The naked mole-rat known as the sand puppy, is a burrowing rodent native to parts of East Africa. It is related to the blesmols and is the only species in the genus Heterocephalus of the family Heterocephalidae; the naked mole-rat and the Damaraland mole-rat are the only known eusocial mammals, the highest classification of sociality. It has a unusual set of physical traits that allow it to thrive in a harsh underground environment and is the only mammalian thermoconformer entirely ectothermic in how it regulates body temperature; the naked-mole rat lacks pain sensitivity in its skin, has low metabolic and respiratory rates. While considered to belong to the same family as other African mole-rats, more recent investigation places it in a separate family, Heterocephalidae; the naked mole-rat is remarkable for its longevity and its resistance to cancer and oxygen deprivation. Typical individuals weigh 30 to 35 grams. Queens may weigh well over 50 grams, the largest reaching 80 grams, they are well-adapted to their underground existence.
Their eyes are quite small, their visual acuity is poor. Their legs are short, their large, protruding teeth are used to dig and their lips are sealed just behind the teeth, preventing soil from filling their mouths while digging. About a quarter of their musculature is used in the closing of their jaws while they dig—about the same proportion, utilized in the human leg, they wrinkled pink or yellowish skin. They lack an insulating layer in the skin; the naked mole-rat is well adapted to the limited availability of oxygen within the tunnels of its typical habitat. It has underdeveloped lungs and its hemoglobin has a high affinity for oxygen, increasing the efficiency of oxygen uptake, it has a low respiration and metabolic rate for an animal of its size, about 70% that of a mouse, thus using oxygen minimally. In response to long periods of hunger, its metabolic rate can be reduced by up to 25 percent; the naked mole-rat survives for at least 5 hours in air. It can live in an atmosphere of 20 % oxygen.
In zero-oxygen atmosphere, it can survive 18 minutes without suffering any harm. During the anoxic period it loses consciousness, its heart rate drops from about 200 to 50 beats per minute, breathing stops apart from sporadic breathing attempts; when deprived of oxygen, the animal uses fructose in its anaerobic glycolysis, producing lactic acid. This pathway is not inhibited by acidosis; as of April 2017, it was not known. The naked mole-rat does not regulate its body temperature in typical mammalian fashion, they are thermoconformers rather than thermoregulators in that, unlike other mammals, body temperature tracks ambient temperatures. However, it has been claimed that "the Naked Mole-Rat has a distinct temperature and activity rhythm, not coupled to environmental conditions." The relationship between oxygen consumption and ambient temperature switches from a typical poikilothermic pattern to a homeothermic mode when temperature is at 28 °C or higher. At lower temperatures, naked mole-rats can use behavioral thermoregulation.
For example, cold naked mole-rats huddle together or seek shallow parts of the burrows that are warmed by the sun. Conversely, when they get too hot, naked mole-rats retreat to the deeper, cooler parts of the burrows; the skin of naked mole-rats lacks neurotransmitters in their cutaneous sensory fibers. As a result, the naked mole-rats feel no pain when they are exposed to capsaicin; when they are injected with substance P, a type of neurotransmitter, the pain signaling works as it does in other mammals but only with capsaicin and not with acids. This is proposed to be an adaptation to the animal living in high levels of carbon dioxide due to poorly ventilated living spaces which would cause acid to build up in their body tissues. Naked mole-rats' substance P deficiency has been tied to their lack of the histamine-induced itching and scratching behavior typical of rodents. Naked mole-rats have a high resistance to tumours, although it is that they are not immune to related disorders. A potential mechanism that averts cancer is an "over-crowding" gene, p16, which prevents cell division once individual cells come into contact.
The cells of most mammals, including naked mole-rats, undergo contact inhibition via the gene p27 which prevents cellular reproduction at a much higher cell density than p16 does. The combination of p16 and p27 in naked mole-rat cells is a double barrier to uncontrolled cell proliferation, one of the hallmarks of cancer. In 2013, scientists reported that the reason naked mole-rats do not get cancer can be attributed to an "extremely high-molecular-mass hyaluronan", over "five times larger" than that in cancer-prone humans and cancer-susceptible laboratory animals; the scientific report was published a month as the cover story of the journal Nature. A few months the same University of Rochester research team announced that naked mole-rats have ribosomes that produce error-free proteins; because of both of these discoveries, the journal Science named the naked mole-rat "Vertebrate of the Year" for 2013. In 2016, a report w
Invertebrates are animals that neither possess nor develop a vertebral column, derived from the notochord. This includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include arthropods, mollusks and cnidarians; the majority of animal species are invertebrates. Many invertebrate taxa have a greater number and variety of species than the entire subphylum of Vertebrata; some of the so-called invertebrates, such as the Tunicata and Cephalochordata are more related to the vertebrates than to other invertebrates. This makes the invertebrates paraphyletic, so the term has little meaning in taxonomy; the word "invertebrate" comes from the Latin word vertebra, which means a joint in general, sometimes a joint from the spinal column of a vertebrate. The jointed aspect of vertebra is derived from the concept of turning, expressed in the root verto or vorto, to turn; the prefix in- means "not" or "without". The term invertebrates is not always precise among non-biologists since it does not describe a taxon in the same way that Arthropoda, Vertebrata or Manidae do.
Each of these terms describes a valid taxon, subphylum or family. "Invertebrata" is a term of convenience, not a taxon. The Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a cladogram, for example, the presence of a notochord; that would at least circumscribe the Chordata. However the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or bauplan. Despite this, the concept of invertebrates as a taxon of animals has persisted for over a century among the laity, within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata; the following text reflects earlier scientific understanding of the term and of those animals which have constituted it.
According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied body plans. Many have like jellyfish or worms. Others have outer shells like those of insects and crustaceans; the most familiar invertebrates include the Protozoa, Coelenterata, Nematoda, Echinodermata and Arthropoda. Arthropoda include insects and arachnids. By far the largest number of described invertebrate species are insects; the following table lists the number of described extant species for major invertebrate groups as estimated in the IUCN Red List of Threatened Species, 2014.3. The IUCN estimates that 66,178 extant vertebrate species have been described, which means that over 95% of the described animal species in the world are invertebrates; the trait, common to all invertebrates is the absence of a vertebral column: this creates a distinction between invertebrates and vertebrates. The distinction is one of convenience only. Being animals, invertebrates are heterotrophs, require sustenance in the form of the consumption of other organisms.
With a few exceptions, such as the Porifera, invertebrates have bodies composed of differentiated tissues. There is typically a digestive chamber with one or two openings to the exterior; the body plans of most multicellular organisms exhibit some form of symmetry, whether radial, bilateral, or spherical. A minority, exhibit no symmetry. One example of asymmetric invertebrates includes all gastropod species; this is seen in snails and sea snails, which have helical shells. Slugs appear externally symmetrical. Other gastropods develop external asymmetry, such as Glaucus atlanticus that develops asymmetrical cerata as they mature; the origin of gastropod asymmetry is a subject of scientific debate. Other examples of asymmetry are found in hermit crabs, they have one claw much larger than the other. If a male fiddler loses its large claw, it will grow another on the opposite side after moulting. Sessile animals such as sponges are asymmetrical alongside coral colonies. Neurons differ in invertebrates from mammalian cells.
Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinal leech, Hirudo medicinalis. Learning and memory using nociceptors in the sea hare, Aplysia has been described. Mollusk neurons are able to detect tissue trauma. Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs and arthropods. One type of invertebrate respi
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
A mouse, plural mice, is a small rodent characteristically having a pointed snout, small rounded ears, a body-length scaly tail and a high breeding rate. The best known mouse species is the common house mouse, it is a popular pet. In some places, certain kinds of field mice are locally common, they are known to invade homes for shelter. Species of mice are found in Rodentia, are present throughout the order. Typical mice are found in the genus Mus. Mice are distinguished from rats by their size; when someone discovers a smaller muroid rodent, its common name includes the term mouse, while if it is larger, the name includes the term rat. Common terms rat and mouse are not taxonomically specific. Scientifically, the term mouse is not confined to members of Mus for the deer mouse. Domestic mice sold as pets differ in size from the common house mouse; this is attributable both to different conditions in the wild. The best-known strain, the white lab mouse, has more uniform traits that are appropriate to its use in research.
Cats, wild dogs, birds of prey and certain kinds of arthropods have been known to prey upon mice. Because of its remarkable adaptability to any environment, the mouse is one of the most successful mammalian genera living on Earth today. Mice, in certain contexts, can be considered vermin which are a major source of crop damage, causing structural damage and spreading diseases through their parasites and feces. In North America, breathing dust that has come in contact with mouse excrement has been linked to hantavirus, which may lead to hantavirus pulmonary syndrome. Nocturnal animals, mice compensate for their poor eyesight with a keen sense of hearing, rely on their sense of smell to locate food and avoid predators. Mice build long intricate burrows in the wild; these have long entrances and are equipped with escape tunnels or routes. In at least one species, the architectural design of a burrow is a genetic trait. Order Dasyuromorphia marsupial mice, smaller species of Dasyuridae order Rodentia suborder Castorimorpha family Heteromyidae Kangaroo mouse, genus Microdipodops Pocket mouse, tribe Perognathinae Spiny pocket mouse, genus Heteromys suborder Anomaluromorpha family Anomaluridae flying mouse suborder Myomorpha family Cricetidae Brush mouse, Peromyscus boylii Florida mouse Golden mouse American Harvest mouse, genus Reithrodontomys family Muridae typical mice, the genus Mus Field mice, genus Apodemus Wood mouse, Apodemus sylvaticus Yellow-necked mouse, Apodemus flavicollis Large Mindoro forest mouse Big-eared hopping mouse Luzon montane forest mouse Forrest's mouse Pebble-mound mouse Bolam's mouse Eurasian Harvest mouse, genus Micromys Mice are common experimental animals in laboratory research of biology and psychology fields because they are mammals, because they share a high degree of homology with humans.
They are the most used mammalian model organism, more common than rats. The mouse genome has been sequenced, all mouse genes have human homologs; the mouse has 2.7 billion base pairs and 20 pairs of chromosomes. They can be manipulated in ways that are illegal with humans, although animal rights activists object. A knockout mouse is a genetically modified mouse that has had one or more of its genes made inoperable through a gene knockout. Reasons for common selection of mice are small size, inexpensive varied diet maintained, can reproduce quickly. Several generations of mice can be observed in a short time. Mice are very docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental. Mice and rats have the same organs in the same places, with the difference of size. Many people buy mice as companion pets, they can be playful and can grow used to being handled. Like pet rats, pet mice should not be left unsupervised outside as they have many natural predators, including birds, lizards and dogs.
Male mice tend to have a stronger odor than the females. However, mice are as pets they never need bathing. Well looked-after mice can make ideal pets; some common mouse care products are: Cage – Usually a hamster or gerbil cage, but a variety of special mouse cages are now available. Most should have a secure door. Food – Special pelleted and seed-based food is available. Mice can eat most rodent food Bedding – Usually made of hardwood pulp, such as aspen, sometimes from shredded, uninked paper or recycled virgin wood pulp. Using corn husk bedding is avoided because it promotes Aspergillus fungus, can grow mold once it gets wet, rough on their feet. In nature, mice are herbivores, consuming any kind of fruit or grain from plants. However, mice adapt well to urban areas and are known for eating all types of food scraps. In captivity, mice are fed commercial pelleted mouse diet; these diets are nutritionally complete. Mice are a staple in the diet of many small carnivores. Humans have eaten mice since prehistoric times and still eat them as a delicacy throughout eastern Zambia and northern Malawi, where they are a seasonal source of protein.
Mice are no longer consumed by humans elsewhere. However, in Victorian Britain, fried mice were still given to children as a folk remedy for bed-wetting. Prescribed cures in Ancient Egypt included mice as medicine. In Ancient Egypt, when infant
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. The brain is located in the head close to the sensory organs for senses such as vision; the brain is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains 14–16 billion neurons, the estimated number of neurons in the cerebellum is 55–70 billion; each neuron is connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. Physiologically, the function of the brain is to exert centralized control over the other organs of the body; the brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows coordinated responses to changes in the environment.
Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. The operations of individual brain cells are now understood in considerable detail but the way they cooperate in ensembles of millions is yet to be solved. Recent models in modern neuroscience treat the brain as a biological computer different in mechanism from an electronic computer, but similar in the sense that it acquires information from the surrounding world, stores it, processes it in a variety of ways; this article compares the properties of brains across the entire range of animal species, with the greatest attention to vertebrates. It deals with the human brain insofar; the ways in which the human brain differs from other brains are covered in the human brain article. Several topics that might be covered here are instead covered there because much more can be said about them in a human context.
The most important is brain disease and the effects of brain damage, that are covered in the human brain article. The shape and size of the brain varies between species, identifying common features is difficult. There are a number of principles of brain architecture that apply across a wide range of species; some aspects of brain structure are common to the entire range of animal species. The simplest way to gain information about brain anatomy is by visual inspection, but many more sophisticated techniques have been developed. Brain tissue in its natural state is too soft to work with, but it can be hardened by immersion in alcohol or other fixatives, sliced apart for examination of the interior. Visually, the interior of the brain consists of areas of so-called grey matter, with a dark color, separated by areas of white matter, with a lighter color. Further information can be gained by staining slices of brain tissue with a variety of chemicals that bring out areas where specific types of molecules are present in high concentrations.
It is possible to examine the microstructure of brain tissue using a microscope, to trace the pattern of connections from one brain area to another. The brains of all species are composed of two broad classes of cells: neurons and glial cells. Glial cells come in several types, perform a number of critical functions, including structural support, metabolic support and guidance of development. Neurons, are considered the most important cells in the brain; the property that makes neurons unique is their ability to send signals to specific target cells over long distances. They send these signals by means of an axon, a thin protoplasmic fiber that extends from the cell body and projects with numerous branches, to other areas, sometimes nearby, sometimes in distant parts of the brain or body; the length of an axon can be extraordinary: for example, if a pyramidal cell of the cerebral cortex were magnified so that its cell body became the size of a human body, its axon magnified, would become a cable a few centimeters in diameter, extending more than a kilometer.
These axons transmit signals in the form of electrochemical pulses called action potentials, which last less than a thousandth of a second and travel along the axon at speeds of 1–100 meters per second. Some neurons emit action potentials at rates of 10–100 per second in irregular patterns. Axons transmit signals to other neurons by means of specialized junctions called synapses. A single axon may make as many as several thousand synaptic connections with other cells; when an action potential, traveling along an axon, arrives at a synapse, it causes a chemical called a neurotransmitter to be released. The neurotransmitter binds to receptor molecules in the membrane of the target cell. Synapses are the key functional elements of the brain; the essential function of the brain is cell-to-cell communication, synapses are the points at which communication occurs. The human brain has been estimated to contain 100 trillion synapses; the functions of these synapses are diverse: some are excitatory.
The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is roughly divided into the Archaic period, Classical period, Hellenistic period, it is succeeded by medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earliest form it resembled Attic Greek and in its latest form it approaches Medieval Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects. Ancient Greek was the language of Homer and of fifth-century Athenian historians and philosophers, it has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article contains information about the Epic and Classical periods of the language. Ancient Greek was a pluricentric language, divided into many dialects; the main dialect groups are Attic and Ionic, Aeolic and Doric, many of them with several subdivisions.
Some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are several historical forms. Homeric Greek is a literary form of Archaic Greek used in the epic poems, the "Iliad" and "Odyssey", in poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects; the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period, they differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not than 1120 BCE, at the time of the Dorian invasion—and that their first appearances as precise alphabetic writing began in the 8th century BCE.
The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians; the Greeks of this period believed there were three major divisions of all Greek people—Dorians and Ionians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-west Greek is the strongest marked and earliest division, with non-west in subsets of Ionic-Attic and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Non-west is called East Greek. Arcadocypriot descended more from the Mycenaean Greek of the Bronze Age.
Boeotian had come under a strong Northwest Greek influence, can in some respects be considered a transitional dialect. Thessalian had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek, spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Most of the dialect sub-groups listed above had further subdivisions equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, Northern Peloponnesus Doric; the Lesbian dialect was Aeolic Greek. All the groups were represented by colonies beyond Greece proper as well, these colonies developed local characteristics under the influence of settlers or neighbors speaking different Greek dialects; the dialects outside the Ionic group are known from inscriptions, notable exceptions being: fragments of the works of the poet Sappho from the island of Lesbos, in Aeolian, the poems of the Boeotian poet Pindar and other lyric poets in Doric.
After the conquests of Alexander the Great in the late 4th century BCE, a new international dialect known as Koine or Common Greek developed based on Attic Greek, but with influence from other dialects. This dialect replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, spoken in the region of modern Sparta. Doric has passed down its aorist terminations into most verbs of Demotic Greek. By about the 6th century CE, the Koine had metamorphosized into Medieval Greek. Ancient Macedonian was an Indo-European language at least related to Greek, but its exact relationship is unclear because of insufficient data: a dialect of Greek; the Macedonian dialect (or l
A frog is any member of a diverse and carnivorous group of short-bodied, tailless amphibians composing the order Anura. The oldest fossil "proto-frog" appeared in the early Triassic of Madagascar, but molecular clock dating suggests their origins may extend further back to the Permian, 265 million years ago. Frogs are distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforests. There are accounting for over 85 % of extant amphibian species, they are one of the five most diverse vertebrate orders. Warty frog species tend to be called toads, but the distinction between frogs and toads is informal, not from taxonomy or evolutionary history. An adult frog has a stout body, protruding eyes, anteriorly-attached tongue, limbs folded underneath, no tail. Frogs have glandular skin, with secretions ranging from distasteful to toxic, their skin varies in colour from well-camouflaged dappled brown and green to vivid patterns of bright red or yellow and black to show toxicity and ward off predators.
Adult frogs live on dry land. Frogs lay their eggs in water; the eggs hatch into aquatic larvae called tadpoles that have internal gills. They have specialized rasping mouth parts suitable for herbivorous, omnivorous or planktivorous diets; the life cycle is completed. A few species bypass the tadpole stage. Adult frogs have a carnivorous diet consisting of small invertebrates, but omnivorous species exist and a few feed on fruit. Frog skin has a rich microbiome, important to their health. Frogs are efficient at converting what they eat into body mass, they are an important food source for predators and part of the food web dynamics of many of the world's ecosystems. The skin is semi-permeable, making them susceptible to dehydration, so they either live in moist places or have special adaptations to deal with dry habitats. Frogs produce a wide range of vocalizations in their breeding season, exhibit many different kinds of complex behaviours to attract mates, to fend off predators and to survive.
Frogs are valued as food by humans and have many cultural roles in literature and religion. Frog populations have declined since the 1950s. More than one third of species are considered to be threatened with extinction and over 120 are believed to have become extinct since the 1980s; the number of malformations among frogs is on the rise and an emerging fungal disease, has spread around the world. Conservation biologists are working to resolve them; the use of the common names "frog" and "toad" has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads"; the use of the term "frog" in common names refers to species that are aquatic or semi-aquatic and have smooth, moist skins. There are numerous exceptions to this rule; the European fire-bellied toad has a warty skin and prefers a watery habitat whereas the Panamanian golden frog is in the toad family Bufonidae and has a smooth skin.
The origin of the order name Anura — and its original spelling Anoures — is the Ancient Greek "alpha privative" prefix ἀν- "without", οὐρά, meaning "animal tail". It refers to the tailless character of these amphibians; the origins of the word frog are debated. The word is first attested in Old English as frogga, but the usual Old English word for the frog was frosc, it is agreed that the word frog is somehow related to this. Old English frosc remained in dialectal use in English as frosh and frosk into the nineteenth century, is paralleled in other Germanic languages, with examples in the modern languages including German Frosch, Icelandic froskur, Dutch vors; these words allow us to reconstruct a Common Germanic ancestor *froskaz. The third edition of the Oxford English Dictionary finds that the etymology of *froskaz is uncertain, but agrees with arguments that it could plausibly derive from a Proto-Indo-European base along the lines of *preu = "jump". How Old English frosc gave rise to frogga is, uncertain, as the development does not involve a regular sound-change.
Instead, it seems that there was a trend in Old English to coin nicknames for animals ending in -g, with examples—themselves all of uncertain etymology—including dog, pig and wig. Frog appears to have been adapted from frosc as part of this trend. Meanwhile, the word toad, first attested as Old English tādige, is unique to English and is of uncertain etymology, it is the basis for the word tadpole, first attested as Middle English taddepol meaning'toad-head'. About 88% of amphibian species are classified in the order Anura; these include over 7,000 species in 56 families, of which the Craugastoridae, Hylidae and Bufonidae are the richest in species. The Anura include any fossil species that fit within the anuran definition; the characteristics of anuran adults include: 9 or fewer presacral vertebrae, the presence of a urostyle formed of fused vertebrae, no tail, a long and forward-sloping ilium, shorter fore limbs than hind limbs and ulna fused and fibula fused, elongated an