Why are some reptiles born alive instead of hatching from eggs?

Some reptiles give birth to live young through a process called viviparity, which has evolved independently many times within the squamates (lizards and snakes). This reproductive strategy is often observed in species living in cold environments, such as those at high altitudes or extreme latitudes, where the ability to retain developing embryos allows the mother to regulate their temperature more effectively than if they were deposited in a nest.12

The transition from egg-laying (oviparity) to live-bearing involves a range of biological mechanisms:12

• Egg Retention: In some species, the only difference is that the female retains the eggs in her oviduct until they are fully developed, sometimes without the formation of a shell.12
• Nutrient Exchange: While some live-bearing reptiles rely solely on the yolk for nourishment, others have evolved placentae that allow for the transfer of nutrients, oxygen, and waste between the mother and the fetus. This ranges from simple gas exchange to complex systems where the mother provides nearly all the necessary nutrition for development.132

Unlike turtles, crocodiles, and tuatara, which are exclusively egg-layers, squamates demonstrate a diverse evolutionary spectrum of reproductive modes.1