All about frogs and toads

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Did you know that a single toad will devour about 10,000 insects over a summer? Or that some frogs gobble 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 animals? Well, yeah!
Frogs are unique in numerous, fascinating ways. Read on to learn lots of surprising facts 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 is an accurate description of their lives—they spend part of it on land and part in water. There are about 7,000 amphibian species worldwide, and frogs make up 90 percent of them.
Along with newts, salamanders, and caecilians, they’re in the order Anura (uh-NOO-ruh), a word that comes 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 Ranidae species and about 500 in Bufonidae.

Origin

Frogs evolved from fish that moved out of the water about 400 million years ago, during the Devonian Period, to spend part of their time on land. Frog- and salamander-like animals appeared around 110 million years later, during the Permian Period. The earliest modern frog, one containing the recognizable frog features of today, showed up about 125 million years ago in the Cretaceous Period.

Itty-bitty to colossal (on a frog scale, that is)

Frogs are remarkably varied in size. The largest in the world 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 end of the scale is the world’s smallest frog, as well as the tiniest known vertebrate, Paedophryne amauensis. Found in Papua Guinea, it’s only 0.30 inches long (7.6 mm), the size of a housefly! Only slightly larger are three species discovered in Madagascar with the humorous Latin names Mini mum, Mini ature, and Mini scule! The largest, Mini mum, could easily fit on your thumbnail.

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

A frog can be distinguished from a toad. Once you know, it’s easy: Frogs have moist, smooth skin; a slender body; visible eardrums on the sides of their head; and no parotid glands. 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, broad head, with big, bulging eyes at the top, a nose, two eardrums, glands, and a mouth with a long tongue. The head contains the brain, spinal cord, and a highly developed nervous system.
Close up of a frog eye's as it sits in water.

A frog’s large bulbous eyes are involved in 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 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. 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 situated at the top of the 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 it’s true! 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 the 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 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

Ever thought of frogs having teeth? They do, at least most species. 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 an unusual thing about frogs. It’s long, and the base of it is fastened at the front of the mouth rather than the back, like that of a human. Because of this, they hold their tongue folded with the tip facing the throat! There’s a purpose for that, of course, as is nature’s way. When an insect happens by, the frog can flick it out quickly and accurately. And, 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. Pipidae, a small family of frogs that inhabit South America and sub–Saharan Africa, lacks a tongue altogether. They use their fingers to catch prey and place it in their mouth. 

Breathing

You’ve probably noticed that frogs pulsate their throat. It’s pretty conspicuous and demonstrates their unusual method of breathing, which is 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 draws air in through its nostrils while simultaneously pulling 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

Frogs have a 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.

Color illustration of a frog's internal anatomy as seen from the belly side of the frog.

(Jonathan McIntosh / Wiki; cc by 2.0)



Frogs have two arms and two legs made up of the same kind of bones 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 are shaped similar to our’s, also.

 Webbing

Their hind legs are long, with webbing that stretches between the toes. Frogs that spend a lot of time in water have longer legs and more webbing than more terrestrial species. Webbing is valuable for helping them to paddle through water. There’s an unusual family of frogs called “gliding” frogs that stretch their toes wide, and the webbing acts as wings, allowing them to jump from high places to lower. They “fly” from tree to tree in this manner. Watch them fly

Tree frogs are an example of those with less webbing. They spend all their time on land once they become adults and it 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.

Most frogs lack claws, but spadefoot toads, which live in dry habitats, have claw-like growths on their hind feet, 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, 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 still-unbroken 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.   Watch a walking toad.

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 is usually dry and bumpy, but it secretes moisture, too, and they’re able to retain it longer. Still, their skin is drier than that of frogs, and that enables them to live farther from water. Some even live in arid habitats, where they dig burrows to escape skin-drying conditions.

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.

The skin does more than provide moisture. Frogs absorb oxygen directly into their bloodstream through pores in it, 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 them so victimized by pollutants—they have little protection.
 

Coloration

It’d be pretty hard for any animal to top the flashy colors of the Red-eyed Tree Frog and the poison dart frogs. Certainly, most other frogs can’t, as they typically wear camouflage colors 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

There are frogs that can change their color, as the Chameleon Lizard does. Some species simply become paler or darker, but others change their color altogether. The change is brought about by chromatophores, which are star-shaped pigment cells in the skin that contain various granules of colors. Some cells have red, others have yellow, or black, and so on. They’re triggered by specific stimuli, such as excitement, humidity, light, temperature, and surroundings.
A dramatic example of color change is the Pacific Tree Frog, Hyla cinerea, which changes its color to match the color of its 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)



Another example is the Gray Tree Frog, normally colored green, gray or brown,, depending on its surroundings. But some are reportedly almost black and 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 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 loathsome morsel 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 is a warning to stay away from them, in the same way that the striking colors of a Monarch Butterfly warn of its toxicity. 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.” 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.

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 estivation, 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 because, like insects and snakes, they can’t control it themselves. They prefer warmth, and they certainly 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 in frigid temperatures. 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 in the spring and the ice melts, the organs spring into action! Voilà! Hello, spring!  Watch video

Reproduction

There are three stages in a frog’s life: egg, tadpole, and adult.
Colorful illustration of frog life cycle, from mating to adulthood.

Frog life cycle. (Orin Zebest / Flickr / orig. artist unk; cc by 2.0)

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, his appearance.
Usually, the couple mates 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 and they don’t actually copulate. Instead, both the male and female have an opening at the end of their torso called a cloaca, which is an exit for body waste, her eggs and his semen. Coupling, for this couple, simply consists of his semen being released onto eggs as they exit her body.
Eggs
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 the 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.

 

Tadpoles

Tadpoles, 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. Then 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 the body. The gills are absorbed, too, as the lungs develop. Tadpoles move through water the same way fish do. When they become adults, they leave the water for a more terrestrial life or continue their water lifestyle, depending on the species. Watch development from egg to frog    

Lifespan

Captive frogs are known to live 15 to 20 years, but just how long they live in the wild isn’t known, because they’re hard to track. Some have lived at least five years, but predators probably eat most within days of their leaving the water.

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.

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
 

Predators

Humans are the biggest threat to frogs as a result of habitat destruction, pollution and pesticide use. Also, many people eat them. Millions of frogs 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 frogs in the spring and fall as they cross roads while migrating between shallow, summer breeding ponds and deeper ponds for hibernating.

Habitat

Most frog species live in tropical rainforests, but they’re native everywhere, except Antarctica and some islands. They’re found in all inhabits where there’s water of some kind, from deep water to quiet creeks and ponds.

Toads are willing to move farther from water, and some species thumb their little toad noses at water pretty much altogether and live in arid places where they bury themselves during dry spells and emerge only when it rains. But otherwise, frogs live close to water where they can be spotted on the ground or in the water. Tree frogs spend most of their time in trees or tall vegetation nearby.

Frogs as bio-indicators

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 the water or on land. When they start dying off or develop mutations, scientists take note. Broadly speaking, where frogs are safe, humans are safe.

Environmental threats to frogs

Cute little brown Fringe-limbed Tree Frog sitting on a green, mossy surface.

“Toughie,” the Fringe-limbed Tree Frog, Ecnomiohyla rabborum. (Brian Gratwicke / Wikimedia; CC BY 2.0)



“Toughie,” shown above, was given his name by the Atlanta Botanical Garden where he lived. He was the last of several dozen rescued in 2005 from an outbreak of Chytridiomycosis in Panama. His species became extinct in the wild. When he died on September 26, 2016, he was the very last one of his kind.

Chytridiomycosis, a fungus disease that sometimes kills off entire populations of frogs in an area, is just one of a long list of threats to the future of frogs. The most significant is ongoing habitat loss and fragmentation. Plus, there’s air pollution, and chemicals, too—particularly pesticide run-off into water that causes hormone disruptions and even kills. Humans are over-harvesting frogs for food. Even lights are damaging because they reduce mating behavior in some species.

Nearly 200 species have gone extinct in our own lifetime! Dozens more are on the brink, and some only exist because they’re in protective captivity. Frogs worldwide are essentially huddled at the end of a dead-end street with a steamroller barreling toward them.

What you can do to help frogs

Avoid pesticide use in your yard (this helps all other wildlife, too), turn off porch lights, at least in the spring. Build a frog pond, provide plantings for them to cool off and hide under. Use native plants because tadpoles need their preferred plants for eating and hiding. Don’t add American Bullfrogs to your pond, they’re thought to contribute to Chytridiomycosis infections. Don’t flush medicines and chemicals down the toilet. On a wider scale, donate to conservation programs and always put trash in proper receptacles.

Top photo: Liz West / Flickr; cc by 2.0

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