Frog Knowledge For Sleep

Cold-blooded and weirdly talented, frogs have been hopping quietly through evolution with their sticky tongues and surprise screams. This episode covers the amphibians’ bizarre lives in enough detail to cure even the most persistent insomnia.

Perfect for sleep, relaxation, and background bedtime stories.

Transcript

Welcome to the I Can't Sleep Podcast,

Where I bore you to sleep with my soothing voice,

One fact at a time.

I'm your host,

Benjamin Boster,

And tonight let's fall asleep learning about frogs.

A frog is any member of a diverse and largely semi-aquatic group of short-bodied,

Tailless amphibian vertebrates,

Composing the order Anura,

Which comes from the ancient Greek Anura,

Literally,

Without tail.

Frog species with rough skin texture due to wart-like paratoid glands tend to be called toads,

But the distinction between frogs and toads is informal and purely cosmetic,

Not from taxonomy or evolutionary history.

Frogs are widely distributed,

Ranging from the tropics to subarctic regions,

But the greatest concentration of species diversity is in tropical rainforest and associated wetlands.

They account for around 88% of extant amphibian species,

And are one of the five most diverse vertebrate orders.

The oldest fossil,

Proto-frog,

Is known from the early Triassic of Madagascar,

250 million years ago,

But molecular clock dating suggests their divergence from other amphibians may extend further back to the Permian,

265 million years ago.

Adult frogs have a stout body,

Protruding eyes,

Anteriorly attached tongue,

Limbs folded underneath,

And no tail.

Their skin varies in color,

From well-camouflaged,

Dappled brown,

Gray,

And green,

To vivid patterns of bright red or yellow and black,

To show toxicity and ward off predators.

Frog skin also has a rich microbiome,

Which is important to their health.

Adult frogs live in both fresh water and on dry land.

Some species are adapted for living underground or in trees.

As their skin is semi-permeable,

Making them susceptible to dehydration,

They either live in moist niches or have special adaptations to deal with drier habitats.

Frogs produce a wide range of vocalizations and exhibit many different kinds of complex behaviors to attract mates,

To fend off predators,

And to generally survive.

Frogs typically spawn their eggs in bodies of water.

The eggs then hatch into fully aquatic larvae called tadpoles,

Which have tails and internal gills.

A few species lay eggs on land or bypass the tadpole stage altogether.

Tadpoles have highly specialized rasping mouthparts suitable for herbivorous,

Omnivorous,

Or planktivorous diets.

The life cycle is completed when they metamorphose into semi-aquatic adults.

Capable of terrestrial locomotion and hybrid respiration,

Using both lungs,

Aided by buccal pumping and gas exchange across the skin,

And the larval tail regresses into an internal eurostyle.

Adult frogs generally have a carnivorous diet consisting of small invertebrates,

Especially insects,

But omnivorous species exist,

And a few feed on plant matter.

The use of the common names frog and toad has no taxonomic justification.

From a classification perspective,

All members of the order Anura are frogs,

But only members of the family Bufonidae are considered true toads.

The use of the term frog in common names usually refers to species that are aquatic or semi-aquatic and have smooth,

Moist skins.

The term toad generally refers to species that are terrestrial with dry,

Warty skins.

There are numerous exceptions to this rule.

The European fire-bellied toad has a slightly warty skin and prefers a watery habitat,

Whereas the Panamanian golden frog is in the toad family Bufonidae and has a smooth skin.

The origin of the order name Anura and its original spelling A-N-O-U-R-E-S is the ancient Greek alpha privative prefix A-N,

From a before a vowel,

Meaning without,

And ura meaning animal tail.

It refers to the tailless character of these amphibians.

The origins of the word frog are uncertain and debated.

The word is first attested in Old English as fraga,

But the usual Old English word for the frog was frosk,

With variants such as frox and forsk,

Spelled F-O-R-S-C,

And it is agreed that the word frog is somehow related to this.

Old English frosk,

F-R-O-S-C,

Remained in dialectal use in English as frosh and frosk into the 19th century,

And is paralleled widely in other Germanic languages,

With examples in the modern languages including German Frosch,

Norwegian Frosk,

Icelandic Froskur,

And Dutch Kikfors.

These words allow reconstruction of a common Germanic ancestor,

Froskats.

The third edition of the Oxford English Dictionary finds that the etymology of Froskats is uncertain,

But agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of proi,

Meaning jump,

But agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of proi,

Meaning jump.

How Old English frosk gave rise to froga is,

However,

Uncertain,

As the development does not involve a regular sound change.

Instead,

It seems that there was a trend in Old English to coin nicknames for animals ending in g,

With examples themselves all of uncertain etymology,

Including dog,

Hog,

Pig,

Stag,

And earwig.

Frog appears to have been adapted from frosk as part of this trend.

Meanwhile,

The word toad,

First attested as Old English tadia,

Is unique to English and is likewise of uncertain etymology.

It is the basis for the word tadpole,

First attested as Middle English tadpole,

T-a-d-d-e-p-o-l,

Apparently meaning toadhead.

About 88% of amphibian species are classified in the order Anura.

These include over 7,

700 species in 59 families.

The Anura include all modern frogs and any fossil species that fit within the Anuran definition.

The characteristics of Anuran adults include 9 or fewer presacral vertebrae,

The presence of urastyle formed of fused vertebrae,

No tail,

A long and forward sloping ilium,

Shorter forelimbs than hindlimbs,

Radius and ulna fused,

Tibia and fibula fused,

Elongated ankle bones,

Absence of a prefrontal bone,

Presence of a hyoid plate,

A lower jaw without teeth,

Consisting of three pairs of bones,

An unsupported tongue,

Lymph spaces underneath the skin,

And a muscle,

The protractor lentis,

Attached to the lens of the eye.

The Anuran larva,

Or tadpole,

Has a single central respiratory spherical and mouth parts consisting of keratinous beaks and denticles.

The origins and evolutionary relationships between the three main groups of amphibians are hotly debated.

A molecular phylogeny based on rDNA analysis dating from 2005 suggests that salamanders and caecilians are more closely related to each other than they are to frogs,

And the divergence of the three groups took place in the Paleozoic or early Mesozoic before the breakup of the supercondent Pangaea,

And soon after their divergence from the lobe-finned fishes.

This would help account for the relative scarcity of amphibian fossils from the period before the group split.

Another molecular phylogenetic analysis conducted about the same time concluded that lizamphibians first appeared about 330 million years ago,

And that the Temnospondyl origin hypothesis is more credible than other theories.

The Neobotrachians seem to have originated in Africa or India,

The salamanders in East Asia,

And the caecilians in tropical Pangaea.

Other researchers,

While agreeing with the main thrust of this study,

Question the choice of calibration points used to synchronize the data.

They propose that the date of lizamphibian diversification should be placed in the Permian,

Rather less than 300 million years ago,

A date in better agreement with the paleontological data.

A further study in 2011,

Using both extinct and living taxa sampled for morphological as well as molecular data,

Came to the conclusion that lizamphibia is monophyletic,

And should be nested within Lepospondylae,

Rather than within Temnospondylae.

The study postulated that lizamphibia originated no earlier than the late Carboniferous,

Some 290 to 305 million years ago.

The split between Anura and Caudata was estimated as taking place 292 million years ago,

Rather later than most molecular studies suggest,

With the caecilians splitting off 239 million years ago.

Frogs have no tail,

Except as larvae,

And most have long hind legs,

Elongated ankle bones,

Webbed toes,

No claws,

Large eyes,

And a smooth or warty skin.

They have short vertebral columns,

With no more than 10 free vertebrae,

And fused tail bones.

Frogs range in size from Paedophrinia maoensis of Papua New Guinea,

That is 7.

7 mm in snout length,

To the up to about 35 cm and 3.

3 kg Goliath frog of Central Africa.

There are prehistoric extinct species that reached even larger sizes.

The structure of the feet and legs varies greatly among frog species,

Depending in part on whether they live primarily on the ground,

In water,

In trees,

Or in burrows.

Adult anurans have 4 fingers on the hands and 5 toes on the feet,

But the smallest species have hands and feet where some of the digits are vestigial.

Frogs must be able to move quickly through their environment to catch prey and escape predators,

And numerous adaptations help them to do so.

Most frogs are either proficient at jumping,

Or are descended from ancestors that were,

With much of the musculoskeletal morphology,

Modified for this purpose.

The tibia,

Fibula,

And tarsals have been fused into a single strong bone,

As have the radius and ulna in the forelimbs,

Which must absorb the impact on landing.

The metatarsals have become elongated to add to the leg length and allow frogs to push against the ground for a longer period on takeoff.

The muscular system has been similarly modified.

The hind limbs of ancestral frogs presumably contain pairs of muscles which would act in opposition,

One muscle to flex the knee,

A different muscle to extend it,

As is seen in most other limbed animals.

However,

In modern frogs,

Almost all muscles have been modified to contribute to the action of jumping,

With only a few small muscles remaining to bring the limb back to the starting position and maintain posture.

The muscles have also been greatly enlarged,

The main leg muscles accounting for over 17% of the total mass of frogs.

Many frogs have webbed feet,

And the degree of webbing is directly proportional to the amount of time the species spends in the water.

The completely aquatic African dwarf frog has fully webbed toes,

Whereas those of white's tree frog and arboreal species are only a quarter or half webbed.

Exceptions include flying frogs,

Which also have fully webbed toes used in gliding.

Arboreal frogs have pads located on the ends of their toes to help grip vertical surfaces.

These are not suction pads,

The surface consisting instead of columnar cells with flat tops with small gaps between them,

Lubricated by mucous glands.

When the frog applies pressure,

The cells adhere to irregularities on the surface,

And the grip is maintained through surface tension.

This allows the frog to climb on smooth surfaces,

But the system does not function efficiently when the pads are excessively wet.

In many arboreal frogs,

A small intercalary structure on each toe increases the surface area touching the substrate.

Furthermore,

Many arboreal frogs have hip joints that allow both hopping and walking.

Some frogs that live high in trees even possess an elaborate degree of webbing between their toes.

This allows the frogs to parachute or make a controlled glide from one position in the canopy to another.

Ground-dwelling frogs generally lack the adaptations of aquatic and arboreal frogs.

Most have smaller toe pads,

If any,

And little webbing.

Some burrowing frogs,

Such as Kooch's spadefoot,

Have a flap-like toe extension on the hind feet,

A keratinized tubercle often referred to as a spade,

That helps them burrow.

Like other amphibians,

Oxygen can pass through their highly permeable skins.

This unique feature allows them to remain in places without access to the air,

Respiring through their skins.

Ribs are generally absent,

So the lungs are filled by buccal pumping.

Frogs have three-chambered hearts,

A feature they share with lizards.

Oxygenated blood from the lungs and deoxygenated blood from the respiring tissues enter the heart through separate atria.

When these chambers contract,

The two bloodstreams pass into a common ventricle before being pumped via a spiral valve to the appropriate vessel,

The aorta for oxygenated blood and pulmonary artery for deoxygenated blood.

Some species of frog have adaptations that allow them to survive in oxygen-deficient water.

Frogs have a highly developed nervous system that consists of a brain,

Spinal cord,

And nerves.

Many parts of frog brains correspond with those of humans.

It consists of two olfactory lobes,

Two cerebral hemispheres,

A pineal body,

Two optic lobes,

A cerebellum,

And a medulla oblongata.

Muscular coordination and posture are controlled by the cerebellum,

And the medulla oblongata regulates respiration,

Digestion,

And other automatic functions.

The relative size of the cerebrum in frogs is much smaller than it is in humans.

Frogs have 10 pairs of cranial nerves which pass information from the outside directly to the brain,

And 10 pairs of spinal nerves which pass information from the extremities to the brain through the spinal cord.

By contrast,

All amniotes,

Mammals,

Birds,

And reptiles have 12 pairs of cranial nerves.

The eyes of most frogs are located on either side of the head,

Near the top,

And project outwards as hemispherical bulges.

They provide binocular vision over a field of 100 degrees to the front,

And a total visual field of almost 360 degrees.

They may be the only part of an otherwise submerged frog to protrude from the water.

Each eye has closable upper and lower lids,

And a nictitating membrane which provides further protection,

Especially when the frog is swimming.

Members of the aquatic family,

Pipetii,

Have the eyes located at the top of the head,

A position better suited for detecting prey in the water above.

The irises come in a range of colors,

And the pupils in a range of shapes.

The common toad has golden irises and horizontal slit-like pupils.

The red-eyed tree frog has vertical slit pupils.

The poison dart frog has dark irises.

The fire-bellied toad has triangular pupils.

And the tomato frog has circular ones.

The irises of the southern toad are patterned so as to blend in with the surrounding camouflaged skin.

The distant vision of a frog is better than its near vision.

Calling frogs will quickly become silent when they see an intruder,

Or even a moving shadow,

But the closer an object is,

The less well it is seen.

When a frog shoots out its tongue to catch an insect,

It is reacting to a small moving object that it cannot see well,

And must line it up precisely beforehand,

Because it shuts its eyes as the tongue is extended.

Although it was formally debated,

More recent research has shown that frogs can see in color,

Even in very low light.

Frogs can hear both in the air and below water.

They do not have external ears.

The eardrums are directly exposed,

Or may be covered by a layer of skin,

And are visible as a circular area just behind the eye.

The size and distance apart of the eardrums is related to the frequency and wavelength at which the frog calls.

A noise causes the tympanum to vibrate,

And the sound is transmitted to the middle and inner ear.

The middle ear contains semicircular canals,

Which help control balance and orientation.

In the inner ear,

The auditory hair cells are arranged in two areas of the cochlea,

The basilar papilla and the amphibian papilla.

The former detects high frequencies,

And the latter low frequencies.

Because the cochlea is short,

Frogs use electrical tuning to extend their hearing.

Frogs have a range of audible frequencies,

And help discriminate different sounds.

In some species that inhabit arid regions,

The sound of thunder or heavy rain may arouse them from a dormant state.

A frog may be startled by an unexpected noise,

But it will not usually take any action until it has located the source of the sound by sight.

The call or croak of a frog is unique to its species.

Frogs create this sound by passing air through the larynx in the throat.

In most calling frogs,

The sound is amplified by one or more membranes of skin under the throat or on the corner of the mouth that extend during the amplification of the call.

Some frog calls are so loud that they can be heard up to a mile away.

Additionally,

Some species have been found to use man-made structures,

Such as drain pipes,

For artificial amplification of their call.

The coastal-tailed frog lives in mountain streams in North America and does not vocalize.

During extreme conditions,

Some frogs enter a state of torpor and remain inactive for months.

In colder regions,

Many species of frog hibernate in winter.

Those that live on land,

Such as the American toad,

Dig a burrow and make a hibernaculum in which to lie dormant.

Others,

Less proficient at digging,

Find a crevice or bury themselves in dead leaves.

Aquatic species,

Such as the American bullfrog,

Normally sink to the bottom of the pond.

They lie semi-immersed in mud,

But still able to access the oxygen dissolved in the water.

Their metabolism slows down and they live on their low energy reserves.

Some frogs,

Such as the wood frog,

Moor frog,

Or spring peeper,

Can even survive being frozen.

Ice crystals form under the skin and in the body cavity,

But the essential organs are protected from freezing by a high concentration of glucose.

An apparently lifeless frozen frog can resume respiration and its heartbeat can restart when conditions warm up.

At the other extreme,

The striped burrowing frog regularly estivates during the hot,

Dry season in Australia,

Surviving in a dormant state without access to food and water for nine or ten months of the year.

It burrows underground and curls up inside a protective cocoon formed by its shed skin.

Researchers at the University of Queensland have found that during estivation,

The metabolism of the frog is altered and the operational efficiency of the mitochondria is increased.

This means that the limited amount of energy available to the comatose frog is used in a more efficient manner.

This survival mechanism is only useful to animals that remain completely unconscious for an extended period of time and whose energy requirements are low because they are cold-blooded and have no need to generate heat.

Other research showed that to provide these energy requirements,

Muscles atrophy,

But hindlimb muscles are preferentially unaffected.

Frogs have been found to have upper critical temperatures of around 41 degrees Celsius.

Reviews

I fell asleep soon after the story ended. Thank you.

Super boring but not a good night for me, apparently.