ecdysis
The ability to regenerate lost limbs, as well as other body parts.
hyoid bone
A U-shaped bone at the root of the tongue; in salamanders it is used to help catch prey.
tympanum
Equivalent to the middle ear; used in hearing.
valarian respiration
Respiration in which the capillary beds are spread throughout the epidermis, so that gases can be exchanged through the skin.
Points to Consider
Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among frog families. These studies will also clarify relationships among families belonging to the rest of vertebrates.
Toxins obtained from some frog species may have potential as therapeutic drugs. The alkaloid epibatidine, a painkiller 200 times more potent than morphine, is found in some species of poison dart frogs. Other chemicals isolated from frog skin may offer resistance to HIV infection. As we will see in the next lesson, reptiles also possess chemicals and unique genes that are useful for medical purposes.
Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species care for their young, and that there is a great diversity of parental behaviors. As you begin to examine the reproductive system of reptiles in the next lesson, think about what kinds of parental behaviors reptiles might have and how they compare to that of amphibians.
Lesson 13.4: Reptiles
Lesson Objectives
List reptile traits.
Describe the general features of lizards and snakes.
List the characteristics of alligators and crocodiles.
Describe the traits of turtles.
Explain the importance of reptiles.
Check Your Understanding
What are some adaptations for living on land that are present in the amphibians?
What features present in amphibians are also useful to reptiles for an aquatic lifestyle?
Introduction
While some types of reptiles, like snakes, alligators, and crocodiles, often have a bad reputation due to their venom, as in snakes, or their aggressive behavior, as in all three groups, reptiles are important both ecologically and economically, as we will see later in this lesson. They also possess some unique traits and interesting behaviors, which we will also explore in greater detail.
Reptiles are tetrapods and amniotes, whose embryos are surrounded by an amniotic membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and are represented by four living orders: Squamata (lizards, snakes and amphisbaenids or “worm-lizards”), Crocodilia (crocodiles, gharials (Figure below), caimans, and alligators) Testudines (turtles and tortoises) and Sphenodontia (tuatara) (Figure below).
Figure 13.19
An Indian gharial crocodile.
Figure 13.20
A tuatara.
Traits of Reptiles
Reptiles are air-breathing, cold-blooded vertebrates that have skin covered in scales. The majority of species are oviparous (egg-laying) although certain species of squamates are capable of giving birth to live young. This is achieved, either by ovoviviparity (egg retention within the female until birth), or viviparity (offspring born without use of calcified eggs).
Many of the viviparous species feed their fetuses through various forms of placenta, similar to those of mammals, with some providing initial care for their hatchlings. The degree of viviparity varies: some species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, while still others lack any yolk and provide all nutrients via a placenta.
All reproductive activity occurs with the cloaca, the single exit/entrance at the base of the tail, where waste is also eliminated. Most reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion (the innermost of the embryonic membranes), chorion (the outermost of the membranes surrounding the embryo) and allantois (a vascular embryonic membrane) are present during embryonic life. There are no larval stages of development.
Most reptiles reproduce sexually, although six families of lizards and one snake are capable of asexual reproduction. In some species of squamates, a population of females is able to produce a nonsexual diploid clone of the mother. This asexual reproduction called parthenogenesis occurs in several species of gecko, and is particularly widespread in the teiids and lacertids.
Extant reptiles range in size from the newly-discovered Jaragua Sphaero, at 1.6 cm (0.6 in), to the saltwater crocodile, at up to 7 m (23 ft).
Most reptiles have a closed circulatory system with a three-chambered heart consisting of two atria and one ventricle. All reptiles breathe using lungs, although aquatic turtles have developed more permeable skin, and some species have modified their cloacas to increase the area for gas exchange. Excretion is performed mainly by two small kidneys.
The reptilian brain is similar to that of amphibians, except the cerebrum and cerebellum are slightly larger. Most typical sense organs are well developed with certain exceptions most notably the snakes lack of external ears (middle and inner ears are present). All reptilians have advanced visual depth perception compared to other animals.
Lizards and Snakes
Lizards and snakes belong to the largest recent order of reptiles (Squamata). Members of the order are distinguished by their skin, which bears horny scales or shields. They also possess movable quadrate bones, making it possible to move the upper jaw relative to the braincase. This is particularly visible in snakes, which are really just legless lizards, http://wps.prenhall.com/esm_freeman_evol_3/12/3321/850294.cw/index.html and are able to open their mouths very widely to accommodate comparatively large prey (Figure below).
Figure 13.21
A corn snake swallowing a mouse.
Lizards are a large and widespread group of reptiles, with nearly 5,000 species, ranging across all continents except Antarctica. Most lizards have four limbs, external ears, movable eyelids, a short neck, a long tail, and are insectivores. Many can shed their tails in order to escape from predators.
Vision, including color vision, is particularly well developed in lizards, and most communicate with body language, bright colors, or pheromones. Adults range from a few cm ( 1 in) in length (some Caribbean geckos) to nearly 3 m (10 ft) (Figure below), although most species are less than 220 g (0.5 lb).
Figure 13.22
A Komodo dragon, the largest of the lizards, attaining a length of 10 ft.
Encompassing 40 families, there is tremendous variety in color, appearance and size of lizards. Most lizards are oviparous, although a few species are viviparous. Many are also capable of regeneration of lost limbs or tails. Almost all lizards are carnivorous, although most are so small that insects are their primary prey. A few species are omnivorous or herbivorous, and others have reached sizes where they can prey on other vertebrates, such as birds and mammals.
Many lizards are good climbers or fast sprinters. Some can run bipedally, such as the collared lizard, and some, like the basilisk, can even run across the surface of water to escape. Many lizards can change color in response to their environments or in times of stress (Figure below). The most familiar example is the chameleon, but more subtle color changes occur in other lizard species, such as the anole, as well.
Figure 13.23
A species of lizard, showing general body form and camouflage against background.
Some lizard species, including the glass lizard and flap-footed lizards, have lost their legs or reduced them to the point they are non-functional. However, some vestigial structures remain. While some legless lizards, like flap-footed lizards, are similar in appearance to snakes, they can be distinguished by such features as their external ears.
All snakes are carnivorous and can be distinguished from legless lizards by lack of eyelids, limbs, external ears, and vestiges of forelimbs. The 2,700+ species of snakes occur in every continent except Antarctica
and range in size from the tiny, 10 cm (4 in) long thread snake to pythons and anacondas over 5 m (17 ft) long (Figure below). In order to accommodate snakes’ narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side.
Figure 13.24
A species of anaconda, one of the largest snakes, getting as long as 17 ft.
While venomous snakes comprise a minority of the species, some possess potent venom capable of causing painful injury or death to humans. However, snake venom is primarily for killing and subduing prey rather than for self-defense. All snakes are strictly carnivorous, eating small animals including lizards, other snakes, small mammals, birds, eggs, fish, snails or insects.
Because snakes cannot bite or tear their food to pieces, prey must be swallowed whole. The body size of a snake has a major influence on its eating habits. The snake’s jaw is unique in the animal kingdom. Snakes have a very flexible lower jaw, the two halves of which are not rigidly attached, and numerous other joints in their skull, allowing them to open their mouths wide enough to swallow their prey whole.
Some snakes have a venomous bite, which they use to kill their prey before eating it; others kill their prey by constriction, and still others swallow their prey whole and alive. After eating, snakes become dormant while the process of digestion takes place. The process is highly efficient, with the snake’s digestive enzymes dissolving and absorbing everything but the prey’s hair and claws.
Most snakes use specialized belly scales to travel, gripping surfaces. The body scales may be smooth, keeled or granular (Figure below). Snakes’ eyelids are transparent “spectacle” scales which remain permanently closed. In the shedding of scales, or molting, the complete outer layer of skin is shed in one layer (Figure below). Molting replaces old and worn skin, allows the snake to grow and helps it get rid of parasites such as mites and ticks.
Figure 13.25
A close up of snake scales of a banded krait, , showing Black and yellow alternating bands and spaces between scales.
Figure 13.26
A northern water snake shedding its skin.
Although a wide range of reproductive modes are used by snakes, all snakes employ internal fertilization, accomplished by means of paired, forked hemipenes, which are stored inverted in the male’s tail. Most species of snakes lay eggs and most species abandon them shortly after laying.
Alligators and Crocodiles
Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia are the closest living relatives of birds, as the two groups are the only known living descendants of the Archosauria, a subclass of reptiles, including the dinosaurs. The basic crocodilian body plan (Figure below) is a very successful one that has changed little over time; modern species closely resemble their Cretaceous ancestors of 84 million years ago. Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk in low, sprawled “belly walk,” or hold their legs more directly underneath them to perform the “high walk.” Most other reptiles can only walk in a sprawled position.
Figure 13.27
Two Nile crocodiles, showing the basic crocodilian body plan.
All crocodilians have, like humans, thecodont dentition, (teeth set in bony sockets), but unlike mammals, they replace their teeth throughout life. Crocodilians also have a secondary bony palate that enables them to breathe when partially submerged, even if the mouth is full of water. Their internal nostrils open in the back of their throat, where a special part of the tongue called the “palatal valve” closes off their respiratory system when they are underwater, allowing them to breathe when submerged.
Crocodiles and gharials (large crocodilians having elongated jaws) have modified salivary glands on their tongue (salt glands), which are used for excreting excess salt ions from their bodies. Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down, but it may also have a social function.
Like mammals and birds and unlike other reptiles, crocodiles have a four-chambered heart; however, unlike mammals, oxygenated and deoxygenated blood can be mixed. Crocodilians are known to swallow stones, known as gastroliths, which act as a ballast in addition to aiding post-digestion processing of their prey. The crocodilian stomach is divided into two chambers, the first is powerful and muscular, like a bird gizzard, where the gastroliths are found. The other stomach has the most acidic digestive system of any animal and can digest mostly everything from their prey: bones, feathers and horns.
The sex of developing crocodilians is determined by the incubation temperature of the eggs. This means crocodilians do not have genetic sex determination, but instead have a form of environmental sex determination, which is based on the temperature that embryos are subjected to early in their development.
Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of other reptiles. As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils are all located on the same plane. They see well during the day and may even have color vision, plus the eyes have a vertical, cat-like pupil, which gives them excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater.
While birds and most reptiles have a ring of bones around each eye which supports the eyeball (the sclerotic ring), the crocodiles lack these bones, just like mammals and snakes. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive, preventing water from entering their external head openings. The middle ear cavity has a complex of bony air-filled passages and a branching Eustachian tube. Eustachian tubes will be discussed in the chapter titled Controlling the Body.
The upper and lower jaws are covered with sensory pits, which encase bundles of nerve fibers that respond to the slightest disturbance in surface water. Thus they can detect vibrations and small pressure changes in water, making it possible for them to detect prey, danger and intruders even in total darkness.
Turtles
Turtles are reptiles of the order Testudines, most of whose bodies are shielded by a special bony or cartilaginous shell developed from their ribs. About 300 species are alive today and some are highly endangered. Turtles cannot breathe in water, but can hold their breath for various periods of time. Like other reptiles, turtles are poikilothermic (or “of varying temperature”). Like other amniotes, they breathe air and don’t lay eggs underwater, although many species live in or around water.
The largest chelonian (all living species) is the great leatherback sea turtle (Figure below), which reaches a shell length of 200 cm (7 ft) and can reach a weight of over 900 kg (2,000 lb). Freshwater turtles are generally smaller, but the largest species, the Asian softshell turtle, has been reported up to 200 cm (7 ft). The only surviving giant tortoises are on the Seychelles and Galapagos Islands and can grow to over 130 cm (4 ft) in length and weigh about 300 kg (670 lb) (Figure below).
Figure 13.28
The largest living chelonian, the leatherback turtle, which can reach up to 7 ft in length and over 2,000 lb.
Figure 13.29
A Galapagos giant tortoise, pictured here, can grow to over 4 ft in length and weigh about 670 lb.
The smallest turtle is the speckled padloper tortoise of South Africa, measuring no more than 8 cm (3 in) in length, and weighing about 140 g (5 oz). Turtles are broken down into two groups, according to how they evolved a solution to the problem of withdrawing their neck into the shell: the Cryptodira, which can draw their neck in while contracting it under their spine, and the Pleurodira, which contract their neck to the side.
Most turtles that spend most of their life on land have their eyes looking down at objects in front of them. Some aquatic turtles, such as snapping turtles and soft-shelled turtles, have eyes closer to the top of the head. These species of turtles can hide from predators i
n shallow water where they lie entirely submerged except for their eyes and nostrils. Sea turtles (Figure below) possess glands near their eyes that produce salty tears that rid their body of excess salt taken in from the water they drink.
Figure 13.30
A species of sea turtle, showing placement of eyes, shell shape, and flippers.
Turtles are thought to have exceptional night vision due to the unusually large number of rod cells in their retinas. Turtles have color vision with a wealth of cone subtypes with sensitivities ranging from the near ultraviolet to red. (For a description of rods and cones, see chapter entitled Controlling the Body). Turtles have a rigid beak and use their jaws to cut and chew food. Instead of teeth, the upper and lower jaws of the turtle are covered by horny ridges. Carnivorous turtles usually have knife-sharp ridges for slicing through their prey. Herbivorous turtles have serrated-edged ridges that help them cut through tough plants.
Although many turtles spend large amounts of their lives underwater, all turtles and tortoises breathe air, and must surface at regular intervals to refill their lungs. They can also spend much of their lives on dry land. Turtles lay eggs, like other reptiles, and which are slightly soft and leathery. The eggs of the largest species are spherical, while the eggs of the rest are elongated. In some species, temperature determines whether an egg develops into a male or female. Large numbers of eggs are deposited in holes dug into mud or sand. They are then covered and left to incubate by themselves. When the turtles hatch, they squirm their way to the surface and head toward the water.
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