One of the most interesting things about reptiles is their reproduction. Not only the genetics and gender determination, but also whether or not they lay eggs or give what most call “live birth.” This one factor alone is a major determining factor in where they fall in scientific nomenclature.
Egg laying reptiles are what most people are familiar with since they seem to make up the largest groups of reptiles. These animals are referred to as oviparous. Ovoviviparous is the term used for reptiles that seem to give live birth. We will discuss the differences in these terms first.
Oviporous is the correct scientific term for animals that lay eggs with a shell from which offspring emerge. Not only do many reptiles fall into this category but also birds and even platypus are oviparous. The eggs of most reptiles have a leather-like shell that thins as the hatch date looms near. Some reptiles, such as turtles and tortoises produce eggs with hard shells. Eggs of reptiles come in all different sizes and shapes to best accommodate the size and shape of the offspring.
Ovoviviporous is the correct scientific term for animals that carry the eggs internally and upon delivery of the offspring the entire shell structure has thinned to the point that only the thin mucous membrane remains from which the offspring emerges. Because no whitish shell structure is seen and the membrane is sometimes broken through by the time the baby is fully delivered by the mother, people often assume that these babies are born live.
True live-bearing animals are called viviparous and the major difference is that no internal egg structure is present at anytime during the development of the offspring. Mammals are viviparous, as are some fish.
Snakes can be oviparous or ovoviviparous. One of the easiest ways to find out about a particular snake is to learn it’s scientific name and know what family it belongs to, either colubridae, boidae, viperidae or another. Colubrids are generally oviparous with few exceptions. So, knowing this, you can expect eggs from snakes such as corns, kings, pines, milks and numerous others. The most noted exception to this is the garter snake group, which is ovoviviparous. Females do not maternally care for the eggs in the wild and act no differently in captivity. Keepers need to artificially incubate the eggs if offspring are actually desired.
Boids are divided into 2 categories based primarily on the way they reproduce. Areas of the world in which they are found naturally plays a small part of their classification, but there are some species, which defy this single factor. Generally, pythons are found in more tropical regions while boas can be found farther north or south of these ranges.
Pythons are, by nature, egg layers. A major difference between the reproductive nature of pythons and colubrids is that pythons maternally incubate the eggs. The female will keep the eggs within her coils and use muscle contractions to raise the temperature around the eggs to an ideal level. Pythons, because of the more temperate climates they inhabit developed no need to carry their eggs internally to allow their offspring a better chance of survival. This is a primary reason that scientists have not reclassified the Calabar Burrowing Python of the western regions of Africa. While appearing like a typical sand boa and other related boas, the burrowing python does in fact lay eggs.
Boas give birth to offspring that must only extricate themselves from the thin mucous membranous sack upon arrival into the world. The egg structure they developed in while being carried by the female has already thinned and disappeared prior to their birth. This may have developed in slightly cooler times in history when the survival of the species required the females to hold the eggs internally in order to allow higher survival rates in the offspring. Because the temperatures developing offspring were exposed to was determined by the body temperature of the gravid (pregnant) female, changing basking sites, even in cooler temperatures, allowed healthy development. If these offspring had been required to develop inside of an egg exposed to nature, cold temperatures could have killed offspring before they had a chance to hatch. An example of boas adapting to cold temperatures is the North American rubber boa. Individuals of this species have been seen basking on snow banks within their range in temperatures ranging in the low 50 degree F range.
Venomous species are not the preferred snakes of this author, but a basic knowledge is always useful. For more in depth questions, I always defer to more knowledgeable keepers of a specific species. It is fair to say that venomous species vary in the delivery of offspring, just like we find in non-venomous species. A simple way to think of it is to know whether a species is a type of viper or not. Typically all vipers, even pit vipers, are ovoviviparous. Most elapids (which include cobras, coral snakes and all other fixed-fang snakes) produce eggs, which are not maternally incubated or protected. However, the King cobra does in fact guard the egg laying site/mound until the offspring hatch. Another exception to the rule, in regards to elapids, is the tiger snake, which is ovoviviparous. This could be due to the total extent of their range, which includes the island nation of Tasmania where cooler temperatures are experienced. The Australian death adder may be another exception to the rule for elapids but this author does not have enough information available at this time.
This is almost an easy section to discuss. Most lizards do in fact lay eggs, and that makes them oviparous. However, there are a few species that defy this simple statement. The monkey-tailed skink from the Solomon Islands is just such a lizard. While these give birth to only 1, and occasionally 2, offspring a couple of times per year, the babies are large and better-developed when compared to other lizard offspring. It is possible that these may in fact be completely viviparous. But it is likely that they are ovoviviparous. More study is needed by us here to find out all of the facts about this species.
So, the answer to the question is: “No, not all lizards lay eggs.”
To the best of this author’s knowledge, all turtles and tortoises lay eggs. This included sea turtles that venture onto beaches to deposit their eggs in the sand before returning to the oceans. Turtle eggs are commonly round in shape with shells that are soft upon laying but harden within a couple of days to provide maximum stability to the developing offspring.
Amphibians are not at the top of the knowledge list here at Double “D” Reptiles, however, we know that most frogs and toads lay eggs and a select small number of salamanders and newts are live-bearing to the point of possibly being viviparous.