Reptiles lay amniotic eggs that protect and nourish the embryo. They also contain a rich source of nutrition in the yolk.
Most reptiles reproduce sexually. Males have one or two penises that pass sperm into the female’s cloaca where fertilization occurs.
However, some reptiles (such as this house gecko) reproduce asexually through parthenogenesis.
Embryonic Development
Reptiles are tetrapod vertebrates, or creatures that have four limbs. Most of them lay eggs, while some — including snakes — give birth to live young. As in mammals, reptiles have a system of XY chromosomes, with XX individuals being female and XY individuals being male. However, sex can also be determined by the environment — called temperature-dependent sex determination (TSD). This non-genetic process, which is common among vertebrates, occurs in some lizard and snake species and all crocodiles.
In most oviparous reptiles, females produce hormones that make them sexually receptive to sperm. The male mounts the female and then discharges sperm through an opening in the female’s body cavity, called her cloaca. The sperm enters the cloaca and is fertilized by her egg cells. The resulting embryo is known as a blastula, a hollow ball of cells that will later develop into the gut, skeleton, and other organs.
Temperature plays an important role in TSD, and a 2008 study by Daniel Warner at Iowa State University in Ames, Iowa, and Richard Shine at the University of Sydney in Australia suggests that temperature-dependent sex determination optimizes reproductive success in some reptile species that exhibit it. This is because TSD allows a smaller number of eggs to be fertilized by the sperm of a single female. As the sperm fertilizes an embryo, it also releases molecules that promote rapid growth and development of the fetus.
Fertilization
Fertilization is the union of sperm and eggs to create a diploid zygote. In reptiles, fertilization occurs internally. Male sperm penetrates the egg and is absorbed by the embryo’s chorion, which contains cell-like structures called test cells. The chorion also prevents self-fertilization, which would allow the zygote to develop into two different species of offspring. In mammals, fertilization happens after contact between sperm and an opening for the digestive, excretory and reproductive systems called the cloaca.
In some reptiles, such as crocodilians, the cloaca is located in the tail. In other species, such as turtles, it is a separate structure from the body. The test cells are located near the cloaca. These test cells can absorb sperm even when the oviducts are closed, and can allow for fertilization without direct male contact.
Most reptiles are ovoviparous, laying eggs that are hatched outside their bodies, but some lizards and snakes are ovoviviparous, giving birth to live young that they nurse inside their own bodies. The newborn animals emerge covered in amniotic fluids.
The sex of most reptiles’ offspring is determined by the chromosomes in the fertilized eggs, but researchers have found that the sex of some turtles and crocodiles can be changed if the environment is controlled. For example, injecting eggs with inhibitors of estrogen production can produce male offspring, even when the eggs are incubated at temperatures that normally produce females.
Ovulation
Reptiles reproduce sexually, but they also have the ability to lay eggs without fertilization. The eggs of oviparous reptiles have membranes, shells and other structures that protect them. In the laying process, an egg goes from being an inactive (pre-vitellogenic) ovary to becoming a mature egg. During this time, estrogen stimulates the liver to convert fat stores into vitellogenin. The resulting substance is released from the follicles and is added to the shell as the eggs pass through the oviducts and into the coelomic cavity.
During mating, male reptiles transfer sperm into the female’s cloacae via a genital opening called the cloaca, which is not a true uterus but a pair of oviducts. Female lizards and snakes have right and left oviducts and can shed their eggs through one or both of these structures.
Once the semen is deposited in the cloaca, the sperm swims up through each oviduct to an opening adjacent to an ovary. The eggs of oviparous squamates are normally fertilized, but the eggs of some oviparous species can become malformed or dead during this process. It is possible that infertile and abnormal eggs are resorbed from the oviducts of some oviparous lizards and snakes, although such resorption has not been demonstrated to occur with any regularity. If it did, the resorption of infertile eggs might allow a female to minimize the loss of her reproductive investment in unsuitable conditions such as unseasonal climatic change or depletion of maternal fat reserves.
Birth
Reptiles that lay eggs are called oviparous, but some give birth to live young. This is called vivipary.
Like mammals, lizards and snakes have evolved to allow embryos to develop inside the mother for a period of time after fertilization. This is called extended embryo retention, and it has important ecological advantages. Embryos retained inside the mother are protected from predators, and they can be fed by her through a placenta. These developments may have been important factors in the evolution of a mammalian uterus and placenta, which allowed mammals to carry their offspring until they are ready to hatch.
Most reptiles use internal fertilization when mating, and most have amniotic eggs that are surrounded by extra-embryonic membranes (the amnion and allantois) and a calciferous shell to protect them from predation. The egg contains a yolk that provides rich, fatty food for the developing embryo.
Most oviparous reptiles, including turtles and crocodiles, have either a penis or a structure called a hemipenis, which is used to deliver sperm into the female’s cloaca during mating. Some, such as the tuatara (Sphenodon), lack a penis and instead copulate by closing their cloacas together until the male discharges sperm. This is called cloacal apposition, and it is also the way that tuataras store their sperm for future fertilization. This stored sperm is often the reason that tuataras can reproduce so rapidly.