This is an audio version of the Wikipedia Article: 00:03:19 1 Etymology and taxonomy 00:04:15 1.1 Etymology 00:05:29 1.2 …
A frog is any member of a diverse and largely
carnivorous group of short-bodied, tailless
amphibians composing the order Anura (literally
without tail in Ancient Greek). The oldest
fossil "proto-frog" appeared in the early
Triassic of Madagascar, but molecular clock
dating suggests their origins may extend further
back to the Permian, 265 million years ago.
Frogs are widely distributed, ranging from
the tropics to subarctic regions, but the
greatest concentration of species diversity
is in tropical rainforests. There are over
6,300 recorded species, accounting for around
88% of extant amphibian species. They are
also one of the five most diverse vertebrate
orders. Warty frog species tend to be called
toads, but the distinction between frogs and
toads is informal, not from taxonomy or evolutionary
history.
An adult frog has a stout body, protruding
eyes, anteriorly-attached tongue, limbs folded
underneath, and no tail (except in tailed
frogs). Frogs have glandular skin, with secretions
ranging from distasteful to toxic. Their skin
varies in colour from well-camouflaged dappled
brown, grey and green to vivid patterns of
bright red or yellow and black to show toxicity
and ward off predators. Adult frogs live in
fresh water and on dry land; some species
are adapted for living underground or in trees.
Frogs typically lay their eggs in water. The
eggs hatch into aquatic larvae called tadpoles
that have tails and internal gills. They have
highly specialized rasping mouth parts suitable
for herbivorous, omnivorous or planktivorous
diets. The life cycle is completed when they
metamorphose into adults. A few species deposit
eggs on land or bypass the tadpole stage.
Adult frogs generally have a carnivorous diet
consisting of small invertebrates, but omnivorous
species exist and a few feed on fruit. Frog
skin has a rich microbiome which is important
to their health. Frogs are extremely efficient
at converting what they eat into body mass.
They are an important food source for predators
and part of the food web dynamics of many
of the world's ecosystems. The skin is semi-permeable,
making them susceptible to dehydration, so
they either live in moist places or have special
adaptations to deal with dry habitats. Frogs
produce a wide range of vocalizations, particularly
in their breeding season, and exhibit many
different kinds of complex behaviours to attract
mates, to fend off predators and to generally
survive.
Frogs are valued as food by humans and also
have many cultural roles in literature, symbolism
and religion. They are also seen as environmental
bellwethers, with declines in frog populations
often viewed as early warning signs of environmental
damage. Frog populations have declined significantly
since the 1950s. More than one third of species
are considered to be threatened with extinction
and over 120 are believed to have become extinct
since the 1980s. The number of malformations
among frogs is on the rise and an emerging
fungal disease, chytridiomycosis, has spread
around the world. Conservation biologists
are working to understand the causes of these
problems and to resolve them.
== Etymology and taxonomy ==
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 (Bombina bombina) has a slightly warty
skin and prefers a watery habitat whereas
the Panamanian golden frog (Atelopus zeteki)
is in the toad family Bufonidae and has a
smooth skin.
=== Etymology ===
The origin of the order name Anura — and
its original spelling Anoures — is the Ancient
Greek "alpha privative" prefix ἀν- (an-)
"without", and οὐρά (ourá), 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 frogga, but
the usual Old English word for the frog was
frosc (with variants such as frox and forsc),
and it is agreed that the word frog is somehow
related to this. Old English frosc remained
in dialectal use in English as frosh and frosk
into the nineteenth century, and is paralleled
widely in other Germanic languages, with examples
in the modern languages including German Frosch,
Icelandic froskur, and Dutch (kik)vors. These
words allow us to reconstruct a Common Germanic
ancestor *froskaz. The third edition of the
Oxford English Dictionary finds that the etymology
of *froskaz is uncertain, but agrees with
arguments that it could plausibly derive from
a Proto-Indo-European base along the lines
of *preu = "jump".How Old English frosc gave
rise to frogga 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 (ear)wig. Frog appears
to have been adapted from frosc as part of
this trend.Meanwhile, the word toad, first
attested as Old English tādige, is unique
to English and is likewise of uncertain etymology.
It is the basis for the word tadpole, first
attested as Middle English taddepol, apparently
meaning 'toad-head'.
===
Taxonomy ===
About 88% of amphibian species are classified
in the order Anura. These include over 7,000
species in 56 families, of which the Craugastoridae
(831 spp.), Hylidae (720 spp.), Microhylidae
(670 spp.), and Bufonidae (610 spp.) are the
richest in species.
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 a urostyle formed of fused
vertebrae, no tail, a long and forward-sloping
ilium, shorter fore limbs than hind limbs,
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 (with the exception of Gastrotheca
guentheri) consisting of three pairs of bones
(angulosplenial, dentary, and mentomeckelian,
with the last pair being absent in Pipoidea),
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
spiracle and mouthparts consisting of keratinous
beaks and denticles.
Frogs and toads are broadly classified into
three suborders: Archaeobatrachia, which includes
four families of primitive frogs; Mesobatrachia,
which includes five families of more evolutionary
intermediate frogs; and Neobatrachia, by far
the largest group, which contains the remaining
families of modern frogs, including most common
species throughout the world. The Neobatrachia
suborder is further divided into the two superfamilies
Hyloidea and Ranoidea. This classification
is based on such morphological features as
the number of vertebrae, the structure of
the pectoral girdle, and the morphology of
tadpoles. While this classification is largely
accepted, relationships among families of
frogs are still debated.Some species of anurans
hybridize readily. For instance, the edible
frog (Pelophylax esculentus) is a hybrid between
the pool frog (P. lessonae) and the marsh
frog (P. ridibundus). The fire-bellied toads
Bombina bombina and B. variegata are similar
in forming hybrids. These are less fertile
than their parents, giving rise to a hybrid
zone where the hybrids are prevalent.
== Evolution ==
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 supercontinent 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 groups split. Another molecular
phylogenetic analysis conducted about the
same time concluded that lissamphibians first
appeared about 330 million years ago and that
the temnospondyl-origin hypothesis is more
credible than other theories. The neobatrachians
seemed to have originated in Africa/India,
the salamanders in East Asia and the caecilians
in tropical Pangaea. Other researchers, while
agreeing with the main thrust of this study,
questioned the choice of calibration points
used to synchronise the data. They proposed
that the date of lissamphibian diversification
should be placed in the Permian, rather less
than 300 million years ago, a date in better
agreement with the palaeontological 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 Lissamphibia is monophyletic and that
it should be nested within Lepospondyli rather
than within Temnospondyli. The study postulated
that Lissamphibia 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.
In 2008, Gerobatrachus hottoni, a temnospondyl
with many frog- and salamander-like characteristics,
was discovered in Texas. It dated back 290
million years and was hailed as a missing
link, a stem batrachian close to the common
ancestor of frogs and salamanders, consistent
with the widely accepted hypothesis that frogs
and salamanders are more closely related to
each other (forming a clade called Batrachia)
than they are to caecilians. However, others
have suggested that Gerobatrachus hottoni
was only a dissorophoid temnospondyl unrelated
to extant amphibians.Salientia (Latin salere
(salio), "to jump") is the name of the total
group that includes modern frogs in the order
Anura as well as their close fossil relatives,
the "proto-frogs" or "stem-frogs". The common
features possessed by these proto-frogs include
14 presacral vertebrae (modern frogs have
eight or 9), a long and forward-sloping ilium
in the pelvis, the presence of a frontoparietal
bone, and a lower jaw without teeth. The earliest
known amphibians that were more closely related
to frogs than to salamanders are Triadobatrachus
massinoti, from the early Triassic period
of Madagascar (about 250 million years ago),
and Czatkobatrachus polonicus, from the Early
Triassic of Poland (about the same age as
Triadobatrachus). The skull of Triadobatrachus
is frog-like, being broad with large eye sockets,
but the fossil has features diverging from
modern frogs. These include a longer body
with more vertebrae. The tail has separate
vertebrae unlike the fused urostyle or coccyx
in modern frogs. The tibia and fibula bones
are also separate, making it probable that
Triadobatrachus was not an efficient leaper.The
earliest known "true frogs" that fall into
the anuran lineage proper all lived in the
early Jurassic period. One such early frog
species, Prosalirus bitis, was discovered
in 1995 in the Kayenta Formation of Arizona
and dates back to the Early Jurassic epoch
(199.6 to 175 million years ago), making Prosalirus
somewhat more recent than Triadobatrachus.
Like the latter, Prosalirus did not have greatly
enlarged legs, but had the typical three-pronged
pelvic structure of modern frogs. Unlike Triadobatrachus,
Prosalirus had already lost nearly all of
its tail and was well adapted for jumping.
Another Early Jurassic frog is Vieraella herbsti,
which is known only from dorsal and ventral
impressions of a single animal and was estimated
to be 33 mm (1.3 in) from snout to vent. Notobatrachus
degiustoi from the middle Jurassic is slightly
younger, about 155–170 million years old.
The main evolutionary changes in this species
involved the shortening of the body and the
loss of the tail. The evolution of modern
Anura likely was complete by the Jurassic
period. Since then, evolutionary changes in
chromosome numbers have taken place about
20 times faster in mammals than in frogs,
which means speciation is occurring more rapidly
in mammals.According to genetic studies, the
families Hyloidea, Microhylidae, and the clade
Natatanura (comprising about 88% of living
frogs) diversified simultaneously some 66
million years ago, soon after the Cretaceous–Paleogene
extinction event associated with the Chicxulub
impactor. All origins of arboreality (e.g.
in Hyloidea and Natatanura) follow from that
time and the resurgence of forest that occurred
afterwards.Frog fossils have been found on
all continents except Antarctica, but biogeographic
evidence suggests they also inhabited Antarctica
in an earlier era when the climate was warmer.
A cladogram showing the relationships of the
different families of frogs in the clade Anura
can be seen in the table above. This diagram,
in the form of a tree, shows how each frog
family is related to other families, with
each node representing a point of common ancestry.
It is based on Frost et al. (2006), Heinicke
et al. (2009) and Pyron and Wiens (2011).
== Morphology and physiology ==
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 tailbones (urostyle or
coccyx). 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
and a frog deprived of its lungs can maintain
its body functions without them. For the skin
to serve as a respiratory organ, it must remain
moist. This makes frogs susceptible to various
substances they may encounter in the environment,
some of which may be toxic and can dissolve
in the water film and be passed into their
bloodstream. This may be one of the causes
of the worldwide decline in frog populations.Frogs
range in size from the recently discovered
7.7-millimetre (0.30 in) Paedophryne amauensis
of Papua New Guinea to the 30-centimetre (12
in) goliath frog (Conraua goliath) of Cameroon.
The skin hangs loosely on the body because
of the lack of loose connective tissue. Frogs
have three eyelid membranes: one is transparent
to protect the eyes underwater, and two vary
from translucent to opaque. They have a tympanum
on each side of their heads which is involved
in hearing and, in some species, is covered
by skin. True toads completely lack teeth,
but most frogs have them, specifically pedicellate
teeth in which the crown is separated from
the root by fibrous tissue. These are on the
edge of the upper jaw and vomerine teeth are
also on the roof of their mouths. No teeth
are in the lower jaw and frogs usually swallow
their food whole. The teeth are mainly used
to grip the prey and keep it in place until
swallowed, a process assisted by retracting
the eyes into the head. The African bullfrog
(Pyxicephalus), which preys on relatively
large animals such as mice and other frogs,
has cone shaped bony projections called odontoid
processes at the front of the lower jaw which
function like teeth.
=== Feet and legs ===
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. 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 fore limbs
(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
take-off. The illium has elongated and formed
a mobile joint with the sacrum which, in specialist
jumpers such as ranids and hylids, functions
as an additional limb joint to further power
the leaps. The tail vertebrae have fused into
a urostyle which is retracted inside the pelvis.
This enables the force to be transferred from
the legs to the body during a leap.
The muscular system has been similarly modified.
The hind limbs of ancestral frogs presumably
contained 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, with 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 (Hymenochirus sp.) has fully webbed
toes, whereas those of White's tree frog (Litoria
caerulea), an arboreal species, are only a
quarter or half webbed. Exceptions include
flying frogs in the Hylidae and Rhacophoridae,
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 Couch's spadefoot (Scaphiopus couchii)
have a flap-like toe extension on the hind
feet, a keratinised tubercle often referred
to as a spade, that helps them to burrow.Sometimes
during the tadpole stage, one of the developing
rear legs is eaten by a predator such as a
dragonfly nymph. In some cases, the full leg
still grows, but in others it does not, although
the frog may still live out its normal lifespan
with only three limbs. Occasionally, a parasitic
flatworm (Ribeiroia ondatrae) digs into the
rear of a tadpole, causing a rearrangement
of the limb bud cells and the frog develops
one or more extra legs.
=== Skin ===
A frog's skin is protective, has a respiratory
function, can absorb water and helps control
body temperature. It has many glands, particularly
on the head and back, which often exude distasteful
and toxic substances (granular glands). The
secretion is often sticky and helps keep the
skin moist, protects against the entry of
moulds and bacteria, and make the animal slippery
and more able to escape from predators. The
skin is shed every few weeks. It usually splits
down the middle of the back and across the
belly, and the frog pulls its arms and legs
free. The sloughed skin is then worked towards
the head where it is quickly eaten.Being cold-blooded,
frogs have to adopt suitable behaviour patterns
to regulate their temperature. To warm up,
they can move into the sun or onto a warm
surface; if they overheat, they can move into
the shade or adopt a stance that exposes the
minimum area of skin to the air. This posture
is also used to prevent water loss and involves
the frog squatting close to the substrate
with its hands and feet tucked under its chin
and body. The colour of a frog's skin is used
for thermoregulation. In cool damp conditions,
the colour will be darker than on a hot dry
day. The grey foam-nest tree frog (Chiromantis
xerampelina) is even able to turn white to
minimize the chance of overheating.Many frogs
are able to absorb water and oxygen directly
through the skin, especially around the pelvic
area, but the permeability of a frog's skin
can also result in water loss. Glands located
all over the body exude mucus which helps
keep the skin moist and reduces evaporation.
Some glands on the hands and chest of males
are specialized to produce sticky secretions
to aid in amplexus. Similar glands in tree
frogs produce a glue-like substance on the
adhesive discs of the feet. Some arboreal
frogs reduce water loss by having a waterproof
layer of skin, and several South American
species coat their skin with a waxy secretion.
Other frogs have adopted behaviours to conserve
water, including becoming nocturnal and resting
in a water-conserving position. Some frogs
may also rest in large groups with each frog
pressed against its neighbours. This reduces
the amount of skin exposed to the air or a
dry surface, and thus reduces water loss.
Woodhouse's toad (Bufo woodhousii), if given
access to water after confinement in a dry
location, sits in the shallows to rehydrate.
The male hairy frog (Trichobatrachus robustus)
has dermal papillae projecting from its lower
back and thighs, giving it a bristly appearance.
They contain blood vessels and are thought
to increase the area of the skin available
for respiration.Some species have bony plates
embedded in their skin, a trait that appears
to have evolved independently several times.
In certain other species, the skin at the
top of the head is compacted and the connective
tissue of the dermis is co-ossified with the
bones of the skull (exostosis).Camouflage
is a common defensive mechanism in frogs.
Most camouflaged frogs are nocturnal; during
the day, they seek out a position where they
can blend into the background and remain undetected.
Some frogs have the ability to change colour,
but this is usually restricted to a small
range of colours. For example, White's tree
frog (Litoria caerulea) varies between pale
green and dull brown according to the temperature,
and the Pacific tree frog (Pseudacris regilla)
has green and brown morphs, plain or spotted,
and changes colour depending on the time of
year and general background colour. Features
such as warts and skin folds are usually on
ground-dwelling frogs, for whom smooth skin
would not provide such effective camouflage.
Certain frogs change colour between night
and day, as light and moisture stimulate the
pigment cells and cause them to expand or
contract. Some are even able to control their
skin texture.
=== Respiration and circulation ===
The skin of a frog is permeable to oxygen
and carbon dioxide, as well as to water. There
are blood vessels near the surface of the
skin and when a frog is underwater, oxygen
diffuses directly into the blood. When not
submerged, a frog breathes by a process known
as buccal pumping. Its lungs are similar to
those of humans, but the chest muscles are
not involved in respiration, and no ribs or
diaphragm exist to help move air in and out.
Instead, it puffs out its throat and draws
air in through the nostrils, which in many
species can then be closed by valves. When
the floor of the mouth is compressed, air
is forced into the lungs. The fully aquatic
Bornean flat-headed frog (Barbourula kalimantanensis)
is the first frog known to lack lungs entirely.Frogs
have three-chambered hearts, a feature they
share with lizards. Oxygenated blood from
the lungs and de-oxygenated blood from the
respiring tissues enter the heart through
separate atria. When these chambers contract,
the two blood streams 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. The ventricle is partially divided
into narrow cavities which minimizes the mixing
of the two types of blood. These features
enable frogs to have a higher metabolic rate
and be more active than would otherwise be
possible.Some species of frog have adaptations
that allow them to survive in oxygen deficient
water. The Titicaca water frog (Telmatobius
culeus) is one such species and has wrinkly
skin that increases its surface area to enhance
gas exchange. It normally makes no use of
its rudimentary lungs but will sometimes raise
and lower its body rhythmically while on the
lake bed to increase the flow of water around
it.
=== Digestion and excretion ===
Frogs have maxillary teeth along their upper
jaw which are used to hold food before it
is swallowed. These teeth are very weak, and
cannot be used to chew or catch and harm agile
prey. Instead, the frog uses its sticky, cleft
tongue to catch flies and other small moving
prey. The tongue normally lies coiled in the
mouth, free at the back and attached to the
mandible at the front. It can be shot out
and retracted at great speed. Some frogs have
no tongue and just stuff food into their mouths
with their hands. The eyes assist in the swallowing
of food as they can be retracted through holes
in the skull and help push food down the throat.
The food then moves through the oesophagus
into the stomach where digestive enzymes are
added and it is churned up. It then proceeds
to the small intestine (duodenum and ileum)
where most digestion occurs. Pancreatic juice
from the pancreas, and bile, produced by the
liver and stored in the gallbladder, are secreted
into the small intestine, where the fluids
digest the food and the nutrients are absorbed.
The food residue passes into the large intestine
where excess water is removed and the wastes
are passed out through the cloaca.Although
adapted to terrestrial life, frogs resemble
freshwater fish in their inability to conserve
body water effectively. When they are on land,
much water is lost by evaporation from the
skin. The excretory system is similar to that
of mammals and there are two kidneys that
remove nitrogenous products from the blood.
Frogs produce large quantities of dilute urine
in order to flush out toxic products from
the kidney tubules. The nitrogen is excreted
as ammonia by tadpoles and aquatic frogs but
mainly as urea, a less toxic product, by most
terrestrial adults. A few species of tree
frog with little access to water excrete the
even less toxic uric acid. The urine passes
along paired ureters to the urinary bladder
from which it is vented periodically into
the cloaca. All bodily wastes exit the body
through the cloaca which terminates in a cloacal
vent.
=== Reproductive system ===
In the male frog, the two testes are attached
to the kidneys and semen passes into the kidneys
through fine tubes called efferent ducts.
It then travels on through the ureters, which
are consequently known as urinogenital ducts.
There is no penis, and sperm is ejected from
the cloaca directly onto the eggs as the female
lays them. The ovaries of the female frog
are beside the kidneys and the eggs pass down
a pair of oviducts and through the cloaca
to the exterior.When frogs mate, the male
climbs on the back of the female and wraps
his fore limbs round her body, either behind
the front legs or just in front of the hind
legs. This position is called amplexus and
may be held for several days. The male frog
has certain hormone-dependent secondary sexual
characteristics. These include the development
of special pads on his thumbs in the breeding
season, to give him a firm hold. The grip
of the male frog during amplexus stimulates
the female to release eggs, usually wrapped
in jelly, as spawn. In many species the male
is smaller and slimmer than the female. Males
have vocal cords and make a range of croaks,
particularly in the breeding season, and in
some species they also have vocal sacs to
amplify the sound.
=== Nervous system ===
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
ten pairs of cranial nerves which pass information
from the outside directly to the brain, and
ten 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 twelve
pairs of cranial nerves.
=== Sight ===
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° to
the front and a total visual field of almost
360°. 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
Pipidae 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 colours and the pupils in a range
of shapes. The common toad (Bufo bufo) has
golden irises and horizontal slit-like pupils,
the red-eyed tree frog (Agalychnis callidryas)
has vertical slit pupils, the poison dart
frog has dark irises, the fire-bellied toad
(Bombina spp.) has triangular pupils and the
tomato frog (Dyscophus spp.) has circular
ones. The irises of the southern toad (Anaxyrus
terrestris) 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 formerly debated,
more recent research has shown that frogs
can see in colour, even in very low light.
=== Hearing ===
Frogs can hear both in the air and below water.
They do not have external ears; the eardrums
(tympanic membranes) 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. In some species such
as the bullfrog, the size of the tympanum
indicates the sex of the frog; males have
tympani that are larger than their eyes while
in females, the eyes and tympani are much
the same size. 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 range of audible frequencies
and help discriminate different sounds. This
arrangement enables detection of the territorial
and breeding calls of their conspecifics.
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.
=== Call ===
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 vocal sacs, membranes of skin under
the throat or on the corner of the mouth,
that distend 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.Frogs in the genera
Heleioporus and Neobatrachus lack vocal sacs
but can still produce a loud call. Their buccal
cavity is enlarged and dome-shaped, acting
as a resonance chamber that amplifies the
sound. Species of frog that lack vocal sacs
and that do not have a loud call tend to inhabit
areas close to constantly noisy, flowing water.
They need to use an alternative means to communicate.
The coastal tailed frog (Ascaphus truei) lives
in mountain streams in North America and does
not vocalize.The main reason for calling is
to allow male frogs to attract a mate. Males
may call individually or there may be a chorus
of sound where numerous males have converged
on breeding sites. Females of many frog species,
such as the common tree frog (Polypedates
leucomystax), reply to the male calls, which
acts to reinforce reproductive activity in
a breeding colony. Female frogs prefer males
that produce sounds of greater intensity and
lower frequency, attributes that stand out
in a crowd. The rationale for this is thought
to be that by demonstrating his prowess, the
male shows his fitness to produce superior
offspring.A different call is emitted by a
male frog or unreceptive female when mounted
by another male. This is a distinct chirruping
sound and is accompanied by a vibration of
the body. Tree frogs and some non-aquatic
species have a rain call that they make on
the basis of humidity cues prior to a shower.
Many species also have a territorial call
that is used to drive away other males. All
of these calls are emitted with the mouth
of the frog closed. A distress call, emitted
by some frogs when they are in danger, is
produced with the mouth open resulting in
a higher-pitched call. It is typically used
when the frog has been grabbed by a predator
and may serve to distract or disorientate
the attacker so that it releases the frog.
Many species of frog have deep calls. The
croak of the American bullfrog (Rana catesbiana)
is sometimes written as "jug o' rum". The
Pacific tree frog (Pseudacris regilla) produces
the onomatopoeic "ribbit" often heard in films.
Other renderings of frog calls into speech
include "brekekekex koax koax", the call of
the marsh frog (Pelophylax ridibundus) in
The Frogs, an Ancient Greek comic drama by
Aristophanes.
=== Torpor ===
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 (Bufo americanus)
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 (Rana catesbeiana) normally
sink to the bottom of the pond where 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 energy reserves. Some frogs 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 the
heart beat can restart when conditions warm
up.At the other extreme, the striped burrowing
frog (Cyclorana alboguttata) regularly aestivates
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 aestivation, 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 hind limb muscles are preferentially
unaffected. Frogs have been found to have
upper critical temperatures of around 41 degrees
Celsius.
== Locomotion ==
Different species of frog use a number of
methods of moving around including jumping,
running, walking, swimming, burrowing, climbing
and gliding.
JumpingFrogs are generally recognized as exceptional
jumpers and, relative to their size, the best
jumpers of all vertebrates. The striped rocket
frog, Litoria nasuta, can leap over 2 metres
(6 ft 7 in), a distance that is more than
fifty times its body length of 5.5 centimetres
(2.2 in). There are tremendous differences
between species in jumping capability. Within
a species, jump distance increases with increasing
size, but relative jumping distance (body-lengths
jumped) decreases. The Indian skipper frog
(Euphlyctis cyanophlyctis) has the ability
to leap out of the water from a position floating
on the surface. The tiny northern cricket
frog (Acris crepitans) can "skitter" across
the surface of a pond with a series of short
rapid jumps.Slow-motion photography shows
that the muscles have passive flexibility.
They are first stretched while the frog is
still in the crouched position, then they
are contracted before being stretched again
to launch the frog into the air. The fore
legs are folded against the chest and the
hind legs remain in the extended, streamlined
position for the duration of the jump. In
some extremely capable jumpers, such as the
Cuban tree frog (Osteopilus septentrionalis)
and the northern leopard frog (Rana pipiens),
the peak power exerted during a jump can exceed
that which the muscle is theoretically capable
of producing. When the muscles contract, the
energy is first transferred into the stretched
tendon which is wrapped around the ankle bone.
Then the muscles stretch again at the same
time as the tendon releases its energy like
a catapult to produce a powerful acceleration
beyond the limits of muscle-powered acceleration.
A similar mechanism has been documented in
locusts and grasshoppers.
Walking and running
Frogs in the families Bufonidae, Rhinophrynidae,
and Microhylidae have short back legs and
tend to walk rather than jump. When they try
to move rapidly, they speed up the rate of
movement of their limbs or resort to an ungainly
hopping gait. The Great Plains narrow-mouthed
toad (Gastrophryne olivacea) has been described
as having a gait that is "a combination of
running and short hops that are usually only
an inch or two in length". In an experiment,
Fowler's toad (Bufo fowleri) was placed on
a treadmill which was turned at varying speeds.
By measuring the toad's uptake of oxygen it
was found that hopping was an inefficient
use of resources during sustained locomotion
but was a useful strategy during short bursts
of high-intensity activity.The red-legged
running frog (Kassina maculata) has short,
slim hind limbs unsuited to jumping. It can
move fast by using a running gait in which
the two hind legs are used alternately. Slow-motion
photography shows, unlike a horse that can
trot or gallop, the frog's gait remained similar
at slow, medium, and fast speeds. This species
can also climb trees and shrubs, and does
so at night to catch insects. The Indian skipper
frog (Euphlyctis cyanophlyctis) has broad
feet and can run across the surface of the
water for several metres (yards).
Swimming
Frogs that live in or visit water have adaptations
that improve their swimming abilities. The
hind limbs are heavily muscled and strong.
The webbing between the toes of the hind feet
increases the area of the foot and helps propel
the frog powerfully through the water. Members
of the family Pipidae are wholly aquatic and
show the most marked specialization. They
have inflexible vertebral columns, flattened,
streamlined bodies, lateral line systems,
and powerful hind limbs with large webbed
feet. Tadpoles mostly have large tail fins
which provide thrust when the tail is moved
from side to side.
BurrowingSome frogs have become adapted for
burrowing and a life underground. They tend
to have rounded bodies, short limbs, small
heads with bulging eyes, and hind feet adapted
for excavation. An extreme example of this
is the purple frog (Nasikabatrachus sahyadrensis)
from southern India which feeds on termites
and spends almost its whole life underground.
It emerges briefly during the monsoon to mate
and breed in temporary pools. It has a tiny
head with a pointed snout and a plump, rounded
body. Because of this fossorial existence,
it was first described in 2003, being new
to the scientific community at that time,
although previously known to local people.
The spadefoot toads of North America are also
adapted to underground life. The Plains spadefoot
toad (Spea bombifrons) is typical and has
a flap of keratinised bone attached to one
of the metatarsals of the hind feet which
it uses to dig itself backwards into the ground.
As it digs, the toad wriggles its hips from
side to side to sink into the loose soil.
It has a shallow burrow in the summer from
which it emerges at night to forage. In winter,
it digs much deeper and has been recorded
at a depth of 4.5 m (15 ft). The tunnel is
filled with soil and the toad hibernates in
a small chamber at the end. During this time,
urea accumulates in its tissues and water
is drawn in from the surrounding damp soil
by osmosis to supply the toad's needs. Spadefoot
toads are "explosive breeders", all emerging
from their burrows at the same time and converging
on temporary pools, attracted to one of these
by the calling of the first male to find a
suitable breeding location.The burrowing frogs
of Australia have a rather different lifestyle.
The western spotted frog (Heleioporus albopunctatus)
digs a burrow beside a river or in the bed
of an ephemeral stream and regularly emerges
to forage. Mating takes place and eggs are
laid in a foam nest inside the burrow. The
eggs partially develop there, but do not hatch
until they are submerged following heavy rainfall.
The tadpoles then swim out into the open water
and rapidly complete their development. Madagascan
burrowing frogs are less fossorial and mostly
bury themselves in leaf litter. One of these,
the green burrowing frog (Scaphiophryne marmorata),
has a flattened head with a short snout and
well-developed metatarsal tubercles on its
hind feet to help with excavation. It also
has greatly enlarged terminal discs on its
fore feet that help it to clamber around in
bushes. It breeds in temporary pools that
form after rains.
Climbing
Tree frogs live high in the canopy, where
they scramble around on the branches, twigs,
and leaves, sometimes never coming down to
earth. The "true" tree frogs belong to the
family Hylidae, but members of other frog
families have independently adopted an arboreal
habit, a case of convergent evolution. These
include the glass frogs (Centrolenidae), the
bush frogs (Hyperoliidae), some of the narrow-mouthed
frogs (Microhylidae), and the shrub frogs
(Rhacophoridae). Most tree frogs are under
10 cm (4 in) in length, with long legs and
long toes with adhesive pads on the tips.
The surface of the toe pads is formed from
a closely packed layer of flat-topped, hexagonal
epidermal cells separated by grooves into
which glands secrete mucus. These toe pads,
moistened by the mucus, provide the grip on
any wet or dry surface, including glass. The
forces involved include boundary friction
of the toe pad epidermis on the surface and
also surface tension and viscosity. Tree frogs
are very acrobatic and can catch insects while
hanging by one toe from a twig or clutching
onto the blade of a windswept reed. Some members
of the subfamily Phyllomedusinae have opposable
toes on their feet. The reticulated leaf frog
(Phyllomedusa ayeaye) has a single opposed
digit on each fore foot and two opposed digits
on its hind feet. This allows it to grasp
the stems of bushes as it clambers around
in its riverside habitat.
GlidingDuring the evolutionary history of
frogs, several different groups have independently
taken to the air. Some frogs in the tropical
rainforest are specially adapted for gliding
from tree to tree or parachuting to the forest
floor. Typical of them is Wallace's flying
frog (Rhacophorus nigropalmatus) from Malaysia
and Borneo. It has large feet with the fingertips
expanded into flat adhesive discs and the
digits fully webbed. Flaps of skin occur on
the lateral margins of the limbs and across
the tail region. With the digits splayed,
the limbs outstretched, and these flaps spread,
it can glide considerable distances, but is
unable to undertake powered flight. It can
alter its direction of travel and navigate
distances of up to 15 m (49 ft) between trees.
== Life history ==
Like other amphibians, the life cycle of a
frog normally starts in water with an egg
that hatches into a limbless larva with gills,
commonly known as a tadpole. After further
growth, during which it develops limbs and
lungs, the tadpole undergoes metamorphosis
in which its appearance and internal organs
are rearranged. After this it is able to leave
the water as a miniature, air-breathing frog.
=== Reproduction ===
Two main types of reproduction occur in frogs,
prolonged breeding and explosive breeding.
In the former, adopted by the majority of
species, adult frogs at certain times of year
assemble at a pond, lake or stream to breed.
Many frogs return to the bodies of water in
which they developed as larvae. This often
results in annual migrations involving thousands
of individuals. In explosive breeders, mature
adult frogs arrive at breeding sites in response
to certain trigger factors such as rainfall
occurring in an arid area. In these frogs,
mating and spawning take place promptly and
the speed of larval growth is rapid in order
to make use of the ephemeral pools before
they dry up.Among prolonged breeders, males
usually arrive at the breeding site first
and remain there for some time whereas females
tend to arrive later and depart soon after
they have spawned. This means that males outnumber
females at the water's edge and defend territories
from which they expel other males. They advertise
their presence by calling, often alternating
their croaks with neighbouring frogs. Larger,
stronger males tend to have deeper calls and
maintain higher quality territories. Females
select their mates at least partly on the
basis of the depth of their voice. In some
species there are satellite males who have
no territory and do not call. They may intercept
females that are approaching a calling male
or take over a vacated territory. Calling
is an energy-sapping activity. Sometimes the
two roles are reversed and a calling male
gives up its territory and becomes a satellite.
In explosive breeders, the first male that
finds a suitable breeding location, such as
a temporary pool, calls loudly and other frogs
of both sexes converge on the pool. Explosive
breeders tend to call in unison creating a
chorus that can be heard from far away. The
spadefoot toads (Scaphiopus spp.) of North
America fall into this category. Mate selection
and courtship is not as important as speed
in reproduction. In some years, suitable conditions
may not occur and the frogs may go for two
or more years without breeding. Some female
New Mexico spadefoot toads (Spea multiplicata)
only spawn half of the available eggs at a
time, perhaps retaining some in case a better
reproductive opportunity arises later.At the
breeding site, the male mounts the female
and grips her tightly round the body. Typically,
amplexus takes place in the water, the female
releases her eggs and the male covers them
with sperm; fertilization is external. In
many species such as the Great Plains toad
(Bufo cognatus), the male restrains the eggs
with his back feet, holding them in place
for about three minutes. Members of the West
African genus Nimbaphrynoides are unique among
frogs in that they are viviparous; Limnonectes
larvaepartus, Eleutherodactylus jasperi and
members of the Tanzanian genus Nectophrynoides
are the only frogs known to be ovoviviparous.
In these species, fertilization is internal
and females give birth to fully developed
juvenile frogs, except L. larvaepartus, which
give birth to tadpoles.
=== Life cycle ===
====
Eggs / frogspawn ====
Frogs' embryos are typically surrounded by
several layers of gelatinous material. When
several eggs are clumped together, they are
collectively known as frogspawn. The jelly
provides support and protection while allowing
the passage of oxygen, carbon dioxide and
ammonia. It absorbs moisture and swells on
contact with water. After fertilization, the
innermost portion liquifies to allow free
movement of the developing embryo. In certain
species, such as the Northern red-legged frog
(Rana aurora) and the wood frog (Rana sylvatica),
symbiotic unicellular green algae are present
in the gelatinous material. It is thought
that these may benefit the developing larvae
by providing them with extra oxygen through
photosynthesis. Most eggs are black or dark
brown and this has the advantage of absorbing
warmth from the sun which the insulating capsule
retains. The interior of globular egg clusters
of the wood frog (Rana sylvatica) has been
found to be up to 6 °C (11 °F) warmer than
the surrounding water and this speeds up the
development of the larvae.The shape and size
of the egg mass is characteristic of the species.
Ranids tend to produce globular clusters containing
large numbers of eggs whereas bufonids produce
long, cylindrical strings. The tiny yellow-striped
pygmy eleuth (Eleutherodactylus limbatus)
lays eggs singly, burying them in moist soil.
The smoky jungle frog (Leptodactylus pentadactylus)
makes a nest of foam in a hollow. The eggs
hatch when the nest is flooded, or the tadpoles
may complete their development in the foam
if flooding does not occur. The red-eyed treefrog
(Agalychnis callidryas) deposits its eggs
on a leaf above a pool and when they hatch,
the larvae fall into the water below. The
larvae developing in the eggs can detect vibrations
caused by nearby predatory wasps or snakes,
and will hatch early to avoid being eaten.
In general, the length of the egg stage depends
on the species and the environmental conditions.
Aquatic eggs normally hatch within one week
when the capsule splits as a result of enzymes
released by the developing larvae.
==== Tadpoles ====
The larvae that emerge from the eggs, known
as tadpoles (or occasionally polliwogs), typically
have oval bodies and long, vertically flattened
tails. As a general rule, free-living larvae
are fully aquatic, but at least one species
(Nannophrys ceylonensis) has semiterrestrial
tadpoles which live among wet rocks. Tadpoles
lack eyelids and have cartilaginous skeletons,
lateral line systems, gills for respiration
(external gills at first, internal gills later),
and vertically flattened tails they use for
swimming.From early in its development, a
gill pouch covers the tadpole's gills and
front legs. The lungs soon start to develop
and are used as an accessory breathing organ.
Some species go through metamorphosis while
still inside the egg and hatch directly into
small frogs. Tadpoles lack true teeth, but
the jaws in most species have two elongated,
parallel rows of small, keratinized structures
called keradonts in their upper jaws. Their
lower jaws usually have three rows of keradonts
surrounded by a horny beak, but the number
of rows can vary and the exact arrangements
of mouth parts provide a means for species
identification. In the Pipidae, with the exception
of Hymenochirus, the tadpoles have paired
anterior barbels, which make them resemble
small catfish. Their tails are stiffened by
a notochord, but does not contain any bony
or cartilaginous elements except for a few
vertebrae at the base which forms the urostyle
during metamorphosis. This has been suggested
as an adaptation to their lifestyles; because
the transformation into frogs happens very
fast, the tail is made of soft tissue only,
as bone and cartilage take a much longer time
to be broken down and absorbed. The tail fin
and tip is fragile and will easily tear, which
is seen as an adaptation to escape from predators
which tries to grasp them by the tail.Tadpoles
are typically herbivorous, feeding mostly
on algae, including diatoms filtered from
the water through the gills. Some species
are carnivorous at the tadpole stage, eating
insects, smaller tadpoles, and fish. The Cuban
tree frog (Osteopilus septentrionalis) is
one of a number of species in which the tadpoles
can be cannibalistic. Tadpoles that develop
legs early may be eaten by the others, so
late developers may have better long-term
survival prospects.Tadpoles are highly vulnerable
to being eaten by fish, newts, predatory diving
beetles, and birds, such as kingfishers. Some
tadpoles, including those of the cane toad
(Bufo marinus), are poisonous. The tadpole
stage may be as short as a week in explosive
breeders or it may last through one or more
winters followed by metamorphosis in the spring.
==== Metamorphosis ====
At the end of the tadpole stage, a frog undergoes
metamorphosis in which its body makes a sudden
transition into the adult form. This metamorphosis
typically lasts only 24 hours, and is initiated
by production of the hormone thyroxine. This
causes different tissues to develop in different
ways. The principal changes that take place
include the development of the lungs and the
disappearance of the gills and gill pouch,
making the front legs visible. The lower jaw
transforms into the big mandible of the carnivorous
adult, and the long, spiral gut of the herbivorous
tadpole is replaced by the typical short gut
of a predator. The nervous system becomes
adapted for hearing and stereoscopic vision,
and for new methods of locomotion and feeding.
The eyes are repositioned higher up on the
head and the eyelids and associated glands
are formed. The eardrum, middle ear, and inner
ear are developed. The skin becomes thicker
and tougher, the lateral line system is lost,
and skin glands are developed. The final stage
is the disappearance of the tail, but this
takes place rather later, the tissue being
used to produce a spurt of growth in the limbs.
Frogs are at their most vulnerable to predators
when they are undergoing metamorphosis. At
this time, the tail is being lost and locomotion
by means of limbs is only just becoming established.
==== Adults ====
After metamorphosis, young adults may disperse
into terrestrial habitats or continue to live
in water. Almost all frog species are carnivorous
as adults, preying on invertebrates, including
arthropods, worms, snails, and slugs. A few
of the larger ones may eat other frogs, small
mammals, and fish. Some frogs use their sticky
tongues to catch fast-moving prey, while others
push food into their mouths with their hands.
A few species also eat plant matter; the tree
frog Xenohyla truncata is partly herbivorous,
its diet including a large proportion of fruit,
Leptodactylus mystaceus has been found to
eat plants, and folivory occurs in Euphlyctis
hexadactylus, with plants constituting 79.5%
of its diet by volume. Adult frogs are themselves
attacked by many predators. The northern leopard
frog (Rana pipiens) is eaten by herons, hawks,
fish, large salamanders, snakes, raccoons,
skunks, mink, bullfrogs, and other animals.
Frogs are primary predators and an important
part of the food web. Being cold-blooded,
they make efficient use of the food they eat
with little energy being used for metabolic
processes, while the rest is transformed into
biomass. They are themselves eaten by secondary
predators and are the primary terrestrial
consumers of invertebrates, most of which
feed on plants. By reducing herbivory, they
play a part in increasing the growth of plants
and are thus part of a delicately balanced
ecosystem.Little is known about the longevity
of frogs and toads in the wild, but some can
live for many years. Skeletochronology is
a method of examining bones to determine age.
Using this method, the ages of mountain yellow-legged
frogs (Rana muscosa) were studied, the phalanges
of the toes showing seasonal lines where growth
slows in winter. The oldest frogs had ten
bands, so their age was believed to be 14
years, including the four-year tadpole stage.
Captive frogs and toads have been recorded
as living for up to 40 years, an age achieved
by a European common toad (Bufo bufo). The
cane toad (Bufo marinus) has been known to
survive 24 years in captivity, and the American
bullfrog (Rana catesbeiana) 14 years. Frogs
from temperate climates hibernate during the
winter, and four species are known to be able
to withstand freezing during this time, including
the wood frog (Rana sylvatica).
=== Parental care ===
Although care of offspring is poorly understood
in frogs, up to an estimated 20% of amphibian
species may care for their young in some way.
The evolution of parental care in frogs is
driven primarily by the size of the water
body in which they breed. Those that breed
in smaller water bodies tend to have greater
and more complex parental care behaviour.
Because predation of eggs and larvae is high
in large water bodies, some frog species started
to lay their eggs on land. Once this happened,
the desiccating terrestrial environment demands
that one or both parents keep them moist to
ensure their survival. The subsequent need
to transport hatched tadpoles to a water body
required an even more intense form of parental
care.In small pools, predators are mostly
absent and competition between tadpoles becomes
the variable that constrains their survival.
Certain frog species avoid this competition
by making use of smaller phytotelmata (water-filled
leaf axils or small woody cavities) as sites
for depositing a few tadpoles. While these
smaller rearing sites are free from competition,
they also lack sufficient nutrients to support
a tadpole without parental assistance. Frog
species that changed from the use of larger
to smaller phytotelmata have evolved a strategy
of providing their offspring with nutritive
but unfertilized eggs. The female strawberry
poison-dart frog (Oophaga pumilio) lays her
eggs on the forest floor. The male frog guards
them from predation and carries water in his
cloaca to keep them moist. When they hatch,
the female moves the tadpoles on her back
to a water-holding bromeliad or other similar
water body, depositing just one in each location.
She visits them regularly and feeds them by
laying one or two unfertilized eggs in the
phytotelma, continuing to do this until the
young are large enough to undergo metamorphosis.
The granular poison frog (Oophaga granulifera)
looks after its tadpoles in a similar way.Many
other diverse forms of parental care are seen
in frogs. The tiny male Colostethus subpunctatus
stands guard over his egg cluster, laid under
a stone or log. When the eggs hatch, he transports
the tadpoles on his back to a temporary pool,
where he partially immerses himself in the
water and one or more tadpoles drop off. He
then moves on to another pool. The male common
midwife toad (Alytes obstetricans) carries
the eggs around with him attached to his hind
legs. He keeps them damp in dry weather by
immersing himself in a pond, and prevents
them from getting too wet in soggy vegetation
by raising his hindquarters. After three to
six weeks, he travels to a pond and the eggs
hatch into tadpoles. The tungara frog (Physalaemus
pustulosus) builds a floating nest from foam
to protect its eggs from predation. The foam
is made from proteins and lectins, and seems
to have antimicrobial properties. Several
pairs of frogs may form a colonial nest on
a previously built raft. The eggs are laid
in the centre, followed by alternate layers
of foam and eggs, finishing with a foam capping.Some
frogs protect their offspring inside their
own bodies. Both male and female pouched frogs
(Assa darlingtoni) guard their eggs, which
are laid on the ground. When the eggs hatch,
the male lubricates his body with the jelly
surrounding them and immerses himself in the
egg mass. The tadpoles wriggle into skin pouches
on his side, where they develop until they
metamorphose into juvenile frogs. The female
gastric-brooding frog (Rheobatrachus sp.)
from Australia, now probably extinct, swallows
her fertilized eggs, which then develop inside
her stomach. She ceases to feed and stops
secreting stomach acid. The tadpoles rely
on the yolks of the eggs for nourishment.
After six or seven weeks, they are ready for
metamorphosis. The mother regurgitates the
tiny frogs, which hop away from her mouth.
The female Darwin's frog (Rhinoderma darwinii)
from Chile lays up to 40 eggs on the ground,
where they are guarded by the male. When the
tadpoles are about to hatch, they are engulfed
by the male, which carries them around inside
his much-enlarged vocal sac. Here they are
immersed in a frothy, viscous liquid that
contains some nourishment to supplement what
they obtain from the yolks of the eggs. They
remain in the sac for seven to ten weeks before
undergoing metamorphosis, after which they
move into the male's mouth and emerge.
== Defence ==
At first sight, frogs seem rather defenceless
because of their small size, slow movement,
thin skin, and lack of defensive structures,
such as spines, claws or teeth. Many use camouflage
to avoid detection, the skin often being spotted
or streaked in neutral colours that allow
a stationary frog to merge into its surroundings.
Some can make prodigious leaps, often into
water, that help them to evade potential attackers,
while many have other defensive adaptations
and strategies.The skin of many frogs contains
mild toxic substances called bufotoxins to
make them unpalatable to potential predators.
Most toads and some frogs have large poison
glands, the parotoid glands, located on the
sides of their heads behind the eyes and other
glands elsewhere on their bodies. These glands
secrete mucus and a range of toxins that make
frogs slippery to hold and distasteful or
poisonous. If the noxious effect is immediate,
the predator may cease its action and the
frog may escape. If the effect develops more
slowly, the predator may learn to avoid that
species in future. Poisonous frogs tend to
advertise their toxicity with bright colours,
an adaptive strategy known as aposematism.
The poison dart frogs in the family Dendrobatidae
do this. They are typically red, orange, or
yellow, often with contrasting black markings
on their bodies. Allobates zaparo is not poisonous,
but mimics the appearance of two different
toxic species with which it shares a common
range in an effort to deceive predators. Other
species, such as the European fire-bellied
toad (Bombina bombina), have their warning
colour underneath. They "flash" this when
attacked, adopting a pose that exposes the
vivid colouring on their bellies.
Some frogs, such as the poison dart frogs,
are especially toxic. The native peoples of
South America extract poison from these frogs
to apply to their weapons for hunting, although
few species are toxic enough to be used for
this purpose. At least two non-poisonous frog
species in tropical America (Eleutherodactylus
gaigei and Lithodytes lineatus) mimic the
colouration of dart poison frogs for self-protection.
Some frogs obtain poisons from the ants and
other arthropods they eat. Others, such as
the Australian corroboree frogs (Pseudophryne
corroboree and Pseudophryne pengilleyi), can
synthesize the alkaloids themselves. The chemicals
involved may be irritants, hallucinogens,
convulsants, nerve poisons or vasoconstrictors.
Many predators of frogs have become adapted
to tolerate high levels of these poisons,
but other creatures, including humans who
handle the frogs, may be severely affected.Some
frogs use bluff or deception. The European
common toad (Bufo bufo) adopts a characteristic
stance when attacked, inflating its body and
standing with its hindquarters raised and
its head lowered. The bullfrog (Rana catesbeiana)
crouches down with eyes closed and head tipped
forward when threatened. This places the parotoid
glands in the most effective position, the
other glands on its back begin to ooze noxious
secretions and the most vulnerable parts of
its body are protected. Another tactic used
by some frogs is to "scream", the sudden loud
noise tending to startle the predator. The
gray tree frog (Hyla versicolor) makes an
explosive sound that sometimes repels the
shrew Blarina brevicauda. Although toads are
avoided by many predators, the common garter
snake (Thamnophis sirtalis) regularly feeds
on them. The strategy employed by juvenile
American toads (Bufo americanus) on being
approached by a snake is to crouch down and
remain immobile. This is usually successful,
with the snake passing by and the toad remaining
undetected. If it is encountered by the snake's
head, however, the toad hops away before crouching
defensively.
== Distribution ==
Frogs live on all the continents except Antarctica,
but they are not present on certain islands,
especially those far away from continental
land masses. Many species are isolated in
restricted ranges by changes of climate or
inhospitable territory, such as stretches
of sea, mountain ridges, deserts, forest clearance,
road construction, or other man-made barriers.
Usually, a greater diversity of frogs occurs
in tropical areas than in temperate regions,
such as Europe. Some frogs inhabit arid areas,
such as deserts, and rely on specific adaptations
to survive. Members of the Australian genus
Cyclorana bury themselves underground where
they create a water-impervious cocoon in which
to aestivate during dry periods. Once it rains,
they emerge, find a temporary pool, and breed.
Egg and tadpole development is very fast in
comparison to those of most other frogs, so
breeding can be completed before the pond
dries up. Some frog species are adapted to
a cold environment. The wood frog (Rana sylvatica),
whose habitat extends into the Arctic Circle,
buries itself in the ground during winter.
Although much of its body freezes during this
time, it maintains a high concentration of
glucose in its vital organs, which protects
them from damage.
== Conservation ==
In 2006, of 4,035 species of amphibians that
depend on water during some lifecycle stage,
1,356 (33.6%) were considered to be threatened.
This is likely to be an underestimate because
it excludes 1,427 species for which evidence
was insufficient to assess their status. Frog
populations have declined dramatically since
the 1950s. More than one-third of frog species
are considered to be threatened with extinction,
and more than 120 species are believed to
have become extinct since the 1980s. Among
these species are the gastric-brooding frogs
of Australia and the golden toad of Costa
Rica. The latter is of particular concern
to scientists because it inhabited the pristine
Monteverde Cloud Forest Reserve and its population
crashed in 1987, along with about 20 other
frog species in the area. This could not be
linked directly to human activities, such
as deforestation, and was outside the range
of normal fluctuations in population size.
Elsewhere, habitat loss is a significant cause
of frog population decline, as are pollutants,
climate change, increased UVB radiation, and
the introduction of non-native predators and
competitors. A Canadian study conducted in
2006 suggested heavy traffic in their environment
was a larger threat to frog populations than
was habitat loss. Emerging infectious diseases,
including chytridiomycosis and ranavirus,
are also devastating populations.Many environmental
scientists believe amphibians, including frogs,
are good biological indicators of broader
ecosystem health because of their intermediate
positions in food chains, their permeable
skins, and typically biphasic lives (aquatic
larvae and terrestrial adults). It appears
that species with both aquatic eggs and larvae
are most affected by the decline, while those
with direct development are the most resistant.
Frog mutations and genetic defects have increased
since the 1990s. These often include missing
legs or extra legs. Various causes have been
identified or hypothesized, including an increase
in ultraviolet radiation affecting the spawn
on the surface of ponds, chemical contamination
from pesticides and fertilizers, and parasites
such as the trematode Ribeiroia ondatrae.
Probably all these are involved in a complex
way as stressors, environmental factors contributing
to rates of disease, and vulnerability to
attack by parasites. Malformations impair
mobility and the individuals may not survive
to adulthood. An increase in the number of
frogs eaten by birds may actually increase
the likelihood of parasitism of other frogs,
because the trematode's complex lifecycle
includes the ramshorn snail and several intermediate
hosts such as birds.In a few cases, captive
breeding programs have been established and
have largely been successful. The World Association
of Zoos and Aquariums named 2008 as the "Year
of the Frog" in order to draw attention to
the conservation issues faced by them.The
cane toad (Bufo marinus) is a very adaptable
species native to South and Central America.
In the 1930s, it was introduced into Puerto
Rico, and later various other islands in the
Pacific and Caribbean region, as a biological
pest control agent. In 1935, 3000 toads were
liberated in the sugar cane fields of Queensland,
Australia, in an attempt to control cane beetles
such as Dermolepida albohirtum, the larvae
of which damage and kill the canes. Initial
results in many of these countries were positive,
but it later became apparent that the toads
upset the ecological balance in their new
environments. They bred freely, competed with
native frog species, ate bees and other harmless
native invertebrates, had few predators in
their adopted habitats, and poisoned pets,
carnivorous birds, and mammals. In many of
these countries, they are now regarded both
as pests and invasive species, and scientists
are looking for a biological method to control
them.
== Uses ==
=== Culinary ===
Frog legs are eaten by humans in many parts
of the world. French cuisses de grenouille
or frog legs dish is a traditional dish particularly
served in the region of the Dombes (département
of Ain). The dish is also common in French-speaking
parts of Louisiana, particularly the Cajun
areas of Southern Louisiana as well as New
Orleans, United States. In Asia, frog legs
are consumed in China, Vietnam, Thailand and
Indonesia. Chinese edible frog and pig frogs
are farmed and consumed on a large scale in
some areas of China. Frog legs are part of
Chinese Sichuan and Cantonese cuisine. In
Indonesia, frog-leg soup is known as swikee
or swike. Indonesia is the world's largest
exporter of frog meat, exporting more than
5,000 tonnes of frog meat each year, mostly
to France, Belgium and Luxembourg.Originally,
they were supplied from local wild populations,
but overexploitation led to a diminution in
the supply. This resulted in the development
of frog farming and a global trade in frogs.
The main importing countries are France, Belgium,
Luxembourg, and the United States, while the
chief exporting nations are Indonesia and
China. The annual global trade in the American
bullfrog (Rana catesbeiana), mostly farmed
in China, varies between 1200 and 2400 tonnes.The
mountain chicken frog, so-called as it tastes
of chicken is now endangered, in part due
to human consumption and was a major food
choice of the Dominicans.Coon, possum, partridges,
prairie hen, and frogs were among the fare
Mark Twain recorded as part of American cuisine.
=== Scientific research ===
Frogs are used for dissections in high school
and university anatomy classes, often first
being injected with coloured substances to
enhance contrasts among the biological systems.
This practice is declining due to animal welfare
concerns, and "digital frogs" are now available
for virtual dissection.Frogs have served as
experimental animals throughout the history
of science. Eighteenth-century biologist Luigi
Galvani discovered the link between electricity
and the nervous system by studying frogs.
In 1852, H. F. Stannius used a frog's heart
in a procedure called a Stannius ligature
to demonstrate the ventricle and atria beat
independently of each other and at different
rates. The African clawed frog or platanna
(Xenopus laevis) was first widely used in
laboratories in pregnancy tests in the first
half of the 20th century. A sample of urine
from a pregnant woman injected into a female
frog induces it to lay eggs, a discovery made
by English zoologist Lancelot Hogben. This
is because a hormone, human chorionic gonadotropin,
is present in substantial quantities in the
urine of women during pregnancy. In 1952,
Robert Briggs and Thomas J. King cloned a
frog by somatic cell nuclear transfer. This
same technique was later used to create Dolly
the sheep, and their experiment was the first
time a successful nuclear transplantation
had been accomplished in higher animals.Frogs
are used in cloning research and other branches
of embryology. Although alternative pregnancy
tests have been developed, biologists continue
to use Xenopus as a model organism in developmental
biology because their embryos are large and
easy to manipulate, they are readily obtainable,
and can easily be kept in the laboratory.
Xenopus laevis is increasingly being displaced
by its smaller relative, Xenopus tropicalis,
which reaches its reproductive age in five
months rather than the one to two years for
X. laevis, thus facilitating faster studies
across generations.
Genomes of Xenopus laevis, X. tropicalis,
Rana catesbeiana, Rhinella marina, and Nanorana
parkeri have been sequenced and deposited
in the NCBI Genome database.
=== Pharmaceutical ===
Because frog toxins are extraordinarily diverse,
they have raised the interest of biochemists
as a "natural pharmacy". The alkaloid epibatidine,
a painkiller 200 times more potent than morphine
is made by some species of poison dart frogs,
although it can also cause death by lung paralysis.
Other chemicals isolated from the skins of
frogs may offer resistance to HIV infection.
Dart poisons are under active investigation
for their potential as therapeutic drugs.It
has long been suspected that pre-Columbian
Mesoamericans used a toxic secretion produced
by the cane toad as a hallucinogen, but more
likely they used substances secreted by the
Colorado River toad (Bufo alvarius). These
contain bufotenin (5-MeO-DMT), a psychoactive
compound that has been used in modern times
as a recreational drug. Typically, the skin
secretions are dried and then smoked. Illicit
drug use by licking the skin of a toad has
been reported in the media, but this may be
an urban myth.
Exudations from the skin of the golden poison
frog (Phyllobates terribilis) are traditionally
used by native Colombians to poison the darts
they use for hunting. The tip of the projectile
is rubbed over the back of the frog and the
dart is launched from a blowgun. The combination
of the two alkaloid toxins batrachotoxin and
homobatrachotoxin is so powerful, one frog
contains enough poison to kill an estimated
22,000 mice. Two other species, the Kokoe
poison dart frog (Phyllobates aurotaenia)
and the black-legged dart frog (Phyllobates
bicolor) are also used for this purpose. These
are less toxic and less abundant than the
golden poison frog. They are impaled on pointed
sticks and may be heated over a fire to maximise
the quantity of poison that can be transferred
to the dart.
== Cultural beliefs ==
Frogs feature prominently in folklore, fairy
tales, and popular culture. They tend to be
portrayed as benign, ugly, and clumsy, but
with hidden talents. Examples include Michigan
J. Frog, "The Frog Prince", and Kermit the
Frog. The Warner Brothers cartoon One Froggy
Evening features Michigan J. Frog, that will
only dance and sing for the demolition worker
who opens his time capsule, but will not perform
in public. "The Frog Prince" is a fairy tale
about a frog that turns into a handsome prince
after he has rescued a princess's golden ball
and she has taken him into her palace. Kermit
the Frog is a conscientious and disciplined
character from The Muppet Show and Sesame
Street; while openly friendly and greatly
talented, he is often portrayed as cringing
at the fanciful behavior of more flamboyant
characters.The Moche people of ancient Peru
worshipped animals, and often depicted frogs
in their art. In Panama, local legend held
that good fortune would come to anyone who
spotted a Panamanian golden frog. Some believed
when one of these frogs died, it would turn
into a golden talisman known as a huaca. Today,
despite being extinct in the wild, Panamanian
golden frogs remain an important cultural
symbol and are illustrated on decorative cloth
molas made by the Kuna people. They also appear
as part of the inlaid design on a new overpass
in Panama City, on T-shirts, and even on lottery
tickets.
== See also ==
Frogs portal

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