Introduction to How Frogs Work

Whether it's a bright blue poison dart frog or a brown, warty toad, a frog's survival is all about water. Most frogs begin their lives as jelly-like eggs that need moisture to develop. The eggs hatch into aquatic tadpoles, which breathe through gills. As the tadpoles grow, they develop lungs and gain the ability to breathe air.

But frogs' need for water doesn't dry up once they're on land. Frogs get virtually all of their water and part of their oxygen through their skin, and this process only works if their skin stays moist. If a frog's skin dries out, it can't get enough oxygen or get rid of enough carbon dioxide, and it dies.

So it should come as no surprise that frogs have developed some interesting adaptations to cope with their lifelong need for water. For example, gastric brooding frogs, which are now extinct, swallowed their eggs. They stopped eating and digesting food while their young developed in their stomachs instead of in water. The baby frogs then exited through their parent's mouth. Strawberry poison dart frogs, which live in Central America and Puerto Rico, lay their eggs on land, and males keep them moist with urine. Once the eggs hatch, the mother carries each tadpole on her back to its own tiny pool of water that has collected between the stem and leaves of a plant. While the tadpoles grow, their mother feeds them her own eggs.

But frogs' adaptations aren't all about water or reproduction. They've lived on Earth for about 200 million years, and in that time they've developed everything from deadly toxins to the ability to survive being frozen. You'll learn about these and other adaptations in this article. We'll start with a look at frogs' appearance and behavior, including why toads are frogs, but not all frogs are toads and why frogs molt but don't leave the skin they shed behind.

This African pyxie frog (Pyxicephalus adspersa) has bumpy, toad-like skin.

Photo by Jeff Foott, © Discovery Communications, LLC

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Frogs and Toads, Warts and All

It's tricky to make generalizations about frogs. The world's smallest frog, the gold frog, is less than a centimeter long. Goliath frogs, the biggest frogs, are thirty times bigger -- about 12.5 inches (32 cm) long from head to tail. While many frog species are nocturnal, some are most active in the morning or afternoon. Life spans vary from species to species, although most live for a few years in the wild and a little longer in captivity. And while many frogs croak, chirp or ribbit, some species are silent.

Although many people think of frogs as green, some are orange, yellow, blue or bright pink. The texture of a frog's skin and the shape of its body and legs vary from species to species, and these are also the traits that separate frogs from toads. Frogs have smooth, glossy, moist skin and legs that are built for swimming and leaping. Toads are frogs that have bumpy, drier skin and have legs that are built for walking instead of jumping.

This northern red salamander (Pseudotriton rubber rube) has a tail, which differentiates it from frogs.

Photo by Jeff Foott, © Discovery Communications, LLC

But the distinction between frogs and toads is mostly arbitrary -- some frogs look like toads, and some toads look like frogs. Toads can generally stay farther away from water, since their skin doesn't dry out as quickly as frogs' skin. But some frogs live in very dry areas and have to go to great lengths to stay hydrated, and some toads live in water.

There are even exceptions to frogs' most defining trait. Frogs are tailless. This is so central to what makes a frog a frog that their scientific order, Anura, means "without tail." This lack of a tail unites frogs, and it sets them apart from the two other orders of amphibians: salamanders and caecilians. All amphibians start their lives breathing water before developing the ability to breathe air, but only frogs grow into land animals that don't have tails.

But two frog species have tails. The coastal tailed frog, Ascaphus truei, and the mountain tailed frog, Ascaphus montanus, both have tails that match the color of their bodies. These tails are reproductive organs. Because of this appendage, tailed frogs are the only type of frog in which a male can fertilize eggs while they're still inside a female's body.

But in spite of all the differences in their appearance and behavior, all frog species have roughly the same anatomical structure.

The Frog Skeletons vs. Human Skeletons

As anyone who has dissected one in biology class knows, a frog's internal organs look a lot like what you'd find inside a much larger animal. Just like mammals -- including people -- a frog's body has a heart and lungs as well as a stomach, pancreas, liver, gallbladder and intestines.

Even though frogs don't look much like people on the outside, their skeletons are similar to people's skeletons, especially when it comes to their limbs. Just like in a person's arms, in a frog's front legs are bones called the humerus, the radius and the ulna. However, a frog's radius and ulna are fused into one bone. The same is true for a frog's legs -- the femur supports its upper leg, and the bones of the lower leg, the tibia and fibula, are fused. A frog has two scapulae, or shoulder blades, and clavicles, or collarbones, that are shaped a lot like the same bones in a person's body.

A collection of small bones makes up a frog's digits, or its fingers and toes. Most of the time, a frog has five toes on its back legs and four toes on its front legs. The length and shape of the toes has a big impact on how the frog moves. Tree frogs have long, flexible toes that allow them to grasp stems and branches as they climb around. Aquatic frogs also have long toes -- the spaces between them are webbed so they can use their feet like flippers. Some frogs burrow into the soil in the summer or winter. Often, their feet are shorter and wider, like shovels or spades.

But a frog's skeleton isn't so similar to a human's once you get past the extremities. Frogs have skulls but don't have necks, so they can't turn, lift or lower their heads like people can. A frog also doesn't have ribs. The rib-like structures you can see in the picture above are part of its spine. A frog's pelvis can slide up and down its spine, which may help it jump. The vertebrae at the bottom end of the spine are fused into one bone called the urostyle.

Read on to find out how frogs hear without ears and why their eyes are perched on top of their heads.

This red eyed frog (Agalychnis callidryas) has large, bulging eyes near the top of its head.

Don Farrall/Getty Images

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Ear Rings, Sinking Eyes and Other Frog Anatomy

Frogs rely on their vision and hearing to catch prey and avoid predators. They have good hearing and vision, although their ears and eyes aren't situated quite like those of most other animals. Frogs don't have external ears. Instead, they have an eardrum called the tympanum that sits just behind each eye. Often, you can see the eardrum -- it's a flat area surrounded by a ring of cartilage. In some species, scientists can tell whether a frog is male or female by comparing the size of the eardrums to the size of the eyes.

Most frogs have wide, bulging eyes that sit on the top of their skulls. This gives the frog a wide field of view and helps compensate for its inability to turn its head. There's not a lot of overlap between what a frog can see with its left eye and what it can see with its right, though. This may mean that frogs don't have as much depth perception as other animals do, which makes some species' ability to catch flying prey with their tongues even more amazing. Most frogs also have a nictitating membrane, or a sturdy film that covers and protects the eye while the frog is underwater.

Frogs call by inflating their vocal sacs.

Jeremy Woodhouse/Getty Images

Frogs' eyes also play an important role in eating. Frogs don't have the skull structure or the necessary muscles to chew their food. Instead, they have to swallow their prey in a couple of gulps. This is tricky since, unlike people, their tongues aren't usually anchored in the back of their mouths. That means a frog can't use its tongue to push food down its throat and toward its stomach. For this reason, when a frog swallows, its eyes sink down into its skull to help push the food along.

Many frogs, particularly males, have vocal cords and a flap of skin called a vocal sac in the front of their throats. It's this sac that allows frogs to croak, trill and ribbit. The frog inhales, and fills the vocal sac with air, causing it to stretch out like a balloon. With its mouth closed, the frog forces air from this sac back and forth over its vocal cords, producing a loud, repetitive sound. Frogs that don't have vocal cords can also use a sharp intake of air to make a clicking sound.

Most of the time, this noise has something to do with mating. Next, we'll look at the frog mating process, which can last for days, and the metamorphosis that turns tadpoles into frogs.

Red-eyed tree frogs mate on a leaf in the rain forest. Note the difference in size and color between the male and female.

Photo by Mattias Klum/National Geographic/Getty Images

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Frog Reproduction, from Mating to Metamorphosis

Most people learn about the basics of frog reproduction in elementary school. Frogs lay eggs in water, and the eggs hatch into tadpoles that grow into frogs. Only about half of all frogs follow these exact steps, but there are a few rules of thumb about frog reproduction. All frogs reproduce sexually, and all hatch from eggs.

In almost all frogs, egg fertilization happens outside the female's body instead of inside. The female releases her eggs and the male releases his sperm at the same time. In order to make sure that the sperm reach the eggs, the male and female get into a mating posture called amplexus. The male climbs onto the female's back and clasps his forelegs around her middle. Frogs can stay in amplexus for hours or even days as the female releases as few as one or as many as several hundred eggs.

Sometimes, it's easy to tell male frogs from female frogs. Many species are sexually dimorphic, meaning that there are differences between the bodies and colors of males and females. But in some species, males and females are hard to tell apart. In such species, male frogs often produce a release call when clasped by another male. During mating season, researchers can use release calls to tell which frogs are male and which are female.

All frogs' eggs require moisture to develop, and most frogs abandon their eggs once they're fertilized. But not all eggs incubate underwater or without parental care. A few species carry their eggs in their vocal sacs or their abdomens. Others lay eggs in dry areas and keep the eggs moist with water or urine. Depending on the frog's species and the climate in which it typically lives, the eggs can hatch in a few days to a few weeks.

In a few species, fully formed froglets hatch from the eggs, but most of the time the frog starts its life as a tadpole. While adult frogs are carnivores, tadpoles can be vegetarians or omnivores. Some are filter feeders that eat algae, and others have teeth and can eat anything from rotting vegetation to other tadpoles. Either way, tadpoles tend to be voracious eaters -- it takes a lot of energy to complete their metamorphosis into frogs.

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Tadpoles that live in temporary rainwater ponds often become frogs in a couple of weeks. The process can take months in species that live in permanent lakes, rivers and ponds. But most of the time, the transformation follows the same basic steps. First, the back legs begin to grow. Then, as the front legs are forming, the tadpole's internal organs began to change. It develops a pair of lungs so it will be able to breathe air, and its digestive system changes to accommodate its adult diet. The tail gradually disappears as it's absorbed into the body. When the froglet is ready to live on land, it usually has a little bit of tail left, but that gradually disappears.

Frog eggs and tadpoles are food for fish, birds and other animals, so most eggs don't survive to adulthood. Adult frogs have several enemies as well, including those that are microscopic. Next, we'll look at some of the threats to frogs' survival and how the absence of frogs could affect human life.

A great blue heron eating a frog

Jeremy Woodhouse/Getty Images

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Eating and Being Eaten: The Ecological Role of Frogs

Frogs play a central role in many ecosystems. They control the insect population, and they're a food source for many larger animals. To keep things in balance, frogs use lots of survival tools. Some run away, play dead or swell up when threatened. Others have more sophisticated defenses. Many use color as a warning or as camouflage. For example, the four-eyed frog, Physalaemus nattereri, has two spots that look like eyes near its back legs. It shows these spots to predators, making it look like a more threatening animal.

Tina Bayden sells skinned frogs at the Laoag public market in Ilocos Norte in the northern Philippines, October 2, 2005. The frogs are considered a delicacy in the region.

Photo credit should read JOEL NITO/AFP/Getty Images

Frogs can also secrete substances through their skin. Some secretions are beneficial -- researchers have used some of them to create new antibiotics and painkillers. But some frogs secrete toxins so powerful that just touching the frog can be fatal. Poison dart frogs, also known as dart poison and poison arrow frogs, are the most famous example. Their toxicity is partially due to their diet -- in captivity, without the right ants to feed on, they're not as dangerous. But even common toads can be toxic, which is why dogs can get very sick or die from eating toads.

But not all of the threats come from wild animals. In many parts of the world, frogs are a delicacy. Since it can be difficult to farm raise frogs, this requires people to hunt and capture them. In some parts of the world, this has led to a steep decline in frog population.

Shown here are two normal frogs and four deformed frogs with missing limbs and limb duplications.

Joseph Kiesecker/Penn State/Getty Images

Because frogs drink through their skin, they're susceptible to several manmade threats. In some parts of the world, frog population has declined due to habitat destruction during development and the damming of rivers and streams. Frogs are also sensitive to water pollution and acid rain. Chemicals, drug residues and other pollutants in the water appear to be causing everything from deformed limbs to feminized male sexual organs in frogs.

Global warming may threaten frogs as well. Frogs are largely ectothermic -- they rely on their environment to control their body temperature. Some frogs can survive temperature extremes. For example, the wood frog can live north of the Arctic Circle. Up to 45 percent of its body freezes in the winter, and proteins and glucose protect its tissues from damage. Wood frogs thaw out again in the spring. Other frogs hibernate or go into a hibernation-like state called aestivation to survive hot weather in the summer.

But not all frogs have these adaptations, so climate change could affect frogs' abilities to keep their bodies at the right temperature. Changes in the global climate may have made it easier for a deadly fungus to spread through the frog population around the world. Next, we'll look at this fungus and what researchers are trying to do to stop it.

Reptile keeper Stuart Kozlowski inspects containers housing corroboree frogs at Taronga Zoo in Sydney, Australia. Around 130 of the critically endangered frogs are housed in a quarantine facility at the zoo. Because of the chytrid fungus, scientists fear these frogs will be extinct in the wild in five years.

Ian Waldie/Getty Images

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Chytrid Fungus and Disappearing Frogs

Today, a major threat to frogs is the chytrid fungus. This fungus feeds on keratin, a component of skin that makes it sturdy and tough. Tadpoles have only a little keratin around their mouths. But as frogs grow, their skin thickens on the soles of their feet and anywhere else their bodies repeatedly come in contact with the ground, such as the lower abdomen. So while the fungus doesn't usually affect tadpoles, it can kill adult frogs. Researchers aren't yet sure of exactly how the fungus kills the frogs. The two prevailing theories are that the fungus produces lethal toxins or that its presence interferes with the exchange of oxygen and carbon dioxide through the skin.

At present, scientists believe that this fungus came from African clawed frogs. These frogs carry the fungus on their skin but suffer no ill effects from it. However, African clawed frogs have spread far beyond their native habitat, carrying the fungus with them, due to global trade. Scientists around the world have used these frogs for research and, from the 1930s to the 50s, to conduct pregnancy tests. People have also kept the frogs as pets. The fungus isn't just riding on the backs of one species of frog today, though. Researchers suspect that it may now be spreading through airborne spores.

Researchers believe that about a third of the frogs in the world may face extinction due to the chytrid fungus. Some species have already disappeared. There is no effective treatment for the fungus, so researchers around the world are trying to quarantine as many frog species as possible. One such quarantine facility is the $500 million Amphibian Ark project.

Although the chytrid fungus is a global problem, there are a couple of things you can do to help stop its spread. First, report the appearance of sick frogs or frogs that seem to have died without an obvious cause to your local authorities. Second, never, ever release pet frogs into the wild. Released pet frogs can breed, overwhelming the local frog population, and they can spread disease. Again, contact your local authorities to inquire about what to do.

You can learn more about frogs, amphibians and related topics by browsing the links on the next page.

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