Reptiles are fascinating creatures who are so different from other types of animals that many people have questions about how deep those differences go. One common question is “Do reptiles lay eggs?”.
Most reptiles lay eggs (called “oviparity) rather than give live birth. All turtles and crocodilians lay eggs, as do most lizards and snakes. The females deposit fertilized eggs into the environment and usually abandon them. The embryos are dependent on the nutrients and water stored in the eggs.
The evolution of the reptile egg was an incredibly important development in the history of life and reptile eggs themselves are life support system marvels.
Read on to learn about what makes reptile eggs so special and how the ability to lay eggs is vital to reptiles’ success.
What is an egg?
The term “egg” can be confusing because it is used to describe two related, but different, biological structures.
First, “egg” can refer to the female gamete or the female reproductive cell that unites with the male gamete to produce a zygote. A zygote is the cell containing both sets of chromosomes – one from each gamete – that develops into a new individual (Allaby 1988).
Second, “egg” can refer to the round or oval, shelled structure that female oviparous reptiles deposit and which develops outside their bodies. The shell contains an embryo, several membranes, and an egg yolk that provides nourishment.
This kind of egg is technically called an “amniotic egg” and is the focus of this post because it is the type of egg reptiles evolved to use for their reproduction.
What is the structure of an amniotic egg?
An amniotic egg is made up of three membranes (the amnion, the allantois, and the chorion), a yolk sac, and an outer egg shell, all of which surround the developing embryo and keep it hydrated, nourished, and protected. (Johnson 2012).
The amnion surrounds the embryo and secretes fluid that both cushions the embryo and prevents it from drying out.
The allantois is an extension of the embryo’s digestive tube (Martof et al. 1980), has a good blood supply, and is partly fused with the chorion. The allantois stores metabolic waste produced by the growing embryo and – along with the chorion – lets the embryo breathe through pores in the egg shell (Halliday and Adler 1987).
The yolk sac is the source of food for the growing embryo. It contains protein and fat and initially takes up most of the room in a freshly laid egg. The yolk shrinks as the embryo feeds off the stored nutrients, allowing the growing embryo more space within the restricted space of the egg shell.
Reptile egg shells are either leathery or parchment-like and pliable, or calcified and hard. In general, snakes, lizards, and aquatic turtles lay leathery eggs that can be dented with gentle pressure while terrestrial turtles, crocodilians, and geckos lay hard, brittle eggs (Halliday and Adler 1987).
As with everything in nature, the evolution of brittle versus pliable eggs within a reptile species involves trade-offs.
The internal environment within hard-shelled eggs tends to be more stable for the embryos because the shell loses and absorbs water slower than soft-shelled eggs.
However, embryos within pliable shells seem to develop faster. Turtle species who live in areas prone to flooding or which offer only a short season in which the eggs can mature tend to lay eggs with pliable shells (Halliday and Adler 1987).
Most reptilian egg shells are spherical but some turtles, like pond and mud turtles, lay elongated eggs. Spherical shells pack closely together when many eggs are laid in the same nest which offers two advantages to species that lay round eggs.
First, the reduced air space between spherical eggs reduces water loss and helps protect the embryo from drying out.
Second, female reptiles need to lay as many eggs as possible to ensure that enough embryos survive to hatch and hopefully grow to adulthood. Very few reptiles provide any care for their eggs after they are laid; usually the females lay the eggs then abandon the nests.
The most well-known exceptions to this behavior are crocodilians like the American alligator (Alligator mississippiensis) and certain species of lizards like the five-lined skink (Eumeces fasciatus).
Alligators guard their eggs and eventual hatchlings aggressively, which is an important exception since alligators can grow to nearly 20 feet (7m) and do not tolerate threats to their nests.
Five-lined skinks are much smaller creatures, growing only to about 8 inches (20.5cm), but work just as hard to protect their next generation. Females will fight or chase off predators to the best of their ability, rotate the eggs in the nest, and even urinate on them if they get too dry.
Reptile eggs are rich sources of fat, protein, and other nutrients. Many creatures eat reptile eggs; some actively hunt for nests to raid while other predators are more opportunistic and will consume any eggs they stumble across. Such predators can decimate any reptile nest they can find.
Animals that prey on reptile eggs in the southeastern United States include:
- Striped skunks (Mephitis mephitis)
- Virginia opossums (Didelphis virginiana)
- Long-tailed weasels (Mustela frenata)
- Minks (Mustela vison)
- Red foxes (Vulpes vulpes)
- American crows (Corvus brachyrhynchos)
- Other reptiles, especially snakes
Spherical eggs have the lowest surface-to-volume ratio of any shape (Halliday and Adler 1987), which means that the highest number of eggs can be packed into a given space if the eggs are round. The nests of reptiles that lay round eggs can be slightly smaller for the the same number of eggs than could be accomplished if they laid oblong eggs. This makes the nests slightly less obvious to predators and more likely that the embryos will survive to hatch.
The egg shell is the primary line of defense for the developing embryo, protecting the embryo from injury, parasites, and infectious microbes.
What is so special about reptile eggs?
Amniotic eggs are special because they are self-contained, aqueous environments on dry land. Despite living on land and breathing through lungs, reptile embryos still have to develop in a watery environment, which is provided by the protective membranes within the shell.
When reptiles developed amniotic eggs, they freed themselves from having to reproduce in water like amphibians and became fully terrestrial.
Since most reptiles do not care for their eggs after they are laid, eggs must provide everything the growing embryos need from the start. Apart from absorbing some water from the surrounding environment and allowing oxygen and carbon dioxide to exchange across the shell, an egg represents its own little world, one that is perfectly designed to support the life it contains.
Conclusion
Reptiles the world over owe much to their distant ancestors who created the first amniotic egg. They would never have evolved into so many species or distributed around the world so widely without the wonderful, humble egg.
Related Now I Wonder Posts
To learn more about reptiles in general, check out these other Now I Wonder posts:
- Do reptiles have cold blood?
- Do reptiles have hearts?
- Can reptiles get too hot?
- Can reptiles get too cold?
- Why can’t reptiles chew?
- Do reptiles give birth?
References
Allaby M, editor. 1988. Illustrated dictionary of science, Andromeda. Andromeda International: Windmill Books.
Halliday T, Adler K, editors. 1987. The encyclopedia of reptiles and amphibians. New York: Facts on File Inc.
Johnson GB. 2012. The living world. 7th ed. New York (NY): McGraw-Hill Companies Inc.
Martof BS, Palmer WM, Bailey JR, Harrison III JR. 1980. Amphibians and reptiles of the Carolinas and Virginia. Chapel Hill (NC): The University of North Carolina Press.
