Birds known as Aves, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, a strong yet lightweight skeleton. Birds range in size from the 5 cm bee hummingbird to the 2.75 m ostrich. They rank as the world's most numerically-successful class of tetrapods, with ten thousand living species, more than half of these being passerines, sometimes known as perching birds. Birds have wings which are less developed depending on the species. Wings, which evolved from forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in flightless birds, including ratites and diverse endemic island species of birds; the digestive and respiratory systems of birds are uniquely adapted for flight. Some bird species of aquatic environments seabirds and some waterbirds, have further evolved for swimming; the fossil record demonstrates that birds are modern feathered dinosaurs, having evolved from earlier feathered dinosaurs within the theropod group, which are traditionally placed within the saurischian dinosaurs.
The closest living relatives of birds are the crocodilians. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of powered flight, many retained primitive characteristics like toothy jaws in place of beaks, long bony tails. DNA-based evidence finds that birds diversified around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages, but birds those in the southern continents, survived this event and migrated to other parts of the world while diversifying during periods of global cooling. This makes them the sole surviving dinosaurs according to cladistics; some birds corvids and parrots, are among the most intelligent animals. Many species annually migrate great distances. Birds are social, communicating with visual signals and bird songs, participating in such social behaviours as cooperative breeding and hunting and mobbing of predators.
The vast majority of bird species are monogamous for one breeding season at a time, sometimes for years, but for life. Other species have breeding systems that are polygynous or polyandrous. Birds produce offspring by laying eggs, they are laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching; some birds, such as hens, lay eggs when not fertilised, though unfertilised eggs do not produce offspring. Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs and feathers. Songbirds and other species are popular as pets. Guano is harvested for use as a fertiliser. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.
Recreational birdwatching is an important part of the ecotourism industry. The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system in use. Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds; this was done by excluding most groups known only from fossils, assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier identified four different definitions for the same biological name "Aves", a problem. Gauthier proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below, he assigned other names to the other groups. Aves can mean all archosaurs closer to birds than to crocodiles Aves can mean those advanced archosaurs with feathers Aves can mean those feathered dinosaurs that fly Aves can mean the last common ancestor of all the living birds and all of its descendants (a "c
The cat is a small carnivorous mammal. It is the only domesticated species in the family Felidae and referred to as the domestic cat to distinguish it from wild members of the family; the cat is either a house cat, kept as a pet, or a feral cat ranging and avoiding human contact. A house cat is valued for its ability to hunt rodents. About 60 cat breeds are recognized by various cat registries. Cats are similar in anatomy to the other felid species, with a strong flexible body, quick reflexes, sharp teeth and retractable claws adapted to killing small prey, they are predators who are most active at dusk. Cats can hear sounds too faint or too high in frequency for human ears, such as those made by mice and other small animals. Compared to humans, they see better in the dark and have a better sense of smell, but poorer color vision. Cats, despite being solitary hunters, are a social species. Cat communication includes the use of vocalizations including mewing, trilling, hissing and grunting as well as cat-specific body language.
Cats communicate by secreting and perceiving pheromones. Female domestic cats can have kittens from spring to late autumn, with litter sizes ranging from two to five kittens. Domestic cats can be shown as registered pedigreed cats, a hobby known as cat fancy. Failure to control the breeding of pet cats by spaying and neutering, as well as abandonment of pets, has resulted in large numbers of feral cats worldwide, contributing to the extinction of entire bird species, evoking population control, it was long thought that cat domestication was initiated in Egypt, because cats in ancient Egypt were venerated since around 3100 BC. However, the earliest indication for the taming of an African wildcat was found in Cyprus, where a cat skeleton was excavated close by a human Neolithic grave dating to around 7500 BC. African wildcats were first domesticated in the Near East; the leopard cat was tamed independently in China around 5500 BC, though this line of domesticated cats leaves no trace in the domestic cat populations of today.
As of 2017, the domestic cat was the second-most popular pet in the U. S. by number of pets owned, with 95 million cats owned. As of 2017, it was ranked the third-most popular pet in the UK, after fish and dogs, with around 8 million being owned; the number of cats in the UK has nearly doubled since 1965. The origin of the English word cat and its counterparts in other Germanic languages, descended from Proto-Germanic *kattōn-, is controversial, it has traditionally thought to be a borrowing from Late Latin cattus,'domestic cat', from catta, compare Byzantine Greek κάττα, Portuguese and Spanish gato, French chat, Maltese qattus, Lithuanian katė, Old Church Slavonic kotъ, among others. The Late Latin word is thought to originate from an Afro-Asiatic language, but every proposed source word has presented problems. Many references refer to "Berber" kaddîska,'wildcat', Nubian kadīs as possible sources or cognates, but M. Lionel Bender suggests the Nubian term is a loan from Arabic قِطَّة qiṭṭa. Jean-Paul Savignac suggests the Latin word is from an Ancient Egyptian precursor of Coptic ϣⲁⲩ šau,'tomcat', or its feminine form suffixed with -t, but John Huehnergard says "the source was not Egyptian itself, where no analogous form is attested."
Huehnergard opines it is "equally that the forms might derive from an ancient Germanic word, imported into Latin and thence to Greek and to Syriac and Arabic". Guus Kroonen considers the word to be native to Germanic and Northern Europe, suggests that it might be borrowed from Uralic, cf. Northern Sami gáđfi,'female stoat', Hungarian hölgy,'stoat'. In any case, cat is a classic example of a word that has spread as a loanword among numerous languages and cultures: a Wanderwort. An alternative word is English puss. Attested only from the 16th century, it may have been introduced from Dutch poes or from Low German puuskatte, related to Swedish kattepus, or Norwegian pus, pusekatt. Similar forms exist in Irish puisín or puiscín; the etymology of this word is unknown, but it may have arisen from a sound used to attract a cat. A group of cats can be referred to a glaring. A male cat is called a tom or tomcat An unspayed female is called a queen in a cat-breeding context. A juvenile cat is referred to as a kitten.
In Early Modern English, the word kitten was interchangeable with the now-obsolete word catling. The male progenitor of a cat a pedigreed cat, is its sire and its mother is its dam. A pedigreed cat is one. A purebred cat is one. Many pedigreed and purebred cats are exhibited as show cats. Cats of unrecorded, mixed ancestry are referred to as domestic short-haired or domestic long-haired cats, or as random-bred, moggies, or mongrels or mutt-cats; the semi-feral cat, a outdoor cat, is not owned by any one individual, but is friendly to people and may be fed by several households. Feral cats are associated with human habitation areas, foraging for food and sometimes intermittently fed by people, but are wary of human interaction. Domesti
Owls are birds from the order Strigiformes, which includes about 200 species of solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, feathers adapted for silent flight. Exceptions include the gregarious burrowing owl. Owls hunt small mammals and other birds, although a few species specialize in hunting fish, they are found in all regions of the Earth except some remote islands. Owls are divided into two families: the true owl family and the barn-owl family, Tytonidae. Owls possess large, forward-facing eyes and ear-holes, a hawk-like beak, a flat face, a conspicuous circle of feathers, a facial disc, around each eye; the feathers making up this disc can be adjusted to focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Most birds of prey have eyes on the sides of their heads, 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 most other birds—so they must turn their entire heads to change views. As owls are farsighted, they are unable to see anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes—hairlike feathers on the beak and feet that act as "feelers", their far vision in low light, is exceptionally good. Owls can rotate their heads and necks as much as 270°. Owls have 14 neck vertebrae compared to seven in humans, they have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about 10 times the diameter of the artery, instead of about the same size as the artery as in humans. Other anastomoses between the carotid and vertebral arteries support this effect; the smallest owl—weighing as little as 31 g and measuring some 13.5 cm —is the elf owl.
Around the same diminutive length, although heavier, are the lesser known long-whiskered owlet and Tamaulipas pygmy owl. The largest owls are two sized eagle owls; the largest females of these species are 71 cm long, have 54 cm long wings, weigh 4.2 kg. Different species of owls produce different sounds; as noted above, their facial discs help owls to funnel the sound of prey to their ears. In many species, these discs are placed asymmetrically, for better directional location. Owl plumage is cryptic, although several species have facial and head markings, including face masks, ear tufts, brightly coloured irises; these markings are more common in species inhabiting open habitats, are thought to be used in signaling with other owls in low-light conditions. Sexual dimorphism is a physical difference between females of a species. Reverse sexual dimorphism, when females are larger than males, has been observed across multiple owl species; the degree of size dimorphism varies across multiple populations and species, is measured through various traits, such as wing span and body mass.
Overall, female owls tend to be larger than males. The exact explanation for this development in owls is unknown. However, several theories explain the development of sexual dimorphism in owls. One theory suggests that selection has led males to be smaller because it allows them to be efficient foragers; the ability to obtain more food is advantageous during breeding season. In some species, female owls stay at their nest with their eggs while it is the responsibility of the male to bring back food to the nest. However, if food is scarce, the male first feeds himself before feeding the female. Small birds, which are agile, are an important source of food for owls. Male burrowing owls have been observed to have longer wing chords than females, despite being smaller than females. Furthermore, owls have been observed to be the same size as their prey; this has been observed in other predatory birds, which suggests that owls with smaller bodies and long wing chords have been selected for because of the increased agility and speed that allows them to catch their prey.
Another popular theory suggests that females have not been selected to be smaller like male owls because of their sexual roles. In many species, female owls may not leave the nest. Therefore, females may have a larger mass to allow them to go for a longer period of time without starving. For example, one hypothesized sexual role is that larger females are more capable of dismembering prey and feeding it to their young, hence female owls are larger than their male counterparts. A different theory suggests that the size difference between male and females is due to sexual selection: since large females can choose their mate and may violently reject a male's sexual advances, smaller male owls that have the ability to escape unreceptive females are more to have been selected. All owls are carnivorous bi
Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents, they are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments. Species can be fossorial, or semiaquatic. Well-known rodents include mice, squirrels, prairie dogs, chinchillas, beavers, guinea pigs, hamsters and capybaras. Other animals such as rabbits and pikas, whose incisors grow continually, were once included with them, but are now considered to be in a separate order, the Lagomorpha. Nonetheless and Lagomorpha are sister groups, sharing a most recent common ancestor and forming the clade of Glires. Most rodents are small animals with robust bodies, short limbs, long tails, they use their sharp incisors to gnaw food, excavate burrows, defend themselves. Most eat seeds or other plant material, they tend to be social animals and many species live in societies with complex ways of communicating with each other.
Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of altricial young, while others are precocial at birth; the rodent fossil record dates back to the Paleocene on the supercontinent of Laurasia. Rodents diversified in the Eocene, as they spread across continents, sometimes crossing oceans. Rodents reached both South America and Madagascar from Africa and were the only terrestrial placental mammals to reach and colonize Australia. Rodents have been used as food, for clothing, as pets, as laboratory animals in research; some species, in particular, the brown rat, the black rat, the house mouse, are serious pests and spoiling food stored by humans and spreading diseases. Accidentally introduced species of rodents are considered to be invasive and have caused the extinction of numerous species, such as island birds isolated from land-based predators; the distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted incisors.
These incisors little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull; as the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a chisel. Most species have up to 22 teeth with no canines or anterior premolars. A gap, or diastema, occurs between the cheek teeth in most species; this allows rodents to suck in their cheeks or lips to shield their mouth and throat from wood shavings and other inedible material, discarding this waste from the sides of their mouths. Chinchillas and guinea pigs have a high-fiber diet. In many species, the molars are large, intricately structured, cusped or ridged. Rodent molars are well equipped to grind food into small particles; the jaw musculature is strong. The lower jaw is pulled backwards during chewing. Rodent groups differ in the arrangement of the jaw muscles and associated skull structures, both from other mammals and amongst themselves.
The Sciuromorpha, such as the eastern grey squirrel, have a large deep masseter, making them efficient at biting with the incisors. The Myomorpha, such as the brown rat, have enlarged temporalis muscles, making them able to chew powerfully with their molars; the Hystricomorpha, such as the guinea pig, have larger superficial masseter muscles and smaller deep masseter muscles than rats or squirrels making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscles may allow them to move the jaw further sideways when chewing. The cheek pouch is a specific morphological feature used for storing food and is evident in particular subgroups of rodents like kangaroo rats, hamsters and gophers which have two bags that may range from the mouth to the front of the shoulders. True mice and rats do not contain this structure but their cheeks are elastic due to a high degree of musculature and innervation in the region. While the largest species, the capybara, can weigh as much as 66 kg, most rodents weigh less than 100 g.
The smallest rodent is the Baluchistan pygmy jerboa, which averages only 4.4 cm in head and body length, with adult females weighing only 3.75 g. Rodents have wide-ranging morphologies, but have squat bodies and short limbs; the fore limbs have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility; the majority of species are plantigrade, walking on both the palms and soles of their feet, have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including quadrupedal walking, burrowing, bipedal hopping and gliding. Scaly-tailed squirrels and flying squirrels, although not related, can both glide from tree to tree using parachute-like membranes that stretch from the fore to the hind limbs; the agouti is antelope-like, being digitigrade and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and siz
The cuckoos are a family of birds, the sole taxon in the order Cuculiformes. The cuckoo family includes the common or European cuckoo, koels, couas and anis; the coucals and anis are sometimes separated as distinct families, the Centropodidae and Crotophagidae respectively. The cuckoo order Cuculiformes is one of three that make up the Otidimorphae, the other two being the turacos and the bustards; the cuckoos are medium-sized slender birds. Most species live in trees; the family has a cosmopolitan distribution, with the majority of species being tropical. Some species are migratory; the cuckoos feed on insect larvae and a variety of other animals, as well as fruit. Some species are brood parasites, laying their eggs in the nests of other species, but the majority of species raise their own young. Cuckoos have played a role in human culture for thousands of years, appearing in Greek mythology as sacred to the goddess Hera. In Europe, the cuckoo is associated with spring, with cuckoldry, for example in Shakespeare's Love's Labour's Lost.
In India, cuckoos are sacred to Kamadeva, the god of desire and longing, whereas in Japan, the cuckoo symbolises unrequited love. Cuckoos are medium-sized birds that range in size from the little bronze cuckoo, at 17 g and 15 cm, to the channel-billed cuckoo, at 630 g and 63 cm. There is little sexual dimorphism in size, but where it exists, it can be either the male or the female, larger. One of the most important distinguishing features of the family are the feet, which are zygodactyl, meaning that the two inner toes point forward and the two outer backward. There are two basic body forms, arboreal species which are slender and have short tarsi, terrestrial species which are more heavy set and have long tarsi. All species have long tails which are used for steering in terrestrial species and as a rudder during flight in the arboreal species; the wing shape varies with lifestyle, with the more migratory species like the black-billed cuckoo possessing long narrow wings capable of strong direct flight, the more terrestrial and sedentary cuckoos like the coucals and malkohas having shorter rounded wings and a more laboured gliding flight.
The subfamily Cuculinae are the brood-parasitic cuckoos of the Old World. They tend to conform to the classic shape, with long tails, short legs, long narrow wings and an arboreal lifestyle; the largest species, the channel-billed cuckoo has the most outsized bill in the family, resembling that of a hornbill. The subfamily Phaenicophaeinae are the non-parasitic cuckoos of the Old World, include the couas and ground-cuckoos, they are more terrestrial cuckoos, with strong and long legs and short rounded wings. The subfamily has brighter plumage and brightly coloured bare skin around the eye; the coucals are another terrestrial Old World subfamily of long tailed long legged and short winged cuckoos. They are large heavyset birds with the largest, the greater black coucal, being around the same size as the channel-billed cuckoo; the subfamily Coccyzinae are long tailed as well, with a number of large insular forms. The New World ground cuckoos are similar to the Asian ground-cuckoos in being long legged and terrestrial, includes the long billed roadrunner, which can reach speeds of 30 km/h when chasing prey.
The final subfamily are the atypical anis, which include the small clumsy anis and the larger guira cuckoo. The anis have smooth glossy feathers; the feathers of the cuckoos are soft, become waterlogged in heavy rain. Cuckoos sun themselves after rain, the anis hold their wings open in the manner of a vulture or cormorant while drying. There is considerable variation in the plumage exhibited by the family; some species the brood parasites have cryptic plumage, whereas others have bright and elaborate plumage. This is true of the Chrysococcyx or glossy cuckoos, which have iridescent plumage; some cuckoos have a resemblance to hawks in the genus Accipiter with barring on the underside. The young of some brood parasites are coloured so as to resemble the young of the host. For example, the Asian koels breeding in India have black offspring to resemble their crow hosts, whereas in the Australian koels the chicks are brown like the honeyeater hosts. Sexual dimorphism in plumage is uncommon in the cuckoos, being most common in the parasitic Old World species.
Cuckoo genera differ in the number of primary wing feathers as below. Coccycua, Phaenicophaeus, Piaya – 9 Cuculus – 9 or 10 Pachycoccyx, Clamator levaillantii, Centropus – 10 Microdynamis, Clamator glandarius – 11 Some coucals – 12 Scythrops novaehollandiae – 13 The cuckoos have a cosmopolitan distribution, ranging across all the world's continents except Antarctica, they are absent from the south west of South America, the far north and north west of North America, the driest areas of the Middle East and North Africa. They only occur as vagrants in the oceanic islands of the Atlantic and Indian Oceans, but one species breeds on a number of Pacific islands and another is a winter migrant across much of the Pacific. Cuculinae is the most widespread subfamily of cuckoos, is distributed across Europe, Africa and Oceania. Amongst the Phaenicophaeinae cuckoos the malkohas and Asian ground-cuckoos are restricted to southern Asia, the couas are endemic to Madagascar and the yellowbill widespread across Africa.
The coucals are distribut
Humans are the only extant members of the subtribe Hominina. Together with chimpanzees and orangutans, they are part of the family Hominidae. A terrestrial animal, humans are characterized by their erect bipedal locomotion. Early hominins—particularly the australopithecines, whose brains and anatomy are in many ways more similar to ancestral non-human apes—are less referred to as "human" than hominins of the genus Homo. Several of these hominins used fire, occupied much of Eurasia, gave rise to anatomically modern Homo sapiens in Africa about 315,000 years ago. Humans began to exhibit evidence of behavioral modernity around 50,000 years ago, in several waves of migration, they ventured out of Africa and populated most of the world; the spread of the large and increasing population of humans has profoundly affected much of the biosphere and millions of species worldwide. Advantages that explain this evolutionary success include a larger brain with a well-developed neocortex, prefrontal cortex and temporal lobes, which enable advanced abstract reasoning, problem solving and culture through social learning.
Humans use tools better than any other animal. Humans uniquely use such systems of symbolic communication as language and art to express themselves and exchange ideas, organize themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an wide variety of values, social norms, rituals, which together undergird human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, mythology, religion and numerous other fields of knowledge. Though most of human existence has been sustained by hunting and gathering in band societies many human societies transitioned to sedentary agriculture some 10,000 years ago, domesticating plants and animals, thus enabling the growth of civilization; these human societies subsequently expanded, establishing various forms of government and culture around the world, unifying people within regions to form states and empires.
The rapid advancement of scientific and medical understanding in the 19th and 20th centuries permitted the development of fuel-driven technologies and increased lifespans, causing the human population to rise exponentially. The global human population was estimated to be near 7.7 billion in 2015. In common usage, the word "human" refers to the only extant species of the genus Homo—anatomically and behaviorally modern Homo sapiens. In scientific terms, the meanings of "hominid" and "hominin" have changed during the recent decades with advances in the discovery and study of the fossil ancestors of modern humans; the clear boundary between humans and apes has blurred, resulting in now acknowledging the hominids as encompassing multiple species, Homo and close relatives since the split from chimpanzees as the only hominins. There is a distinction between anatomically modern humans and Archaic Homo sapiens, the earliest fossil members of the species; the English adjective human is a Middle English loanword from Old French humain from Latin hūmānus, the adjective form of homō "man."
The word's use as a noun dates to the 16th century. The native English term man can refer to the species as well as to human males, or individuals of either sex; the species binomial "Homo sapiens" was coined by Carl Linnaeus in his 18th-century work Systema Naturae. The generic name "Homo" is a learned 18th-century derivation from Latin homō "man," "earthly being"; the species-name "sapiens" means "wise" or "sapient". Note that the Latin word homo refers to humans of either gender, that "sapiens" is the singular form; the genus Homo evolved and diverged from other hominins in Africa, after the human clade split from the chimpanzee lineage of the hominids branch of the primates. Modern humans, defined as the species Homo sapiens or to the single extant subspecies Homo sapiens sapiens, proceeded to colonize all the continents and larger islands, arriving in Eurasia 125,000–60,000 years ago, Australia around 40,000 years ago, the Americas around 15,000 years ago, remote islands such as Hawaii, Easter Island and New Zealand between the years 300 and 1280.
The closest living relatives of humans are gorillas. With the sequencing of the human and chimpanzee genomes, current estimates of similarity between human and chimpanzee DNA sequences range between 95% and 99%. By using the technique called a molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated; the gibbons and orangutans were the first groups to split from the line leading to the h
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the