MammalsPosted on May 18, 2018 - Last modified: September 8, 2018
Mammals (class Mammalia) are any member of the group of Vertebrate animals in which the young are fed milk from the mother's mammary glands. In addition to these characteristic milk glands, mammals are distinguished by several other unique characteristics. Hair is a typical feature of mammals, although in many whales it has disappeared except in the fetal stage. The lower jaw of the mammal articulates directly to the skull, rather than through a separate bone as in all other vertebrates. A chain of three tiny bones transmits sound waves through the middle ear. A muscular diaphragm separates the heart and lungs from the abdominal cavity. Only the left aortic arch remains. (In birds the right aortic arch persists; in reptiles, amphibians, and fish both arches are retained. Mature red blood cells (erythrocytes) in all mammals lack a nucleus; all other vertebrates have nucleated red blood cells.
With the exception of monotremes (a mammalian egg-laying order comprising echidnas and the platypus), all mammals are viviparous. In placental mammals (which have a placenta to facilitate the exchange of nutrients and wastes between the mother and the developing fetus), the young are transported within the mother's uterus, reaching a relatively advanced stage of development before birth. In marsupials (eg, kangaroos, opossums, and wallabies), newborns are incompletely developed at birth and continue to develop outside the uterus, attaching to the female body in the area of her mammary glands. Some marsupials have a pouch-like structure or fold, the pouch, that houses suckling young.
The Mammalia class is worldwide distributed. Mammals have been said to be more widely distributed and more adaptable than any other class of animals, with the exception of certain less complex forms such as arachnids and insects. This versatility in the exploitation of the Earth is attributed in large part to the ability of mammals to regulate body temperature and the internal environment, both in conditions of excessive heat and aridity and intense cold.
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The evolution of mammals has produced a tremendous diversity of forms and habits. The types range in size from a bat that weighs less than a gram and tiny shrews that weigh a few grams to the largest animal that has ever existed, the blue whale, which reaches a length of more than 30 meters and a weight of 180 tons. Every major habitat has been exploited by mammals that swim, fly, run, burrow, glide, or climb.
There are more than 5.500 species of living mammals, distributed in some 125 families and up to 27-29 orders (family and ordinal groupings sometimes vary between authorities). Rodents (order Rodentia) are the most numerous mammals, both in number of species and in number of individuals, and are one of the most diverse living lineages. Instead, the order Tubulidentata is represented by a single living species, the anteater. The Uranotheria (elephants and their relatives) and Perissodactyla (horses, rhinos and their relatives) are examples of orders in which much greater diversity occurred in the late Paleogene and Neogene periods (about 30 million to about 3 million years ago) that nowadays.
The greatest current diversity is observed in continental tropical regions, although members of the Mammal class live in (or in seas adjacent to) large land masses. Mammals can also be found on many oceanic islands, which are primarily, but not exclusively, inhabited by bats. The main regional fauna can be identified; These resulted in much of the evolution in the comparative isolation of the populations of the first mammals to arrive in these areas.
Feeding of mammalian animals
Mammals are very diverse in their eating habits; as a whole they eat practically anything. Proteins fuel your energy, while fat insulates you from harsh weather, and carbohydrates store energy and provide vitamins, minerals, and fiber. Mammals rarely adhere to a strict diet. Their environment often dictates what mammals will eat. If a certain food becomes scarce, they have to change their habits to survive.
Importance with humans
Throughout our own evolution, for example, humans have depended on other mammals for food and clothing. The domestication of mammals helped provide a source of protein for ever-increasing human populations, as well as providing means of transportation and heavy labor. Today, domesticated strains of the house mouse, European rabbit, guinea pig, hamster, gerbil, and other species provide much-needed laboratory subjects for the study of physiology, psychology, and a variety of related diseases. the human being, from dental caries to cancer. The study of non-human primates (monkeys and apes) has opened up vast new areas of research relevant to human well-being. The care of domestic and captive mammals is, of course, the foundation of the practice of veterinary medicine.
Mature female mammals have an interaction of hormones from the pituitary gland and the ovaries that produces a phenomenon known as the estrous cycle. Heat, or "heat," usually coincides with ovulation, and during this time the female is receptive to the male. Estrus is preceded by proestrus, during which ovarian follicles mature under the influence of an anterior pituitary follicle-stimulating hormone. Follicular cells produce estrogen, a hormone that stimulates the growth of the uterine lining or endometrium. After ovulation, in late heat, the ruptured ovarian follicle forms a temporary endocrine gland known as the corpus luteum. Another hormone, progesterone, secreted by the corpus luteum, causes the endometrium to stay still and ready for implantation of the developing egg (blastocyst), if fertilization occurs. In members of the Eutheria infraclass (placental mammals), the placenta, in addition to transmitting nutrition to the embryo, has an endocrine function, producing hormones that maintain the endometrium throughout pregnancy.
If fertilization and implantation do not occur, a phase called metestrum occurs, in which the reproductive system assumes its normal state. The mestrus can be followed by the anestrus, a non-reproductive period characterized by the inactivity or involution of the reproductive tract. On the other hand, heat may be followed by a short period of rest (diestrus) and another preparatory pro-est phase. Mammals that reproduce only once a year are called monstrous and exhibit a long anestrum; those that reproduce more than once a year are called polyesters. In many polyester species the estrous cycle ceases during gestation and lactation (milk production), but some rodents are in postpartum heat and mate immediately after giving birth.
The menstrual cycle of higher primates is derived from the estrous cycle, but differs from heat in that when progesterone secretion from the corpus luteum ceases, in the absence of fertilization, the uterine lining slips. In anthropoids other than humans, a different period of "heat" occurs around the time of ovulation.
Monotremes lay shelled eggs, but the ovarian cycle is similar to that of other mammals. The eggs are predominantly yolk (telolecitales), like those of reptiles and birds. Young monotremes hatch at a relatively early stage of development and are dependent on the parent (altricial). They reach sexual maturity in about a year.
The reproduction of marsupials differs from that of placentas in that the uterine wall is not specialized for the implantation of embryos. The period of intrauterine development varies between 8 and 40 days. After this period, the young migrate through the vagina to adhere to the nipples and develop further. The bag, or pouch, is structured in several ways. Many species, such as kangaroos and opossums, have a single, well-developed pouch; in some phalangerids (cusps and brush-tailed possums), the pouch is compartmentalized, with a single nipple in each compartment. South American caenolesids, or rat possums, do not have a pouch. The young of most marsupials depend on maternal care through the pouch for considerable periods, from 13 to 14 weeks in the North American opossum or Virginia opossum (Didelphis virginiana). Young koalas are carried in the bag for almost 8 months, kangaroos up to 10 months.
Gestation and birth
Reproductive patterns in placental mammals are diverse, but in all cases a secretory phase is present in the uterine cycle, and the endometrium is maintained by progesterone secretions from the corpus luteum. The blastocyst implants itself in the uterine wall. The villi are embedded in the lining of the uterus. The resulting complex of embryonic and maternal tissues is a true placenta. The uterine lining may shed with the fetal membranes as "postpartum" (a condition called deciduate) or it may be reabsorbed by the female (not deciduate). The placentas have been classified based on the relationship between maternal and embryonic tissues. In the simplest undecided placental arrangement, the chorionic villi are in contact with the uterine epithelium (the inner surface layer). In the "deciduous intimate" types, seen in primates, bats, insectivores, and rodents, the capillary endothelium (the layer containing tiny blood vessels) of the uterine wall breaks down, and the chorionic epithelium is in direct contact with maternal blood. In the advanced stages of pregnancy in rabbits, even the chorionic epithelium erodes, and the embryonic endothelium comes into contact with the mother's blood supply. However, in no case is there a real exchange of blood between mother and fetus; nutrients and gases must pass through the walls of the fetal blood vessels.
The period of intrauterine development, or gestation, varies widely among Euterians, usually depending on the size of the animal, but is also influenced by the number of pups per litter and the condition of the pups at birth. The gestation period of the golden hamster is about 2 weeks, while that of the blue whale is 11 months and that of the African elephant 21 to 22 months.
At birth, young may be well developed and able to move immediately (early), or they may be blind, glabrous, and essentially defenseless (altricial). In general, precocial pups are born after a relatively long gestation period and in a small litter. Hares and many large grazing mammals give birth to their young early. Rabbits, carnivores, and most rodents have altricial offspring.
After birth, young mammals feed on the milk secreted by the female's mammary glands. The development of milk-producing tissue in female breasts is triggered by conception, and the stimulation of the newborn's lactation causes copious lactation. In therians (marsupials and placentals) the glands open through specialized nipples. Newborn baby marsupials are unable to breastfeed, and milk is "pumped" to the young by the mother.
Milk consists of fat, protein (especially casein), and lactose (milk sugar), as well as vitamins and salts. The actual composition of mammalian milk varies widely between species. The milk of whales and seals is 12 times higher in fat and 4 times higher in protein than that of domestic cows, but it contains almost no sugar. Milk provides an efficient energy source for the rapid growth of young mammals; the birth weight of some marine mammals doubles in five days.
Skin and hair
Mammalian skin is made up of two layers, a superficial non-vascular epidermis and an inner layer, the dermis, or chorion. The two layers are intertwined by finger-shaped projections (dermal papillae), which consist of a sensitive vascular dermis projecting into the epidermis.
The outermost layers of the epidermis are cornified (impregnated with various hard proteins), and its cells are enucleated (they lack cell nuclei). The epidermis is made up of layered flattened cells and is the interface between the individual and the environment. Its main function is defensive, and it is cornified to resist abrasion. The surface of the skin is coated with lipids and organic salts, the so-called "acid mantle," which is believed to have antifungal and antibacterial properties. Deep in the epidermis is an electronegative (electron attractor) layer, an additional deterrent to foreign organic or ionic agents.
Most mammals have three different types of hair. The guard hairs protect the rest of the coat from abrasion and often moisture, and usually provide a characteristic color pattern. The thicker subfloor is primarily insulating and may differ in color from the guard hairs. The third common hair type is the vibrissa, or mustache, a stiff, typically elongated hair that works on tactile sensation. The hairs can be modified to form stiff feathers. The rhinoceros 'horn' is made of a fibrous keratin material derived from hair. Examples of keratinized derivatives of the integument other than hair are horns, hooves, nails, claws, and beards.
Although the main function of the skin is defensive, it has been modified in mammals to serve functions as diverse as thermoregulation and nutrition of the young. Sweat gland secretions promote cooling due to evaporation on the surface of the body, and the mammary glands are a type of apocrine gland (that is, a sweat gland associated with hair follicles.
Specialization in eating habits has led to profound dental changes. The primitive tooth of mammals had high and sharp cusps and served to tear the meat or crush chitinous material (mainly the exoskeletons of terrestrial arthropods, such as insects). Herbivores tend to have specialized cheek teeth with complex contact patterns (occlusion) and various ways to expand the crowns of the teeth and avoid the problem of wear. Omnivorous mammals, such as bears, pigs, and humans, tend to have molars with low, rounded cusps.
A good example of convergence in conjunction with dietary specialization is observed in those mammals adapted to feed on ants and termites, a specialization generally called myometophagia. Tendencies frequently associated with myometophagia include strong claws, an elongated rounded skull, an extendable worm-like tongue, marked reduction in the jaw (lower jaw), and extreme loss or simplification of teeth (dentition). This habit has led to remarkably similar morphology among animals as diverse as the echidna (a monotreme), the numbat (a marsupial), the anteater (a xenarthran), and the pangolin (a pholidotan).
The skeletal system of mammals shows a series of advances with respect to that of lower vertebrates. The mode of ossification (bone formation) of long bones is characteristic. In lower vertebrates, each long bone has a single center of ossification (the diaphysis), and the replacement of cartilage by bone proceeds from the center to the ends, which can remain cartilaginous, even in adults. In mammals, secondary centers of ossification (epiphysis) develop at the ends of the bones.
Bone growth occurs in areas of cartilage between the diaphysis and the epiphysis. The skeletal growth of mammals is called determined, since once the zone of active growth of the cartilage has been erased, the growth in length ceases. As in all bony vertebrates, of course, there is a continuous renewal of bone throughout life. The advantage of epiphyseal ossification lies in the fact that the bones have strong articular surfaces (related to the joints) before the skeleton matures. In general, the skeleton of the adult mammal has less structural cartilage than that of a reptile.
The basic structure of the spinal column is comparable in all mammals, although in many cases modifications have been made to specialized locomotor modes to obtain particular mechanical advantages.
The skull is of compound origin and complex in function. Functionally, the bones of the head can be separated into the brain box and the jaws. In general, it is the head of the animal that meets the environment. The skull protects the brain and sensory capsules (the parts of the skeleton that facilitate the senses of sight, hearing, taste, and smell), houses the teeth and tongue, and contains the entrance to the pharynx.
Thus, the head functions in sensory reception, acquisition of food, defense, respiration and (in higher groups) communication. To fulfill these functions, bony elements have been recruited from the visceral skeleton, the endochondral skeleton (the parts of the skeleton that are formed from cartilage), and the dermal skeleton of lower vertebrates.