Reptiles Reptiles were one of the first animals to invade dry land. Important adaptations to skin, reproduction, and the evolution of the amniotic egg allowed this. Reptiles today represent a varied group of animals. In many ways, grouping them together is artificial and misleading.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
The structure and function of reptile skin An important adaptation that enabled reptiles to invade land was their impermeable skin. A fold develops in the epidermis to become a keratinized scale. This hard layer prevents desiccation of the body fluids, and thus enabled them to survive in land environments. This hardened skin also affords some protection against other animals. Some reptiles, such as crocodiles, go even further and have sections where the bone directly supports the skin, forming `osteoderms'. Turtles have developed a hard layer of `scutes' atop the extension of their ribs that comprises their shell. This is still part of the skin, however, and a turtle can thus bleed from damage to the shell. Some reptiles have glands in their skin. These glands are usually concentrated in a particular area, such as the mouth or rectum and are usually for producing scent for intraspecific social contact, or for protection. Many turtles, for example, have glands that produce a stink that probably serves to deter predators.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
Reproductive biology and adaptations in reptiles Another important factor that allowed reptiles to conquer the terrestrial environment was modifications in their reproductive cycle. The first of these modifications is the habit many reptiles have of internal fertilization. While many rely on pressing of the cloacas, in other groups the males have actually developed a penile organ (or two), to transmit sperm directly. After successful mating, the female usually digs a hole into a substrate into which she will lay her eggs. The type of egg the female reptile laid is the second of these important evolutionary modifications.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
Variations on the reptilian reproductive cycle As the reptiles consist of many varied groups, there are variations on this reproductive strategy. The pythons (family Pythonidae), have the unusual strategy of incubating their eggs by curling around them and creating the warmth by shivering. The closely related boas that were once placed in the same family, are an example of snakes that give birth to live young (Torn 2000). The true sea snakes also give birth to live young, possibly as an adaptation to their marine lifestyle. Also many other species of snakes in the more temperate, colder areas of Australia have also evolved a live bearing strategy. Crocodiles are noted for another variation in their reproductive life cycle compared to other extant reptiles. Female crocodiles build a nest of which she then protects. The mother goes even further and exhibits parental care of her newly hatched young.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
The Amniotic egg These eggs are covered with a hard shell that is secreted by the mother as it develops through the body. Within this exterior casing, is contained a separate environment in which the embryo can develop. Food is provided, as it was in the earlier fish and amphibian eggs, by the yolk. A layer called the `amnion' provides support for the developing embryo, and gives this type of egg its name. Being in a closed environment, means that waste has to be collected and stored somewhere, and this requirement is served by the 'allontois'. Both this organ and the 'chorion' serve in allowing exchange of gases for respiration of the embryo. There is also store of water and protein in the `albumin' There are various factors that effect development of the egg during incubation. These include moisture, carbon dioxide levels, and temperature. In many reptiles this last factor is of dominant importance and the phenomenon is called `temperature dependent sex determination', or TSD. Essentially, a warmer temperature during the middle phase of incubation results in larger animals. If the animal is sexually dimorphic with larger females, then higher temperatures will generally result in females. This is the case with turtles where the female is larger, perhaps to aid in supporting the male on top of her shell during mating. However, in the lizards, snake, tuatara and the crocodilians, where the male is bigger, higher incubation temperatures result in males.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
The Evolution of the Amniotic Egg The amniotic egg is viewed an important step in the evolution of animals. It freed vertebrates from using a watery environment as a vital part in their reproduction and thus allowed dry land to be fully colonized. Reptiles, birds, and mammals form a natural group called the amniotes. Therefore, it is thought that the amniotic egg must have developed from a previous amphibian-like ancestor. There are some important points to remember in considering the evolution of the amniote egg. An important part of the change from a permeable amphibian egg to the closed reptile egg was the co-evolutionary development of uric acid. This meant waste of the developing animal could be stored in a benign form within the egg. The amniote egg appears with several important modifications that separated it from previous water dependent eggs; however these ancestral eggs already possessed yolk, the food reserve for the developing embryo. This is evident by looking at extant fish and amphibian eggs that contain yolk. There are several theories of how the rest of the features of the amniotic egg may have evolved. The standard theory on the evolution of the amniotic egg suggests it evolved from, logically enough, an amphibian-like or `anamniotic' egg. This may have evolved from a small amphibian perhaps similar to the extant plethodont salamanders that have no lungs but use only cutaneous respiration. This animal has been hypothesized to have had very tiny eggs that were laid in a moist environment and were small enough to allow diffusion of gases. While there are no ancestors that small in the fossil record to provide evidence for this idea, there are certainly extant examples. Alternatively, larger eggs may have been naturally selected for and over time a hard shell membrane developed, as the existing jelly capsule provided only limited support and gaseous diffusion. Presumably, air spaces were present between the fibers of the shell to allow diffusion of gases. Once a hard casing evolved over a jelly covering, eggs could become much bigger. However, there is nothing in modern amphibians analogous to the components of the amniote egg. Another, more recent theory on the evolution of the amniotic egg suggests an ancestor that gave birth to live young. Since the embryo developing inside the mother was already in an embryonic membrane, it was thus pre-adapted to a similar environment in an egg. Although egg laying is usually considered the original, more primitive condition, there are certainly many examples of live-bearing in all the major extant groups of animals.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
Diseases, parasites and disabilities of reptiles in the wild and in captivity Reptiles, like all other animals, also suffer from parasites and the subsequent diseases and disabilities. As many reptiles are terrestrial, they can pick up various nematode round worms, or mites and ticks. The aquatic and marine turtles and snakes can be infected by the more water-reliant parasites such as platyhelminth flatworms. Captive animals seem to be more susceptible to parasites than wild specimens. Reptiles are popular as pets as they do not require much food or attention, and seem to suffer from minimal boredom. However, these captive reptiles are often looked after by non-professionals and thus suffer from various conditions unknown to the keeper. Many of these conditions are simply a reflection of an inadequate or inappropriate diet. For example, lack of calcium in the diet may ultimately manifest itself in weak bones in the captive animal. This could simply be avoided by simulating the natural lifestyle and providing the animal with a prey animal complete with bones, rather than processed food. Being kept in captivity may deprive the animal of natural sunlight, which may also lead to similar future disabilities. Reptiles both in the wild and in captivity can be susceptible to various factors during the incubation time, when the embryo is developing in the egg. These can include differences in temperature, carbon dioxide levels and moisture. Temperature differences in many reptiles may determine developmental effects such as the actual sex of the future hatchling. If these conditions vary to extremes however, deformities can occur. During the captive breeding programs of crocodiles, managers have to be careful with possible bacterial and fungal infections of eggs.
Script: Courtesy of Damon Ramsey BSc.(Zool) Biologist Guide
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