While many reptiles are known for laying eggs, a significant number of species give birth to live young. These exceptions highlight the diverse reproductive adaptations within the reptilian lineage.
Reptiles That Give Live Birth
A notable portion of snakes and lizards give live birth. Most vipers, boas, and anacondas, for example, produce live young. Boa constrictors, native to South American tropical forests, can deliver up to 60 snakelets at a time, with newborns often over a foot long. Garter snakes, common across North America, also produce live offspring, typically ranging from 10 to 40 babies per litter.
Beyond snakes, approximately 20% of lizard species reproduce via live birth. These include skinks such as Solomon Island, blue-tongue, and shingleback skinks. The viviparous lizard, Zootoca vivipara, found across Eurasia, is notable for living farther north than any other non-marine reptile, though some populations can still lay eggs. Jackson’s chameleons, primarily found in Kenya, are also among the few chameleon species that give live birth. All true sea snakes, except for a single genus, are exclusively live-bearing, delivering their young directly into the water.
How Reptiles Reproduce
All reptile reproduction involves internal fertilization. The most common method is oviparity, where the female lays eggs with leathery or calcified shells, and the embryos develop outside her body, relying on the yolk for nourishment.
Another strategy is ovoviviparity, where eggs hatch inside the mother’s body, and live young are born. While the eggs are retained internally, the embryos typically receive most of their nutrients from the yolk sac. A third method is viviparity, representing true live birth. In this case, the embryo develops inside the mother’s body, receiving direct nourishment from her, often through a placenta-like structure.
The Process of Viviparous Birth
After fertilization, embryos develop within the mother’s oviducts. Unlike egg-laying species, where a protective shell forms around the egg, viviparous reptiles do not form a hard external shell. Instead, embryonic membranes within the oviduct facilitate exchange processes.
The developing embryo receives nourishment, oxygen, and water from the mother. While a yolk sac provides initial nutrition, analogous placenta-like structures, derived from fetal and maternal tissues, develop to support the growing embryo. These structures, such as the chorioallantoic placenta, transfer nutrients and gases between the mother and the offspring. Upon completion of development, the fully formed young emerge from the mother’s body. They are typically born encased only in an embryonic sac, from which they quickly break free.
Why Some Reptiles Give Live Birth
The evolution of live birth in reptiles is often linked to environmental and ecological pressures. Protection from predators is a significant factor; eggs laid in nests are highly vulnerable. By retaining embryos internally, the mother offers a protective environment, which significantly increases the survival rate of offspring.
Temperature regulation also plays a major role, particularly in colder climates or environments with fluctuating temperatures. A pregnant female can actively thermoregulate her body by basking in the sun, maintaining a warmer and more stable temperature for her developing embryos than would be available in an external nest. This maternal control over temperature can accelerate embryonic development and improve offspring quality, especially where incubation periods might be too long for eggs to hatch before cold weather sets in. Live-born young are also immediately mobile, allowing them to escape danger or find resources quickly after birth.