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Frogs
American Bullfrog
Physical description
HabitatAll except for polar regions.
Lifespan18 months-40 years
Average Size0.95-40 cm (0.3-16 inches); some species up to 89 cm (35 in) long with legs extended
Average weightLess than one gram to 2.7 kg
DietInvertebrates, smaller vertebrates including other frogs, and plants.
Scientific classification
PhylumChordata
ClassAmphibia
OrderAnura
Distribution
Distribution of speciesNorth, Central, and South America, Europe, Asia, Africa, Oceania, Australia
Frog range

Frogs are amphibians in the order Anura (meaning "tail-less", from Greek an-, without + oura, tail), formerly referred to as Salientia (Latin salere (salio), "to jump"). Most frogs are characterized by a short body, webbed digits (fingers or toes), protruding eyes and the absence of a tail. Frogs are widely known as exceptional jumpers, and many of the anatomical characteristics of frogs, particularly their long, powerful legs, are adaptations to improve jumping performance. Due to their permeable skin, frogs are often semi-aquatic or inhabit humid areas, but move easily on land. They typically lay their eggs in puddles, ponds or lakes, and their larvae, called tadpoles, have gills and develop in water. Adult frogs follow a carnivorous diet, mostly of arthropods, annelids and gastropods. Frogs are most noticeable by their call, which can be widely heard during the night or day, mainly in their mating season.

The distribution of frogs ranges from tropic to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species described, they are among the most diverse groups of vertebrates. However, populations of certain frog species are declining significantly.

A popular distinction is often made between frogs and toads on the basis of their appearance, but this has no taxonomic basis. In addition to their ecological importance, frogs have many cultural roles, such as in literature, symbolism and religion, and they are also valued as food and as pets.

Classification

For more information on this topic, see List of anuran families.

Green Frog

A green frog (Lithobates clamitans) and a northern leopard frog (Lithobates pipiens).

Frogs are members of the class Amphibia, which also includes salamanders and caecilians. Within the amphibians, frogs are part of their own order, Anura. The order is divided into three suborders, Archaeobatrachia, Mesobatrachia, and Neobatrachia, each of which has at least two superfamilies consisting of two or more families.

The distinction between frogs and toads has no taxonomic basis. Strictly speaking, all members of the order Anura are frogs, while only members of the family Bufonidae are toads. Frogs in other families of the Hyloidea group have also been referred to as toads, as have any frogs with dry, warty skin.

Evolutionary history

The origin of frogs has been the subject of a great deal of debate. Fossil frogs have been found in many fossil searches for decades, and yet only recently have they been studied in detail. These extinct frogs are almost identical to living ones, and prove that the frog body plan is a successful one.

Frog fossil

A fossilized frog.

Fossils have recently confirmed that frogs are descended from an ancient group of amphibians known as labyrinthodonts, meaning "maze toothed", after the mazelike patterns in their tooth enamel. Many labyrinthodonts were large, tailed animals that would have looked much like crocodiles today. A well-known example of a labyrinthodont is Mastodonsaurus, a predator from the Middle Triassic period. Over time, a group of labyrinthodonts gradually changed to meet a different ecological niche: their bodies in general became smaller and their bones became more lightweight. This new group, the Lissamphibia, consists of all living amphibians and many extinct species. Lissamphibians are characterized by light bones, a reduced number of vertebrae, and poorly developed ribs. The modern lissamphibians are classified in three orders: Gymnophiona (caecilians), Caudata (salamanders and newts), and Anura (frogs and toads).

The taxonomy of Lissamphibia reveals that salamanders and frogs are descended from a common ancestor that they do not share with caecilians. Salamanders are the most recently evolved group of amphibians, not appearing until Early Jurassic Asia.

The first frog-like organism, Triadobatrachus massinoti, was found in Early Triassic rocks in Madagascar. It was surprisingly similar to the tailed frogs, the most primitive genus of frogs alive today. In the case of Triadobatrachus, however, the tail appeared to be an actual tail, whereas in the so-called tailed frogs of modern times, the "tail" is an extension of the male cloaca.

The next fossil frog to appear was Vieraella herbsti, hailing from Argentina during the Early Jurassic period. Other frogs were present in the Early Jurassic as well, including Prosalarius bitis, which resembled a modern true (or pond) frog. The Argentinian Notobatrachus, which hailed from the Late Jurassic, also was similar to the true frogs of today.

More advanced frogs also appeared in the Jurassic. A fossil frog resembling a modern tree frog, or possibly a sedge frog, was discovered in northern Africa, dated from around the Jurassic-Cretaceous boundary. The fossil of a true toad was uncovered in rocks from Middle Cretaceous South America. The largest fossil frog found to date, Beelzebufo, a giant horned frog, was discovered in Late Cretaceous rocks from Madagascar.

Frogs appear to have been affected by the K-Pg extinction event, but not to the degree that many other animal groups were. Fossil evidence indicates that they were among the first groups to recover from the devastation, and by the early Eocene fossil frogs are numerous. The first parsley frogsneotropical frogs and true toads are of Eocene age. Recognizable tree frogs appear during the Oligocene period (although possible tree frogs have been recovered from Paleocene rocks). 

Anatomy

Frogs are often viewed as the quintessential amphibians, although their anatomy amongst other amphibians is unique. They lack the almost reptilian appearence of salamanders or the caecilians' resemblance to snakes or earthworms, and are immediately recognizeable.

Like salamanders, caecilians, and most other amphibians, frogs are unique among vertebrates in that they possess a free-living larval stage (tadpoles) that are significantly different from adult frogs. Tadpoles are completely aquatic and have gills, with small forward-facing eyes and are herbivorous. Adult frogs breathe via lungs and through their permeable skins, lack tails, have eyes positioned on the sides of their heads and are carnivorous (though toads and their relatives tend to have more forward-facing eyes than frogs).

The term toad has little taxonomic meaning. Taxonomically all members of the order Anura are frogs, while many dry-skinned, warty anurans have been called toads. Therefore it can be accurately stated that all toads are frogs, but not all frogs are toads.

Skeletal structure

Ceratophrys Cornuta Skeleton

Surinam horned frog skeleton.

Most of a frog's skeletal features are centred around their unique method of jumping by kicking out with both of their hind legs simaltaneously. Their bones are very lightweight and porous in order to reduce the animal's overall weight, and in many aquatic or arboreal species the hind legs may be twice or even three times the length of the forelegs. In terrestrial frogs and toads, the difference is less pronounced and may not be easily visible upon casual observation. The skull of most frogs are large and consist mostly of mouth, though hyloid frogs have larger brain cavities and larger brains than most other frogs.

Ribs and vertebrae

Lithobates Catesbeianus

American bullfrog, Lithobates catesbeianus skeleton.

While extinct frogs in the fossil record frequently have ribs, modern frogs generally lack any kind of ribs. The reason for this loss may be to prevent damage during landing from great heights, as many frogs do fairly regularly.

Frogs' vertebrae have also been reduced over time. The vertebrae are above the sacrum and are spread at surprisingly irregular intervals. Primitive frogs, notably tailed frogs, have nine ribs, while most frogs have a maximum of eight. Some advanced frogs have only seven vertebrae. It is of note that most frogs lack necks; their skulls are joined to their spines with small protrusions called condiles. While they can look from side to side, frogs are incapable of looking up or down. To compensate, most frogs prop their bodies upwards to look up or lean forwards to look down.

Jaws and teeth

The skulls of most frogs are adapted for eating. Frog jaws are possess powerful muscles and are linked by elastic ligaments that allow them to accomodate proportionately large prey. Like most verterbates, the upper jaw in frogs is fixed to the skull while the lower jaw is a moveable, separate unit. Frogs do not have strong bites as they do not usually eat particularly hard food- the insects that most of them feed on are easily crushed and what remains dissolves in their stomach acids.

All frogs have sharp teeth in their upper jaws designed to grab, hold, and qickly crush prey. The lower jaw is usually toothless, although hyloid frogs tend to have fang-like points in their lower jaw. Some true toads and a few frogs are entirely toothless. These include pipid and microhylid frogs. While hyloids possess sharp points in the lower jaw, these are not true teeth; South American and Australian marsupial frogs are the only frogs to have teeth in the lower jaw.

Feet and jumping

Red Eyed Tree Frog Foot

View of the foot of Agalychnis callidryas

The feet of frogs are some of the most prominent features in many species. Many frogs get about by hopping, thrusting out with both of their hind legs to cover a surprising distance with little effort. The features of frog feet vary from species to species and on their role.

Most frogs have four toes on their front feet. The "thumb", "index", "middle", and "ring" fingers have remained largely intact, while the "little finger" has minimized and eventually vanished over time. The reason for this loss, like the loss of their vertebrae, is unknown. All but a select few frogs have five toes on their hind feet. The joints in frog digits are noticeably different from those of humans. Human fingers are (from the thumb to the fifth finger) jointed 2-3-3-3-3, while frog "fingers" are jointed 2-2-3-3. Frog "toes", meanwhile, are jointed 2-2-3-4-3.

Aside from the skeletal features, frog feet vary. Primarily aquatic frogs have webbed feet; arboreal frogs possess suckerlike discs at the tips of the fingers and toes to give them an adhesive grip. Males of many species develop hardened patches on their toes during breeding to assist them in clasping their mates.

Digestive system

Mmmdragonflies

European tree frog, Hyla arborea, consuming a damselfly.

The digestive system of frogs is fairly simple. Most frogs have large mouths for capturing fairly large prey. Those with teeth will quickly crunch their prey before swallowing; those without teeth will crush prey between their jaws. Frogs possess a short esophagus. The stomach; however, is quite large and the acids it produces are especially powerful, among them hydrochloric acid. What remains of the prey after being chewed is quickly dissolved in these potent acids and, as in humans, passed into the intestines for digestion. Frogs' small intestines, like those of all terrestial vertebrates, have a distinctive duodenum and extract the vast majority of the nutrients. In fish, most of the nutrients are absorbed in the large intestine. Frogs are unique in that they do not have a true large intestine, but rather an elongated sac that removes the last nutrients. Like most animals, frogs remove waste through their cloacal openings.

Senses

Sight

Sight is the primary sense for most frogs. Frog eyes are protected by eyelids and possess glands that moisturize and lubricate the eyeball. The eyes also possess a nictitating membrane, or "third eyelid". The sense of sight varies from species to species. Most frogs are nearsighted, with the background being little more than a colourful blur. This assists the frogs in locating prey and detecting predators; motion against the background will reveal the presence of an animal. Hyloid frogs, especially tree frogs, can see clearly for a distance and have very good binocular vision.

Callidryas 01

The South American red-eyed tree frog has effective binocular vision.

The shape of the pupil is important to frogs. Horizontal pupils typically indicate that the frog is diurnal, whereas vertical pupils are a sign of nocturnal activities. Frogs that are both diurnal and nocturnal will normally have round pupils.

Most North American frogs have oblong pupils that allow both diurnal and nocturnal activities. Many European frogs have round or roughly diamond-shaped pupils; they can be active at any time of day, but prefer to be nocturnal.

Hyloid frogs have oval-shaped pupils which form very clear, non-distorted images. Many hyloids also have forward-facing eyes to give them better binocular vision when climbing trees or crossing dangerous terrain. True toads (Bufonidae) and poison dart frogs (Dendrobatidae) have horizontal pupils. Many tree frogs, including the charismatic red-eyed tree frogs have vertical pupils.

Fire-bellied toads (Bombinatoridae) have heart-shaped pupils and are largely diurnal. Nocturnal microhylid frogs have round or triangular pupils. Studies have shown that most frogs are unable to see food items that are not moving. Why this feature would evolve is a mystery.

Hearing

All frogs have ears located behind the eye and above the jaws. As with the sense of sight, frogs' ability to hear varies. Frogs that are vulnerable to predation typically have a very sensitive sense of hearing; frogs that are not may have a fairly poor sense of hearing. The inner ears are protected by a very thin membrane of skin known as the tympanum.

Intelligence

2nd Bakhuis

The dyeing poison dart frog is one of the more intelligent frogs.

Frog intelligence has been a matter of debate. Few frogs approach mammals or birds in terms of intelligence, though many do rival or surpass reptiles, and the most intelligent are thought to come close to the intellectual level of a bird or lower mammal.

Wood frogs, Rana sylvatica, have demonstrated a basic capacity to learn. In captivity wood frogs that attack distasteful caterpillars will learn to avoid that particular species, although they may attack similar species. Over time, however, wood frogs will begin to avoid all caterpillars that look similar to the distasteful ones. Many frogs do have the capacity to learn, but a few learn more readily than others.

The cane toad, Rhinella marina, demonstrates surprising intelligence. Groups gather around street lamps, attracted to the insects that gather there. When the insects fall, the toads hastily devour them. Cane toads have also learned to raid garbage cans, steal dog food, and can learn traffic patterns.

Skin

Poison

Reproduction and parental care

Conservation

In captivity

Beardiehatching
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