Reproductive Anatomy & Physiology in Reptiles: Nine Key Facts

Introduction

Although anatomy and physiology varies greatly among the diverse class Reptilia, the basic structure of the reproductive tract is fairly consistent. The paired ovaries and testes, which range in color from yellow to grayish-pink, are located dorsomedially within the coelom although their exact location is species-specific. The right gonad sits cranial to the left, particularly in snakes. Females possess a right and left oviduct, but no true uterus. The oviduct empties directly into the cloaca through a genital papillae

1. Secondary sex characteristics

   The very first step of any signalment, determining your patient’s gender, can come to a screeching halt with many reptiles. Observation of secondary sex characteristics (Box 1) can prove invaluable in determining gender.

Figure 1. The Jackson’s or three-horned chameleon (Chamaeleo jacksoni xantholophus) is an example of sexual dimorphism. Only the male possesses the three rostral horns shown here. Click to enlarge.

Figure 2. Femoral pores (arrows) are larger and more prominent in some male lizards such as this green iguana (Iguana iguana) shown here. Click image to enlarge.

In many instances, clinical techniques (Box 2) must be performed to determine a patient’s sex. Cloacal probing is the most common technique performed in snakes.

1. Oviparity or egg laying

   All chelonians (turtles and tortoises), pythons, most colubrids, monitors, and most iguanas lay eggs. Eggs can often be detected during visual examination in many species of snakes, although larger species like pythons require palpation. Viviparous or live bearing reptiles include many skinks, chameleons, boas, all rattlesnakes, and most vipers.

2. Egg production

   The process of egg production in reptiles is very similar to that described in birds.

   • Rising estrogen levels promote vitellogenesis or yolk production within the liver
   • Yolk is then transported via the blood stream to the reproductive tract where it is deposited in oocytes. During this time, lab results in female reptiles frequently reveals high levels of triglycerides, hypercholesterolemia, and hypercalcemia.
   • Triggers for ovulation are poorly understood in the reptile, but when ovulation > does occur, oocytes enter the oviduct and become fertilized. (Females of some species can store sperm for an extended period and produce fertile eggs after being separated from males for a long time).
   • As oocytes move through the oviduct, albumen is added and the shell is layed down.

   Depending on the species, the egg can normally remain within the oviduct anywhere from 9 days to 6 months, however most reptiles retain eggs for 1 to 2 months.

3. Many females can lay eggs in the absence of a male

   Many lizard species, including iguanas, geckos, and some snakes such as the corn snake can begin folliculogenesis and even lay eggs without the presence of a mate. Although these eggs are frequently infertile, this is not always the case since some species can store sperm for extended periods of time. This fact is important to keep in mind, since it means egg-related problems cannot be ruled out in many “single”, pet reptiles.

4. Thinly shelled eggs

   The thinly shelled eggs produced by many normal reptiles may surprise anyone familiar with the hard, strong avian eggshell. All snakes, most lizards, and some turtles produce thinly shelled eggs that are normally pliant. The eggs of tortoises, many geckos, and crocodilians are more rigid.

5. Phallus or hemipenes

   The chelonian phallus sits in a groove on the cloacal floor. When engorged, the phallus extends cranioventrally (Fig 3).

   

   Figure 3. Normal extended phallus in a box turtle (Genus Terrapene). Photo credit: Dr. Ed Ramsay. Click image to enlarge.

   Male snakes and lizards posses paired copulatory organs called hemipenes that normally range in color from pink to black. Hemipenes sit within pouches at the base of the tail just caudal to the cloaca (Fig 4, Fig 5). They are connected to the testes by the ductus deferens, and each hemipenis is functionally complete. During copulation, only one hemipenis everts into the female’s cloaca. Sperm travels along a groove on the outside of the everted organ.

Figure 4. Hemipenes of a Western diamondback rattlesnake (Crotalus atrox). Photo credit: Tess Thornton via Wikimedia Commons. Click image to enlarge.

Figure 5. Hemipenes in a green iguana (Iguana iguana) cadaver. Hemipenes are generally pink in the live specimen. Photo credit: Sahaquiel9102 via Wikimedia Commons. Click image to enlarge.

1. Sexual maturity

   Sexual maturity is determined primarily by size. The time it takes to reach a particular size can vary dramatically with husbandry, therefore sexual maturity is determined primarily by size as opposed to age in reptiles. Denardo (2006) has stated that under “optimal conditions”:

   • Snakes usually reach sexual maturity at 2-3 years
   • Small lizards at 1-2 years
   • Large lizards at 3-4 years
   • Chelonians at 5-7 years

2. Breeding season

   Most reptiles possess a distinct breeding season. Although some species reproduce throughout the year, most reptiles have a distinct breeding season. Although rainfall can be an important trigger for equatorial species, a change in temperature serves as the most important environmental stimulus in temperate zone species, with reproductive activity beginning after a period of seasonal cooling. Tropical boids are a notable exception to this rule of thumb since they tend to breed during cooler periods of the year. In the United States, reproductive activity in the green iguana (Iguana iguana) usually occurs between October and April.

3. The normal gravid lizard

   The gravid lizard frequently produces anywhere from 20 to 40 eggs. As ovarian follicles enlarge, they eventually fill the entire coelom compressing the gastrointestinal tract and frequently resulting in anorexia. Prominent coelomic distension may be seen and individual eggs may even become visible against the body wall.

   Iguana owners frequently describe loss of appetite 1 to 4 weeks before egg laying. Weight loss and some loss of body condition (reduced muscle mass over the pelvis, shoulders and limbs) can also be detected as eggs continue to mature. A behavioral change is also common. The lizard first becomes hyperactive, scratching and digging in various substrates as she intensely searches for an appropriate nesting site. Eventually she will become less active, and will spend more time basking. Aggression can also be observed (Box 3).

Box 3. Possible findings in the gravid lizard

Differences in the clinical appearance of a normal gravid lizard and one suffering from follicular stasis or dystocia can be subtle. Frequently the most obvious difference between a gravid lizard and one in dystocia is her attitude. The gravid female is alert, aware, and active (as possible) while the female in follicular stasis or dystocia may be lethargic, persistently anorectic, and possibly emaciated.

Summary

Like other taxa, many reptiles display secondary sex characteristics that can assist in confirmation of gender. Like Class Aves, many reptiles are also oviparous and the production of eggs is very similar to that seen in birds. Unlike the avian egg, the normal reptile egg is thin-shelled and pliant. The male intromittent reproductive organ also displays diversity in Class Reptilia. Chelonians possess a large, fleshy phallus, while lizards and snakes paired hemipenes. Reptiles are also unique in that puberty is size-related and therefore primarily influenced by environmental factors. Finally, the significant reduced metabolic rate of reptiles means that unlike gravid birds or mammals, the gravid reptile can normally display anorexia.

References