Many people assume that all reptiles reproduce by laying eggs, a reproductive strategy known as oviparity. While this is true for a significant majority of reptile species, the diverse world of reptiles holds a fascinating exception. Some reptiles defy this common perception by giving birth to live young, showcasing a remarkable evolutionary adaptation.
Reptiles That Give Live Birth
Live birth in reptiles, known as viviparity, is found across various groups, particularly within snakes and lizards. Examples among snakes include all species of boas and anacondas. Garter snakes, common across North America, are also live-bearers, as are many vipers and pit vipers like rattlesnakes and Gaboon vipers. Sea snakes also give birth to live young, an adaptation well-suited to their aquatic lifestyle where laying eggs on land would be challenging.
Among lizards, several species have evolved live birth. Skinks, a diverse group found globally, include species like the Solomon Island skink, blue-tongue skink, and shingleback skink that give birth to live young. The common lizard (Zootoca vivipara) is found farther north than any other non-marine reptile, with most populations being viviparous. This Eurasian lizard can produce between three and ten young in July.
Diverse Reproductive Strategies
The term “live birth” in reptiles actually encompasses two distinct reproductive strategies: ovoviviparity and true viviparity. Ovoviviparous reptiles produce eggs that develop and hatch inside the mother’s body, with the young emerging alive. In this method, the embryos primarily receive nourishment from the yolk sac within their egg, and the mother provides physical protection and a stable internal environment. Many vipers, such as rattlesnakes, and certain skinks exhibit ovoviviparity.
True viviparity involves fertilized eggs developing inside the mother’s body with direct nutritional support, often through a placental-like structure. This maternal nourishment goes beyond gas exchange, involving the transfer of water and nutrients. While less common than ovoviviparity, some skinks and certain sea snakes demonstrate true viviparity, with embryos having a more intimate connection to the maternal system, similar to mammals. Some species, like the common lizard, can even exhibit both ovoviviparity and viviparity depending on environmental conditions.
Evolutionary Reasons for Live Birth
The evolution of live birth in reptiles has occurred independently over 100 times, suggesting significant adaptive advantages. One primary reason is enhanced protection for developing embryos from predators. Eggs laid externally are vulnerable to consumption by various animals. By retaining embryos internally, the mother can actively defend her offspring, increasing their chances of survival.
Another significant advantage, especially in cooler climates, is thermal regulation. Reptiles are ectothermic, meaning they rely on external heat sources to regulate their body temperature. A live-bearing mother can move to warmer or cooler areas, such as basking in the sun, to maintain an optimal temperature for her developing young, something static eggs cannot do. This allows viviparous reptiles to colonize regions too cold for egg incubation, such as higher latitudes or mountainous areas. While live birth places a higher metabolic demand on the mother, the increased offspring survival rates often outweigh this cost.