Reptiles & Amphibians

Reptiles & Amphibians Science Lesson

Reptile & Amphibian Basics

Reptiles and amphibians have many physical differences, but they can be grouped together for a couple of reasons. Both are cold-blooded or ‘ectothermic,’ which means their body temperatures adjust to the temperatures of their surroundings, rather than maintaining one set body temperature like humans do. So when they get too hot, they jump in cool water or hide in the shade. When they need to warm up, they bask in the sun or burrow into mud. In colder climates, reptiles and amphibains hibernate during the winter.

Amphibians and reptiles are also vertebrates: they have a backbone. The joints in between the individual vertebrae allow the backbone to be somewhat flexible.

All reptiles and amphibians, except crocodilians, have three-chambered hearts. The crocodilians need more efficient four-chambered hearts like ours, because of their size.

Reptiles

What comes to mind when you think of reptiles? An alligator poking its head out of a bayou? A snake sliding through the grass? A pet turtle? There’s a lot of variety in the reptile world; in addition to the ones just named, there are tortoises; lizards (including Komodo dragons, iguanas, geckos, and skinks); gharials, caimans, and crocodiles; amphisbaenians or ‘worm lizards’; and iguana-like tuataras.

Lizards are the largest group of reptiles, with about 4500 species. Geckos are lizards with tiny pads on their feet that help them cling to walls. They eat spiders and mosquitoes, making them helpful critters to have in your house if you live in a tropical climate! Some species make a clicking or squeaking noise that sounds like their name, ‘gecko.’ Chameleons, another kind of lizard, are most well-known for their ability to change color to blend into their environment. They have protruding eyes that move in different directions at once, so they can see up, down, and to the right and left at the same time!

Snakes are in the same order as lizards, Squamata, but they are limbless and have double-hinged jaws designed for swallowing food whole. Although there aren’t as many species of snakes as lizards, they are more widespread.

There are only two living species of tuatara, although at one time there were many more that are now extinct. This animal lives on islands near New Zealand. It has a third ‘eye’ on the top of its head, thought to be a group of light-sensitive cells which help the tuatara know when it’s had enough sun.

Turtles and tortoises (the general term for land-dwelling turtles) have hard shells, usually made out of bony plates covered with keratin – the same substance that forms your fingernails and hair. The top part of a turtle’s shell is the carapace; the flat underneath part is the plastron. Leatherback sea turtles and softshell turtles don’t have a hard bony plate, but instead have a rubbery skin. Sea turtles spend all of their adult lives in the water – although they still have to surface to get air – except when the females come to land to lay their eggs. Scientists believe they return to the same beach they hatched at themselves. Leatherbacks are the largest turtles, growing up to six feet and weighing 1000 pounds! They are unusual among cold-blooded reptiles in their ability to survive in frigid ocean water.

Giant tortoises live on the Seychelles islands off the east coast of Africa and on the Galapagos Islands, which were named after the Spanish word for tortoise. These huge land turtles can weigh up to 400-500 pounds and can live for perhaps as long as 150 years. The Galapagos tortoises, made famous by Darwin’s observations, are thought to belong to one species, with 11 sub-species. Some live where there is lush vegetation in easy reach; these ones are larger with dome-shaped shells. Others are smaller, with saddle-shaped shells and longer necks and legs which allow them to reach sparser overhead vegetation.

The crocodilian group of reptiles includes 23 species of crocodiles, alligators, gharials, and caimans. There is only one living species of gharial, found in India. Although it has sharp teeth, its main diet is fish that it gulps down whole. Caimans are similar to American alligators; they’re actually both in the same family. Caimans are found wild only in South America. Even if most crocodilians look like man-eating monsters, several species don’t grow past 4-5 feet. How can you tell an alligator from a crocodile? An alligator’s lower teeth are hidden inside when its mouth is closed, but a croc’s teeth show. Crocodiles usually have more pointed snouts, as well.

Reptiles, as well as many other animals, have a Jacobson’s organ as part of their senses. The animal’s tongue picks up airborne particles and chemicals, then sticks its tongue in the Jacobson’s organ in the roof of its mouth. The Jacobson’s organ analyzes the particles and tells the brain what the animal ‘smelled.’ Some snakes – boas, pythons, and pit vipers – also have heat-sensitive receptors (‘pit’ openings) on the sides of their heads.

Did you know that about 4/5 of the world’s snakes aren’t harmful to people? If you don’t live in an Asian or African country, your chances of ever getting bit by a venomous snake are slim – in the U.S., more people die from insect bites than snake bites. Most species even of venomous snakes, such as the black mamba, are not aggressive. Other reptiles can be dangerous, too, even if they don’t have venom. Crocodilians are strong and have sharp teeth, while Komodo dragons (a large monitor lizard) can bring down a deer!

The majority of reptiles are oviparous, meaning that they lay eggs. But some snake species are viviparous – the eggs are fertilized inside the female’s body and the baby snakes crawl out after they hatch, a sort of ‘live birth.’ There’s also one species of snake (the Brahminy blind snake) that can reproduce parthogenetically, without fertilization by the male. All the snakes that hatch this way are females with genetic code identical to their mother.

Many reptiles, including turtles, tuataras, and crocodiles, are male or female because of the temperature their eggs were in the nest. Hotter temperatures produce males, cooler produce females, and in-between temperatures can result in a mix of male and female.

Amphibians & Metamorphosis

Amphibians, the class of animals that frogs and toads belong to, are unique in their ability to live both on land and in the water. They have moist skin that water can pass in and out of, and most of them spend at least part of their life – the tadpole-like larval stage before they mature – in the water. The name Amphibian literally means ‘both life.’ In addition to frogs and toads, amphibians include salamanders (newts are in that group) and caecilians, burrowing worm-like animals that have jaws and teeth.

Metamorphosis is the scientific term for a life cycle which includes a larval stage that looks very different from the adult stage; the word means ‘change form.’ (You can watch metamorphosis firsthand with an inexpensive Frog Hatchery Kit!) An amphibian begins life as a fertilized egg. The female usually lays eggs in water in a string or mass that sticks to vegetation. The male fertilizes the eggs as they are laid. The outer layer of the eggs is a jelly-like material that swells in water, forming a protective coating. The fertilized egg is a single cell that rapidly divides again and again, producing new cells that become the organs of the amphibian embryo. The amphibian hatches into its first larval stage (within 2 to 25 days for frogs, depending on water temperature), as a tadpole or polliwog. At this stage, the animal looks more like a fish than anything else. As the larvae develops, gills form that allow it to breathe efficiently underwater. Its tail grows longer and a fin forms, which allows the tadpole to swim effectively.

The tadpole continues to swim, eat, and grow for at least several weeks before it matures to the next stage. In frogs, the first sign of further development is the appearance of hind legs. Then front legs develop and the tail becomes shorter. Internally, the tadpole’s gills are replaced with lungs until finally the tadpole has become an adult amphibian. The mature adult then starts the cycle again by laying or fertilizing eggs.

Some amphibians, especially salamanders, become adults without ever outgrowing the last larval stage. They retain their gills instead of developing lungs, but do have the ability to reproduce. This is called neotony, and can be caused by environmental factors like a lack of iodine.

Reptiles & Amphibians Science Projects

Snake Dissection

One practical way to see for yourself how an amphibian and reptile differ is to compare the anatomy of two species, such as a frog and a snake. Although you can observe the external anatomy on live ones at a pet store or in the wild, it’s hard to discover anything about their insides without doing a dissection.

Even if you don’t want to dissect a snake (or a turtle, for another interesting reptile specimen), you might find the following description of a snake’s anatomy helpful. You can compare it to our online frog dissection guide, with pictures and detailed instructions. (You can also find virtual frog dissections online, instead of doing your own.) This is an ideal project for junior high and high school students who are studying animal classification.

Snake Anatomy

Click this link for a printable snake dissection diagram with labeled parts (.pdf). Use this as a guide for locating organs.

1. Inspect the external anatomy of the snake. Why do you think the scales on the specimen’s back (dorsal side) are different than the scales on its belly (ventral side)? The ventral scales correspond to the number of ribs that the snake has and allow greater flexibility of movement.

2. Make a long incision down the center of the ventral surface, from the cloacal opening to the throat. Carefully pull back the skin and pin it down on either side, cutting the membrane layer underneath as necessary. Once the snake is opened, observe how it looks inside. Then slice the membrane away until it separates from the organs. (This membrane holds the organs securely in place in the living snake.) This is the longest part of the dissection process, but go slowly and cut carefully.

3. Identify the major organs listed below. The heart, stomach, gallbladder, liver, and large and small intestine will probably be the easiest to find. These are listed in order roughly from head to tail.

Trachea: A dark-colored tube in throat that brings air to the lung.

Heart: Located just below trachea. It is a three-chambered like a frog heart.

Right lung: Most snakes have only one functioning lung, a long narrow sac starting near the heart. Look for it between the liver and stomach. The specimen might also have another, smaller lung.

Liver: A long, thin orangish-colored organ on the left side (as you look at it).

Stomach: A long sac that connects to the esophagus in the throat and small intestine lower down. Is your specimen’s stomach empty or full? If full, you might want to check out the contents to discover what the snake was eating.

Gallbladder & pancreas: The gallbladder is small and round, usually greenish-colored from the bile for digestion stored in it. You might have to remove some of the yellow fat bodies to see it. (A healthy snake will have many fat bodies.) The pancreas looks just like an extension of the intestine, right next to the stomach.

Small & large intestine: The small intestine starts right below the stomach and forms many coils.

Gonads: These might look similar to the intestine, but are not connected to it. Male snakes are identifiable by testes inside and hemipenes at the cloacal opening. Females have a pair of ovaries and might have eggs. (If both the frog and snake specimens have eggs, be sure to compare them!)

Kidneys: These are located near the end of the large intestine; they should be similar in color to the intestine, but if you look closely you’ll see that they are a different kind of tissue.

Print out this diagram and fill in the labels yourself to test your knowledge of snake anatomy:

• Diagram of dissected snake to label (.pdf)

See our other free dissection guides with photos and printable PDFs. Click here

Comparing Frogs & Snakes

How do you tell the difference between a reptile and an amphibian? To help your young children understand the similarities and differences between reptiles and amphibians, make a chart. For older elementary and junior high kids, let them create charts listing differences between different kinds of reptiles or amphibians (e.g., how is a frog different from a salamander?).

Since reptiles and amphibians can be hard to observe in the wild, take a field trip to a zoo or pet store. You might call ahead to see if your kids will be allowed to handle any of the animals. Before you go, ask younger kids what they think a snake feels like. What about a frog? A turtle? Will a frog or a snake feel more ‘slimy’? After the visit, discuss the experience. Encourage older kids to investigate feeding habits, make sketches, observe how the animal moves, etc.

If you are handling frogs in the wild, be sure that you have wet hands or hold the animal with a plastic bag. Your bare hands can quickly cause the frog’s skin to dry out. After touching any amphibian or reptile, be sure to wash your hands well with soap and water!

To find out more about frogs, do research on one of these topics: what kinds of frogs live in your area? Can you find more than one species of tadpole locally? If so, compare them. What do local frogs eat? How would the mosquito population be affected if there were few or no frogs in a swampy region? Pick a frog or frog characteristic that is interesting to you, and see what you can find out about it. Look for close-up frog pictures in a magazine like National Geographic or on a website.

To see for yourself how an amphibian and reptile differ, compare the external anatomy of two species such as a frog and a snake. You can observe the external anatomy of live ones at a pet store. Or do an online image search: try ‘bufo’ (part of the scientific name for some frog species), ‘bullfrog,’ ‘poison dart frog,’ ‘tree frog,’ ‘garter snake,’ ‘elipidae’ (the main family of poisonous snakes), ‘colubridae’ (the family with common snakes), and ‘boidae’ (constrictors).

Want to really dig in? Try a frog dissection! Dissections are a great way to see an animal’s internal anatomy up close, and we provide a variety of frog specimens to explore and learn from:

• 3.5″-4″ Plain Grass Frog Specimen
• 4″-4.5″ Plain Grass Frog Specimen
• 4.5″-5.5″ Plain Grass Frog Specimen
• 3.5″-4″ Single Injected Grass Frog Specimen
• 4″-4.5″ Double Injected Grass Frog Specimen
• 4.5″-5.5″ Double Injected Grass Frog Specimen
• 5″-6″ Bullfrog Specimen
• 6″-7″ Bullfrog Specimen

This is just an overview of some features of frogs and snakes – there’s a lot more to learn about them!

• Skin: How does the skin of a frog look (and feel) compared to a snake or other reptile? One is typically smooth and moist, one is dry and scaly. Frogs can exchange oxygen and carbon dioxide through their skin. They have mucous glands that secrete a waterproof coating to keep their skin moist and slippery. Snakes have a tough coating of scales made of keratin, the same protein that forms your hair and fingernails. Each species either has smooth scales or rougher keeled ones, with a unique pattern of scales and coloring. They also have long horizontal scales on their belly that help them move across surfaces.Both frogs and snakes (as well as other reptiles) molt, or shed their skin. Frogs change their skin about once a week! Although all reptiles shed their skin as they grow, snakes lose their skin in a whole piece rather than pieces flaking off.
• Head & mouth: Sensory organs on frogs and snakes differ quite a lot. Frogs have bulging eyes that provide them a wide range of view. Snakes, on the other hand, generally have poor vision.

Frogs have a tympanic membrane that can detect sound waves in water or the air and transfer the sound to the inner ear. Snakes have no external ear openings, but they can pick up vibrations through their jaw bones, which transfer to their internal ear bones.

Frog tongues are broad and specially attached so they can be thrust out and catch insects. Snake tongues are narrow and forked, to ‘taste’ chemical particles in the air.

• Respiration: Frog tadpoles breathe through gills in the water, but adult frogs get oxygen through wet skin and their lungs. Watch a frog closely and you can see its throat moving in and out as it breathes. Snake respiration is unusual, too. The left lung in most snakes – except boas and pythons – is very small or else absent. They do fill their right lung with air, however.
• Feeding: Snakes and frogs are both carnivorous – they eat ‘meat,’ ranging from tiny insects to large mammals. Many frogs have two raised bumps in the roof of their mouth, vomerine teeth, plus a bony ridge in their gums. The roof of a snake’s mouth is covered with rows of tiny teeth to help it grasp its prey so it can swallow it whole. Some species have more than 200 teeth! (Neither reptiles nor amphibians really use their teeth to chew – even alligators and crocodiles swallow their food down whole or in large pieces!). Snakes’ jaws are also hinged so that they can open their mouths wide enough to swallow their food whole.
• Limbs & movement: Frogs have webbed feet (perfect for swimming) and their back legs are much stronger than their forelegs, because they are designed for swimming and jumping.
• With their powerful legs, frogs can leap 20 times their body length! Tree frogs also have suction cups on their toes that allow them to cling to the bark of tress. Snakes do not have limbs, but their bodies are still designed for movement. Large, heavy snakes use rectilinear movement, traveling in almost a straight line by pulling themselves along with their scales. Other snakes travel by S-shaped serpentine motion or throwing themselves along by sidewinding. Tree-climbing snakes use concertina movement, bunching up one end of their body to propel themselves forward. The fastest snake in the world, the black mamba, can move at 10-12 miles per hour!
• Reproduction: Snake eggs are more like chicken eggs than like the spawn of frogs and other amphibians. Amphibian eggs lack a shell or other hard protective layer. The female usually lays hundreds or thousands of eggs in the water, which the male fertilizes. Although both kinds of animals ‘hatch,’ some kinds of snakes keep their eggs inside their bodies until the babies are born and slither out of the birth opening.
• Skeleton: Both amphibians and reptiles have a backbone. Most frogs have no ribs, however. Depending on the species, a snake will have between 180 and 400 vertebrae and almost as many ribs!
• Defenses: Different snakes have different venoms made up of toxins that affect their victims in different ways. There are some toxins that attack the nervous system, and same that destroy cells, damage the heart, or affect red blood cells. Other snakes, constrictors, kill their prey by grabbing it with their mouth and then coiling their body around so tightly that the prey can no longer breathe. Frogs secrete poison in their skin, although only some species are toxic to people. Some also have the ability to ‘spit’ poison at predators or prey. Usually the most toxic frogs are brightly colored, to warn off predators. Other frogs have camouflage skin colorings that help them to blend in with their surroundings. Special pigment cells in their skin control the camouflage pattern and colors.