. Males are often very virile and will engage in relentless courtship (shell butting, leg biting). In some cases a female may become traumatized and need separating from an amorous male.

     3. Snakes. Many species require multiple males to encourage mock combat and copulation.

Fertilization is internal. Viviparous or ovoviviparous females (some lizards and snakes) produce live young and require: sufficient laying/birth sites for all females, and a secluded, warm, humid retreat for laying/birth. Oviparous species lay leathery or calcified eggs, which best removed and artificially incubated at 28-32° C and 90-100% humidity. Temperature dependent sex determination occurs in chelonia, crocodilia and some lizards. Neonates will usually start eating within 3-7 days of birth/hatching (after the yolk sac has been fully absorbed).