MAMMALS

From Big Medical Encyclopedia

MAMMALS (Mammalia) — a class like chordates, a subtype of vertebrata. M are characterized by abundance and a variety of skin glands (stalemate, the lacrimal, grease, lacteal — derivatives of sweat glands; the scent glands allocating twisted and polospetsifichesky smells), indumentum, development of horn educations (claws, nails, hoofs, horns etc.) are developed. Bodies of vertebrae flat with the intervertebral disks located between them. Rudimentary cervical edges grow together with vertebrae, forming cross shoots. Cervical vertebrae 7 (it is very rare 6 — 9); chest — 9 — 24 (13 is more often), lumbar — 2 — 9; the sacrum is formed by accrete vertebrae (the number varies them from 1 to 10); tail vertebrae — 3 — 46. A skull for the second time platibazalny type — with the wide basis, brain department of a skull of the big sizes.

The number of bones of a skull at M. in comparison with reptiles is reduced (there are no front and zadnelobny bones). The secondary bony palate separates an oral cavity from nasal. The mandible is formed by the only bone. The carrying-out device of a middle ear is formed by three acoustical stones. There are slozhnorazvity bone nasal sinks performing function of heating of an incoming air and serving as a skeleton of an olfactor organ.

A brain with the developed hemispheres of a neoncephalon. A small mesencephalon with chetverokholmiy. Olfactory shares of a brain are usually well developed. Cerebellum large. XI and XII couples of cranial nerves appearing at amniot are well developed only at M. Acoustic organs and sight reach the highest development. The oral device is also well developed. Existence of lips (body of capture of food) with a rich innervation is characteristic of M. M have complex tooth system. Teeth on the chemical structure (availability of enamel and dentine) most of all remind scales of cartilaginous fishes. Besides, only M. in the basis of teeth can have cement; teeth are in alveoluses and are differentiated on different types: cutters, canines, preradical (or lozhnokorenny) and radical. There are 4 couples of sialadens. Saliva possesses enzymatic and other functions. The throat is difficult developed, phonatory bands appear. Bronchial tubes are branchy, lungs are usually subdivided into shares that increases their surface. Existence of a diaphragm is characteristic that is connected with an intensification of respiratory movements. Erythrocytes of a discal form, in a mature state — nuclear-free. Heart four-chamber. Arterial and venous blood flows are separated from each other completely not only in heart, but also in vessels. Full division of arterial and venous blood flows is closely connected with formation of homoiothermy (see. Gomoyotermny animals ).

The amount of DNA on a cellular kernel at M. are on average twice more, than at birds, and 30% more, than at reptiles, but is 40% less, than at tailless amphibians. System of sex determination — men's heterogamety (gonosomes like XY at males, XX — at females). The known cases of a deviation from system XX — XY are connected by translocations of sites of autosomes on gonosomes that leads to formation of multiple gonosomes.

Homoiothermy, care of posterity and development higher nervous activity (see) allowed M. to win almost all areas of sushi and the sea: they are widespread from the North Pole (a polar bear, nek-ry Pinnipedia) to coast of Antarctica, live in the air environment (wing-handed animals) and the seas (pinniped, sirenovy, cetacea).

Fig. 1. The scheme of an origin of mammals (on E. Thenius, 1969, with changes). Fat black lines designated borders of evolutionary steps of transition from theroid reptiles (lower than the line I) to mammals (higher than the line III). Shaped lines — lack of exact paleontologic data. Percent showed a share of mammalny signs (the signs inherent to mammals) in various fossil groups (on E. Kuhn-Schnyder, 1967). At the left down — the geochronological table. Mammalny signs appear at the end of the Paleozoic at theroid reptiles as in groups of direct ancestors of mammals (Therocephala, Ictido-sauria), and in collateral groups (Oorgonopsia, Cynodontia, etc.). Groups of mammals (Multituberculata, Docodonta, Symmetrodonta, Panto-theria) arise at the end of the Triassic and Yura's beginning, creating further two infraklass (Allotheria, Prototheria) and a subclass of Theria. Now (Cenozoic) there are groups of single-pass (Monotremata), ascigerous (Marsupialia) and placental (Placentalia) of mammals.

The origin of mammals is in many respects not clear though it is indisputable that they come from the reptiles which kept still many lines of the organization of amphibians. Most closer to M. there are theroid reptiles of a subclass of Theromorpha from a superorder of teriodont. The question of where it is necessary to draw line between theroid reptiles and primitive M., is beyond a problem of an origin of M. and belongs to questions of the principles of creation of system. If to consider necessary for natural group its origin from one root, then after G. G. Simpson and other researchers of M. it is necessary to consider as combined group. If to consider M. from the point of view of the level of evolutionary development, then M. form, since upper chalk, accurately outlined group having many the general, independently acquired characters (tsvetn. fig. 1). The first signs characteristic of the organization of mammals are found in the reptiles living at a boundary of Carbon and Perm; in different groups of reptiles by the end of the Triassic it is possible to find up to 75% of the signs inherent to mammals. Process of acquisition of signs of mammals — a mammalization — began at some reptiles at the end of the Paleozoic, and evolution towards a mammalization was undergone not only by direct ancestors of mammals, but also some groups of reptiles which do not have a direct bearing on the basis of group of mammals. It is supposed that in the course of a mammalization of theroid reptiles the following sequence of events was observed: acquisition of upper olfactory sinks; development of three-tubercular structure of molars; expansion of big parencephalons; acquisition of soft lips with own muscles; development of a new joint of a mandible with a skull between a tooth and scaly bone; acquisition of the sound carrying out device of a middle ear from three acoustical stones. It is traditionally considered to be that M. arose at the end of the Triassic. Cretaceous multitubercular (Multituberculata), carried by all authors to M., as well as cloacal (their fossil finds are unknown before a pleistocene), undoubtedly have an origin, independent of viviparous M. In the upper Triassic contain 3 groups M. (Triconodonta, Docodonta, Symmetrodonta), 2 more groups are added to Jure to them (Pantotheria, Multituberculata), to the middle of chalk all of them, except for multitubercular, die out. In upper chalk not only marsupials, but also placental are known already (insectivorous and «kreodonta»). In a paleotsenoza and Eocene all other groups M were created. Since the end of the Triassic to the present existed, according to different data, 33 — 43 groups which combined 250 — 320 families about more than 3000 childbirth and 12 — 13 thousand types. Number of types of modern M. not less than 4000.

Classification

division into 2 subclasses Is accepted: Atheria about 2 infraklassam of Prototheria (group cloacal — Monotremata) and Allotheria (group multitubercular — Multituberculata, Yura — Eocene) and Theria — viviparous M., to the Crimea belong 3 infraklass: Trituberculata (groups: simmetrodonta, the upper Triassic — the lower chalk and pantoteriya, the upper Triassic — Yura), metateriya (group marsupials, upper chalk — sovr.) and placental (Eutheria-Placentalia, swept — sovr.).

Fig. 2. Some species of modern mammals. Single-pass: 1 — a duck-bill; 2 — an echidna. Marsupials: 3 — a koala bear; 4 — an ascigerous marten; 5 — an ascigerous mole; 6 — a kangaroo; 7 — an opossum; 8 — tsenolest. Insectivorous: 9 — a shrew; 10 — a muskrat; 11 — a hedgehog; 12 — tenrek; 13 — a prygunchik. Wing-handed animals: 14 — ushan; 15 — a flying fox. Sherstokryla: 16 — sherstokryl. Predatory: 17 — a civet; 18 — a sable; 19 — a wolf; 20 — an ermine; 21 — a bear; 22 — a hyena; 23 — a lynx. Primacies: 24 — tupaya; 25 — dolgopit; 26 — a lemur of a koat; 27 — a thin lory; 28 — a monkey; 29 — the tamarin; 30 — the howler; 37 — a baboon; 32 — a chimpanzee. Rodents: 33 — a porcupine; 34 — a squirrel; 35 — a blind rat-mole; 36 — a jerboa; 37 — a beaver; 38 — a mouse. Zaytseobraznye: 39 — a hare. Pinnipedia: 40 — a seal; 41 — a seal; 42 — a walrus.
Fig. 3. Some types modern mammals (continuation). Cetacea: 43 — a whale (a small fin whale); 44 — a dolphin a belobochka; 45 — the Gangetic dolphin. Pangolins: 46 — a pangolin. Nepolnozubye: 47 — a wood anteater; 48 — nine-poyasny a battleship; 49 — a three-fingered idler. Trubkozubye: 50 — trubkozub. Sirens: 51 — a manatee. Damen: 52 — a daman. Hemipterous: 53 — the African elephant. One-hoofed: 54 — a rhinoceros; 55 — a cheprachny tapir; 56 — a zebra. Mozolenogiye: 57 — a two-humped camel. Artiodactyl: 58 — a warthog; 59 — a hippopotamus; 60 — a deer; 61 — a buffalo; 62 — a Middle Asian gazelle.

The majority of M.'s types existing nowadays concerns to placental animals. They are united in the following groups: insectivorous (In-sectivora), sherstokryly (Dermoptera), wing-handed animals (Chiroptera), primacies (Primates), nepolnozuby (Edentata), pangolins (Pholidota), zaytseobrazny (Lagomorpha), rodents (Rodentia), cetacea (Cetacea), predatory (Carnivora), pinniped (Pinnipedia), trubkozuby (Tubulidentata), damen (Hyracoidea), hemipterous (Proboscidea), sirenovy (Sirenia), one-hoofed (Perissodactyla), artiodactyl (Artiodactyla) (tsvetn. fig. 2, 3). Treats placental also 12 become extinct groups.

Biological types

Universal biol, the type is characteristic of the majority of the land mammals using all 4 extremities using holes or other shelters as temporary shelters for movement (ascigerous mice, ascigerous rats, ascigerous martens, shrews, land squirrel, mice and other rodents, the majority of land predatory). At the herbivorous M. which do not have the shelters using alternately one pastures, others hoofs develop, the sizes of a body increase, gregarious instinct quite often develops, there is biol, a type of hoofed animals connected with a reduction side and strengthening of long fingers (Mara, agut among rodents, damen, hemipterous, litopterna, one-hoofed, artiodactyl). Development of dry steppes and semi-deserts leads to transition to the ricocheting type of biped run and is connected with a reduction of front extremities, a strong organogeny of hearing (a kangaroo, jerboas, kangaroo «rats», tushkanchikovidny Madagascar hamsters, kafrskiya dolgonog). Transition to a wood way of life leads to emergence of a tsepkokhvostost, opposition of the I finger (or I and II fingers) extremities the rest quite often develops (tupay, lemurs, many monkeys, wood kangaroos, couscouses). The following stage of development of upper tiers of the wood — transition to the planning flight from a tree to a tree that is connected with development of flying membranes (an ascigerous flying squirrel, sherstokryl, the real flying squirrels) or a parusovidny hair fringe of a body (a fat-bodied monkey). Development of the air space, adaptation to the waving flight, acquisition of wings is characteristic of wing-handed animals, and representatives of a suborder of bats are adapted for food by the flying insects. The semi-water way of life (getting of food in an aqueous medium at the device of shelters on coast of small reservoirs etc.) is connected with development of a nonwettable fur cover, swimming membranes or the hair fringe of paws increasing a rowing surface, development of sphincters — a zamykately nostril, adaptive reorganization of circulatory system (a shrew-kutora, a muskrat, a muskrat, a beaver, a mink, an otter, etc.). Full development of an aqueous medium leads to development of a powerful fatty layer, reduction of back extremities, transformation of lobbies (sometimes and back) extremities in flippers, to development of sphincters — a zamykately nostril, ears, to increase in oxygen capacity of blood, acquisition of other adaptive features (pinniped, sirenovy, cetacea). Transition to an underground way of life leads to development of extremities of the digging type, a reduction of eyes, a tail, loss of the direction of hairiness of a fur cover (moles, the blind rat-moles, tsokor digging battleships, etc.). Independent development of similar ecological niches leads to similarity in biol, types M. of unrelated systematic groups, to broad development of parallelizm and convergence in different systematic groups.

Practical and epidemiological value M. in human life is huge. The vast majority treats M. domestic animals (see), giving main products of meat, skin, all products of wool, a large amount of biologically active drugs. A number of diseases of domestic animals (a brucellosis, a malignant anthrax, etc.) is given to the person. At the beginning of 20 century industrial cultivation of fur-bearing animals (the American silver-black fox, the American mink, a polar fox, a nutria, etc.), to-rye along with hunting-trade types began (a sable, a squirrel, a marten, an ermine, etc.) give the ground mass of fur products. Plagues (groundhogs) can be Nek-rye from these M. keepers and sources of causative agents of rage (predatory). Sea M. are got as a source of technical fats, spermaceti, food and fodder meat, bone meal. Antler reindeer breeding (a maral in Altai, a dappled deer on D. Vostoke) gives Pantocrinum and other biologically active agents. Wild M. — important subject to trade and sports hunting. Many types of M. play an important role in the nature, can cause damage to crop production, forestry (many species of rodents, nek-ry hoofed animals), livestock production (a wolf, the run wild dogs), at the same time predators (marten, a fox, a wolf) are the important regulator of number of harmful types, play a role of natural hospital attendants, exterminating the got sick and weakened animals. M serve as the main owners of causative agents of many diseases with natural ochagovost (see). One of the reasons of infection of the person Seleznyami of wild mammals is stay in the territory of the natural centers that is especially often observed at development of uninhabited areas. Other way of infection of people is connected with synanthropic M. (a house mouse, gray and black rats) and semi-synanthropic (temporarily migrating to dwellings of the person M.). The expressed specificity of certain infections to M. of nek-ry groups is found: plague is connected with rodents, a tularemia — with * rodents and zaytseobrazny, rage — with predatory and wing-handed animals, a brucellosis — with artiodactyl, tick-borne rickettsioses — with rodents, a malignant anthrax and a tsetse-fly disease — with artiodactyl, a hay fever — with artiodactyl and rodents. For experimental work from number M. constantly growing number of types is used laboratory animals (see).

Possessing difficult organized genotype, all types of M. can be potentially used by the person as carriers of unique genes for experiments on genetic and cellular engineering, for experimental work both with the whole animal, and with his culture of fabrics. Steady reduction of number of many types of M. (elimination nek-ry under the influence of activity of the person began in a paleolith when mammoths in Eurasia were exterminated, mastodons and huge idlers in America) continued during tens of centuries. So, a tour, tarpan, a stellerova the cow, an ascigerous wolf and other types of M. were exterminated (all since the beginning of 17 century 62 types of M. are exterminated completely already). A number of types of M. disappeared not as a result of direct destruction, and as a result of economic activity of the person. In the International Red List 226 endangered species of M., and are brought in the Red List of the USSR (The book of rare animals and plants of the USSR) — 50 types of M. (for 1978). At the same time there is also a positive experience of recovery of number of endangered species (in the USSR — a saiga, a sable, a beaver, a bison) that allows to combine economic activity of the person with protection and recovery of number of M rationally.

The section of zoologiya studying M. is called teriologiya (see), or mammology. In the world there are several national teriologichesky societies (in the USSR — All-Union teriologichesky society at Academy of Sciences of the USSR), about 10 magazines are issued; since 1974 time in 4 years the International teriologichesky congresses (1974 — Moscow, 1978 are carried out — Brno), on to-rykh sections of a medical teriologiya work.



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H. H. Vorontsov.

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