Reptiles are oviparous, meaning that they lay eggs. Some squamates, such as snakes and crocodilians, give birth to live young.
During copulation male reptiles discharge sperm from one of their two hemipenes into the female’s cloaca. The sperm then moves up into each of her oviducts to be fertilized.
Many species of snake and turtle can store sperm for up to six years to fertilize future clutches. This is called non-genetic sex determination.
Fertilization
Reptiles may reproduce in a number of ways. Most lay eggs, and this method of reproduction is known as oviparous reproduction. However, some species of lizards and snakes are ovoviviparous, meaning that they carry the eggs inside their bodies until they hatch, and then give birth to the young.
In ovoviviparous reptiles, fertilization occurs when the male and female produce hormones that cause them to become sexually receptive. The male then mounts the female and inserts his reproductive organ into her cloaca. After the sperm enters her body cavity, it meets the egg cytoplasm and causes fertilization.
Once the sperm has entered an egg, it swells and its chromosomal material disperses. A new envelope rapidly forms around the male pronucleus, and a membrane separates it from the egg nucleus. The egg nucleus then begins to duplicate the chromosomal material of the sperm, creating two identical copies of the sperm molecule.
The duplicate molecule of the sperm is then passed to the egg, and it begins to develop the structures that will form an outer shell, or exoskeleton. This calcification, called ovoviviparous ossification or ovulation ossification, usually occurs at least several days before the reptile is due to lay its eggs. The amount of ossification is dependent on the temperature, and it is why some ovoviviparous reptiles will not ovulate in hot weather.
There are a number of other factors that can influence the development of the egg, including the number of follicles present, the ovarian environment and the type of ovulation hormone produced. It is also possible for a reptile to be polygynous, and this can lead to multiple paternity. However, the results of studies examining this issue have been mixed. It is likely that more research will be needed before conclusions can be drawn.
Eggs
Most reptiles reproduce sexually, but a small number of species are able to produce offspring without fertilization. These eggs are often buried in the ground or in nests, and many reptile parents have little to do with them once they have been laid. The most famous example is the leatherback turtle, whose eggs can take decades to hatch.
Reptiles that lay eggs are called amniotes because they have an amniotic membrane that encloses their embryos. This membrane eliminates waste and allows gas exchange between the embryo and the outside world. It also cushions and protects the developing embryos, and provides nutrition from an egg yolk.
The outer layer of the egg is composed of a hard shell that is either flexible or inflexible. The inner portion is made of a jello-like substance called albumin, which provides water and protein for the embryos, and a fatty membrane that carries oxygen and nutrients to the eggs. In addition to these membranes, eggs of reptiles contain three extra-embryonic membranes called chorion, amnion, and allantois. The chorion provides protection and support for the embryo, while the amnion absorbs shock and prevents excessive movement of the embryos. The allantois collects metabolic waste products and facilitates respiration.
Because of their amniotic structure, reptiles’ eggs do not have to be kept constantly moist. In fact, they can be buried in dry sand or soil and still develop normally. In captivity, eggs should be placed in a container of vermiculite that is lightly moistened with bottled water. Rotating the eggs is generally discouraged, as this can cause them to crack or rupture.
Among vertebrates that are capable of laying eggs, only reptiles and some birds have evolved to do so successfully. Some mammals, including platypuses and echidnas (spiny anteaters), lay live young. These offspring, however, do not have the specialized tissues that characterize amniotes, and they are not considered true reptiles.
Males
Most reptiles are oviparous, meaning they lay eggs. However, some species are ovoviviparous, like crocodiles and alligators, giving birth to live young. In this case, the sex of the offspring is determined at fertilization.
Male reptiles do not have external genitalia to help identify their sex, but they do possess copulatory organs. These may be a single penis (turtles and tortoises, crocodilians) or a pair of hemipenes, seen as two bulges behind the cloaca at the base of the tail (lizards and snakes). When the reptile is ready for mating, the hemipenes or penis are inserted into an opening adjacent to the female genital opening, termed a cloaca. Sperm then enters the cloaca and fertilizes the eggs within it. Once fertilized, the sperm can remain intact for years, and can be used to fertilize subsequent clutches by the female without the aid of another male.
During development, reptile embryos produce sex hormones that control egg formation. These sex hormones can result from either genotypic, or chromosomal, determination or from environmental factors. Temperature is one environmental factor that influences the production of sex hormones in an embryo. Eggs laid at lower temperatures produce a higher percentage of male offspring, while eggs incubated at higher temperatures produce more female offspring.
As eggs mature, they are deposited in the oviducts or cloaca and develop into amniotic sacs. Occasionally, an embryo fails to develop or misdevelops, causing the patient to suffer from dystocia, a condition that causes difficulty in giving birth or laying eggs. Dystocia in reptiles can be obstructive or non-obstructive and is associated with poor husbandry, stress and disease.
It is important to recognize the nuances of reptile reproductive physiology. As with dogs and cats, reptile patients can present a variety of symptoms that are difficult to interpret. The diagnosis of these patients requires careful consideration of their morphology, environment and diet.