Though often associated with birds, eggs with shells were "invented" by reptiles. The evolutionary adaptation allows reptile eggs to be laid on land and gives them some degree of protection from the environment and potential predators at the same time – traits that shell-lacking amphibian eggs lack. While not all of them practice parental care of their young, a wide variety of reptile species lay eggs, from snakes and lizards to turtles and crocodiles: As humans continue to build and change the environments that reptiles live in, it's important to know where these eggs are laid, so people can avoid harming the reptiles' ability to reproduce.
TL;DR (Too Long; Didn't Read)
The amniotic, or shelled egg, is among the more famous of the evolutionary reptile adaptations. While not all reptile species lay eggs, those that do generally do not take care of their eggs: Many reptile eggs are simply laid in appropriately warm areas or quickly built nests and left to fend for themselves. These locations can range from warm dips in sand to holes dug into the ground to areas of beach loosely protected by rocks, depending on the type of reptile.
Revolutionary Reptile Eggs
The eggs you're familiar with, laid by animals like chickens and iguanas, are incredibly special tools: These shelled eggs, referred to as amniotic eggs, are an evolutionary adaptation developed when amphibians were starting to evolve into the earliest species of reptiles. Unlike the eggs of animals like frogs, which must be laid in water and defended because water offers no protection to the growing embryo, shelled eggs wrap everything required for the embryo to grow into a fluid-filled, protected package that can be laid on land and left alone. While this may not seem like much today, this adaptation allowed creatures to live entirely or almost entirely on land, giving them access to more food – and minimizing the risk presented by water-based predators that could eat an amphibian's eggs while the parents were on land. The only downside to the amniotic egg is that it is designed strictly for land use: If one of these eggs is submerged in water, the embryo inside will drown.
Reptile Reproduction Habits
Not all reptiles lay eggs; on the contrary, some species of snakes and lizards give live birth. But regardless of whether a reptile lays eggs or gives live birth, only crocodiles and related reptiles practice parental care of young – in other words, when most reptiles lay eggs, the eggs are laid in a location and left to either hatch or be eaten. Any newborns that survive long enough to hatch will have to fend for themselves immediately. As a result of this, reptiles will lay anywhere from five to 100 eggs at a time (depending on species) – though the location where the eggs are laid is not always given much mind. Curiously, while reptiles are generally solitary, many species will lay their eggs in communal nests for the sake of convenience (though this can actually reduce the chance of a newborn reptile surviving to adulthood). Eggs are laid in warm areas where there is no risk of an egg's interior freezing or drying out.
Dedicated Nests and Shallow Holes
Generally, snakes pay the least attention to where their eggs are laid: Many species of snake will deposit their eggs into dips or shallow holes found in sand or warm grass, or in small holes that are sometimes covered with grass or leaves in order to hide the eggs from potential predators like raccoons. Lizards will dig holes in warm, secure spaces to lay a clutch of eggs, and turtles will, famously, lay their eggs under the sand in safe spaces under rocks, in sites the turtles return to year after year. Crocodiles, as the only reptile type that maintains a nest after laying eggs, will either dig small holes to lay eggs or create mound nests, where eggs are covered in dirt and mud that hardens, creating a protective layer that the mother must later crack open to allow her newborn children to exit.
- Stanford University: Parental Care
- Stanford University: Eggs and Their Evolution
- North Carolina State University: Reptiles
- Fernbank Science Center: Reptiles
- University of Illinois Wildlife Epidemiology Laboratory: Week 6: Digging Holes and Dropping Eggs
- UCSB ScienceLine: How Do Iguanas Reproduce? Do They Lay Eggs or Give Live Births?
- NCSU Extension: Reptiles and Amphibians in Your Backyard
- EurekaAlert: Why Solitary Reptiles Lay Eggs in Communal Nests
- University of Florida: The Croc Docs: Nesting Crocodiles and Hatchlings
- Wikimedia Commons