Reptiles: Born Alive Or In Eggs? It Depends On The Strategy

One of the key characteristics that people often associate with reptiles is egg-laying behaviors, but not all reptiles will lay eggs and incubate them externally, some actually give birth to live young!

Different species of reptiles have different reproductive strategies that determine whether they lay eggs or give live birth. Reptiles that lay eggs include bearded dragons, many geckos, turtles, and crocodiles. Reptiles that give birth include blue-tongued skinks, boas, vipers, and some geckos.

There are benefits to both egg-laying and live birth reproductive strategies and lifestyle and environmental evolution is what drives this. Read on to learn more about these two different reproductive strategies and factors that influence reptiles’ reproductions.

Reptiles that are born alive and reptiles that lay eggs

Check out the below table to see examples of common reptiles and how they reproduce.

Reptiles that lay eggsNo. of offspring in a clutchReptiles that give live birthNo. of offspring in a clutch
Leopard Geckos2 eggs (average 5 clutches)Blue-tounged skink 6 – 20 young
Crested Geckos2 eggs (6-10 clutches)Shingle-backed skink 2 or 3 large young
Bearded Dragons 20 eggs (up tp 5 clutches)Boasup to 60 young
Iguanas20 – 71 eggsVipers 20 – 40 young
Water Dragons6 – 18Garter snakes10 – 40 young
Monitorsup to 50 eggsAll geckos & skinks native to New ZealandGeckos – 2 young
Skinks – up to 10
Turtles & Tortoises species-dependent (can reach up to 110 eggs!)Some geckos native to New Caledonia2 young
Crocodilians30 – 60 eggsJackson’s chameleon30 young

All species of reptiles are precocial, meaning they are born fully physically capable. Basically a mini version of the adult of the species! This is beneficial because once born or hatched reptiles receive no parental care from their parents. Some species will guard nests during incubation, but many reptiles will lay their eggs or give birth then leave their offspring to fend for themselves.

Egg-laying in reptiles

The scientific name for egg-laying is Oviparous. Ovi meaning and relating to egg and parous meaning giving birth.

Reptiles that hatch from eggs need to be able to break through the shell as they hatch.

Most reptiles will have an egg tooth to help them break out of the tough egg but turtles, crocodiles, and tuatara have a caruncle, a fleshy growth, on their snout. This resembles a horn and they use this to bust out from the egg.

Advantages of egg-laying in reptiles

  • Less stress – carrying offspring to maturity can come with a great physical toll. Egg production and partition are not as stressful to the mother and her body.
  • Higher possibility of multiple clutches – egg production inside a reptile takes less time than producing a full-sized offspring as the rest of the development will occur inside the egg once it is laid. This means the body has more resources to create more clutches
  • Survival – in the case that the mother dies, eggs can survive without her.
  • Genetic diversity – fertilization of eggs for laying allows for multiple matings from different males meaning there’s a higher chance to produce a greater diversity of young

Disadvantages of egg-laying in reptiles

  • Vulnerable nests – eggs laid and left to incubate make a tasty treat to passing predators.
  • Development of young – eggs are usually laid in large amounts to increase survival, with less energy put into each individual. Hatchlings are often small and have little defense, making their survival rate lower.
  • Environmental needs – mothers need to search for the perfect environmental conditions to lay their eggs to ensure their survival, this can be increasingly difficult in an everchanging climate!

Not all nests of eggs or freshly hatched young are left alone, this bullfrog dad does an amazing job of protecting his young, even digging a trench to some fresh water to ensure they stay moist, which is vital for the amphibian lifecycle.

Live birth in reptiles

Live birth in reptiles is called ovoviviparous. You will see this word is very similar to the name for egg-laying. That is because the process is much the same except instead of laying the eggs, they are retained inside the mother where they hatch then are birthed.

Reptiles’ live birth is different from a mammalian live birth as the offspring are still fed and develop from egg yolk and do not have a placenta.

Advantages of live birth in reptiles

  • Protection from the environment – since development happens inside the mother, environmental conditions can be more favorable as the female can move to optimal positions. Laid eggs stay in the same spot they are laid regardless of environmental change.
  • Thermoregulation – the internal temperature of the female can be managed to stay stable and adjusted as needed to provide optimum growing conditions for young
  • Developed young – since less young are in a clutch of oviviviparous reptiles over oviparous reptiles, the young come out more developed and ready to get moving, increasing the initial chance of survival.

Disadvantages of live birth in reptiles

  • Less young – space is limited inside the mother! giving live birth means on average there are a lot less young than a clutch of externally laid eggs.
  • Reliance on mother – the mother has to ensure she remains in good condition throughout the period which she is carrying young. If her health suffers she will pass that on to young she carries.
  • Birthing complications – young that are birthed live can come with a higher risk of complications, such as impacted and prolapse, in birth as they are larger and more awkward than a small smooth egg.

Check out this incredible video of a chameleon giving birth, look how quickly the young are up and about.

Asexual reproduction in reptiles

Most reproduction in reptiles requires two individuals, one of each sex. But some female reptiles can reproduce all on their own!

This process is called parthenogenesis and there are approximately 50 species of lizard and 1 species of snake that reproduce exclusively this way and many others who can reproduce asexually in some circumstances.

This is an incredible evolutionary adaption so species can continue to multiply without a suitable mate or wasting precious ectotherm energy on mating searching and courting behavior.

The disadvantage to this particular strategy is that all offspring are basically clones of the mother meaning there is no opportunity for mutations to drive evolution over time, nor is their genetic diversity to create resilience in a population.

If all the genes are the same then a disease that kills one individual has the potential to wipe out whole populations very quickly!

Factors of reptile reproduction

Temperature

As ectotherms who rely on external sources of heat to thermoregulate, reptiles’ lives are largely revolved around temperatures and reasons.

Some reptiles have a phenomenon called temperature-dependent sex determination, meaning different temperature ranges force certain sexes to hatch. All crocodiles, some turtles, tuatara, and some lizards possess this trait.

Seems cool, but since climates are changing across the globe, the slightest change in temperate can alter the sex ratios of certain species and poses a significant threat to their survival. After all, if all offspring are female, how would they go on to reproduce!

Health

This one is pretty self-explanatory, an unhealthy animal is not going to have the energy or resources to produce offspring, if they do their offspring may be compromised with development issues or poor immune systems.

Lizards are able to reabsorb fertilized eggs if they find themselves under pressure. This is common when a lizard loses its tail. The stress will cause reabsorption as not to waste energy on offspring when it needs to regrow a tail.

Adult body size is just one aspect associated with the number of offspring; genetic constitution and nutrition are also major factors.

are all reptiles born in eggs

Clutch size

The size of a species clutch is evolution-driven. Those whose eggs are at the most risk tend to produce larger clutches to compensate for the potential losses of offspring and to increase the survival rate. For example, sea turtles can produce more than 100 eggs in a clutch!

Many geckos have fixed clutches of just 2 eggs, one egg per ovary per cycle. This is due to the small size of most geckos, they are incapable of carrying large loads of eggs.

Nutrition

It should go without saying that producing new life demands a lot of resources from the body of any animal!

Since reptiles produce eggs, even the ones that give live birth, they require high levels of calcium to produce eggs that are strong and healthy to protect the young. Calcium is vital for the growth and development of bones of the offspring also.

The demands of reproduction can also cause dehydration, if you have reptiles breeding in your collection you should offer them electrolytes before and after birth or egg-laying to help them replenish their supplies and not fall ill. This electrolyte is awesome for breeding reptiles.

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