The gram is a metric system unit of mass. Defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, at the temperature of melting ice". However, in a reversal of reference and defined units, a gram is now defined as one thousandth of the SI base unit, the kilogram, or 1×10−3 kg, which itself is now defined by the International Bureau of Weights and Measures, not in terms of grams, but by "the amount of electricity needed to counteract its force" The only unit symbol for gram, recognised by the International System of Units is "g" following the numeric value with a space, as in "640 g" to stand for "640 grams" in the English language; the SI does not support the use of abbreviations such as "gr", "gm" or "Gm". The word gramme was adopted by the French National Convention in its 1795 decree revising the metric system as replacing the gravet introduced in 1793, its definition remained that of the weight of a cubic centimetre of water. French gramme was taken from the Late Latin term gramma.
This word—ultimately from Greek γράμμα, "letter"—had adopted a specialised meaning in Late Antiquity of "one twenty-fourth part of an ounce", corresponding to about 1.14 modern grams. This use of the term is found in the carmen de ponderibus et mensuris composed around 400 AD. There is evidence that the Greek γράμμα was used in the same sense at around the same time, in the 4th century, survived in this sense into Medieval Greek, while the Latin term did not remain current in Medieval Latin and was recovered in Renaissance scholarship; the gram was the fundamental unit of mass in the 19th-century centimetre–gram–second system of units. The CGS system co-existed with the MKS system of units, first proposed in 1901, during much of the 20th century, but the gram has been displaced by the kilogram as the fundamental unit for mass when the MKS system was chosen for the SI base units in 1960; the gram is today the most used unit of measurement for non-liquid ingredients in cooking and grocery shopping worldwide.
Most standards and legal requirements for nutrition labels on food products require relative contents to be stated per 100 g of the product, such that the resulting figure can be read as a percentage by weight. 1 gram = 15.4323583529 grains 1 grain = 0.06479891 grams 1 avoirdupois ounce = 28.349523125 grams 1 troy ounce = 31.1034768 grams 100 grams = 3.527396195 ounces 1 gram = 5 carats 1 gram = 8.98755179×1013 joules 1 undecimogramme = 1 "eleventh-gram" = 10−11 grams in the historic quadrant–eleventh-gram–second system a.k.a. hebdometre–undecimogramme–second system 500 grams = 1 Jin in the Chinese units of measurement. 1 gram is equal to 1 small paper clip or pen cap. The Japanese 1 yen coin has a mass of one gram, lighter than the British penny, the United States cent, the Euro cent, the 5 cent Australian coins. Conversion of units Duella Gold gram Orders of magnitude Gram at Encyclopædia Britannica
In animal anatomy, the mouth known as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth; this cavity is known as the buccal cavity, from the Latin bucca. Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostomes and deuterostomes. In the first multicellular animals, there was no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as endocytosis; the particles became enclosed in vacuoles into which enzymes were secreted and digestion took place intracellularly. The digestive products were diffused into other cells; this form of digestion is used nowadays by simple organisms such as Amoeba and Paramecium and by sponges which, despite their large size, have no mouth or gut and capture their food by endocytosis.
The vast majority of other multicellular organisms have a mouth and a gut, the lining of, continuous with the epithelial cells on the surface of the body. A few animals which live parasitically had guts but have secondarily lost these structures; the original gut of multicellular organisms consisted of a simple sac with a single opening, the mouth. Many modern invertebrates have such a system, food being ingested through the mouth broken down by enzymes secreted in the gut, the resulting particles engulfed by the other cells in the gut lining. Indigestible waste is ejected through the mouth. In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut. In deuterostomes, the blastopore becomes the anus while the gut tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth while the anus formed as an opening made by the other end of the gut.
More recent research, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus. Apart from sponges and placozoans all animals have an internal gut cavity, lined with gastrodermal cells. In less advanced invertebrates such as the sea anemone, the mouth acts as an anus. Circular muscles around the mouth are able to contract in order to open or close it. A fringe of tentacles thrusts food into the cavity and it can gape enough to accommodate large prey items. Food passes first into a pharynx and digestion occurs extracellularly in the gastrovascular cavity. Annelids have simple tube-like gets and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients. Many molluscs have a radula, used to scrape microscopic particles off surfaces. In invertebrates with hard exoskeletons, various mouthparts may be involved in feeding behaviour. Insects have a range of mouthparts suited to their mode of feeding.
These include mandibles and labium and can be modified into suitable appendages for chewing, piercing and sucking. Decapods have six pairs of mouth appendages, one pair of mandibles, two pairs of maxillae and three of maxillipeds. Sea urchins have a set of five sharp calcareous plates which are used as jaws and are known as Aristotle's lantern. In vertebrates, the first part of the digestive system is the buccal cavity known as the mouth; the buccal cavity of a fish is separated from the opercular cavity by the gills. Water flows in through passes over the gills and exits via the operculum or gill slits. Nearly all fish have jaws and may seize food with them but most feed by opening their jaws, expanding their pharynx and sucking in food items; the food may be held or chewed by teeth located in the jaws, on the roof of the mouth, on the pharynx or on the gill arches. Nearly all amphibians are carnivorous as adults. Many catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth where they hold the prey with their jaws.
They swallow their food whole without much chewing. They have many small hinged pedicellate teeth, the bases of which are attached to the jaws while the crowns break off at intervals and are replaced. Most amphibians have one or two rows of teeth in both jaws but some frogs lack teeth in the lower jaw. In many amphibians there are vomerine teeth attached to the bone in the roof of the mouth; the mouths of reptiles are similar to those of mammals. The crocodilians are the only reptiles to have teeth anchored in sockets in their jaws, they are able to replace each of their 80 teeth up to 50 times during their lives. Most reptiles are either carnivorous or insectivorous but turtles are herbivorous. Lacking teeth that are suitable for efficiently chewing of their food, turtles have gastroliths in their stomach to further grind the plant material. Snakes have a flexible lower jaw, the two halves of which are not rigidly attached, numerous other joints in their skull; these modifications allow them to open their mouths wide enough to swallow their prey whole if it is wider than they are.
Birds do not have teeth, macerating their food. Their beaks have a range of sizes and shapes according to their diet and are compose
Scientific American is an American popular science magazine. Many famous scientists, including Albert Einstein, have contributed articles to it, it is the oldest continuously published monthly magazine in the United States. Scientific American was founded by inventor and publisher Rufus M. Porter in 1845 as a four-page weekly newspaper. Throughout its early years, much emphasis was placed on reports of what was going on at the U. S. Patent Office, it reported on a broad range of inventions including perpetual motion machines, an 1860 device for buoying vessels by Abraham Lincoln, the universal joint which now can be found in nearly every automobile manufactured. Current issues include a "this date in history" section, featuring excerpts from articles published 50, 100, 150 years earlier. Topics include humorous incidents, wrong-headed theories, noteworthy advances in the history of science and technology. Porter sold the publication to Alfred Ely Beach and Orson Desaix Munn a mere ten months after founding it.
Until 1948, it remained owned by Company. Under Munn's grandson, Orson Desaix Munn III, it had evolved into something of a "workbench" publication, similar to the twentieth-century incarnation of Popular Science. In the years after World War II, the magazine fell into decline. In 1948, three partners who were planning on starting a new popular science magazine, to be called The Sciences, purchased the assets of the old Scientific American instead and put its name on the designs they had created for their new magazine, thus the partners—publisher Gerard Piel, editor Dennis Flanagan, general manager Donald H. Miller, Jr.—essentially created a new magazine. Miller retired in 1979, Flanagan and Piel in 1984, when Gerard Piel's son Jonathan became president and editor. In 1986, it was sold to the Holtzbrinck group of Germany. In the fall of 2008, Scientific American was put under the control of Nature Publishing Group, a division of Holtzbrinck. Donald Miller died in December 1998, Gerard Piel in September 2004 and Dennis Flanagan in January 2005.
Mariette DiChristina is the current editor-in-chief, after John Rennie stepped down in June 2009. Scientific American published its first foreign edition in 1890, the Spanish-language La America Cientifica. Publication was suspended in 1905, another 63 years would pass before another foreign-language edition appeared: In 1968, an Italian edition, Le Scienze, was launched, a Japanese edition, Nikkei Science, followed three years later. A new Spanish edition, Investigación y Ciencia was launched in Spain in 1976, followed by a French edition, Pour la Science, in France in 1977, a German edition, Spektrum der Wissenschaft, in Germany in 1978. A Russian edition V Mire Nauki was launched in the Soviet Union in 1983, continues in the present-day Russian Federation. Kexue, a simplified Chinese edition launched in 1979, was the first Western magazine published in the People's Republic of China. Founded in Chongqing, the simplified Chinese magazine was transferred to Beijing in 2001. In 2005, a newer edition, Global Science, was published instead of Kexue, which shut down due to financial problems.
A traditional Chinese edition, known as Scientist, was introduced to Taiwan in 2002. The Hungarian edition Tudomány existed between 1984 and 1992. In 1986, an Arabic edition, Oloom Magazine, was published. In 2002, a Portuguese edition was launched in Brazil. Today, Scientific American publishes 18 foreign-language editions around the globe: Arabic, Brazilian Portuguese, Simplified Chinese, Traditional Chinese, Dutch, German, Hebrew, Japanese, Lithuanian, Romanian and Spanish. From 1902 to 1911, Scientific American supervised the publication of the Encyclopedia Americana, which during some of that period was known as The Americana, it styled itself "The Advocate of Industry and Enterprise" and "Journal of Mechanical and other Improvements". On the front page of the first issue was the engraving of "Improved Rail-Road Cars"; the masthead had a commentary as follows: Scientific American published every Thursday morning at No. 11 Spruce Street, New York, No. 16 State Street, No. 2l Arcade Philadelphia, by Rufus Porter.
Each number will be furnished with from two to five original Engravings, many of them elegant, illustrative of New Inventions, Scientific Principles, Curious Works. Improvements and Inventions; this paper is entitled to the patronage of Mechanics and Manufactures, being the only paper in America, devoted to the interest of those classes. As a family newspaper, it will convey more useful intelligence to children and young people, than five times its cost in school instruction. Another important argument in favor of this paper, is that it will be worth two dollars at the end of the year when the volume is complete, (Old volumes of the New York Mechanic, being now worth double th
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
Types of chocolate
Chocolate is a range of foods derived from cocoa, mixed with fat and finely powdered sugar to produce a solid confectionery. There are several types of chocolate, classified according to the proportion of cocoa used in a particular formulation; the use of particular name designations is sometimes subject to international governmental regulation. Some governments assign chocolate ranges of chocolate differently; the cocoa bean products from which chocolate is made are known under different names in different parts of the world. In the American chocolate industry: chocolate liquor is the ground or melted state of the nib of the cacao bean, containing equal parts cocoa butter and solids. Cocoa butter is the fatty component of the bean. Cocoa solids are the remaining nonfat part of the cocoa bean. Different forms and flavours of chocolate are produced by varying the quantities of the different ingredients. Other flavours can be obtained by varying the temperature when roasting the beans. Milk chocolate is solid chocolate made with milk added in the form of powdered milk, liquid milk, or condensed milk.
In 1875, a Swiss confectioner, Daniel Peter, developed the first solid milk chocolate using condensed milk, invented by Henri Nestlé, Peter's neighbour in Vevey. The US Government requires a 10% concentration of chocolate liquor. EU regulations specify a minimum of 25% cocoa solids. However, an agreement was reached in 2000 that allowed an exception from these regulations in the UK, Malta, where "milk chocolate" can contain only 20% cocoa solids; such chocolate is labelled as "family milk chocolate" elsewhere in the European Union. Cadbury is the leading brand of milk chocolate in the United Kingdom; the Hershey Company is the largest producer in the US. The actual Hershey process is a trade secret, but experts speculate that the milk is lipolyzed, producing butyric acid, the milk is pasteurized, stabilizing it for use; this process gives the product a particular taste, to which the US public has shown to have an affinity, to the extent that some rival manufacturers now add butyric acid to their milk chocolates.
Dark chocolate known as "plain chocolate", is produced using a higher percentage of cocoa with all fat content coming from cocoa butter instead of milk, but there are "dark milk" chocolates and many degrees of hybrids. Dark chocolate can be eaten as is, or used in cooking, for which thicker baking bars with high cocoa percentages ranging from 70% to 100%, are sold. Baking chocolate containing no added sugar may be labeled "unsweetened chocolate". Semisweet and bittersweet are terms for dark chocolate traditionally used in the United States to indicate the amount of added sugar. Bittersweet chocolate has less sugar than semisweet chocolate, but the two are interchangeable when baking. Both must contain a minimum of 35% cocoa solids. Couverture chocolate is a high-quality class of dark chocolate, containing a high percentage of cocoa solids and cocoa butter, tempered. Couverture chocolate is used by professionals for dipping, coating and garnishing. Popular brands of couverture chocolate used by pastry chefs include: Valrhona, Lindt & Sprüngli, Scharffen Berger and Guittard.
White chocolate is made of sugar and cocoa butter, without the cocoa solids. It is pale ivory colour, lacks many of the compounds found in milk and dark chocolates, it remains solid at room temperature as, below the melting point of cocoa butter. Cocoa powder is the pulverized cocoa solids left after extracting all the cocoa butter, it is used to add chocolate flavour in baking, for making chocolate drinks. There are two types of unsweetened cocoa powder: natural cocoa produced by the Broma process, with no additives, Dutch process cocoa, additionally processed with alkali to neutralize its natural acidity. Natural cocoa is light in colour and somewhat, is used in recipes that use baking soda. Dutch cocoa is milder in taste, with a darker colour, it is used for chocolate drinks such as hot chocolate due to its ease in blending with liquids. However, Dutch processing destroys most of the flavonoids present in cocoa. Raw chocolate is chocolate that has not been heated, or mixed with other ingredients.
It is sold in chocolate-growing countries, to a much lesser extent in other countries promoted as healthy. Compound chocolate is the name for a confection combining cocoa with other vegetable fat tropical fats or hydrogenated fats, as a replacement for cocoa butter, it is used for candy bar coatings. In many countries it may not be called "chocolate". Modeling chocolate is a chocolate paste made by melting chocolate and combining it with corn syrup, glucose syrup, or golden syrup, it is used by cakemakers and pâtisseries to add decoration to cakes and pastries. Ruby chocolate is a type of chocolate created by Barry Callebaut; the variety was in development from 2004, was released to the public in 2017. The chocolate type is made from the Ruby cocoa bean, resulting in a distinct red colour and a different flavour, described as "sweet yet sour". Flavours such as mint, coffee, orange, or strawberry are sometimes added to chocolate in a creamy form or in small pieces. Chocolate bars contain added ingredients such as peanuts, fruit and crisped rice.
Pieces of chocolate, in various flavours, are
Alkaloids are a class of occurring organic compounds that contain basic nitrogen atoms. This group includes some related compounds with neutral and weakly acidic properties; some synthetic compounds of similar structure may be termed alkaloids. In addition to carbon and nitrogen, alkaloids may contain oxygen, sulfur and, more other elements such as chlorine and phosphorus. Alkaloids are produced by a large variety of organisms including bacteria, fungi and animals, they can be purified from crude extracts of these organisms by acid-base extraction. Alkaloids have a wide range of pharmacological activities including antimalarial, anticancer, vasodilatory, analgesic and antihyperglycemic activities. Many have found use as starting points for drug discovery. Other alkaloids possess psychotropic and stimulant activities, have been used in entheogenic rituals or as recreational drugs. Alkaloids can be toxic too. Although alkaloids act on a diversity of metabolic systems in humans and other animals, they uniformly evoke a bitter taste.
The boundary between alkaloids and other nitrogen-containing natural compounds is not clear-cut. Compounds like amino acid peptides, nucleotides, nucleic acid and antibiotics are not called alkaloids. Natural compounds containing nitrogen in the exocyclic position are classified as amines rather than as alkaloids; some authors, consider alkaloids a special case of amines. The name "alkaloids" was introduced in 1819 by the German chemist Carl Friedrich Wilhelm Meißner, is derived from late Latin root alkali and the suffix -οειδής – "like". However, the term came into wide use only after the publication of a review article by Oscar Jacobsen in the chemical dictionary of Albert Ladenburg in the 1880s. There is no unique method of naming alkaloids. Many individual names are formed by adding the suffix "ine" to the genus name. For example, atropine is isolated from the plant Atropa belladonna. Where several alkaloids are extracted from one plant their names are distinguished by variations in the suffix: "idine", "anine", "aline", "inine" etc.
There are at least 86 alkaloids whose names contain the root "vin" because they are extracted from vinca plants such as Vinca rosea. Alkaloid-containing plants have been used by humans since ancient times for therapeutic and recreational purposes. For example, medicinal plants have been known in the Mesopotamia at least around 2000 BC; the Odyssey of Homer referred to a gift given to Helen by the Egyptian queen, a drug bringing oblivion. It is believed. A Chinese book on houseplants written in 1st–3rd centuries BC mentioned a medical use of Ephedra and opium poppies. Coca leaves have been used by South American Indians since ancient times. Extracts from plants containing toxic alkaloids, such as aconitine and tubocurarine, were used since antiquity for poisoning arrows. Studies of alkaloids began in the 19th century. In 1804, the German chemist Friedrich Sertürner isolated from opium a "soporific principle", which he called "morphium" in honor of Morpheus, the Greek god of dreams; the term "morphine", used in English and French, was given by the French physicist Joseph Louis Gay-Lussac.
A significant contribution to the chemistry of alkaloids in the early years of its development was made by the French researchers Pierre Joseph Pelletier and Joseph Bienaimé Caventou, who discovered quinine and strychnine. Several other alkaloids were discovered around that time, including xanthine, caffeine, nicotine, colchicine and cocaine; the development of the chemistry of alkaloids was accelerated by the emergence of spectroscopic and chromatographic methods in the 20th century, so that by 2008 more than 12,000 alkaloids had been identified. The first complete synthesis of an alkaloid was achieved in 1886 by the German chemist Albert Ladenburg, he produced coniine by reacting 2-methylpyridine with acetaldehyde and reducing the resulting 2-propenyl pyridine with sodium. Compared with most other classes of natural compounds, alkaloids are characterized by a great structural diversity. There is no uniform classification; when knowledge of chemical structures was lacking, botanical classification of the source plants was relied on.
This classification is now considered obsolete. More recent classifications are based on similarity of the carbon biochemical precursor. However, they require compromises in borderline cases. Alkaloids are divided into the following major groups: "True alkaloids" contain nitrogen in the heterocycle and originate from amino acids, their characteristic examples are atropine and morphine. This group a
Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus Rattus. Other rat genera include Neotoma and Dipodomys. Rats are distinguished from mice by their size; when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonomically specific. In other words, rat is not a scientific term; the best-known rat species are the brown rat. This group known as the Old World rats or true rats, originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but weigh over 500 grams in the wild; the term rat is used in the names of other small mammals that are not true rats. Examples include the North American pack rats, a number of species loosely called kangaroo rats, others. Rats such as the bandicoot rat are murine rodents related to true rats but are not members of the genus Rattus.
Male rats are called bucks. A group of rats is referred to as a mischief; the common species are opportunistic survivors and live with and near humans. They may cause substantial food losses in developing countries. However, the distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat. Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, Campylobacter; the Black Death is traditionally believed to have been caused by the microorganism Yersinia pestis, carried by the tropical rat flea, which preyed on black rats living in European cities during the epidemic outbreaks of the Middle Ages. Another zoonotic disease linked to the rat is foot-and-mouth disease. Rats become sexually reach social maturity at about 5 to 6 months of age; the average lifespan of rats varies by species.
The black and brown rats diverged from other Old World rats in the forests of Asia during the beginning of the Pleistocene. The characteristic long tail of most rodents is a feature, extensively studied in various rat species models, which suggest three primary functions of this structure: thermoregulation, minor proprioception, a nocifensive-mediated degloving response. Rodent tails—particularly in rat models—have been implicated with a thermoregulation function that follows from its anatomical construction; this particular tail morphology is evident across the family Muridae, in contrast to the bushier tails of Sciuridae, the squirrel family. The tail is hairless and thin skinned but vascularized, thus allowing for efficient countercurrent heat exchange with the environment; the high muscular and connective tissue densities of the tail, along with ample muscle attachment sites along its plentiful caudal vertebrae, facilitate specific proprioceptive senses to help orient the rodent in a three-dimensional environment.
Lastly, murids have evolved a unique defense mechanism termed degloving that allows for escape from predation through the loss of the outermost integumentary layer on the tail. However, this mechanism is associated with multiple pathologies that have been the subject of investigation. Multiple studies have explored the thermoregulatory capacity of rodent tails by subjecting test organisms to varying levels of physical activity and quantifying heat conduction via the animals' tails. One study demonstrated a significant disparity in heat dissipation from a rat's tail relative to its abdomen; this observation was attributed to the higher proportion of vascularity in the tail, as well as its higher surface-area-to-volume ratio, which directly relates to heat's ability to dissipate via the skin. These findings were confirmed in a separate study analyzing the relationships of heat storage and mechanical efficiency in rodents that exercise in warm environments. In this study, the tail was a focal point in measuring heat modulation.
On the other hand, the tail's ability to function as a proprioceptive sensor and modulator has been investigated. As aforementioned, the tail demonstrates a high degree of muscularization and subsequent innervation that ostensibly collaborate in orienting the organism; this is accomplished by coordinated flexion and extension of tail muscles to produce slight shifts in the organism's center of mass, etc. which assists it with achieving a state of proprioceptive balance in its environment. Further mechanobiological investigations of the constituent tendons in the tail of the rat have identified multiple factors that influence how the organism navigates its environment with this structure. A particular example is that of a study in which the morphology of these tendons is explicated in detail. Namely, cell viability tests of tendons of the rat's tail demonstrate a higher proportion of living fibroblasts that produce the collagen for these fibers; as in humans, these tendons contain a high density of golgi tendon organs that help the animal assess stretching of muscle in situ and adjust accordingly by relaying the information to higher cortical areas associated with balance and movement.