Getting to Know Amphibians
Three different types of amphibians (am-FIB-ee-uhns) live on Earth today:
- Frogs are the often-slimy creatures almost everyone has seen hopping into a pond or heard calling on a spring evening. The smallest species reach less than one-half an inch (1.3 centimeters) long, while the largest can grow to more than a foot (30.5 centimeters) in length. Frogs are in the order Anura (ann-UR-uh). Toads are included in this order, too. They are simply one kind of frog. Frogs are different from other amphibians because they do not have tails when they are adults. Some frogs, called the tailed frogs, have little taillike bits of tissue, but they are not really tails. Many frogs have long and strong hind legs for hopping, but a few have short hind legs and typically get around by walking or running.
- Salamanders are the four-legged, tailed animals that hikers or gardeners sometimes surprise when they turn over a rock or log. The smallest salamanders are less than 1.2 inches (3 centimeters) long, while the largest can grow to 4 feet 11 inches (150 centimeters) in length, or more. Salamanders have bodies in the shape of a pipe with a tail at the rear. Most have small legs that are all about the same size. They hold their legs out to the side of the body when they are scrambling around on the ground. A few species have only two legs. The name of the salamanders' order is Caudata (kaw-DAY-tuh).
- Caecilians (seh-SILL-ee-uhns) come in many sizes, ranging from just 4.5 inches long to more than 5 feet 3 inches (160 centimeters) in length, but most people have never seen them in the wild. Caecilians look rather like earthworms, even having similar rings around their bodies, but caecilians have many things that earthworms do not, including jaws and teeth. A caecilian's tail is actually quite short, but since it blends into the rest of the body, this can be difficult to see unless the animal is flipped over. The tail in a caecilian begins at the vent, a slitlike opening on its underside. The caecilians are in the order Gymnophiona (jim-no-fee-OH-nuh).
In all, the world holds at least 4,837 species of frogs and toads, 502 of salamanders, and 165 of caecilians. Scientists are still discovering new species, so those numbers grow larger and larger as the years pass.
WHAT MAKES AN AMPHIBIAN AN AMPHIBIAN?
Although frogs, salamanders, and caecilians are usually not mistaken for one another, they still share several features that make them all amphibians.
Some people confuse salamanders with lizards, but lizards are reptiles. An easy way to tell an amphibian from a reptile is to check for scales on the skin. Reptiles have scales, but amphibians do not. The skin of an amphibian is at least a little bit moist, even among the rather-dry toads, and some amphibians are very slippery. Part of the slipperiness comes from the moist or wet places they live, and part of it comes from their mucus (MYOO-kus) glands. Mucus glands are little sacks that ooze a slimy substance.
Amphibians also have another type of glands in their skin that ooze poison instead of mucus. Depending on the species, the poison may be weak or very strong. The poison in some of the poison frogs of South America is even powerful enough to kill a person who gets some in his or her bloodstream. In other species, just a little taste of the poison can turn a person's lips numb or cause extreme sickness.
Like fishes and reptiles, amphibians have body temperatures that become colder when the outside temperature is cold and warmer when the outside temperature is hot. Animals with a changing body temperature like this are known as ectothermic (EK-toe-thur-mik) animals. Sometimes, people call ectothermic animals "cold-blooded," but they are really only cold when the weather is also cold. Many amphibians warm themselves by sunbathing, or basking. Frogs frequently sit on shore in damp but sunny spots to bask. They may also simply swim into the warmer, upper layer of water in a pond to heat themselves up a bit. When they get too hot, they typically move to a cooler place, sometimes even going underground. This not only keeps them cooler but also helps them stay moist, which is important for their breathing.
Amphibians breathe in several different ways. Like reptiles, birds, and mammals, most amphibians breathe in air through their nostrils to fill up their lungs. Caecilians have two lungs, but the left one is much smaller than the right one. This arrangement works well for the caecilians, which would not have room for two large lungs in their long and thin bodies. Some salamanders have very small lungs, and a few, such as the red-backed salamander that is common in North American forests, have no lungs at all.
Small or no lungs does not cause a problem for amphibians, however, because they do much of their breathing through their skin. When a person breathes in through the nose, the air travels into the lungs in the chest, where blood picks up the oxygen from the air and delivers it throughout the body. In amphibians, oxygen can pass right through their moist skin and into blood that is waiting in blood vessels just below the skin. The skin must be moist for this process to work: A dry amphibian is a dead amphibian. Using this through-the-skin breathing, which is called cutaneous respiration (kyoo-TAIN-ee-us res-per-AY-shun), amphibians can even breathe underwater. Oxygen that is dissolved in the water can also cross the skin and enter their blood.
Most amphibians go through a phase in their lives when they breathe underwater through gills, just as a fish does. Gill breathing is like cutaneous respiration, because dissolved oxygen in the water is picked up by blood in vessels that are in the gills. Gills are so full of blood vessels that they are typically bright red. Usually an amphibian breathes through gills only when it is young. Frogs, for instance, use gills when they are still tadpoles. A young salamander, which also has gills, is called a larva (LAR-vuh). The plural of larva is larvae (LAR-vee). Some amphibians, however, skip the gill-breathing phase and hatch right from the egg into a lung- and/or skin-breather. Others, however, keep their gills throughout their entire lives. Mudpuppies are examples of a salamander that has gills even as an adult. Since they live in the water, gills work well for them. In a few species, like the eastern newt, the animal goes through several phases: a gill-breathing larva, then a gill-less juvenile, and finally a gilled adult.
Besides hearing sounds like humans do, frogs and salamanders can hear vibrations in the ground. When the ground vibrates, the movement travels up their front legs to the shoulder blade and then to a muscle that connects to the ear, so the amphibian can hear it. This type of hearing can be very sensitive. Not only can amphibians hear the footsteps of an approaching predator, like a raccoon, but they can also hear something as slight as an insect digging in the soil.
WHERE AMPHIBIANS LIVE
Amphibians live around the world. The only places where they do not live are in the extremely cold polar regions of the Earth, most of the islands in the ocean, and some desert areas. The three major groups of amphibians—the frogs, the caecilians, and the salamanders—each have their own favorite climates. Caecilians stay in warm, tropical climates and nowhere else. Although frogs live just about anywhere an amphibian can live, the greatest number of species make their homes in the tropics. Salamanders, on the other hand, tend toward cooler areas. Most salamanders live north of the Equator, and many exist in areas that have all four seasons, including a cold winter.
Because amphibians must keep their skin moist, they are always tied to water. That water may be a lake or river, a little puddle, a clammy spot under a log, or even a slightly damp burrow underground.
In the water
Most amphibians live at least part of their lives in the water. Many frogs and salamanders lay their eggs in the water. The frog eggs hatch into tadpoles, and the salamander eggs hatch into larvae. Both the tadpoles and the salamander larvae have gills that they use to breathe underwater. Eventually, the tadpoles turn into baby frogs, and the salamander larvae turn into young salamanders, and both can then leave the water to live on land. Scientists do not have all of the details about caecilians, but they think the typical caecilian lays its eggs on land; the eggs hatch into young that are also called larvae and have gills; and the larvae wriggle into water. The caecilian larvae grow in the water before losing their gills and moving onto land.
Those species that live on land for much of the year and only have their young in the water, often choose small pools that are only filled with water part of the year. Such pools are called temporary pools. Temporary pools, since they dry up later in the year, usually do not contain fish, which often eat amphibian eggs and young. The only problem with laying eggs in a temporary pool is that the pools sometimes dry up too fast for the eggs to hatch into the tadpoles or larvae and for these to turn into land-living amphibians. When this happens, the young may die.
In each major group of amphibians, some species remain in the water for their entire lives. These are known as fully aquatic (uh-KWOT-ik) animals. The word aquatic means that an organism lives in the water, and the word fully means that it can always live there. Some caecilians from South America live in the water. Sirens and mudpuppies are types of salamanders that live in the water as eggs, larvae, and adults. As adults, both have bodies that are well-designed for swimming instead of walking on land. They have strong, flattened tails to move swiftly through the water but very tiny legs. The sirens only have two small front legs and have neither back legs nor hip bones.
Many frogs are fully aquatic. The clawed frogs and Surinam toads, for instance, live in just about any kind of freshwater, including swamps, slow streams, and ponds. They have very large and webbed hind feet, which make excellent paddles. One very unusual frog is the hairy frog. Adults of this species live on land most of the year, but the males will stay with the eggs underwater until they hatch. During this time, the male develops "hairs" all over the sides of its body. The hairs are actually thin fringes made of skin. This gives him more skin area and makes it easier for him to breathe. With his "hairs," he is able to stay underwater for days with his eggs without ever coming up for air.
Tadpoles, aquatic larvae, and some aquatic adult amphibians have lateral (LAT-eh-rul) line systems. Fishes have lateral line systems, too. The lateral line system looks like a row of stitchlike marks or dots that runs down each side of the body. Inside each mark or dot are tiny hairs that sway one way or the other with the movements of the water. When another animal swims past or enters the water nearby, the hairs lean and send a message to the amphibian's brain that it is not alone in the water. This helps amphibians to escape predators or, if they eat insects or other water-living prey, to find the next meal.
Along the ground
Many adult frogs and salamanders live on land and along the ground. Since they have to keep their skin moist, they often huddle under a rotting log, inside a crack in a rock, in piles of dead leaves, under the low-lying leaves of plants, or in some other damp place. Once in a while, a caecilian is also found snuggled between a leaf and stem in a low plant. In many cases, amphibians only move about on the ground during or after a heavy rain. Some, like the American toads, can survive under a bit drier conditions than other amphibians and hop or walk around the forest floor even on warm and dry summer days.
Above the ground
Some frogs and salamanders will venture into the trees. Animals that spend part of their lives off the ground and in plants or trees are known as arboreal (ar-BOR-ee-ul) animals. Among the salamanders, only some lungless salamanders are arboreal. One, which is known as the arboreal salamander, may crawl under tree bark or climb into tree holes to escape hot and dry weather. Many more frogs than salamanders are arboreal. Hundreds of these are called treefrogs and have sticky, wide pads on the tips of their toes to help them scramble up plants and trees. Some of the arboreal frogs live in humid forests that are moist enough for them to sit out on leaves most of the time. Others need more moisture and find it in bromeliads (broh-MEE-lee-ads), which are plants that often grow on the sides of trees and have tube-shaped leaves that catch rainwater. There, the frogs find tiny pools where they can dip their bodies or float.
Under the soil
Since amphibians need to keep their skin moist, many of them find dampness under the soil. Animals that live underground are called fossorial (faw-SOR-ee-ul) animals. Most of the caecilians remain underground, only coming up to the surface once in a while to feed. They typically have tiny eyes and are nearly blind, although they can tell light from dark. They make their own burrows, digging headfirst into moist soil. Among the salamanders, the best-known burrowers are the mole salamanders. These salamanders, which live through much of North America, usually do not make their own burrows, instead borrowing them from mice and other small rodents. They stay inside these underground hideaways until rains wet the ground. At that time, they climb out and look for food to eat. Many of the mole salamanders, such as the blue-spotted salamander, also may live under rotting logs. The larger spotted salamander sometimes hides under rocks or deep in a damp well.
Numerous frog species, including the spadefoot toads, live underground for much of their lives. They, like many other burrowing frogs, have a hard bump that looks like the edge of a shovel blade on each of their digging feet. Some burrowing frogs do not have hard bumps on their feet. They do, however, have powerful digging legs and usually wide feet to move away the soil as they burrow.
HOW DO AMPHIBIANS MOVE?
Since amphibians may have four legs, two legs, or no legs at all, and they may spend most of their time on the ground, in the water, or in trees, they move in many different ways. Some walk or run; some hop or leap; some swim; some burrow; and some even glide through the air.
Walking and running
The land-living, or terrestrial (te-REH-stree-uhl), salamanders travel from one place to another by walking or running. They do this with their bodies very close to the ground and their upper legs held out from the body in the same position that a person takes when starting to do a push-up. Lizards, which people often confuse with salamanders, typically hold their bodies higher off the ground. The arboreal salamanders use these same movements to climb trees. Some frogs, especially those frogs with short hind legs, also get around mainly by walking. The Roraima bush toad is an example. This little toad walks slowly over the rocks where it lives. If it needs to escape quickly, it tucks in its legs so it forms a little ball and rolls off the face of the stone.
The frogs and toads are the hoppers and leapers among the amphibians. They have two especially long ankle bones in their hind legs, as well as a long rod of bone in the hip where the jumping muscles attach. These bones give the frog's leaps added boost. They also have a strong but springy chest that can catch the frog safely as it lands on its front feet. Not all frogs and toads hop, but most do. Some, like most of the frogs in the family called true toads, have short hind legs and can only hop a short distance. Others, like most of those in the family called true frogs, have long and powerful hind legs that help them leap several times their body length. Some people even hold frog-leaping contests and bet on the frog they think will jump the farthest.
Adult frogs swim much as they leap, shoving off with both hind feet at the same time. The frogs that are the best swimmers typically have large hind feet with webbing stretched between the toes and to the toe tips or close to the tips. Tadpoles do not have any legs until they start to turn into froglets, but they can swim by swishing their tails. Salamander larvae and the aquatic adult salamanders may or may not have tiny legs, but they all use their tails to swim. The aquatic caecilians swim much as snakes do, waving their bodies back and forth in "s" patterns to slither-swim through the water.
Caecilians burrow head-first into the moist soil where they live. Frogs may burrow head-first or hind feet-first. The spadefoot toads are one of the groups of frogs that dig backwards into the soil, scraping through the soil with their back feet while wriggling backward. This buries the frog deeper and deeper into the soil. The sandhill frog that lives in Australia is one of the frogs that digs head-first by paddling its front feet and making it look as if it is swimming down into the sand.
A few of the frog species, including the flying frogs in the family known as the Asian treefrogs, can soar through the air. They do not flap their front legs or have feathers like a bird, but they do have long toes that are separated by webbing that reaches the toe tips. When they widen their toes, the feet look almost like fans. These treefrogs can leap off a tree branch high above the ground and glide safely to earth by using their fan-shaped feet to keep from falling too fast. They are also able to steer by moving their feet one way or the other.
WHAT DO AMPHIBIANS EAT?
Many amphibians eat meat or are carnivorous (kar-NIH-vor-us). For most of them, their meals are insects, spiders, and other invertebrates (in-VER-teh-brehts), which are animals without backbones. Often, larger species will eat larger prey. Most caecilians eat earthworms, termites, and other invertebrates that live underground. Mexican caecilians, which may grow to 19.7 inches (500 centimeters) in length, sometimes eat other animals, such as small lizards and baby mice that crawl on top of the leaf-covered ground where the caecilians live. Most salamanders eat earthworms or small arthropods (AR-throe-pawds), which are insects and other invertebrates with jointed legs. Adult frogs also usually eat invertebrates, but if they are able to capture a larger prey and swallow it, many will. The bullfrog, which is common in much of North America, will eat anything and nearly everything from other frogs to small snakes, rodents, and even small birds.
Many amphibians hunt by ambush, which means that they stay very still and wait for a prey animal to happen by. Some amphibians hunt by foraging (FOR-ij-ing), when they crawl, hop, or swim about looking for something to eat. Many amphibians simply snap their mouths around the prey and swallow it. Some flick their tongues out to nab it and then reel their tongues and the prey back into their mouths. Many salamanders have especially long tongues.
Tadpoles are usually herbivorous (urh-BIH-vor-us), which means that they eat plants. Many have beaklike mouths that scrape algae (AL-jee) and other scum from rocks and underwater plants. Some, like the tadpoles of spadefoot toads, will eat invertebrates in addition to plants.
AMPHIBIANS AS PREY
A wide variety of animals attack and eat amphibians. Birds, snakes, raccoons and other mammals, fishes, and other amphibians are their predators. Even insects, like diving beetles, can kill a tadpole. For most amphibians, the best defense against their predators is to remain still and let their camouflage colors help them stay out of sight. Frogs, in particular, are often the same color as their surroundings. Some, like the horned frogs, have large and pointy heads that look much like dead leaves. Other amphibians are very brightly colored. The juvenile eastern newt, for example, is bright orange red. This newt also is very poisonous, and its bright colors advertise to predators that they are dangerous to eat.
When numerous amphibians are attacked, they will stiffen their bodies, arch their backs, and hold out their feet. This position is called the unken (OONK-en) reflex. The fire-bellied frogs use this position, which shows off their bright red, yellow, or orange undersides and the similarly colored bottoms of their feet. The colors may remind predators that these frogs have a bad-tasting poison in their skin and convince them to leave the frogs alone.
Although it is not very common, some amphibians will fight back if attacked. Adult African bullfrogs will snap at large predators, even lions or people, who come too close to the frogs or their young. Among salamanders, the large hellbenders can give a painful bite.
In all three groups of amphibians, mating involves both males and females. The females produce the eggs, and the males make a fluid that contains microscopic cells called sperm. An egg will only develop into a baby amphibian if it mixes with sperm. This mixing is called fertilization (FUR-tih-lih-ZAY-shun). In almost all frogs, the male climbs onto the back of the female, and as she lays her eggs, he releases his fluid so that the eggs are fertilized outside. In the caecilians, the male adds his fluid to the eggs while they are still inside the female's body. Salamanders fall in between these two types of fertilization. In most salamanders, the male puts drops of his fluid along the ground, and the female follows along behind to scoop up the droplets and put them inside her body with the eggs. All amphibians either lay their eggs in the water or in a moist place where the eggs will not dry out.
Most amphibian eggs hatch into tadpoles or larvae before becoming miniature versions of the adults. Often, these eggs, tadpoles, and larvae develop in the water. In some species, the adults lay the eggs on land but near water; the eggs hatch into tadpoles or larvae that squirm into the water or scramble onto the parent's back for a ride to the water. A number of species have young that never enter the water. In many of these amphibians, the eggs skip the tadpole or larvae stage and hatch right into miniature adults.
Amphibians often have certain times of day or times of year when they are active. Some may even enter states of deep sleep for parts of the year when the weather is too cold or too dry.
Day and night
Most amphibians are nocturnal (nahk-TER-nuhl), which means they are active at night. Nocturnal animals hide someplace during the day. Sirens, which are the two-legged salamanders, spend their days buried in mud. Many frogs likewise stay out of sight during the day, sometimes hidden underground, in a rock crevice, or in some other hiding place, and come out at night to look for food or to mate. By being active at night instead of the daytime, these amphibians can avoid many predators that rely on their eyesight to find prey. Nights are also usually more humid than days, so the amphibians can keep their skin moist better if they are only active at night.
Some species are diurnal (die-UR-nuhl), which means that they are active during the day. In many cases, these species have especially poisonous or bad-tasting skin that protects them from daytime predators. Many of the poison frogs of South America, for example, are diurnal. On rainy days, some of the nocturnal amphibians will come out of hiding and wander about. With the wet weather, they can keep their skin moist.
During the seasons
Many species of amphibians are active only during some times of year. Those that live in climates with a cold winter often spend the winter underground or in another sheltered spot and enter a state of deep sleep, called hibernation (high-bur-NAY-shun). The bodies of some species, like the wood frog in the family of true frogs, actually freeze in the winter, but they are able to thaw out the following spring and continue living. Many other cold-climate species become active again when the spring arrives. Salamanders in the northern United States, for instance, start to move about on land even before the snow melts. Frequently, in these species, the spring also is the time for mating.
Besides the cold-weather species, some other amphibians enter a state of deep sleep when the weather becomes too dry. For species that live in deserts or dry grasslands, such as the water-holding frog of Australia, many burrow down into the ground and wait there until the next rainy season arrives. A period of deep sleep during a dry period is known as estivation (es-tih-VAY-shun). In these species, the rainy season marks the beginning of the mating period.
Amphibians that live in warm and wet tropical areas usually are active all year long, but they often mate only on rainy days.
AMPHIBIANS AND PEOPLE
Of all the amphibians, frogs are the most familiar to people. Nearly everyone has seen a frog or heard one calling during its mating season. Because neither salamanders nor caecilians have mating calls, and both usually stay out of sight during the day, many people have seen few, if any, of these two types of animals. Frogs are also much more common pets than salamanders or caecilians. In addition, many people eat frogs and some even eat tadpoles, but few people eat caecilians or salamanders.
Scientists are interested in amphibians for many reasons. In some species, their skin poisons or other chemical compounds have been made into or studied as medicines. Scientists also use amphibians to learn how their bodies work and therefore learn more about how human bodies function. Perhaps most importantly, ecologists see amphibians as living alert systems. Since amphibians live on land and in the water, and often are very sensitive to changes in the environment, they are excellent alarms that can warn humans about problems, such as water or air pollution.
Through the World Conservation Union, which goes by the initials IUCN, scientists keep track of how well amphibians, along with other organisms, are surviving on Earth. They separate the species into different categories based on the number of individuals in the species and anything that might make them lose or gain numbers in the future. One of the categories the IUCN uses is called Data Deficient. This category means that scientists do not have enough information to make a judgment about the threat of extinction. The number of amphibians listed as Data Deficient is quite large: 1,165 species of frogs, 62 species of salamanders, and 111 caecilians. Many of these species are rare and/or live underground or in some other hard-to-reach location where they are difficult to study.
Amphibians in danger
The IUCN lists 367 species of frogs and forty-seven species of salamanders as Critically Endangered and facing an extremely high risk of extinction in the wild; 623 frog species, 106 salamanders, and one caecilian are Endangered and facing a very high risk of extinction in the wild; 544 frogs, 86 salamander species, and three caecilians are Vulnerable and facing a high risk of extinction in the wild; and 302 frogs and fifty-nine salamanders are Near Threatened and at risk of becoming threatened with extinction in the future.
Many of these species are at risk because the places where they live or breed are disappearing or changing, perhaps as people cut down trees for lumber or otherwise clear the land to put in farms, homes, or other buildings. Some of the other problems for amphibians come from air and water pollution, infection with a fungus that is killing amphibians around the world, and global warming. Global warming changes weather patterns, sometimes causing especially dry conditions in some places. Since frogs need to keep their skin moist, especially dry weather can be deadly to them.
Saving endangered amphibians
To help many of the at-risk amphibians, governments, scientific organizations, and other groups are protecting some of the areas where the animals live. These may be national parks, preserves, or other natural areas. Many local, state, and national governments have also designed laws to protect the amphibians from being hunted or collected. In a few cases, conservationists are trying to raise amphibians in captivity and then releasing them into the wild with the hopes that they will survive, breed, and increase the size of the natural populations.
Too late to save
The efforts to protect the Earth's amphibians are important, because many species have already become extinct in recent years. An extinct species is one that is no longer in existence. This includes two species of salamanders and thirty-two species of frogs. In addition, the IUCN lists one frog as Extinct in the Wild, which means that it is no longer alive except in captivity or through the aid of humans.
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