Reptiles are cold-blooded vertebrates with dry skin and lay soft-shelled eggs. Most reptiles reproduce sexually but some have the ability to produce asexually in a process called parthenogenesis.
During the mating process, male reptiles pass sperm through their penises or hemipenes into the female’s cloaca. The sperm fertilizes her eggs, creating amniotic embryos that have a shell to protect them and albumin for water, protein, and fatty food sources.
The process of fertilization is when sperm and an egg combine in order to form a single-celled diploid zygote with all the information it needs to develop into a fully-fledged organism. This can either occur inside (internal fertilization) or outside the body of the female (external fertilization).
Reptile eggs have a hard shell that protects them from shock and weather, while also being permeable enough to allow the passage of oxygen and carbon dioxide. They contain albumen, a calcareous layer, and an amniotic fluid called allantois that provides water and energy for the embryo. They are symmetrical and are initially white in color. The eggs also have three extra-embryonic membranes: chorion, amnion, and allantois.
Male reptiles, like lizards and snakes, have a tube-shaped organ that is similar to a mammalian penis. During copulation, sperm is released into the female’s cloaca via this structure. Once in the cloaca, sperm enter each ovary and fertilize them. Most reptiles are oviparous, meaning they lay eggs. However, some species, such as geckos and Komodo dragons, are ovoviviparous, meaning they nurse their eggs and give birth to live young.
Ovulation is the process of a reptile releasing eggs. Most reptiles are oviparous, and females release their eggs after fertilization. However, some lizards, including New Mexico whiptails, reproduce asexually.
Reptiles use pheromones during breeding behavior to attract the opposite sex. Males may also use a courtship dance to attract females. In this dance, the forepart of a male’s body is held high and entwined with the tail of a female to create a swaying wrestling match. This pheromone exchange is believed to encourage cloacal apposition.
In most cases, a female will release her oocytes in spring after a period of cloacal apposition with a male. Males commonly complete spermatogenesis in late summer and store the sperm until the eggs are ovulated.
Reproductive diseases are common in reptiles and should be included on the differential list for any symptomatic patient. Although sex determination in reptile patients can be difficult, it is critical for proper treatment and diagnosis.
Oviposition is the process of depositing eggs. Oviparous reptiles (such as all crocodylians, turtles, and the tuatara) produce amniotic eggs that contain membranes and other structures that protect embryos from contaminants, regulate temperature, and provide nourishment. These eggs are also rich in energy, which provides the embryos with a source of fuel for development.
In order to fertilize the eggs, males must deposit sperm into the female’s cloaca during copulation. Most females have a right and left oviduct that open into an opening adjacent to each ovary. After a male deposits semen into the female cloaca, the sperm travel up through each oviduct and to an opening adjacent to one of the ovaries.
Insects can recognize oviposition-deterring pheromones emitted by their natural enemies to avoid the sites where they may be attacked by the enemy1. In addition, insects can use chemical volatiles to select suitable oviposition sites2.
Most reptiles are oviparous, meaning they lay eggs that hatch outside of the mother. However, some reptiles such as snakes and lizards are also viviparous, giving birth to live young. In these animals, the embryo develops within a hollow egg sac that contains its own yolk and is housed inside the female body until it’s ready to hatch. The egg yolk provides the offspring with nutrition during this stage of development, known as ovovivipary or yolk-sac viviparity.
In most ovoviviparous reptiles, males complete spermatogenesis in the summer and store the sperm until the female is ready to ovulate. In some cases, such as in a snapping turtle (Chelydra serpentine), sperm are released directly into the female’s cloaca. In other reptiles, such as a tuatara (Sphenodon), the male cloaca is aligned with the female’s cloaca through a close connection called a cloacal sphincter or hemitomelanome, which acts like a placenta and provides for gas exchange and a common chamber and outlet for excretion and reproduction.
In some ovoviviparous reptiles, such as the aforementioned tuatara, a prolapse of the cloacal sphincter can occur. This condition is treated through the use of analgesics and lubrication to reduce inflammation and a coeliotomy, which involves the removal of the prolapsed tissue.
The embryo of viviparous animals grows inside the mother’s body until it is mature and can survive independently. This is a common reproductive mode in mammals and some reptiles, such as snakes. Viviparous animals have the advantage of assured maternal protection and nourishment throughout development, known as gestation.
Viviparous reptiles typically have well-developed placentas that allow for direct transport of nutrients from mother to offspring and removal of metabolic wastes. In the case of lizards, such as viviaporus lizards, this is accomplished through an opening in the rear of the pelvis called the cloaca, which is used for both egg production and reproduction.
Viviparity has evolved multiple times in the squamate reptiles (snakes, lizards and geckos). A viviparous fossil was recently discovered of the pachypleurosaur Keichosaurus hui from the Middle Triassic of China (Lin and Rieppel, 1998). Several other dinosaurs are thought to have been viviparous based on the morphology of their eggs and their oviducts. Viviparity may also have been common in many extinct reptiles, including saber-toothed cats, crocodiles, and plesiosaurs. Viviparous females typically produce only one offspring at a time, unlike whales and seals, which usually have multiple fetuses.