All about frogs and toads

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Did you know that a single toad will eat about 10,000 insects over the summer? Or that some frogs eat their entire body weight in insects in a single day? Larger toads and frogs even include tiny rodents or reptiles in their diet. Not only that, but frogs themselves are food for numerous other wildlife, including birds, foxes, and other mammals. So, are they valuable wildlife? Well, yeah!
Frogs are unique in many ways. Read on to learn lots of surprising facts about them and how they live their lives.
Note: Toads are frogs. They’re just classified into different scientific families. So, in this article “frog” also applies to “toad,” in the same way that robins and wrens are both birds, but they belong to different families. Any noteworthy differences between the two are highlighted in red. 

Background

Frogs are amphibians, a word from the Greek amphibion, meaning “double life,” which describes their lives: They spend part of their lives on land and part in water. There are about 7,000 amphibian species worldwide, and frogs and toads make up 90 percent of them.
Frogs, along with newts, salamanders, and caecilians, are in the order Anura (uh-NOO-ruh), which is a word from the Greek a + ura, meaning “tail-less.” Anura is divided into 29 families, including tree frogs, spadefoot frogs, glass frogs, poison-dart frogs, true frogs (Ranidae) and true toads (Bufonidae). The latter two families may be the most familiar to you, as they include two of the most widely distributed anurans in the United States, the American Bullfrog and American Toad. Worldwide, there are about 600 species in Ranidae and about 500 in Bufonidae.

Origin

Frogs evolved from fish that moved out of the water to spend part of their time on land about 400 million years ago, in the Devonian Period. Frog- and salamander-like animals appeared about 290 million years ago, during the Permian Period. The earliest modern frog, one containing all the physical frog features of today, showed up about 125 million years ago in the Cretaceous Period.

Environmental threats

Most frog species live in tropical rainforests, but they’re native everywhere, except Antarctica and some islands. Frogs, as well as other amphibians, are threatened around the world from loss of habitat. Other forces are at play, too, such as chemical pollution of air and water, disease, and over-harvesting for food. Sadly, nearly 200 frog species have already gone extinct. Some now exist only in protective captivity and dozens more are under threat of extinction.

Size

Frogs are extremely varied in size. The world’s largest is the endangered Goliath Frog, Conraua goliath, native to Cameroon and Equatorial Guinea. It grows to 12.6 inches long (32 cm), without its legs extended, and weighs up to 7.17 pounds (3.25 kg). Imagine coming across one of those! Way, way down at the opposite tip of the scale is the world’s smallest frog, as well as tiniest known vertebrate, Paedophryne amauensis, found in Papua Guinea, only 0.30-inch long (7.6 mm).

Illustration of a dime with width measurements and frog species compared to it.

World’s smallest frogs, compared to a dime. (Hd93129/Wikimedia; CC BY-SA 4.0)



In the United States, the largest native frog is one you might guess: the American Bullfrog, Lithobates catesbeianus, which is 3.75–6.0 inches (9.5–15 cm) long and weighs up to 1.13 pounds (513 g). The smallest is the Little Grass Frog, Limnaoedus ocularis, which, at a mere 0.75-inch (19 mm) would fit on a penny. The largest toad in the US is the Sonoran Desert Toad, Incilius alvarius, at 7.5 inches (19 cm) long. The smallest toad is the Oak Toad, Bufo quercicus, from 0.7 to 1.25 inches (18 to 3.2 mm) long.

Physical description

Male and female frogs and toads mostly look alike, except that females are often much larger than their counterparts. But a frog can be distinguished from a toad. Once you know how, it’s easy: Frogs have moist, smooth skin; a slender body; visible eardrums on the sides of their head; no parotid glands; and tree frogs have toe pads. Toads have dry, warty-looking skin, a heavier-looking body; visible eardrums; visible parotid glands; and no toe pads.

Side view of a frog facing to the right.

Frog. (makamuki0 / Pixabay PD)

Side view of a toad facing to the right.

Toad. (© Brian Church / iStock)

FrogToad
Skin smooth, moistSkin dry, warty-looking
Slender bodyHeavier body
Usually lives near waterMay live far away from water
Visible eardrumsVisible eardrums
Tree frogs have toe padsNo toe pads
 
Head
Frogs have a flat, wide head, with big, bulging eyes at the top, a nose, two eardrums, glands, and a mouth having a long tongue. The head contains a brain, spinal cord, and highly developed nervous system. 
Close up of a frog eye's as it sits in water.

A frog’s large bulbous eyes are involved with swallowing. The eardrum is the large circular area below the eye. (LisaAnn_37/Pixabay; PD)



Eyes

Frogs have excellent vision in both daylight and dark, with a wide field of view that allows them to see to the front, sides and partially behind. This is important for spotting predators, because a frog can’t turn its head or move it up and down.

The upper and lower eyelids are underdeveloped and move very little. In order to completely close their eyes, frogs must draw their eyeballs deep into their sockets, which pulls the eyelids together. They have a transparent membrane that can cover their eyes, which protects and cleans them.

Because their eyes are located at the top of their head, species that spend much of their time in water can sit motionless with only their eyes above the surface. All the better for watching for unwary insects they can eat. Toads spend most of their time on land.

Frogs’ eyes have a surprising function beyond vision. It seems far-fetched, but they help frogs to swallow! When a frog catches prey, its eyes close, drop down into the roof of its mouth and help push the food down its throat!

Hearing

Frogs have excellent hearing. Their external ear is a sealed, circular membrane on each side of the head, called a tympanum, or eardrum. (It can easily be seen just below the eye in the photo above.) It both transmits sound waves to the inner ear and keeps water out.

Like humans, frogs have a middle and inner ear. At the end of the inner ear is a membrane thought to help as a protection against deafening noises—which they produce themselves! Some frogs have calls that are so ear-piercing they can be heard a mile away! In 2008, scientists discovered a frog species living near a noisy area in China that can tune its hearing to different frequencies, like a radio knob, effectively tuning out objectionable noises or even the calls of other frog species.

Other senses

Frogs are sensitive to touch and have a good sense of smell. Their sense of taste is excellent—in fact, they’ll reject foods they don’t like.

Mouth
Their mouth is another interesting thing about these animals. Ever thought of them having teeth? They do, at least most of them. But don’t expect a big, toothy smile! They’re very short and located only in the upper jaw and used for grinding food. Toads don’t have teeth and swallow their food whole.
The tongue is long, and the base of it is fastened at the front of their mouth rather than at the back. Because of this, they hold their tongue folded with the tip facing the throat. When an insect happens by, it can be flicked out quickly and accurately. It’s coated with a sticky substance from a mucus gland that “glues” the insect to it—there’s no escaping a frog’s tongue. A small family of frogs, Pipidae, which inhabit South America and sub–Saharan Africa, don’t have tongues and use their fingers to catch prey and place it in their mouth. 
Buccal pumping
You’ve probably noticed that frogs pulsate their throat. It’s pretty conspicuous and demonstrates their unusual method of breathing, called buccal pumping. It’s a “two-stroke” process for most frogs (one small, totally aquatic family uses a four-stroke process). Here’s how it works: First, the frog lowers the floor of its mouth, which pulls air in through its nostrils while simultaneously drawing air from its lungs into its buccal (mouth) cavity. Following that, it raises the floor of its mouth, which causes the opposite action of pushing air out, as well as into the lungs. This is always done with the mouth closed.
Illustration of a frog demonstrating how buccal pumping works.

(Mokele / Wikimedia; CC BY-SA 3.0)



Torso

The frog’s torso contains the heart, two lungs, two kidneys, stomach, liver, small intestine, large intestine, spleen, pancreas, gallbladder, urinary bladder, and ureter. There’s also a urinogenital duct which serves as a passageway for waste products, sperm, and eggs to exit the body through the cloaca (anus). Males have two testes and females have two ovaries, in addition to other organs related to reproduction.

Limbs

Frogs have two arms and two legs made up of the same kind of bones that humans have. Their arms have a humerus, fused radius and ulna bones and four fingers. The legs have a femur, fused tibia and fibula bones and five toes. The shoulder blades and collarbone have a similar shape, too.

Frog skeleton shown in sitting position on a green surface for display.

American Bullfrog. (Ryan Somma / Wiki; cc by-sa 2.0)



Legs

Their hind legs are long, with webbing that stretches between the toes. Those that spend a lot of time in water have longer legs, and more webbing than more terrestrial species. Webbing is important for helping them to paddle through water.

Tree frogs are an example of those with less webbing. They spend all their time on land and webbing covers only half or less the length of their toes. Their toes differ in another way, too: They’re equipped with mucus-tipped, bristly pads that help them stick horizontally or even hang from the underside of leaves. Toads don’t have webbed feet.

There’s an unusual family of frogs (Hylidae, Rhacophoridae, Rhacophoridae) called “gliding” frogs that stretch their toes wide, and the webbing serves as wings, allowing them to jump from high places to lower. They “fly” from tree to tree in this manner. (Watch them fly)

Most frogs lack claws, but spadefoot toads, which live in dry habitats, have claw-like growths on their hind feet that are used for digging burrows. Their method of execution is interesting: As they dig, they move backward in a circle until the hole is deep enough for them to disappear into it.

Some species of frogs walk like a lizard (watch a walking toad). But most hop as a standard method of locomotion on land. Frogs are powerful jumpers with muscles and tendons that perform like springs. Their arms are of little use in jumping, but they help to cushion the landing. An American Bullfrog, cleverly named Rosie the Ribeter, set the all-time record for the longest frog jump in the US in a single bound at the Calaveras County (California) Fair in 1986—21.6 feet (6.6 m). How frogs jump so high.

Skin

Toads usually have dry, bumpy skin. Most frogs have thin, smooth skin, although a few species have spines or tubercles that hide them so well they look like moss or lichen. 
Vietnamese Mossy Frog, Theloderma corticale, looking much like it's covered by moss.

Vietnamese Mossy Frog, Theloderma corticale. (Katie Chan/Wikimedia; CC BY-SA )4.0



The outer skin, or epidermis, of frogs is moist. The moisture comes from mucus that’s secreted by glands in the lower layer of skin, called the dermis. The skin of toads secretes moisture, too, and they’re able to retain it longer. Still, their skin is drier than that of frogs, which enable them to live farther from water. Some even live in arid habitats, where they dig burrows to escape skin-drying conditions.

Moist skin does more than cover tissue and bone. Frogs absorb oxygen directly into their bloodstream through pores, in a process called cutaneous gas exchange. This is crucial when they’re underwater, where 100 percent of their oxygen intake is through the skin. Frogs also “drink” through their skin rather than with their mouth. (This permeability is what makes frogs so victimized by pollutants—their skin offers them little protection.)
 
Frogs shed (molt) their skin about once a week. They loosen old skin by twisting their body all around, then pull it off over their head in one piece, like a sweater. They don’t leave it lying on the ground, like a dirty garment—they eat it.
 

Coloration

It would be hard for any animal to top the flashy colors of the Red-eyed Tree Frog and poison dart frogs. Certainly most other frogs can’t, as they’re typically camouflaged in shades of brown, gray or green.

A colorful Red-eyed Tree Frog, Agalychnis callidryas, which has orange toes, sitting on a green leaf.

Note the round toe pads on this Red-eyed Tree Frog, Agalychnis
callidryas, native to Central America. Lovely and harmless.
(Carey James Balboa / Wiki; PD)

Beautiful poison dart frog Ranitomeya amazonica with orange and black stripes and blue markings.

Poison Dart Frog, Ranitomeya amazonica. It’s beautiful but secretes a dangerous toxin through glands in its skin. Native to  tropical South America. (V2 / Wikimedia CC BY-SA 3.0)



Chromatophores
Some frog species become paler or darker, or change color altogether. The color-changing process is complicated, but it’s triggered by specific stimuli, such as excitement, humidity, light, temperature, and surroundings.
The changes are brought about by chromatophores, star-shaped pigment cells in the skin that contain various granules of colors. Some cells have red, some have yellow, some have black, and so on.
An example is the Pacific Tree Frog, Hyla cinerea, which changes its skin color to match the color of the background, moving from shades of light-green to dark-brown as the need arises.
A bright green-colored Pacific Tree Frog, Pseudacris regilla.

Pacific Tree Frog, Pseudacris regilla. (kjfmartin Wikimedia; CC BY-SA 3.0

A brown-colored Pacific Tree Frog, Pseudacris regilla, lying on a brown twig.

This is the same species of Pacific Tree Frog. (NPS / Wikimedia; PD)



Gray Tree Frogs can be even more dramatic. Their normal coloration varies, with some being yellow and others green, gray or brown. But some almost black ones can change their color to nearly white. And, there’s a small African frog that’s creamy-white and lives in the white blossoms of a particular lily. When the flowers die and turn brown, it turns brown, too.
Toads and their ‘warts’
Toads (and some frogs, too) have warty-looking bumps all over their body. But contrary to common myth, they aren’t warts and they don’t cause humans to get warts by touching them.

Common Toad, Bufo bufo, sitting on it's haunches, showing its lumpy body and poison gland.

Those “warts” aren’t warts! Notice, too, the long, puffy poison gland located beside the eardrum. Common Toad, Bufo bufo. (leoleobobeo / Pixabay PD0

The ‘warts” are a mix of two types of glands: mucous glands, which can sometimes be poisonous, and granular glands, which produce a much more toxic substance, called bufotoxin.
Bufotoxin
Bufotoxin is secreted through two large, puffy glands, one behind each eye. It’s a milky-looking poison that’s released when a toad is squeezed or shaken (as dogs will tend to do). It can kill small predators and in some very rare instances has killed small dogs.
Bufotoxin reacts with the mucus lining of a predator’s mouth and tongue and often causes foaming, making the animal look rabid. It’s very bitter, prompting most of them (and curious pets) to drop the toad immediately. Most will never again try to taste a toad. It’s a toad’s only defense against predators, and effective. 
Bufotoxin doesn’t typically harm human skin, but taken internally it can affect the heart, cause hallucinations and act as a vasoconstrictor. If you handle a toad, don’t squeeze it, and keep your hands away from your eyes, mucous membranes, and any open wounds, until you’ve washed up. 
Poison dart frogs
In the tropics live some of the most eye-catching animals on earth: brightly colored frogs dressed in vivid yellows, blues, reds, oranges and purple, some with lovely patterns of spots or stripes. These beauties have much to admire visually, but they can’t be touched. Their striking coloration serves as a warning to stay away. The frogs tend to be diurnal (active in daytime) because they don’t fear being seen.
Golden Poison Frog, Phyllobates terribilis.

Golden Poison Frog, Phyllobates terribilis, world’s most poisonous. (Lwp Kommunikáció / Flickr; cc by 2.0)



One of the most toxic animals on earth is the Golden Poison Frog, Phyllobates terribilis, that inhabits Central America and South America: Just one milligram of its poison can kill between 10 and 20 people. There are no frogs or toads in the US that are lethal to humans unless ingested. Even the poisonous Cane Toad, Rhinella marina, of the southern US can be safely handled if done with care.

Communication

Frogs communicate with others of their species. Their calls fall into categories of attraction calls and aggression calls, as well as release calls, which are used by both males and females to signal non-readiness to mate. Some species emit their attraction calls separately and others join a group in an immensely loud “chorus” of calls. Some species inflate and pulsate a conspicuous vocal sac located on their throat when they make calls.
Small-headed Tree Frog, Dendropsophus microcephalus, male, with inflated vocal sac while clinging to a vertical plant stem.

Small-headed Tree Frog, Dendropsophus microcephalus, male, with inflated vocal sac. (Brian Gratwicke / Flickr; cc by 2.0)



A lot of people are familiar with the deep croak of American Bullfrogs, but frog calls also include chirps, trills, twitters, peeps, clicks, twangs and other sounds that defy description. In order to attract the right mate, each species has its own special call. As for the frog that goes, “ribbit, ribbit, ribbit,” it’s the Pacific Tree Frog, Pseudacris regillaListen here.

Defenses

Frogs have few defenses, so most are active at night when they’re harder to see. Camouflage coloration is an important defense. It allows them to sometimes hide in plain sight. Some change color to match their background. Another defense is playing dead when approached by a predator that prefers live food. Frogs also hop away from predators or dive into water. Some species puff up to appear bigger. Watch toad playing dead

Estivation

Frogs may become inactive when weather conditions are too dry or too cold. They burrow into mud or sand, or enter cracks or holes in logs and rocks, or hide in such places as leaf litter or in abandoned burrows. Then they go into a dormant state known as aestivation, which differs from hibernation in that it’s a shallower “sleep.” The Northern Burrowing Frog, Neobatrachus aquilonius, an Australian species, even forms a hard cocoon with its outer layer of skin to lock in moisture before estivating.

Hibernation

Frogs are cold-blooded animals, which means their body temperature is equal to the temperature around them. They prefer warmth, and they also don’t want to freeze to death. So, prior to winter weather they bulk up with fat and hibernate below the freezing line.
They dig down into the soil or move into such places as caverns, abandoned burrows, and under rocks, leaf litter or debris, anywhere they think they’ll be safe during freezing weather. Aquatic frogs hibernate underwater, partially burying themselves in the soil at the bottom. They may occasionally swim slowly around. Some species hibernate up to eight long months. 

Frogs that withstand freezing

There are five known exceptions to this in NA, frogs that can withstand freezing: Wood Frog (Lithobates sylvaticus), Cope’s Gray Tree Frog (Hyla chrysoscelis), Eastern Gray Tree Frog (Hyla versicolor), Spring Peeper (Pseudacris crucifer), and Western Chorus Frog (Pseudacris triseriata).
Even though ice crystals may form under their skin, in their bladder and elsewhere, and their heart, respiration, and muscle action stops, these frogs don’t die! When freezing begins to occur, their body produces, through a series of actions, a high glucose concentration that acts like antifreeze. When their body warms up, the ice melts, the organs spring into action! Voilà! Hello, spring!  Watch video

Reproduction

There are three stages in their lives: egg, tadpole, and adult.
Most males become sexually mature at around 10 to 12 months of age, females at two to three years. The mating ritual begins when males start calling for females in the spring. Each species has a specific call. Females come to the callers and choose a mate based on his call and, possibly, appearance.
Usually, they mate in the water, but depending on the species it may occur in trees above water or on the ground. The male climbs on the female’s back and wraps his forelegs around her “waist,” neck or head in an embrace called amplexus. Males don’t have a penis. Instead, both the male and female have openings at the end of their torso called a cloaca—an exit for body waste, and for males, semen. So they don’t actually copulate. Instead, the male releases semen onto eggs as they leave the female’s body.
With the exception of a few toad species, eggs are laid in water, or on a plant just above it, in which case tadpole hatchlings immediately drop into water. When eggs touch water, even if it’s just a tiny puddle in a cupped leaf, they soak it up and swell into a gelatinous mass. Eggs are subject to extreme predation and consequently are laid in huge batches of thousands (sometimes called frogspawn), which increases the chance that a few will survive.  Toads lay their eggs in a long strand.
Gelatinous mass containing dozens of tiny tadpoles that are so small they look like black dots.

Mass of frog eggs. The black “dots” are developing tadpoles. (© Maurice van der Velden / iStock)



Frogs have developed some interesting patterns of parenting. Some males, depending on the species, may watch over the eggs, carry them from a moist place, such as the water in a cupped leaf, to a wetter place, or, like Darwin’s Frogs, Rhinoderma darwinii, of Chile and Argentina, carry eggs in their vocal sac until they hatch into tadpoles! The eggs of Suriname Sea Toads get embedded into the skin on their mother’s back during mating and she carries them around through the tadpole stage. When they surface, they’re full-grown toads!

The time it takes for eggs to hatch varies from two to 40 days—the warmer the temperature, the shorter the time. They hatch into tadpoles, a stage that lasts for days, weeks or years, depending on the species. The American Bullfrog, for example, hatches in just a few days, but spends about three years as a tadpole.

 
Color illustration of the frog's entire life cycle from mating to adult.

Life cycle of frog. (Orin Zebest / Flickr / orig. artist unk; cc by 2.0)



Tadpoles, which are sometimes called pollywogs or froglets, eat voraciously, first on algae and later on tiny water animals, small fish and insects. 

Their appearance is quite different from their parents. At first, tadpoles have an oval-shaped body with gills, a long tail and, like their parents, large eyes. Body parts begin to develop: their tongue, teeth, some of the bones, kidneys, glands and gonads. Their legs start to grow, and finally, the tail is absorbed into their body. The gills are absorbed, too, as the lungs develop. Tadpoles move through water the same way fish do. After their metamorphosis into frogs they leave the water for a more terrestrial life or continue their water lifestyle, depending on the species. Watch development from egg to frog

Food sources
Frogs are carnivores. They eat insects, spiders, snails, worms, small fish, and small land animals, such as mice. A frog hunts by sitting motionless, rarely even blinking. When prey walks or flies by, its mouth opens, and its tongue flicks out and back, faster than a human can see.
 
Habitat
Most frog species live in tropical rainforests, but they’re native everywhere, except Antarctica and some islands. Some arid areas even have toad species living there—they bury themselves during dry spells and emerge when it rains.
 
All frogs live close to ponds, lakes and or streams. Tree frogs spend most of their time in nearby trees or tall vegetation, while other frogs may be seen on the ground or in water. Toads are willing to move farther from water.

Frogs are considered to be bio-indicators because their disappearance in an area indicates that something unhealthy in happening in the environment, something has gone amiss. Their permeable skin absorbs toxins easily, whether they’re in water or on land. When they start dying off or develop mutations, scientists take note. Broadly speaking, where frogs are safe, humans are safe.

 
Lifespan
Captive frogs are known to live 15 to 20 years, but just how long frogs live in the wild isn’t known, because they’re hard to track. Some are known to have lived at least five years, but predators probably eat most within days of their leaving the water.
Predators
Humans are the biggest threat to frogs as a result of habitat destruction, pollution and pesticide use. Also, many people eat them, and millions more die during scientific research or classroom experiments. Some are captured and kept as pets. Other predators are snakes, lizards, large spiders, fish, birds, and foxes and other mammals. Automobiles also kill many frogs in spring and fall as they cross roads while migrating between shallow, summer breeding ponds and deeper ponds for hibernating.
Top photo: Liz West / Flickr; cc by 2.0
2 Encyclopedia Britannica https://www.britannica.com/animal/Ranidae

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