Frog And Owl

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Frogs are amphibians in the order Anura (meaning "tailless", 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, bifid tongue and the absence of a tail. Frogs are widely known as exceptional jumpersSony VAIO PCG-8131M battery, 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 tails to develop in waterSony VAIO PCG-8152M battery. 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 significantlySony VAIO PCG-31311M battery.

A popular distinction is often made between frogs and toads on the basis of their appearance, but this has no taxonomic basis. (Members of the anuran family Bufonidae are called true toads, but many species from other families are also called toads.) 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 petsSony VAIO PCG-31111M battery.

Etymology and terminology

The name frog derives from Old English frogga, (compare Old Norse frauki, German Frosch, older Dutch spelling kikvorsch), cognate with Sanskrit plava (frog), probably deriving from Proto-Indo-European praw = "to jump".[1]

A distinction is often made between frogs and toads on the basis of their appearance, caused by the convergent adaptation among so-called toads to dry environmentsSony VAIO PCG-8112M battery; however, this distinction has no taxonomic basis. The only family exclusively given the common name "toad" is Bufonidae, but many species from other families are also called "toads," and the species within the toad genus Atelopus are referred to as "harlequin frogs".

Taxonomy

For more details on this topic, see List of Anuran families.

The order Anura contains 4,810 species[2] in 33 families, of which the Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. About 88% of amphibian species are frogsSony VAIO PCG-7186M battery.

European Fire-bellied Toad (Bombina bombina)

Young American bullfrog found in a stream in New Jersey

The use of the common names "frog" and "toad" has no taxonomic justification. From a taxonomic 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 with smooth and/or moist skinsSony VAIO PCG-7171M battery, and the term "toad" generally refers to species that tend to be terrestrial with dry, warty skin. An exception is the fire-bellied toad (Bombina bombina): while its skin is slightly warty, it prefers a watery habitat.

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 frogsSony VAIO PCG-9Z1M battery; and Neobatrachia, by far the largest group, which contains the remaining 24 families of "modern" frogs, including most common species throughout the world. Neobatrachia is further divided into the Hyloidea and Ranoidea.[3] This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpolesSony VAIO PCG-5S1M battery. While this classification is largely accepted, relationships among families of frogs are still debated. Future studies of molecular genetics should soon provide further insights to the evolutionary relationships among anuran families.[4]

Some species of anurans hybridise readily. For instance, the Edible Frog (Rana esculenta) is a hybrid of the Pool Frog (R. lessonae) and the Marsh Frog (R. ridibunda). The fire-bellied toads Bombina bombina and Bombina variegata similarly form hybridsSony VAIO PCG-5P1M battery, although these are less fertile, giving rise to a hybrid zone.

Morphology and physiology

Skeleton of Rana esculenta showing bones of the head, vertebral column, ribs, pectoral and pelvic girdles and limbs

The morphology of frogs is unique among amphibians. Compared with the other two groups of amphibians, (salamanders and caecilians), frogs are unusual because they lack tails as adults and their legs are more suited to jumping than walkingSony VAIO PCG-5N2M battery. The physiology of frogs is generally like that of other amphibians (and differs from other terrestrial vertebrates) because oxygen can pass through their highly permeable skin. This unique feature allows frogs to respire largely through their skins.[7] The ribs are poorly developed so the lungs are filled by gulping down air. It has been shown that a frog that has been deprived of its lungs can continue living, respiring entirely through its skin. Sony VAIO PCG-3C2M battery Because the oxygen is dissolved in an aqueous film on the skin and passes from there to the blood, the skin must remain moist at all times; this makes frogs susceptible to many toxins in the environment, some of which can similarly dissolve in the layer of water and be passed into their bloodstream. This may be one of the causes of the decline in frog populationsSony VAIO PCG-8161M battery.

Many characteristics are not shared by all of the approximately 5,250 described frog species. However, some general characteristics distinguish them from other amphibians. Frogs are usually well suited to jumping, with long hind legs and elongated ankle bones. They have a short vertebral column, with no more than ten free vertebrae, followed by a fused tailbone (urostyle or coccyx), typically resulting in a tailless phenotype. Sony VAIO PCG-8141M battery

Frogs range in size from 10 mm (0.39 in) (Brachycephalus didactylus of Brazil and Eleutherodactylus iberia of Cuba) to 300 mm (12 in) (goliath frog, Conraua goliath, of Cameroon). The skin hangs loosely on the body because of the lack of loose connective tissue. Skin texture varies: it can be smooth, warty or folded. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaqueSony VAIO PCG-3J1M battery. Frogs have a tympanum on each side of the head, which is involved in hearing and, in some species, is covered by skin. Most frogs have teeth, specifically pedicellate teeth in which the crown is separated from the root by fibrous tissue. Most only have teeth on the edge of the upper jaw (maxillary teeth) as well as vomerine teeth on the roof of their mouth. They do not have any teeth on their lower jawSony VAIO PCG-3H1M battery, so they usually swallow their food whole. The teeth are mainly used to hold the prey and keep it in place till they can get a good grip on it and swallow their meal, assisted by retracting their eyes into their head.[18] True toads lack any teeth at all. Some species, like the African bull frog (Pyxicephalus), which preys on relatively large organisms including mice and other frogsSony VAIO PCG-3F1M battery, have cone shaped projections of bone called odontoid processes at the front of the lower jaw which function like teeth.[2]

Feet and legs

Tyler's Tree Frog (Litoria tyleri) illustrates large toe pads and webbed feet

A bullfrog skeleton, showing elongate limb bones and extra joints. Red marks indicate bones which have been substantially elongated in frogs and joints which have become mobile. Blue indicates joints and bones which have not been modified or only somewhat elongated.

The structure of the feet and legs varies greatly among frog species
Sony VAIO PCG-3C1M battery
, 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 do so.

Many frogs, especially those that live in water, have webbed toes. The degree to which the toes are webbed is directly proportional to the amount of time the species lives in the water. Sony VAIO PCG-9Z2L battery] For example, the completely aquatic African dwarf frog (Hymenochirus sp.) has fully webbed toes, whereas the toes of White's tree frog (Litoria caerulea), an arboreal species, are only a half or a quarter webbed.

A frog gripping on a palm frond

Arboreal frogs have "toe pads" to help grip vertical surfaces. These pads, located on the ends of the toes, do not work by suction. Rather, the surface of the pad consists of interlocking cells, with a small gap between adjacent cellsSony VAIO PCG-9Z1L battery. When the frog applies pressure to the toe pads, the interlocking cells grip irregularities on the substrate. The small gaps between the cells drain away all but a thin layer of moisture on the pad, and maintain a grip through capillarity. This allows the frog to grip smooth surfaces, and does not function when the pads are excessively wet. Sony VAIO PCG-9131L battery

In many arboreal frogs, a small "intercalary structure" in each toe increases the surface area touching the substrate. Furthermore, since hopping through trees can be dangerous, 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, as do aquatic frogsSony VAIO PCG-8161L battery. In these arboreal frogs, the webs allow the frogs to "parachute" or control their glide from one position in the canopy to another.[21]

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 have a toe extension—a metatarsal tubercle—that helps them to burrow. The hind legs of ground dwellers are more muscular than those of aqueous and tree-dwelling frogsSony VAIO PCG-8152L battery.

Sometimes during the tadpole stage, one of the animal's rear leg stubs is eaten by a dragonfly nymph. In some of these cases, the full leg grows anyway, and in other cases, it does not, although the frog may still live out its normal lifespan with only three legs. Other times, a parasitic flatworm called Riberoria trematodes digs into the rear of a tadpole, where it rearranges the limb bud cells, which sometimes causes the frog to have extra legs. Sony VAIO PCG-8141L battery

Jumping

Rainforest Rocket Frog jumping

Frogs are generally recognized as exceptional jumpers, and the best jumper of all vertebrates. The Australian rocket frog, Litoria nasuta, can leap over 50 times its body length (5.5 cm), resulting in jumps of over 2 meters.[23] The acceleration of the jump may be up to twice gravity. There are tremendous differences between species in jumping capabilitySony VAIO PCG-8131L battery, but 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 from the water from a position floating on the surface.[24]

While frog species can use a variety of locomotor modes (running, walking, gliding, swimming, and climbing), more are either proficient at jumping or descended from ancestors who wereSony VAIO PCG-81312L battery, with much of the musculo-skeletal morphology modified for this purpose. The tibia, fibula and tarsals have been fused into a single, strong bone, as have the radius and ulna in the forelimbs (which must absorb the impact of landing). The metatarsals have become elongated to add to the leg length and allow the frog to push against the ground for longer during a jumpSony VAIO PCG-81214L battery. The illium has elongated and formed a mobile joint with the sacrum which, in specialist jumpers such as Ranids or Hylids, functions as an additional limb joint to further power the leaps. The tail vertebrae fused into a urostyle which retracted inside the pelvis. This enabled the force to be transferred from the legs to the body during a leap. Sony VAIO PCG-81115L battery

The muscular system has been similarly modified. The hind limbs of the ancestor of 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 jumpingSony VAIO PCG-81114L battery, 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 the frog. 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 airSony VAIO PCG-81113L battery. The forelegs are folded against the chest and the hind legs remain in the extended, streamlined position for the duration of the jump.[25] In some species of powerful jumpers, when the muscles contract, the energy is transferred into the stretched tendon which is wrapped around the ankle bone. Finally the muscles stretch again at the same time as the tendon releases its energy like a catapult to produce a powerful accelerations. Sony VAIO PCG-7142L battery

In some extremely capable jumpers, such as the Cuban tree frog, the peak power exerted during a jump can exceed that which the muscle is theoretically capable of producing. It is hypothesized that frogs are storing muscular energy by stretching their tendons like springs, then triggering the release all at once, allowing the frog to increase the energy of its jump beyond the limits of muscle-powered accelerationSony VAIO PCG-7141L battery. A similar mechanism has already been documented in locusts and grasshoppers.[27]

Skin

Pouched Frog (Assa darlingtoni) camouflaged against leaf litter

Microscopic view of frog skin

Being cold-blooded, frogs have to adopt behaviour patterns to regulate their temperature. To warm up they can move into the sun or onto a warm surface and to cool down they can move into the shade or adopt a stance that exposes the minimum area of skin to the air. This involves squatting down, tucking the forefeet under the chin and the hind feet under the belly. Sony VAIO PCG-71111L battery The colour of a frog's skin is used for thermo-regulation. 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.[29]

Many frogs are able to absorb water and oxygen directly through the skin, especially around the pelvic area. However, the permeability of a frog's skin can also result in water lossSony VAIO PCG-61411L battery. Glands located all over the body exude mucous 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 tree frogs reduce water loss by having a waterproof layer of skin and several South American species coat their skin with a waxy secretionSony VAIO PCG-61112L battery. Others frogs have adopted behaviours to conserve water, including becoming nocturnal and resting in a water-conserving position. This posture involves the frog lying with its toes and fingers tucked under its body and chin with no gap between the body and the substrate. Some frog species will also rest in large groups with each frog pressed against its neighboursSony VAIO PCG-61111L battery. This reduces the amount of skin exposed to the air or a dry surface, and thus reduces water loss. The Woodhouse's Toad (Bufo woodhousii), if given access to water after confinement in a dry location, sits in the shallows to rehydrate itself.[30] These water conservation measures only reduce water loss enough for a predominantly arboreal existence and are not suitable for arid conditionsSony VAIO PCG-5T4L battery.

Camouflage is a common defensive mechanism in frogs. Most camouflaged frogs are nocturnal, which adds to their ability to hide. Nocturnal frogs usually find the ideal camouflaged position during the day to sleep. Some frogs have the ability to change colour, but this is usually restricted to shades of one or two colours. For example, White's tree frog varies in shades of green and brownSony VAIO PCG-5T3L battery. Features such as warts and skin folds are usually found on ground-dwelling frogs, where a smooth skin would not disguise them effectively. Arboreal frogs usually have smooth skin, enabling them to disguise themselves as leaves.[citation needed]

Certain frogs change colour between night and day, as light and moisture stimulate the pigment cells and cause them to expand or contract. Sony VAIO PCG-5T2L battery

Poison

Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands—the parotoid glands—located behind the eyes, on the top of the head. Some frogs, such as some poison dart frogs, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictorsSony VAIO PCG-5S3L battery. Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected.

Oophaga pumilio, a poison dart frog, contains numerous alkaloids which deter predators

Some frogs obtain poisons from the ants and other arthropods they eat;[31] others, such as the Australian Corroboree Frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid not derived from their dietSony VAIO PCG-5S2L battery.[32] Some native people of South America extract poison from the poison dart frogs and apply it to their darts for hunting,[33] although few species are toxic enough to be used for this purpose. It was previously a misconception the poison was placed on arrows rather than darts. The common name of these frogs was thus changed from "poison arrow frog" to "poison dart frog" in the early 1980sSony VAIO PCG-5S1L battery. Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as aposematism. There are at least two non-poisonous species of frogs in tropical America (Eleutherodactylus gaigei and Lithodytes lineatus) that mimic the colouration of dart poison frogs' coloration for self-protection (Batesian mimicry). Sony VAIO PCG-5R2L battery

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 found in some species of poison dart frogs. Other chemicals isolated from the skin of frogs may offer resistance to HIV infection.[36] Arrow and dart poisons are under active investigation for their potential as therapeutic drugs. Sony VAIO PCG-5R1L battery

The skin secretions of some toads, such as the Colorado River toad and cane toad, contain bufotoxins, some of which, such as bufotenin, are psychoactive, and have therefore been used as recreational drugs. Typically, the skin secretions are dried and smoked. Skin licking is especially dangerous, and appears to constitute an urban myth. See psychoactive toadSony VAIO PCG-5P4L battery.

Respiration and circulation

The skin of a frog is permeable to oxygen and carbon dioxide, as well as to water. There are a number of blood vessels near the surface of the skin. When a frog is underwater, oxygen is transmitted through the skin directly into the bloodstream. On land, adult frogs use their lungs to breathe. Their lungs are similar to those of humans, but the chest muscles are not involved in respirationSony VAIO PCG-5P2L battery, and there are no ribs or diaphragm to support breathing. Frogs breathe by taking air in through the nostrils (which often have valves which close when the frog is submerged), causing the throat to puff out, then compressing the floor of the mouth, which forces the air into the lungs. In August 2007 an aquatic frog named Barbourula kalimantanensis was discovered in a remote part of IndonesiaSony VAIO PCG-5N4L battery. The Bornean Flat-headed Frog (B. kalimantanensis) is the first species of frog known to science without lungs.

Frogs are known for their three-chambered heart, which they share with all tetrapods except birds, crocodilians and mammals.[38] In the three-chambered heart, oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter by separate atriaSony VAIO PCG-5N2L battery, and are directed via a spiral valve to the appropriate vessel—aorta for oxygenated blood and pulmonary artery for deoxygenated blood. This special structure is essential to keeping the mixing of the two types of blood to a minimum, which enables frogs to have higher metabolic rates, and to be more active than otherwise.

Some species of frog have remarkable adaptations that allow them to survive in oxygen deficient waterSony VAIO PCG-51513L battery. The lake titicaca frog (Telmatobius culeus) is one such species and to survive in the poorly oxygenated waters of Lake Titicaca it has incredibly wrinkly skin that increases its surface area to enhance gas exchange. This frog will also do 'push-ups' on the lake bed to increase the flow of water around its body.[39]

Digestion and excretion

The frog's digestive system begins with the mouth. Frogs have teeth along their upper jaw called maxillary teeth, which are used to hold food before it is swallowedSony VAIO PCG-51511L battery. 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. This normally lies coiled in the mouth, free at the back and attached to the mandibles at the front. It can be shot out and retracted at great speed.[19] Some frogs have no tongue and just stuff food into their mouths with their hands. Sony VAIO PCG-51412L batteryThe eyes assist in the swallowing of food as they are able to be retracted through holes in the skull and help push food down the throat.[19] 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 pancreasSony VAIO PCG-51411L battery, 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 water is absorbed and wastes are routed to the cloaca.

The excretory system is similar to that of mammals. There are two kidneys which remove urea from the blood and convert it into urine. This 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 and the cloacal vent. Sony VAIO PCG-51312L battery

Reproductive system

In frogs the sexes are separate. 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. In the male the two testes are attached to the kidneys and sperm passes into the kidneys through fine tubes called efferent ductsSony VAIO PCG-51311L battery. They then travel through the ureters which are consequently called urinogenital ducts in the male frog. There is no penis and sperm are ejected from the cloaca directly onto the eggs that the female is laying during amplexus. The ovaries of the female frog are also beside the kidneys and the eggs pass down a pair of oviducts to the exterior. During amplexus, the firm grip of the male frog stimulates the release of eggs, usually wrapped in jelly, as spawn. Sony VAIO PCG-51211L battery

Nervous system

The frog has a highly developed nervous system which consists of a brain, spinal cord and nerves. Many parts of the frog's brain correspond with those of humans. The brain 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 respirationSony VAIO PCG-41112L battery, digestion and other automatic functions.[40] The relative size of the cerebrum of a frog is much smaller than that of a human. Frogs have ten cranial nerves (nerves which pass information from the outside directly to the brain) and ten pairs of spinal nerves (nerves which pass information from extremities to the brain through the spinal cord).[40] By contrast, all amniotes (mammals, birds and reptiles) Sony VAIO PCG-3A4L batteryhave twelve cranial nerves. Frogs do not have external ears; the eardrums (tympanic membranes) are directly exposed. As in all animals, the ear contains semicircular canals which help control balance and orientation. Due to their short cochlea, frogs use electrical tuning to expand their range of audible frequenciesSony VAIO PCG-3A3L battery.

Sight

The eyes of frogs are located on or near the top of the head and often project outwards as hemispherical bulges. They have a large field of view and may be the only part of an otherwise submerged frog to protrude from the water. Each eye has a closable upper and lower lid and a nictitating membrane which provides further protection, especially when the frog is swimmingSony VAIO PCG-3A2L battery. 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. Sony VAIO PCG-3A1L battery

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 his shadow but the closer an object is, the less well it is seen.[28] 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. Sony VAIO PCG-394L battery Whether a frog sees in colour is debatable but it has been shown that it responds positively to blue light, perhaps because that colour is associated with bodies of water that can provide refuge when the frog feels threatened.[41]

Natural history

The life cycle of frogs, like that of other amphibians, consists of four main stages: egg, tadpole, metamorphosis and adultSony VAIO PCG-393L battery. The reliance of frogs on an aquatic environment for the egg and tadpole stages gives rise to a variety of breeding behaviours that include the well-known mating calls used by the males of most species to attract females to the bodies of water that they have chosen for breeding. Some frogs also look after their eggs—and in some cases even the tadpoles—for some time after they are laidSony VAIO PCG-391L battery.

Life cycle

Frogspawn

Frogspawn development

Tadpole of Haswell's Froglet (Paracrinia haswelli

The life cycle of a frog starts with an egg. A female generally lays gelatinous egg masses containing thousands of eggs, in water. Each anuran species lays eggs in a distinctive, identifiable manner. An example are the long strings of eggs laid by the common American toad. The eggs are highly vulnerable to predationSony VAIO PCG-384L battery, so frogs have evolved many techniques to ensure the survival of the next generation. In colder areas the embryo is black to absorb more heat from the sun, which speeds up the development. Most commonly, this involves synchronous reproduction. Many individuals will breed at the same time, overwhelming the actions of predators; the majority of the offspring will still die due to predationSony VAIO PCG-383L battery, but there is a greater chance some will survive. Another way in which some species avoid the predators and pathogens eggs are exposed to in ponds is to lay eggs on leaves above the pond, with a gelatinous coating designed to retain moisture. In these species the tadpoles drop into the water upon hatching. The eggs of some species laid out of water can detect vibrations of nearby predatory wasps or snakesSony VAIO PCG-382L battery, and will hatch early to avoid being eaten.[42] Some species, such as the Cane Toad (Bufo marinus), lay poisonous eggs to minimise predation. While the length of the egg stage depends on the species and environmental conditions, aquatic eggs generally hatch within one week. Other species go through their whole larval phase inside the eggs or the mother, or they have direct developmentSony VAIO PCG-381L battery. Unlike salamanders and newts, frogs and toads never become sexually mature while still in their larval stage.

Eggs hatch and continue life as tadpoles (occasionally known as polliwogs), which typically have oval bodies and long, vertically flattened tails. At least one species (Nannophrys ceylonensis) has tadpoles that are semi-terrestrial and live among wet rocks, but as a general rule, free living larvae are fully aquaticSony VAIO PCG-7185L battery. They lack eyelids and have a cartilaginous skeleton, a lateral line system, gills for respiration (external gills at first, internal gills later) and tails with dorsal and ventral folds of skin for swimming.[45] From pretty early onward they develop a gill pouch that covers the gills and the front legs and also the lungs are developed in an early stage as an accessory breathing organSony VAIO PCG-7184L battery. Some species which go through the metamorphosis inside the egg and hatch to small frogs never develop gills, instead there are specialised areas of skin that takes care of the respiration. Tadpoles also lack true teeth, but the jaws in most species usually have two elongate, parallel rows of small keratinized structures called keradonts in the upper jaw while the lower jaw has three rows of keradontsSony VAIO PCG-7183L battery, surrounded by a horny beak, but the number of rows can be lower or absent, or much higher.[46] 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. Cannibalism has been observed among tadpoles. Early developers who gain legs may be eaten by the others, so the late bloomers survive longerSony VAIO PCG-7182L battery. This has been observed in England in the species Rana temporaria (common frog).[47]

Tadpoles are highly vulnerable to predation by fish, newts, predatory diving beetles and birds such as kingfishers. Poisonous tadpoles are present in many species, such as Cane Toads. The tadpole stage may be as short as a week, or tadpoles may overwinter and metamorphose the following year in some species, such as the midwife toad (Alytes obstetricans) and the common spadefoot (Pelobates fuscus) Sony VAIO PCG-7181L battery. In the Pipidae, with the exception for Hymenochirus, the tadpoles have paired anterior barbels which make them resemble small catfish.[48]

With the exception of the base of the tail, where a few vertebral structures develop to give rise to the urostyle later in life, the tail lacks the completely solid, segmental, skeletal elements of cartilage or bony tissue that are so typical for other vertebrates, although it does contain a notochordSony VAIO PCG-7174L battery

At the end of the tadpole stage, frogs undergo metamorphosis, in which they undergo a transition into the adult form. This metamorphosis last typically only 24 hours and consists of:

Larva of the common frog Rana temporaria a day before metamorphisis

Common frog - Metamorphosis stage. Notice the deformed jaws, large eyes and the remains of the gill pouchSony VAIO PCG-7173L battery.

Young frog with tail remains after metamorphosis

Adult leopard frog

The disappearance of the gill pouch, making the front legs visible.

The transformation of the jaws into the big jaws of predatory frogs (most tadpoles are scrapers of algae or are filter feeders)

The transformation of the digestive system: the long spiral gut of the larva is being replaced by the typical short gut of a predator.

An adaptation of the nervous system for stereoscopic vision, locomotion and feeding

A quick growth and movement of the eyes to higher up the skull and the formation of eyelids.

Formation of skin glands, thickening of the skin and loss of the lateral line systemSony VAIO PCG-7172L battery

An eardrum is developed to lock the middle ear.

The disappearance of the tail is somewhat later (occurs at higher thyroxin levels) and after the tail has been resorbed the animals are ready to leave the water. The material of the tail is being used for a quick growth of the legs. The disappearing of the larval structures is a regulated process called apoptosisSony VAIO PCG-7171L battery.

Incident of frog cannibalism

After metamorphosis, young adults may leave the water and disperse into terrestrial habitats, or continue to live in the aquatic habitat as adults. Almost all species of frogs are carnivorous as adults, eating invertebrates such as arthropods, annelids and gastropods. A few of the larger species may eat prey such as small mammals, fish and smaller frogsSony VAIO PCG-7162L battery. Some frogs use their sticky tongues to catch fast-moving prey, while others capture their prey and force it into their mouths with their hands. However, there are a very few species of frogs that primarily eat plants.[49] Adult frogs are themselves preyed upon by birds, large fish, snakes, otters, foxes, badgers, coatis, and other animals. Frogs are also eaten by people (see section on uses in agriculture and research, below) Sony VAIO PCG-7161L battery.

Frogs and toads can live for many years; though little is known about their life span in the wild, captive frogs and toads are recorded living up to 40 years.[50]

Frogs from temperate climates hibernate through the winter, and 4 species are known to freeze during this time, most notably Rana sylvatica.[51]

Reproduction of frogs

Once adult frogs reach maturity, they will assemble at a water source such as a pond or stream to breed. Many frogs return to the bodies of water where they were born, often resulting in annual migrations involving thousands of frogsSony VAIO PCG-7154L battery. In continental Europe, a large proportion of migrating frogs used to die on roads, before special fences and tunnels were built for them.

Male and female Common toad (Bufo bufo) in amplexus

A Male and Female common toad in amplexus. The black strands are eggs released into open water minutes after birth.

Once at the breeding ground, male frogs call to attract a mate, collectively becoming a chorus of frogs. The call is unique to the species, and will attract females of that species. Some species have satellite males who do not call, but intercept females that are approaching a calling maleSony VAIO PCG-7153L battery.

The male and female frogs then undergo amplexus. This involves the male mounting the female and gripping her (sometimes with special nuptial pads) tightly. Fertilization is external: the egg and sperm meet outside of the body. The female releases her eggs, which the male frog covers with a sperm solution. The eggs then swell and develop a protective coatingSony VAIO PCG-7152L battery. The eggs are typically brown or black, with a clear, gelatin-like covering. Members of the West African genus Nimbaphrynoides are the only species known to be viviparous and the Tanzanian genus Nectophrynoides are the only known to be ovoviviparous: In both cases, fertilization is internal and females give birth to fully developed, small toadlets. Sony VAIO PCG-7151L battery
Sony VAIO PCG-7148L battery

Most temperate species of frogs reproduce between late autumn and early spring. In the UK, most common frog populations produce frogspawn in February, although there is wide variation in timing. Water temperatures at this time of year are relatively low, typically between four and 10 degrees Celsius. Reproducing in these conditions helps the developing tadpoles because dissolved oxygen concentrations in the water are highest at cold temperaturesSony VPCW21M2E/WI battery. More importantly, reproducing early in the season ensures that appropriate food is available to the developing frogs at the right time.

Parental care

Colour plate from Ernst Haeckel's 1904 Kunstformen der Natur, depicting frog species that include two examples of parental care.

Although care of offspring is poorly understood in frogs, it is estimated that up to 20% of amphibian species may care for their young in one way or another, and there is a great diversity of parental behaviours. Sony VPCW21C7E battery Some species of poison dart frog lay eggs on the forest floor and protect them, guarding the eggs from predation and keeping them moist. The frog will urinate on them if they become too dry. After hatching, a parent (the sex depends upon the species) will move them, on its back, to a water-holding bromeliad. The parent then feeds them by laying unfertilized eggs in the bromeliad until the young have metamorphosedSony VPCW12S1E/WZ battery. Other frogs carry the eggs and tadpoles on their hind legs or back (e.g. the midwife toads, Alytes spp.). Some frogs even protect their offspring inside their own bodies. The male Australian Pouched Frog (Assa darlingtoni) has pouches along its side in which the tadpoles reside until metamorphosis. The female Gastric-brooding Frogs (genus Rheobatrachus) from AustraliaSony VPCW12S1E/W battery, now probably extinct, swallows its tadpoles, which then develop in the stomach. To do this, the Gastric-brooding Frog must stop secreting stomach acid and suppress peristalsis (contractions of the stomach). Darwin's Frog (Rhinoderma darwinii) from Chile puts the tadpoles in its vocal sac (see next section) for development. Some species of frog will leave a 'babysitter' to watch over the frogspawn until it hatchesSony VPCW12S1E/T battery.

The evolution of parental care in frogs is driven primarily by the size of the water body in which they breed. There is an inverse relationship between the level of parental care in a frog species and the size of the body of water—frogs that breed in smaller water bodies tend to have more complex parental care behaviors. Sony VPCW12S1E/P battery Water body size shows this strong relationship with parental care because it encompasses several important variables that interact to select for parental care: predation, desiccation, competition, and resource limitation. Because predation of eggs and larvae is high in large water bodies, a number of frog species evolved terrestrial oviposition. Once eggs are deposited on landSony VPCW11S1E/W battery, the desiccating terrestrial environment demands uniparental care in the form of egg hydration to ensure egg survival.[56] The subsequent need to transport hatched tadpoles to a water source requires an even more intense form of uniparental care. In small water bodies where predators are mostly absent, such as phytotelmata (water-filled leaf axils or small woody cavities) Sony VPCW11S1E/T battery, inter-tadpole competition becomes the variable that constrains tadpole survival. Certain frogs species avoid this competition by evolving the use of smaller phytotelmata as tadpole deposition sites.[57] However, while these smaller tadpole rearing sites are free of competition, they also lack nutrients. Because they do not have sufficient nutrients to support a tadpole without parental provisioning behaviorSony VPCW11S1E/P battery, frog species that transitioned from the use of larger to smaller phytotelmata have evolved trophic (unfertilized) egg laying. In this complex form of biparental care, the female provides her offspring with nutritive eggs. While each of these variables select for different behaviors, they correlate with the size of a species' tadpole-rearing site and influence the degree of parental care displayed by a speciesSony VPCYA1S1E/B battery.

Call

A male Dendropsophus microcephalus displaying its vocal sac during its call.

The call of a frog is unique to its species. Frogs call 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, they can be heard up to a mile away. Sony VPCYA1V9E/B batteryThe field of neuroethology studies the neurocircuitry that underlies frog audition.

Some frogs lack vocal sacs, such as those from the genera Heleioporus and Neobatrachus, but these species can still produce a loud call. Their buccal cavity is enlarged and dome-shaped, acting as a resonance chamber that amplifies their call. Species of frog without vocal sacs and that do not have a loud call tend to inhabit areas close to flowing water. The noise of flowing water overpowers any call, so they must communicate by other meansSony VPCY22Z5008B battery.

The main reason for calling is to allow males to attract a mate. Males call either individually or in a group called a chorus. Females of many frog species, for example Polypedates leucomystax, produce calls reciprocal to the males', which act as the catalyst for the enhancement of reproductive activity in a breeding colony. Sony VPCY21S1E/SI batteryA male frog emits a release call when mounted by another male. Tropical species also have a rain call that they make on the basis of humidity cues prior to a rain shower. Many species also have a territorial call that is used to chase away other males. All of these calls are emitted with the mouth of the frog closedSony VPCY21S1E/L battery.

A distress call, emitted by some frogs when they are in danger, is produced with the mouth open, resulting in a higher-pitched call. The effectiveness of the call is unknown; however, it is suspected the call intrigues the predator until another animal is attracted, distracting them enough for its escapeSony VPCY21S1E/G battery.

Many species of frog have deep calls, or croaks. The English onomatopoeic spelling is "ribbit". The croak of the American bullfrog (Rana catesbiana) is sometimes spelt "jug o' rum".[60] Other examples are Ancient Greek brekekekex koax koax for probably Rana ridibunda, and the description in Rigveda 7:103.6 gómāyur éko ajámāyur ékaħ = "one has a voice like a cow's, one has a voice like a goat's"Sony VPCY11S1E/S battery.

Distribution and conservation status

The Red-eyed Tree Frog (Litoria chloris) is a species of tree frog native to eastern Australia.

See also: Decline in amphibian populations

The habitat of frogs extends almost worldwide, but they do not occur in Antarctica and are not present on many oceanic islands.[61][62] The greatest diversity of frogs occurs in the tropical areas of the world, where water is readily available, suiting frogs' requirements due to their skin. Some frogs inhabit arid areas such as deserts, where water may not be easily accessibleSony VPCY11S1E battery, and rely on specific adaptations to survive. The Australian genus Cyclorana and the American genus Pternohyla will bury themselves underground, create a water-impervious cocoon and hibernate during dry periods. Once it rains, they emerge, find a temporary pond and breed. Egg and tadpole development is very fast in comparison to most other frogs so that breeding is complete before the pond dries upSony VPCZ11X9E/B battery. Some frog species are adapted to a cold environment; for instance the wood frog (Rana sylvatica), whose habitat extends north of the Arctic Circle, buries itself in the ground during winter when much of its body freezes.[19]

Golden toad (Bufo periglenes) – last seen in 1989

Frog populations have declined dramatically since the 1950s: more than one third of species are believed to be threatened with extinction and more than 120 species are suspected to be extinct since the 1980s. Sony VPCZ11Z9E/B battery Among these species are the golden toad of Costa Rica and the Gastric-brooding frogs of Australia. Habitat loss is a significant cause of frog population decline, as are pollutants, climate change, the introduction of non-indigenous predators/competitors, and emerging infectious diseases including chytridiomycosis. Many environmental scientists believe that amphibiansSony VPCZ12M9E/B battery, including frogs, are excellent biological indicators of broader ecosystem health because of their intermediate position in food webs, permeable skins, and typically biphasic life (aquatic larvae and terrestrial adults).[64] It appears that it is the species with both aquatic eggs and aquatic larvae that are most affected by the decline, while those with direct development are the most resistant. Sony VPCZ12X9E/X battery

A Canadian study conducted in 2006, suggested heavy traffic near frog habitats as a large threat to frog populations.[66] In a few cases, captive breeding programs have been attempted to alleviate the pressure on frog populations, and these have proved successful.[67][68] In 2007, it was reported the application of certain probiotic bacteria could protect amphibians from chytridiomycosis. Sony VPCZ12Z9E/X battery One current project, The Panama Amphibian Rescue and Conservation Project, has subsequently been developed in order to rescue species at risk of chytridiomycosis in eastern Panama, and to develop field applications of this probiotic cure.[70]

Zoos and aquariums around the world named 2008 the Year of the Frog, to draw attention to the conservation issues. Sony VPCZ138GA battery

Evolution

A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas

Until the discovery of the Early Permian Gerobatrachus hottoni in 2008, a stem-batrachian with many salamander-like characteristics, the earliest known proto-frog was Triadobatrachus massinoti, from the 250 million year old early Triassic of Madagascar.[72] The skull is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern amphibiaSony VPCZ13M9E/B battery. These include a different ilium, a longer body with more vertebrae, and separate vertebrae in its tail (whereas in modern frogs, the tail vertebrae are fused, and known as the urostyle or coccyx). The tibia and fibula bones are unfused and separate, making it probable Triadobatrachus was not an efficient leaper.

Another fossil frog, Prosalirus bitis, was discovered in 1995. The remains were recovered from Arizona's Kayenta Formation, which dates back to the Early Jurassic epoch, Sony VPCZ13M9E/X batterysomewhat younger than Triadobatrachus. Like Triadobatrachus, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail[citation needed] and was well adapted for jumping.[74]

The earliest known "true frog" is Vieraella herbsti, from the early Jurassic (188–213 million years ago) Sony VPCZ13V9E battery. It is known only from the 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. It is likely the evolution of modern Anura was completed by the Jurassic period. The main evolutionary changes involved the shortening of the body and the loss of the tailSony VPCZ13V9E/X battery.

The earliest full fossil record of a modern frog is of sanyanlichan, which lived 125 million years ago[75] and had all modern frog features, but bore 9 presacral vertebrae instead of the 8 of modern frogs.[76]

Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they inhabited Antarctica when it was warmer. Sony VPCZ13Z9E/X battery

Uses in agriculture and research

Cooked frog. Jianyang, Fujian, China

For more details on this topic, see Animal testing on frogs.

Frog meat less than one minute old in a market in Haikou City, Hainan Province, China

Frogs are raised commercially for several purposes. Frogs are used as a food source; frog legs are a

delicacy in China, France, Indonesia, the Philippines, the north of Greece and in many parts of the American South, especially Louisiana. Dead frogs are sometimes used for dissections in high school and university anatomy classesSony VPCZ21M9E battery, often after being injected with coloured plastics to enhance the contrast between the organs. This practice has declined in recent years with the increasing concerns about animal welfare.

Frogs have served as important model organisms throughout the history of science. Eighteenth-century biologist Luigi Galvani discovered the link between electricity and the nervous system through studying frogs. The African clawed frog or platanna (Xenopus laevis) was first widely used in laboratories in pregnancy assays in the first half of the 20th centurySony VPCZ21Q9E battery. When human chorionic gonadotropin, a hormone found in substantial quantities in the urine of pregnant women, is injected into a female X. laevis, it induces them to lay eggs. In 1952, Robert Briggs and Thomas J. King cloned a frog by somatic cell nuclear transfer, the same technique later used to create Dolly the Sheep, their experiment was the first time successful nuclear transplantation had been accomplished in metazoans. Sony VPCZ21V9E battery

Frogs are used in cloning research and other branches of embryology because frogs are among the closest living relatives of man to lack egg shells characteristic of most other vertebrates, and therefore facilitate observations of early development. Although alternative pregnancy assays have been developed, biologists continue to use Xenopus as a model organism in developmental biology because it is easy to raise in captivity and has a large and easily manipulatable embryoSony VPCEH3T9E battery. Recently, X. laevis is increasingly being displaced by its smaller relative X. tropicalis, which reaches its reproductive age in five months rather than one to two years (as in X. laevis),[79] facilitating faster studies across generations. The genome sequence of X. tropicalis will probably be completed by 2015 at the latest.[80]

Cultural beliefs

For more details on this topic, see Frogs in popular culture.

Moche Frog 200 A.D. Larco Museum Collection Lima, PeruSony VPCEH3N6E battery

Frogs feature prominently in folklore, fairy tales and popular culture. They tend to be portrayed as benign, ugly, clumsy, but with hidden talents. Examples include Michigan J. Frog, The Frog Prince, and Kermit the Frog. Michigan J. Frog, featured in the Warner Brothers cartoon One Froggy Evening, only performs his singing and dancing routine for his ownerSony VPCEH3N1E battery. Once another person looks at him, he will return to a frog-like pose. "The Frog Prince" is a fairy tale of a frog who turns into a handsome prince once kissed. Kermit the Frog, on the other hand, is a conscientious and disciplined character of 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 charactersSony VPCEH3D0E battery.

The Moche people of ancient Peru worshipped animals and often depicted frogs in their art.[81] In Panama local legend promised luck to anyone who spotted a golden frog in the wild and some believed that when Panamanian Golden Frogs died, they would turn into a gold talisman, known as a huaca. Today, despite being extinct in the wildSony VPCEH3B1E battery, Panamanian Golden Frogs remain an important cultural symbol and can be found on decorative cloth molas made by the Kuna Indians, on T-shirts, as inlaid design on a new overpass in Panama City and even on lottery tickets.[82]

Owls are a group of birds that belong to the order Strigiformes, constituting 200 extant bird of prey species. Most are solitary and nocturnal, with some exceptions (e.g., the Northern Hawk Owl). Owls hunt mostly small mammals, insects, and other birdsSony VPCEH2Z1E battery, although a few species specialize in hunting fish. They are found in all regions of the Earth except Antarctica, most of Greenland and some remote islands. Though owls are typically solitary, the literary collective noun for a group of owls is a parliament. Owls are characterized by their small beaks and wide faces, and are divided into two families: the typical owls, Strigidae; and the barn-owls, TytonidaeSony VPCEH2S9E battery.

Description

Burrowing Owl (Athene cunicularia)

Owls have large forward-facing eyes and ear-holes; a hawk-like beak; a flat face; and usually a conspicuous circle of feathers, a facial disc, around each eye. The feathers making up this disc can be adjusted in order to sharply focus sounds that come from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey sport eyes on the sides of their headsSony VPCEH2Q1E battery, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets—as are those of other birds—so they must turn their entire head to change views. Owls can rotate their heads and necks as much as 270 degrees in either direction. Sony VPCEH2P0E batteryAs owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.

The smallest owl—weighing as little as 31 grams (1 oz) and measuring some 13.5 centimetres (5 in)—is the Elf Owl (Micrathene whitneyi).[4] Around the same diminutive length, although slightly heavier, are the lesser known Long-whiskered Owlet (Xenoglaux loweryi) Sony VPCEH2N1E battery and Tamaulipas Pygmy Owl (Glaucidium sanchezi).[4] The largest owl by length is the Great Grey Owl (Strix nebulosa), which measures around 70 cm (28 in) on average and can attain a length of 84 cm (33 in).[4] However, the heaviest (and largest winged) owls are two similarly-sized eagle owls; the Eurasian Eagle-Owl (Bubo bubo) and Blakiston's Fish Owl (B. blakistoni). These two speciesSony VPCEH2M9E battery, which are on average about 2.53 cm (1.00 in) shorter in length than the Great Grey, can both attain a wingspan of 2 m (6.6 ft) and a weight of 4.5 kg (10 lb) in the largest females.

Different species of owls make different sounds; this wide range of calls aids owls in finding mates or announcing their presence to potential competitors, and also aids ornithologists and birders in locating these birds and recognizing species. As noted above, the facial disc helps owls to funnel the sound of prey to their earsSony VPCEH2M1E battery. In many species, these discs are placed asymmetrically, for better directional location.[9][verification needed]

The plumage of owls is generally cryptic, but many species have facial and head markings, including face masks, ear tufts and brightly coloured irises. These markings are generally more common in species inhabiting open habitats, and are thought to be used in signaling with other owls in low light conditions. Sony VPCEH2L9E battery

Owl eggs usually have a white color and an almost spherical shape, and range in number from a few to a dozen, depending on species. Eggs are laid at intervals of 1 to 3 days and do not hatch at the same time. This fact accounts for the wide variation in the size of sibling nestlings. Owls do not construct nests, but rather look for a sheltered nesting site or an abandoned nest in trees, underground burrows, or in buildings, barns and caves. Sony VPCEH2J1E battery

Behavior

Most owls are nocturnal, actively hunting their prey only in water and darkness. Several types of owl, however, are crepuscular—active during the twilight hours of dawn and dusk; one example is the Pygmy owl (Glaucidium). A few owls are active during the day also; examples are the Burrowing Owl (Speotyto cunicularia) and the Short-eared Owl (Asio flammeus) Sony VPCEH2H1E battery.

The serrations on the leading edge of an owl's flight feathers reduce noise

Owl eyes each have nictitating membranes that can move independently of each other, as seen on this Spotted Eagle-Owl in Johannesburg, South Africa

Owls yawn

Much of the owls' hunting strategy depends on stealth and surprise. Owls have at least two adaptations that aid them in achieving stealth. First, the dull coloration of their' feathers can render them almost invisible under certain conditionsSony VPCEH2F1E battery. Secondly, serrated edges on the leading edge of owls' remiges muffle an owl's wing beats, allowing an owl's flight to be practically silent. Some fish-eating owls, for which silence has no evolutionary advantage, lack this adaptation.

An owl's sharp beak and powerful talons allow it to kill its prey before swallowing it whole (if it is not too big). Scientists studying the diets of owls are helped by their habit of regurgitating the indigestible parts of their prey (such as bones, scales and fur) in the form of pelletsSony VPCEH2E0E battery. These "owl pellets" are plentiful and easy to interpret, and are often sold by companies to schools for dissection by students as a lesson in biology and ecology.

Adaptations for hunting

All owls are carnivorous birds of prey and live mainly on a diet of insects and small rodents such as mice, rats and hares. Some owls are also specifically adapted to hunt fish. They are very adept in hunting in their respective environments. Since owls can be found in nearly all parts of the world and across a multitude of ecosystemsSony VPCEH2D0E battery, their hunting skills and characteristics vary slightly from species to species, though most characteristics are shared among all species.

Flight and feathers

Most owls share an innate ability to fly almost silently and also more slowly in comparison to other birds of prey. Most owls live a mainly nocturnal lifestyle and being able to fly without making any noise gives them a strong advantage over their prey that are listening for any sign of noise in the dark nightSony VPCEH2C0E battery. A silent, slow flight is not as necessary for diurnal and crepuscular owls given that prey can usually see an owl approaching. While the morphological and biological mechanisms of this silent flight are more or less unknown, the structure of the feather has been heavily studied and accredited to a large portion of why they have this ability. Owls’ feathers are generally larger than the average birds’ feathersSony VPCEH1Z1E battery, have fewer radiates, longer pennulum, and achieve smooth edges with different rachis structures.[12] Serrated edges along the owl’s remiges bring the flapping of the wing down to a nearly silent mechanism. Research has shown that the serrations are more likely reducing aerodynamic disturbances, rather than simply reducing noise.[12] The surface of the flight feathers are covered with a velvety structure which absorbs the sound of the wing movingSony VPCEH1S9E battery. These unique structures reduce noise frequencies above 2 kHz,[13] making the sound level emitted drop below the typical hearing spectrum of the owl’s usual prey.[13][14] and also within the owl’s own best hearing range [4,5]. This optimizes the owl’s ability to silently fly in order to capture prey without the prey hearing the owl first as it flies in. It also allows the owl to monitor the sound output from its flight patternSony VPCEH1S8E battery.

Vision

Another characteristic of the owl which aids in their nocturnal prey capture is their eyesight. Owls are part of a small group of birds that live nocturnally, but do not use echolocation to guide them in flight in low-light situations. Owls are known for their disproportionally large eyes in comparison to their skullSony VPCEH1S1E battery. An apparent consequence of the evolution of an absolutely large eye in a relatively small skull is that the eye of the owl has become tubular in shape.[15] This shape is found in other so-called nocturnal eyes, such as the eyes of prosimians and bathypelagic fishes.[16] Since the eyes are fixed into these sclerotic tubes, they are unable to move the eyes in any direction.[17] Instead of moving their eyes, owls swivel their head to visualize their surroundingsSony VPCEH1S0E battery. The swiveling radius of the owl’s head is around 270˚, easily enabling them to see behind them without relocating the torso.[17] This ability keeps bodily movement at a minimum and thus reduces the amount of sound the owl makes as it waits for its prey. Owls are regarded as having the most frontally placed eyes amongst all avian groups, which gives them some of the largest binocular fields of visionSony VPCEH1M9E battery. But owls are farsighted and cannot focus on objects within a few centimeters of their eyes.[16][18] While it is commonly believed that owls have such great nocturnal vision due to their large (and thus very light-gathering) eyes and pupils and/or extremely sensitive rod receptors, the true cause for their ability to see in the night is due to neural mechanisms which mediate the extraction of spatial information gathered from the retinal image throughout the nocturnal luminance rangeSony VPCEH1M1E battery. These mechanisms are only able to function due to the large sized retinal image.[19] Thus, the primary nocturnal function in the vision of the owl is due to its large posterior nodal distance; retinal image brightness is only maximized to the owl within secondary neural functions.[19] These attributes of the owl cause the nocturnal eyesight to be far superior to that of its average prey. Sony VPCEH1L9E battery

Hearing

Owls exhibit specialized hearing functions and ear shapes that also aid in hunting. They are noted for asymmetrical ear placements on the skull in some genera. Owls can have either internal or external ears, but those genera exhibiting asymmetrical ear geometry only have external ear placements. Asymmetry has not been reported to extend to the middle or internal ear of the owlSony VPCEH1L8E battery. Asymmetrical ear placement on the skull allows the owl to pinpoint the location of its prey. This is especially true for strictly nocturnal species such as the barn owls 'Tyto' or Tengmalm’s Owl.[17] With ears set at different places on its skull, an owl is able to determine the direction from which the sound is coming by the minute difference in time that it takes for the sound waves to penetrate the left and right ears. Sony VPCEH1L0E batteryThe owl turns its head until the sound reaches both ears simultaneously, at which point it is directly facing the source of the sound. This time difference between ears is a matter of about 0.00003 seconds, or 30 millionths of a second. In coordination with this left to right calibration, some owls (like the barn owl) have slight up and down asymmetricality of the ears, which allows the owl to recognize whether a sound originates higher or lower in vertical space by the volume of the sound in each earSony VPCEH1J8E battery. If the sound is louder in the ear facing upwards, then the prey is higher up than the owl’s focus.[20] Like the eyes, which utilize feather movements to focus light, the ears are surrounded by feathers to maximize hearing capabilities. Behind the ear openings there are modified, dense feathers, densely packed to form a facial ruff, which creates an anteriorly-facing concave wall that cups the sound into the ear structure. Sony VPCEH1J1E batteryThis facial ruff is poorly defined in some species and prominent, nearly encircling the face, in other species. The facial disk also acts to direct sound into the ears, and a downward-facing, sharply triangular beak minimizes sound reflection away from the face. The shape of the facial disk is adjustable at will to focus sounds more effectively.[17] Owls have an audible range similar to that of humansSony VPCEH1E1E battery, but far more acute in certain frequencies, allowing them to detect even the slightest movements of their prey. Once an owl has determined the location of its prey, it flies towards it according to the last sound perceived. If the prey moves, the owl is able to adjust its flight pattern mid-flight.[20]

Talons

While the auditory and visual capabilities of the owl allow it to locate and pursue its prey, the talons and beak of the owl do the final workSony VPCEH1AJ battery. The owl kills its prey by using these talons to crush the skull and knead the body.[17] The crushing power of an owl’s talons varies according to prey size and type, and by the size of the owl. The Burrowing Owl (Athene cunicularia), a small partly insectivorous owl, has a release pressure of only 500 grams. The larger Barn Owl (Tyto alba) needs pressure of 3000 grams to release its preySony VPCCA1S1E/W battery, and one of the largest owls, the Great Horned Owl (Bubo virginianus) needs pressure of over 13,000 grams to release prey in its talons.[22] An owl’s talons, like those of most birds of prey, can seem massive in comparison to the body size outside of flight. The Masked owl has some of the proportionally longest talons of any bird of prey; they appear enormous in comparison to the body when fully extended to grasp prey. Sony VPCCA1S1E/P battery Owls' foot design differs from that of diurnal birds of prey because owls have a zygodactyl toe arrangement, with two toes directed forward and two toes directed backward during perching. During flight, the outer front toe swivels forward on a flexible joint so that three of the four toes are facing forward. Some owls’ talons have adapted to include filoplume feathers that aid in sensing the prey once it is close or has in fact made contact with the talonSony VPCCA1S1E/G battery, which is useful in capturing small prey at night. The underside of the talon foot is covered in a rough, knobby surface, which allows the owl to grasp its prey and hold it without having to keep the muscles tightly contracted.[20] An owl’s claws are sharp and curved. The family Tytonidae have inner and central toes of about equal length, while the family Strigidae have an inner toe that is distinctly shorter than the central one. Sony VPCCA1S1E/D batteryThese different morphologies allow efficiency in capturing prey specific to the different environments they inhabit.

Beak

The beak of the owl is short, curved and downward-facing, and typically hooked at the tip for gripping and tearing its prey. Once prey is captured, the scissor motion of the top and lower bill is used to tear the tissue and kill. The sharp lower edge of the upper bill works in coordination with the sharp upper edge of the lower bill to deliver this motion. Sony VPCCA1S1E/B battery The downward-facing beak allows the owl’s field of vision to be clear, as well as to direct sound into the ears without deflecting sound waves away from the face.

Snowy Owl blends well with its snowy surroundings

Camouflage

The coloration of the owl’s plumage plays a key role in its ability to sit still and blend into the environment, making it nearly invisible to prey. Owls tend to mimic the colorations and sometimes even the texture patterns of their surroundings, the common barn owl being an exception. Nyctea scandiacaSony VPCCA1S1E battery, or the Snowy Owl, appears nearly bleach-white in color with a few flecks of black, mimicking their snowy surroundings perfectly. Likewise, the Muted Wood-Owl (Strix ocellata) displays shades of brown, tan and black, making the owl nearly invisible in the surrounding trees, especially from behind. Usually, the only tell-tale sign of a perched owl will be its vocalizations or its vividly colored eyesSony VPCCA1C5E battery.

Evolution and systematics

Great Horned Owl (Bubo virginianus) sleeping during daytime in a hollow tree

The systematic placement of owls is disputed. For example, the Sibley-Ahlquist taxonomy finds that, based on DNA-DNA hybridization, owls are more closely related to the nightjars and their allies (Caprimulgiformes) than to the diurnal predators in the order Falconiformes; consequently, the Caprimulgiformes are placed in the StrigiformesSony VPCCA1AHJ battery, and the owls in general become a family Strigidae. A recent study indicates that the drastic rearrangement of the genome of the accipitrids may have obscured any close relationship of theirs with groups such as the owls.[24] In any case, the relationships of the Caprimulgiformes, the owls, the falcons and the accipitrid raptors are not resolved to satisfactionSony VPCCA1AGJ battery; currently there is an increasing trend to consider each group (with the possible exception of the accipitrids) a distinct order.

There are some 220 to 225 extant species of owls, subdivided into two families: typical owls (Strigidae) and barn-owls (Tytonidae). Some entirely extinct families have also been erected based on fossil remains; these differ much from modern owls in being less specialized or specialized in a very different way (such as the terrestrial Sophiornithidae) Sony VPCCA1AFJ battery. The Paleocene genera Berruornis and Ogygoptynx show that owls were already present as a distinct lineage some 60–57 mya (million years ago), and, hence, possibly also some 5 million years earlier, at the extinction of the non-avian dinosaurs. This makes them one of the oldest known groups of non-Galloanserae landbirds. The supposed "Cretaceous owls" Bradycneme and Heptasteornis are apparently non-avialan maniraptors. Sony VPCCA190 battery

During the Paleogene, the Strigiformes radiated into ecological niches now mostly filled by other groups of birds.[clarification needed] The owls as we know them today, on the other hand, evolved their characteristic morphology and adaptations during that time, too. By the early Neogene, the other lineages had been displaced by other bird ordersSony VPCCA18EC battery, leaving only barn-owls and typical owls. The latter at that time were usually a fairly generic type of (probably earless) owl similar to today's North American Spotted Owl or the European Tawny Owl; the diversity in size and ecology found in typical owls today developed only subsequently.

Around the Paleogene-Neogene boundary (some 25 mya) Sony VAIO VPCF135Z1E/B battery, barn-owls were the dominant group of owls in southern Europe and adjacent Asia at least; the distribution of fossil and present-day owl lineages indicates that their decline is contemporary with the evolution of the different major lineages of typical owls, which for the most part seems to have taken place in Eurasia. In the Americas, there was rather an expansion of immigrant lineages of ancestral typical owlsSony VAIO VPCF137HG/BI battery.

The supposed fossil herons "Ardea" perplexa (Middle Miocene of Sansan, France) and "Ardea" lignitum (Late Pliocene of Germany) were more probably owls; the latter was apparently close to the modern genus Bubo. Judging from this, the Late Miocene remains from France described as "Ardea" aureliensis should also be restudied.[26] The Messelasturidae, some of which were initially believed to be basal StrigiformesSony VAIO VPCF136FG/BI battery, are now generally accepted to be diurnal birds of prey showing some convergent evolution towards owls. The taxa often united under Strigogyps[27] were formerly placed in part with the owls, specifically the Sophiornithidae; they appear to be Ameghinornithidae instead.

For fossil species and paleosubspecies of extant taxa, see the genus and species articles.

Long-eared Owl (Asio otus) in erect pose

Unresolved and basal forms (all fossil)

Berruornis (Late Paleocene of France) basal? SophornithidaeSony VAIO VPCF135FG/B battery?

Strigiformes gen. et ap. indet. (Late Paleocene of Zhylga, Kazakhstan)

Palaeoglaux (Middle – Late Eocene of WC Europe) own family Palaeoglaucidae or Strigidae?

Palaeobyas (Late Eocene/Early Oligocene of Quercy, France) Tytonidae? Sophiornithidae?

Palaeotyto (Late Eocene/Early Oligocene of Quercy, France) Tytonidae? Sophiornithidae?

Strigiformes gen. et spp. indet. (Early Oligocene of Wyoming, USA)[29]

OgygoptyngidaeSony VAIO VPCF127HG/BI battery

Ogygoptynx (Middle/Late Paleocene of Colorado, USA)

Among the Kikuyu of Kenya it was believed that owls were harbingers of death. If one saw an owl or heard its hoot, someone was going to die. In general, owls are viewed as harbingers of bad luck, ill health, or death. The belief is widespread even today.[31]

The Little Owl, 1506, by Albrecht Dürer

The Americas

In the culture of the Uto-Aztec tribe, the Hopi, taboos surround owls, which are associated with sorcery and other evilsSony VAIO VPCF11Z1E battery. The Aztecs and Maya, along with other Natives of Mesoamerica, considered the owl a symbol of death and destruction. In fact, the Aztec god of death, Mictlantecuhtli, was often depicted with owls. There is an old saying in Mexico that is still in use[32]: Cuando el tecolote canta, el indio muere ("When the owl cries/sings, the Indian dies"). The Popol VuhSony VAIO VPCF11S1E battery, a Mayan religious text, describes owls as messengers of Xibalba (the Mayan "Place of Fright").[33] The belief that owls are messengers and harbingers of the dark powers is also found among the Hočągara (Winnebago) of Wisconsin.[34] When in earlier days the Hočągara committed the sin of killing enemies while they were within the sanctuary of the chief's lodge, an owl appeared and spoke to them in the voice of a humanSony VAIO VPCF11M1E/H battery, saying, "From now on the Hočągara will have no luck." This marked the beginning of the decline of their tribe.[35] An owl appeared to Glory of the Morning, the only female chief of the Hočąk nation, and uttered her name. Soon afterwards she died.[36] People often allude to the reputation of owls as bearers of supernatural danger when they tell misbehaving children, "the owls will get you." Sony VAIO VPCF11M1E battery Also, in the native Cherokee culture, as well as many other Native American cultures, owls are a very bad omen. It is said that if you are outside in the broad day light and an owl flies over your head a family member or loved one would die within the coming week.

Middle East

In Arab mythology, owls are seen as bad omens.[38]

Hinduism

Lakshmi with the owl

In Hinduism, an owl is the vahana, mount, of Goddess Lakshmi.

Western culture

The modern West generally associates owls with wisdom. This link goes back at least as far as Ancient Greece, where Athens, noted for art and scholarship, and Athena, Athens' patron goddess and the goddess of wisdom, had the owl as a symbol. Sony VAIO VPCF11JFX/B battery Marija Gimbutas traces veneration of the owl as a goddess, among other birds, to the culture of Old Europe, long pre-dating Indo-European cultures.[40]

T. F. Thiselton-Dyer in his Folk-lore of Shakespeare says that "from the earliest period it has been considered a bird of ill-omen, and Pliny tells us how, on one occasion, even Rome itself underwent a lustration, because one of them strayed into the Capitol. He represents it also as a funereal bird, Sony VAIO VPCF119FJ battery a monster of the night, the very abomination of human kind. Virgil describes its death-howl from the top of the temple by night, a circumstance introduced as a precursor of Dido's death. Ovid, too, constantly speaks of this bird's presence as an evil omen; and indeed the same notions respecting it may be found among the writings of most of the ancient poets." [41] A list of "omens drear" in John Keats' Hyperion includes the "gloom-bird's hated screech." Sony VAIO VPCF119FC/BI battery

In France, where owls are divided into eared owls (hiboux) and earless owls (chouettes), the former are seen as symbols of wisdom while the latter are assigned the grimmer meaning.

Three Canadian provinces have owls as provincial symbols: the Great Horned Owl in Alberta, the Great Grey Owl in Manitoba, and the Snowy Owl in QuebecSony VAIO VPCF119FC battery.

Use as rodent control

Encouraging natural predators to control rodent population is a natural form of pest control, along with excluding food sources for rodents. Placing a new box for owls on a property can help control rodent populations (one family of hungry barn owls can consume more than 3,000 rodents in a nesting season) while maintaining the naturally balanced food chain. Sony VAIO VPCF117HG/BI battery

Owl conservation issues

Laughing Owl (Sceloglaux albifacies), last seen in 1914

All owls are listed in Appendix II of the international CITES treaty (the Convention on Illegal Trade in Endangered Species of Wild Fauna and Flora). Although owls have long been hunted, a 2008 news story from Malaysia indicates that the magnitude of owl poaching may be on the rise. In November 2008, TRAFFIC reported the seizure of 900 plucked and "oven-ready" owls in Peninsular MalaysiaSony VAIO VPCF116FG/BI battery. Said Chris Shepherd, Senior Programme Officer for TRAFFIC's Southeast Asia office, "This is the first time we know of where 'ready-prepared' owls have been seized in Malaysia, and it may mark the start of a new trend in wild meat from the region. We will be monitoring developments closely." Traffic commended the Department of Wildlife and National Parks in Malaysia for the raid that exposed the huge haul of owls. Included in the seizure were dead and plucked Barn OwlsSony VAIO VPCF115FG/B battery, Spotted Wood Owls, Crested Serpent Eagles, Barred Eagles, and Brown Wood Owls, as well as 7,000 live lizards.

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