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.
—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
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.
—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.