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 supplementation is an absolute requirement for diurnal, herbivorous or omnivorous reptiles. While it is not an absolute requirement for nocturnal and carnivorous species, some may benefit from it. Unfortunately, this is an essential husbandry component that is still lacking in some captive reptiles and its omission contributes to the occurrence of secondary nutritional hyperparathyroidism. One challenge is the difficulty of obtaining easy to follow and accurate information on how to choose and use UVB lights properly. The abundance of misinformation on internet sites and the lack of training of pet store employees are further contributors to this problem. It is therefore essential that veterinarians working with reptiles are able to properly educate clients and provide easy to follow instructions on the use of UVB lights.
UVB LIGHT AND VITAMIN D3
Ultraviolet light is electromagnetic radiation that exists in three main bands: UVA, UVB, and UVC. Each band is characterized by a specific wavelength. UVA has the longest wavelength at 320‐400nm while UVB has a wavelength of 290‐320nm. Both UVA and UVB have low tissue penetration with most light being absorbed at the epidermis. UVA has a wavelength of less than 290nm, the deepest penetration, and is the most harmful. These wavelengths place ultraviolet somewhere between visible light (369‐769nm) and X‐rays (0.01‐10nm). The ozone layer is responsible for filtering the majority of UV radiation hence protecting living organisms from its harmful exposure. UVC is considered germicidal and can have significant harmful effects to living animals while both UVA and UVB have therapeutic and physiologic applications. Still, as we know from human medicine, too much sun/ultraviolet exposure has a slew of negative implications ranging from sunburn to premature aging, and cancer.
UVB light is essential for the synthesis of vitamin D3. Although commonly referred to as a vitamin, D3 is actually a hormone. UVB light from the sun stimulates conversion of 7‐dehydrocholesterol to cholecalciferol (D3) in the skin. Cholecalciferol is then transported to the liver via the blood stream and hydroxylated into 25‐hydroxycholecalciferol (calcidiol), which is then transported to the kidneys for hydroxylation into 1,25‐dihydroxycholecalciferol(calcitriol), the active form of D3. The 25‐hydroxycholecalciferol in the liver is the storage form of D3 and is a natural source of D3 for animals fed whole prey. Plants are a source of ergocalciferol or ergosterol (D2), which is not readily bioavailable to most reptiles and is therefore not an appropriate source of D3 supplementation. Calcidiol, an animal source of D3, is more bioavailable for reptiles.
There are two main sources of UVB, natural sunlight and UVB bulbs. Exposure to natural sunlight is the most efficient way for reptiles to produce adequate levels of D3. However, this is often not possible year round, outside the tropics and some subtropical regions, necessitating the use of artificial UVB light sources.
CHOOSING UVB LIGHTING
There has been a long‐standing debate about the effectiveness and even potential harmful effects of artificial UVB lights. Like the application of any treatment or therapeutic, the use of UVB lights must be in accordance with certain parameters
