Osmoregulation in reptiles presents unique challenges. A reptile’s body mass is approximately 70% water, similar to mammals but lower than amphibians (75–80%). Total sodium and potassium is similar to mammals but variation exists in larger species. Because of the lack of a loop of Henle, reptiles are unable to concentrate urine beyond their plasma osmolality. Methods of water conservation include the secretion of uric acid, presence of salt glands, fluid reabsorption from the cloaca, the renal portal system, and the ability to tolerate decreases in glomerular filtration rate. All reptiles produce and excrete uric acid, but some may also excrete ammonia and/or urea. Uric acid is the primary or only excretory product of terrestrial species. Uric acid is excreted via the renal tubules, so dehydration does not stop its excretion. It precipitates out of solution in the bladder or cloaca to form urates, which can be composed of potassium salts (herbivores) or sodium salts (carnivores). Uric acid is not a sensitive indicator of renal disease because over 60% of renal function must be lost in order to cause hyperuricemia; thus, dehydration is a more common cause of hyperuricemia. Ammonia and urea excretion occurs primarily in aquatic species. Crocodilians are ammoniotelic while freshwater turtles are ureotelic. Sea turtles excrete variable amounts of urea and ammonia. The cloaca, colon, and urinary bladder are significant sites of osmoregulation and allow active ion transport and passive water absorption through their walls. The bladder actively absorbs sodium while secreting potassium and urates.
The renal portal system is an afferent blood supply from the renal artery to the glomerulus. The renal portal vein (arising near epigastric and iliac veins) bypasses the glomerulus and enters at the tubules; thus, during dehydration the renal portal system perfuses the tubules to prevent necrosis. For this reason, the effects of the renal portal system on drug metabolism is of primary consideration for pharmaceuticals excreted by the tubules and not as critical for those excreted by glomerular filtration. Effects of the renal portal system on drug metabolism and excretion are variable according to the metabolic status of the animal and the pharmacokinetics of the drug.
REPRODUCTION
Reproduction is regulated by the pineal gland, hypothalamus, and pituitary glands, which process environmental stimuli into hormonal changes. In temperate species, gonadal stimulation occurs by increased temperature and longer days while in tropical species increased food availability and rainfall provide gonadal stimulation.
Sex determination may be genotypic (ZZ are males, ZW are female) or temperature dependent. Temperature sex determination is not reported or well known in snakes but occurs in all the other reptile groups. In chelonians, high temperatures yield mostly females while low temperatures yield a majority of males. In most the lizards, temperature sex determination is opposite to chelonians. Some turtles and lizards will have a predominance of females at high and low temperatures and males at intermediate temperatures. For crocodilians, females occur at low temperature (28–30 degrees C) in all species. In caimans and alligators, male and females occur at 31–32 degrees C while males occur at 32–34 degrees C. In crocodiles, more males than females will occur at 31–33 degrees C, while the ratio is reversed at 33–34 degrees C.
The right gonad is located close to vena cava and connected to it by small vessels, while the left is associated with the left adrenal and has its own blood supply. In some male lizards, there is a single urodeum opening for the ureter and vas, while in other reptiles there are two separate openings.
The oviducts are responsible for egg transport and secretion of albumin, proteins, and calcium. The oviduct is divided from proximal to distal into the infundibulum, uterine tube, isthmus, uterus, and vagina. There is a three‐phase ovarian cycle comprising:
1 Quiescent stage—no follicular development.
2 Vitellogenic phase—rapid hypertrophy of the ovary and oviduct, yolk production, increased estrogen, and calcium mobilization.
3 Gravidity/pregnancy—during which fertilization and oviposition occur. Many species have a pre‐lay and/or a pre‐ovulatory shed.
Reptiles may be oviparous or viviparous. All chelonians and crocodilians, most lizards (iguanids, monitors, geckos), all pythons, and most colubrids are oviparous. Oviparous reptiles produce two to three clutches per season, with the yolk being the only source of nutrients for the embryo. All boas and vipers, some skinks and chameleons, the European lizard, and garter snakes are viviparous. Viviparous species maintain the corpus luteum for longer, and produce one clutch per year with a longer gestation time. Viviparous species have a more drastic decrease in body condition of the females because they provide direct nutrition to the embryos.
IMMUNE SYSTEM
The lymphatic system is a wide network of lymphatic ducts, which are pumped by lymph hearts (smooth muscle dilations in lymphatic channels). Their primary connection with the venous system is at the base of the neck where a precardiac sinus transfers lymph to the venous system. A number of lymphatic trunks are described in mammals. The jugular trunk drains the head and neck regions. The subclavian trunk drains the forelimbs. The lumbar trunk drains the hind limbs, and the thoracic trunk drains the trunk and coelom. The lumbar and thoracic trunks form a dilation known as the cisterna chili. Some version of this system is suspected to exist in reptiles, but its specific anatomy and species variability are unknown.
The thymus of reptiles does not involute but does decrease in weight and size over time. In some species, such as crocodilians, there can be well‐defined and widespread gut‐associated lymphoid tissue. Well‐defined lymphoid aggregates may also be found in other tissues such as the lungs.
SPECIAL SENSES
The eyes of reptiles can be quite varied. Reptiles have avascular retinas. Scleral ossicles are present in lizards and chelonians but absent in other reptiles. Reptiles have a conus papillary that is analogous to the pecten of birds. Most lizards have a nasolacrimal duct, but eyelids may be absent, as in the case of some gecko species. Chelonians lack a nasolacrimal duct and have a predominance of cones. Snakes have a nasolacrimal duct that drains the subspectacular space, but they lack a nasolacrimal gland. They are the only group to have scleral cartilage. The number of rods and cones in snakes will vary by species. Although it looks transparent, the spectacle of snakes is very vascular.
A vomeronasal organ is present in tuataras, lizards, and snakes, but absent in crocodilians. Its presence is debated in chelonians. Pit organs are openings in the maxillary and/or mandible of boas, pythons, and pit vipers, which serve to provide infrared detection of environmental cues. There are free nerve endings within the pits or scales, and this is thought to be a very sensitive system.
Suggested Reading
1 O’Malley, B. Clinical Anatomy and Physiology of Exotic Species. Edinburgh, NY: Elsevier Saunders, 2005.
2 Jacobson, ER. Overview of Reptile Biology, Anatomy, and Histology. In: Jacobson ER, ed. Infectious Diseases and Pathology of Reptiles: Color Atlas and Text. Boca Raton, FL: CRC Press; 2007: 1–130.
3 Fowler ME, Miller RE. Fowler’s Zoo and Wild Animal Medicine, Volume 8. St. Louis, MO: Elsevier, 2015: Part II: Reptile Groups.
4 Mader DR. Reptile Medicine and Surgery. 2nd ed. St. Louis, MO: Saunders/Elsevier; 2006.
5 Pet Industry Market Size & Ownership Statistics. The 2019–2020 American Pet Products Association National Pet Owners Survey © American Pet Products Association, Inc. https://www.americanpetproducts.org/press_industrytrends.asp.
UVB Lighting Principles for Captive Reptiles
Exposure to ultraviolet B (UVB) light is a critical aspect of reptile husbandry. UVB light