While the image of a snake or lizard laying eggs is common, not all reptiles reproduce in this way. Contrary to popular belief, a notable number of reptile species give birth to live young, a fascinating adaptation that allows them to thrive in diverse environments.
Beyond Egg Laying
Reptiles that give birth to live young exhibit two primary reproductive modes: ovoviviparity and viviparity. Ovoviviparity involves the development of embryos within eggs that remain inside the mother’s body until hatching. The developing young receive nourishment primarily from the yolk sac, with the mother providing protection and a stable internal environment. Once fully developed, the young hatch inside the mother and are then born alive.
Viviparity, on the other hand, is a process where embryos develop inside the mother’s body and receive direct nourishment from her, often through a placenta-like structure. This direct maternal provisioning, similar to that seen in mammals, allows for a closer physiological connection between the mother and her developing offspring. The young are typically more developed at birth compared to those from ovoviviparous species.
Reptiles That Give Birth Live
Many snakes are known to give live birth, encompassing both ovoviviparous and viviparous species. All four species of anaconda, including the green and yellow anacondas, are viviparous. Similarly, all snake species within the Boidae family, which includes boa constrictors and rubber boas, are viviparous or ovoviviparous.
Vipers are another group of snakes that predominantly give live birth. Most vipers, such as rattlesnakes and eyelash vipers, are ovoviviparous, with their eggs hatching internally before live birth. Some water snakes, belonging to the Colubrid family, are also viviparous, an adaptation likely linked to their aquatic habitats where finding dry nesting sites would be difficult. Almost all sea snakes exhibit viviparity because their marine environment prevents them from laying eggs on land. Garter snakes, common across North America, are also ovoviviparous, producing live young.
Among lizards, live birth is less common but still observed in several groups. The viviparous lizard (Zootoca vivipara), also known as the common lizard, is a notable example found across Eurasia; most populations are viviparous, especially in colder regions. Some skinks, particularly those found in cooler climates, also give live birth, with certain species like the blue-tongued skink being viviparous. The three-toed skink is particularly interesting as it can exhibit both egg-laying and live birth, sometimes even within the same population. Jackson’s chameleons are another example of lizards that are ovoviviparous, with females retaining eggs internally until the young are ready to emerge.
How Live Birth Evolved in Reptiles
The evolution of live birth in reptiles has occurred independently multiple times across various lineages, indicating its adaptive advantages. One primary factor driving this evolution is the protection offered to developing embryos. By retaining eggs or embryos internally, the mother can shield them from external predators and environmental hazards such as fluctuating temperatures or desiccation.
In colder climates, live birth offers an advantage because the mother can actively regulate the embryo’s temperature. She can bask in the sun to warm her body, thereby warming the developing young, a capability external eggs buried in the ground lack. This allows live-bearing reptiles to inhabit regions with shorter warm seasons or higher altitudes where external incubation might be unfeasible. Furthermore, live birth can be beneficial for species in aquatic or arboreal environments, as it eliminates the need to find suitable nesting sites on land or descend from trees to lay eggs, which could expose them to increased risks. The ability to move with developing offspring also enhances their chances of survival.