How Do Reptiles Reproduce?

Reptiles are cold-blooded, scaly, quadrupedal vertebrates that originated 318 million years ago. They are a broad and widespread species with many biological characteristics in common. Some of these are:

• Cannot control their body temperature: they are ectothermic and depend on the environment and their own behavior to keep their bodies at the optimal temperature.
  
  
• Slow metabolism: their metabolism is much slower than that of mammals, so they often rest for long periods after eating, feeding on one large meal for months.
  
  
• Efficient predators: most reptiles are carnivores and have a simple digestive pattern. They are usually efficient predators, equipped with ferocity, sharp teeth, and often venomous or toxic glands that make their bite a deadly attack. Herbivores, on the other hand, use rocks and stones (gastroliths) to crush vegetables because they lack an ideal chewing apparatus.
  
  
• Use their lungs to breathe: unlike amphibians with moist skin, reptiles rely on their lung capacity for gas exchange. Some are anatomically prepared to breathe through the nostrils even when the mouth is open or busy with other tasks. Marine reptiles, such as turtles, can store enormous amounts of oxygen within themselves, which enables them to remain submerged for long periods of time thanks to their slow metabolism.

How are reptiles classified?

Reptiles are a group of vertebrates that can be classified according to two classification systems:

• Cladistics: cladistics is an approach to biological classification in which organisms are divided into groups based on hypotheses about their recent common ancestry. The term "reptile" is not used in this classification. Instead, they are divided into three main groups: Lepidosauria (lizards and snakes), Testudines (turtles), and Archosaurs (crocodiles and birds).
  
  
• Linnean: it is considered the traditional classification and divides the living beings into different levels of hierarchy, starting with the kingdom. According to this classification, the reptiles are assigned to the subphylum vertebrates and form a class of their own, Reptilia.

However, according to current cladistic systematics, this is a paraphyletic group, i.e., it does not include all descendants of the common ancestor (birds and mammals are left out). Therefore, the modern redefinition of Reptilia includes the birds but excludes the synapsids, resulting in the mammals.

You may be interested in this other article where we talk about what are the eight most dangerous reptiles.

Amphibians were the first vertebrates to live a semi-terrestrial life thanks to the evolutionary development of certain characteristics, such as:

• Well-developed legs
• Transformation of the sensory and respiratory organs
• Adaptations of the skeletal system

However, there is one aspect in which amphibians are still totally dependent on water: reproduction. In other words, their eggs and later their larvae require an aqueous medium for their development.

However, the reptile phylogeny has developed a particular reproductive strategy, the development of an egg with a shell, which allowed the first reptiles to become completely independent of water for reproduction. Nevertheless, some authors argue that reptiles still rely on a moist environment to develop their eggs. This is because these stages now take place within membranes that enfold the embryo, and protect it and provide it with nutrients as well as moisture.

Reptile eggs

Reptile eggs are amniotic, can be laid on land rather than in water, and amniotic fluid protects the young from drying out. This allows them to develop and survive in dry environments, just as they do in aquatic environments.

On one side is a hard and porous outer shell that protects the embryo from shock, weather, and predators; however, it is permeable enough to allow the passage of oxygen and carbon dioxide from the outside. On the other side is the interior, with an amniotic sac containing the amniotic fluid that moistens the embryo and another sac that collects the excretions. In addition, the egg contains a large amount of yolk, which provides nutrition for the embryo.

You may be interested in this other article, where we explain in more detail the similarities and differences between reptiles and amphibians.

The animal kingdom is characterized by a great diversity, in which each species has developed different characteristics and strategies to guarantee their biological success. With a group as diverse as the reptiles, it is therefore not surprising that they have developed different reproductive strategies, namely the following:

• Oviparous: they lay eggs that are hatched after they have been laid by the parents, as in birds.
• Viviparous: they do not lay eggs, and the embryo develops in the mother's body.
• Ovoviviparous: the embryos develop in eggs that remain in the mother's body until they can hatch.

Most reptiles are oviparous. Only some lizards and snakes such as boas, rattlesnakes, and water snakes are viviparous.

There are numerous variants that are not limited to the reproductive types previously mentioned, but also have other variations, such as species that can live either in an oviparous or viviparous state depending on their location. An example of this is the common lizard (Zootoca viviparous), which is oviparous in Iberian populations in the far west, while viviparous in France, the British Isles, Scandinavia, Russia, and part of Asia. The same is true for two Australian lizard species, Bougainville's skink (Lerista bougainvillii) and Saiphos (Saiphos equalis), which have different reproductive methods depending on their location.

Reptiles reproduce sexually, that is, the male of the species fertilizes the female, so that embryonic development occurs later. However, there are cases where females do not need to be fertilized for an embryo to develop. This is called parthenogenesis, a process that produces offspring that are genetically exactly like the mother. This last case can be observed in some gecko species, such as the Bynoe gecko (Heteronotia binoei) and the Komodo dragon (Varanus komodoensis).

Another way to look at reproduction in reptile species is whether fertilization is internal or external. In reptiles, fertilization is always internal. Males have one or two penises that pass sperm from their cloaca into the cloaca of a female. Fertilization occurs in the cloaca, and the fertilized eggs leave the female's body through the opening in the cloaca.

You may be interested in this other article, where we explain more fascinating facts about reptiles, such as their breathing.

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