Snakes exhibit diverse reproductive strategies, with some species laying eggs and others giving birth to live young. While egg-laying is common, a significant portion of snake species employ live birth. This process involves internal development of offspring, offering advantages for survival.
Reproductive Strategies in Snakes
Snake reproduction primarily falls into two categories: oviparity and viviparity. Oviparity refers to the process where female snakes lay eggs, which then develop and hatch outside the mother’s body. Approximately 70% of snake species reproduce this way, with the female depositing her eggs in a safe, warm location, often abandoning them after laying. Some oviparous species, such as pythons, exhibit parental care by coiling around their eggs to regulate temperature and humidity.
Viviparity involves live birth, where the young develop entirely inside the mother’s body. This includes two distinct subcategories: ovoviviparity and true viviparity. Ovoviviparity is a common form of live birth in snakes, where eggs develop and hatch internally within the female’s oviduct, and the live young are then expelled. The developing embryos in ovoviviparous species primarily receive nourishment from a yolk sac, though some exchange of nutrients and oxygen can occur through the thin egg membrane from the mother.
True viviparity, less common in snakes, is similar to mammalian reproduction, involving the development of young inside the mother’s body without an eggshell. The mother nourishes the developing young through a placenta-like structure and a yolk sac, facilitating the exchange of oxygen, water, and waste. This method is particularly beneficial in colder climates or aquatic environments, where external eggs might not survive due to temperature fluctuations or drowning risks.
Common Live-Bearing Snake Species
A diverse array of snake species across different families exhibit live birth, primarily through ovoviviparity. Many vipers, including rattlesnakes, copperheads, and water moccasins, are examples of live-bearing snakes. All rattlesnake species are ovoviviparous.
Boas and anacondas are also known for giving live birth. All species within the Boidae family, which includes boa constrictors and all four species of anacondas (green, yellow, darkly-spotted, and Bolivian), are viviparous. Green anacondas can give birth to 20 to 40 live young, typically after a gestation period of about six months. Boa constrictors can deliver litters ranging from 10 to 65 young after a gestation period of approximately 100 to 150 days.
Garter snakes, common in North America, are another widespread example of ovoviviparous snakes. Often found near water, they typically give birth to 15 to 40 live young. Many water snakes in the genus Nerodia are also live-bearing, a trait that evolved due to their wet habitats where egg-laying would be challenging. Most sea snakes, except for a single genus, give live birth, an adaptation given that snake eggs cannot develop underwater. The Death Adder, found in Australia and Papua New Guinea, is ovoviviparous and can produce up to 30 snakelets per birth.
The Process of Live Birth in Snakes
The process of live birth in snakes begins with internal fertilization, where sperm from the male fertilizes eggs within the female’s reproductive tract. In ovoviviparous species, these fertilized eggs develop inside the female’s oviduct, each surrounded by a thin membrane and its own yolk for nutrition. The female retains these eggs internally throughout the developmental period, which can range from a few weeks to several months depending on the species and environmental conditions.
When the young are fully developed, they hatch from their internal egg membranes while still inside the mother. The mother then expels the live young, which are typically born fully formed, complete with scales, eyes, and fangs. Each neonate may still be encased in a thin, transparent membrane that it quickly slits open upon birth. For truly viviparous snakes like some boas and anacondas, the developing young establish a more direct physiological connection with the mother, exchanging materials through placental attachments to the uterus.
Immediately after birth, newborn snakes are generally independent and capable of fending for themselves. They are equipped with a sharp “egg tooth” on their upper lip, which helps them cut through any remaining membrane. Within a short period, often within a few days, the newborn snakes undergo their first shed and the egg tooth is lost. While some maternal care, such as brief coiling around the young, may occur in certain species, most live-born snakelets disperse quickly to begin hunting and surviving on their own.