Form and function
Life expectancy and Maximum life span. February 16, at 1: In adult cetaceans insulation is provided by thick subcutaneous fat deposits, or blubber, with hair limited to a few stiff vibrissae about the mouth. Many give visual anti-predator signals , as when deer and gazelle stot , honestly indicating their fit condition and their ability to escape,   or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected. Meat Stock is your answer. Would you suggest store bought, powdered collagen? Retrieved 17 November
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If fertilization and implantation do not occur, a phase termed metestrus ensues, in which the reproductive tract assumes its normal condition. Metestrus may be followed by anestrus , a nonreproductive period characterized by quiescence or involution of the reproductive tract.
On the other hand, anestrus may be followed by a brief quiescent period diestrus and another preparatory proestrus phase. Mammals that breed only once a year are termed monestrous and exhibit a long anestrus; those that breed more than once a year are termed polyestrous. In many polyestrous species the estrous cycle ceases during gestation and lactation milk production , but some rodents have a postpartum estrus and mate immediately after giving birth. The menstrual cycle of higher primates is derived from the estrous cycle but differs from estrus in that when progesterone secretion from the corpus luteum ceases, in the absence of fertilization, the uterine lining is sloughed.
Monotremes lay shelled eggs, but the ovarian cycle is similar to that of other mammals. The eggs are predominantly yolk telolecithal , like those of reptiles and birds. Young monotremes hatch in a relatively early stage of development and are dependent upon the parent altricial. They reach sexual maturity in about one year. The reproduction of marsupials differs from that of placentals in that the uterine wall is not specialized for the implantation of embryos.
The period of intrauterine development varies from about 8 to 40 days. After this period the young migrate through the vagina to attach to the teats for further development. The pouch, or marsupium , is variously structured. Many species, such as kangaroos and opossums , have a single well-developed pouch; in some phalangerids cuscuses and brush-tailed possums , the pouch is compartmented, with a single teat in each compartment.
The South American caenolestids, or rat opossums , have no marsupium. The young of most marsupials depend on maternal care through the pouch for considerable periods, 13 to 14 weeks in the North American, or Virginia opossum Didelphis virginiana.
Young koalas are carried in the pouch for nearly 8 months, kangaroos to 10 months. 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 secretions of progesterone from the corpus luteum.
The blastocyst implants in the uterine wall. Villi are embedded in the lining of the uterus. The resulting complex of embryonic and maternal tissues is a true placenta.
Placentas have been classified on the basis of the relationship between maternal and embryonic tissues. In the simplest nondeciduate placental arrangement, the chorionic villi are in contact with uterine epithelium the inner surface layer. In advanced stages of pregnancy in rabbits , even the chorionic epithelium is eroded, and the embryonic endothelium contacts the maternal blood supply.
In no case, however, is there actual exchange of blood between mother and fetus; nutrients and gases must still pass through the walls of the fetal blood vessels. The period of intrauterine development, or gestation, varies widely among eutherians, generally depending on the size of the animal but also influenced by the number of young per litter and the condition of young at birth. The gestation period of the golden hamster is about 2 weeks, whereas that of the blue whale is 11 months and that of the African elephant 21 to 22 months.
At birth the young may be well-developed and able to move about at once precocial , or they may be blind, hairless, and essentially helpless altricial. In general, precocial young are born after a relatively long gestation period and in a small litter. Hares and many large grazing mammals bear precocial offspring. Rabbits , carnivores , and most rodents bear altricial young.
After birth young mammals are nourished by milk secreted by the mammary glands of the female. The development of milk-producing tissue in the female mammae is triggered by conception , and the stimulation of suckling the newborn prompts copious lactation. In therians marsupials and placentals the glands open through specialized nipples. Milk consists of fat , protein especially casein , and lactose milk sugar , as well as vitamins and salts.
The actual composition of milk of mammals varies widely among species. The milk of whales and seals is some 12 times as rich in fats and 4 times as rich in protein as that of domestic cows but contains almost no sugar.
Milk provides an efficient energy source for the rapid growth of young mammals; the weight at birth of some marine mammals doubles in five days.
The dependence of the young mammal on its mother for nourishment has made possible a period of training. Such training permits the nongenetic transfer of information between generations.
The ability of young mammals to learn from the experience of their elders has allowed a behavioral plasticity unknown in any other group of organisms and has been a primary reason for the evolutionary success of mammals. The possibility of training is one of the factors that has made increased brain complexity a selective advantage. Increased associational potential and memory extend the possibility of learning from experience, and the individual can make adaptive behavioral responses to environmental change.
Individual response to short-term change is far more efficient than genetic response. Some types of mammals are solitary except for brief periods when the female is in estrus. Others, however, form social groups. Such groups may be reproductive or defensive, or they may serve both functions.
In those cases that have been studied in detail, a more or less strict hierarchy of dominance prevails. Within the social group , the hierarchy may be maintained through physical combat between individuals, but in many cases stereotyped patterns of behaviour evolve to displace actual combat, thereby conserving energy while maintaining the social structure see also animal behaviour , territorial behaviour , and territoriality.
A pronounced difference between sexes sexual dimorphism is frequently extreme in social mammals. In large part this is because dominant males tend to be those that are largest or best-armed. Play extends the period of maternal training and is especially important in social species, providing an opportunity to learn behaviour appropriate to the maintenance of dominance. That area covered by an individual in its general activity is frequently termed the home range.
A territory is a part of the home range defended against other members of the same species. As a generalization it may be said that territoriality is more important in the behaviour of birds than of mammals, but data for the latter are available primarily for diurnal species. This form of territorial labeling is less evident to humans than the singing or visual displays of birds.
Many mammals that do not maintain territories per se nevertheless will not permit unlimited crowding and will fight to maintain individual distance. Such mechanisms result in more economical spacing of individuals over the available habitat. Mammals may react to environmental extremes with acclimatization , compensatory behaviour, or physiological specialization.
One way for a mammal to endure stressful environmental conditions is to become dormant. Dormancy is the general term that relates to the reduced metabolic activity adopted by many organisms under conditions of environmental stress. Physiological responses to adverse conditions include torpor , hibernation in winter , and estivation in summer.
Torpor is a type of dormancy that may occur in the daily cycle or during unfavourable weather; short-term torpor is generally economical only for small mammals that can cool and warm rapidly.
The body temperature of most temperate-zone bats drops near that of the ambient air whenever the animal sleeps. The winter dormancy of bears at high latitudes is an analogous phenomenon and cannot be considered true hibernation. Strictly speaking, hibernation only occurs in warm-blooded vertebrates. True hibernation involves physiological regulation to minimize the expenditure of energy.
The body temperature is lowered, and breathing may be slowed to as low as 1 percent of the rate in an active individual. There is a corresponding slowing of circulation and typically a reduction in the peripheral blood supply. When the body temperature nears the freezing point, spontaneous arousal occurs, although other kinds of stimuli generally elicit only a very slow response.
In mammals that exhibit winter dormancy such as bears, skunks , and raccoons , arousal may be quite rapid. Hibernation has evidently originated independently in a number of mammalian lines, and the comparative physiology of this complex phenomenon is only now beginning to be understood.
Inactivity in response to adverse summer conditions heat , drought , lack of food is termed estivation. Estivation in some species is simply prolonged rest, usually in a favourable microhabitat ; in other species estivating mammals regulate their metabolism , although the effects are typically not as pronounced as in hibernation.
Behavioral response to adverse conditions may involve the selection or construction of a suitable microhabitat, such as the cool, moist burrows of desert rodents.
Migration is a second kind of behavioral response. The most obvious kind of mammalian migration is latitudinal. Many temperate-zone bats, for example, undertake extensive migrations, although other bat species hibernate near their summer foraging grounds in caves or other equable shelters during severe weather when insects are not available. Caribou Rangifer tarandus , or reindeer , migrate from the tundra to the forest edge in search of a suitable winter range, and a number of cetaceans whales , dolphins , and porpoises and pinnipeds walruses and seals undertake long migrations from polar waters to more temperate latitudes.
Gray whales , for example, migrate southward to calving grounds along the coasts of South Korea and Baja California from summer feeding grounds in the northern Pacific Ocean Okhotsk, Bering, and Chukchi seas. Of comparable extent is the dispersive feeding migration of the northern fur seal Callorhinus ursinus. Migrations of lesser extent include the elevational movements from mountains to valleys of some ungulates—the American elk Cervus elaphus canadensis , or wapiti, and bighorn sheep Ovis canadensis , for example—and the local migrations of certain bats from summer roosts to hibernation sites.
Most migratory patterns of mammals are part of a recurrent annual cycle, but the irruptive sudden emigrations of lemmings and snowshoe hares are largely acyclic responses to population pressure on food supplies. The structure and dynamics of a population depend, among other things, on the relative lengths of these ages, the rate of recruitment of individuals either by birth or by immigration , and the rate of emigration or death.
The reproductive potential of some rodents is well known; some mice are reproductively mature at four weeks of age, have gestation periods of three weeks or less, and may experience postpartum estrus, with the result that pregnancy and lactation may overlap.
Litter size, moreover, may average four or more, and breeding may occur throughout the year in favourable localities. The reproductive potential of a species is, of course, a theoretical maximum that is rarely met, inasmuch as, among other reasons, a given female typically does not reproduce throughout the year. Growth of a population depends on the survival of individuals to reproductive age. The absolute age at sexual maturity ranges from less than 4 weeks in some rodents to some 15 years in the African bush elephant Loxodonta africana.
Postreproductive individuals are rare in most mammalian populations. Survival through more than a single reproductive season is probably uncommon in many small mammals, such as mice and shrews.
Larger species typically have longer life spans than do smaller kinds, but some bats are known, on the basis of banding records, to live nearly 20 years. Many species show greater longevity in captivity than in the wild.
Captive echidnas are reported to have lived more than 50 years. Horses have been reported to live more than 60 years, and elephants have lived to more than Various cetaceans survive to more than 90 years of age, and research involving the dating of harpoons embedded in some Greenland right whales Balaena mysticetus , or bowheads, suggests that Greenland right whales can live years or more. Specialization in habitat preference has been accompanied by locomotor adaptations.
Terrestrial mammals have a number of modes of progression. The primitive mammalian stock walked plantigrade —that is, with the digits, bones of the midfoot, and parts of the ankle and wrist in contact with the ground.
The limbs of ambulatory mammals are typically mobile, capable of considerable rotation. Mammals modified for running are termed cursorial. The stance of cursorial species may be digitigrade the complete digits contacting the ground, as in dogs or unguligrade only tips of digits contacting the ground, as in horses. In advanced groups limb movement is forward and backward in a single plane. This mode of locomotion is typically found in mammals living in open habitats.
Jumping mammals typically have elongate, plantigrade hind feet, reduced forelimbs, and long tails. Convergent evolution within a given adaptive mode has contributed to the ecological similarity of regional mammalian faunas. Bats are the only truly flying mammals. Only with active flight have the resources of the aerial habitat been successfully exploited. Mammals belonging to other groups colugos , marsupials , rodents are adapted for gliding.
A gliding habit is frequently accompanied by scansorial climbing locomotion. Many nongliders, such as tree squirrels , are also scansorial. Well-adapted arboreal mammals frequently are plantigrade, five-toed, and equipped with highly mobile limbs. Some species, including many New World monkeys , have a prehensile tail , which is used like a fifth hand. The primitive opposable anthropoid thumb is reduced as a specialization for this method of locomotion. Tarsiers are highly arboreal primates that have expanded pads on the digits to improve grasping, whereas many other arboreal mammals have claws or well-developed nails.
Several mammalian groups sirenians , cetaceans , and pinnipeds have independently assumed fully aquatic habits. In some cases semiaquatic mammals are relatively unmodified representatives of otherwise terrestrial groups otters , muskrats , and water shrews , for example. Other kinds have undergone profound modification for natatorial swimming locomotion for life at sea. Pinniped carnivores walruses and seals give birth to their young on land, but cetaceans are completely helpless out of water, on which they depend for mechanical support and thermal insulation.
The earliest mammals, like their reptilian ancestors, were active predators. From such a basal stock there has been a complex diversification radiation of trophic level adaptations. Modern mammals occupy a wide spectrum of feeding niches. In most terrestrial and some aquatic communities, carnivorous mammals are the top predators. Herbaceous mammals often serve as primary consumers in most ecosystems. The voracious shrews , smallest of mammals, sometimes prey on vertebrates larger than themselves.
They may eat twice their weight in food each day to maintain their active metabolism and compensate for heat loss caused by an unfavourable surface-to-volume ratio. On the other hand, the largest of vertebrates, the blue whale Balaenoptera musculus , feeds on minute planktonic crustaceans called krill. Within a given lineage, the adaptive radiation of food habits may be broad. These are arranged in a single row on each Jaw.
Sensory papillae are present. Tongue is narrow triangular and fleshy. Its surface is covered with horny material and bears thorn-like projections which carry taste buds and mucous glands. Tongue is highly specialized, fleshy and muscular and can be moved in different directions. It can be protruded out. Its surface is rugose being covered with numerous papillae along with taste buds.
A pair of internal nostrils open into the roof of the buccal cavity anteriorly. Hard palate is present. A bony palate is wanting in birds but a pair of palatal folds and palatal groove between the two folds are present. Internal nostrils are located dorsal to the palatal folds. The nasal passages are separated from the buccal cavity by a bony palate. The internal nostrils open into the pharynx nearer to glottis. A bony palate is present covering the roof of the buccal cavity.
A bony palate is absent. But soft palate is formed of two membranous folds. The palate is differentiated into anterior bony hard palate and a soft palate is formed of connective tissue. The soft palate is produced behind into a process — velum palati hanging down from the roof, which prevents the entry of food into nasal passage.
Unicellular mucous glands are present and keep the buccal cavity always wet. Unicellular mucous glands are absent in the epithelium of bucco-pharyngeal region. Uni cellular mucous glands are absent. But multi cellular serous glands are present. Salivary glands are absent. But labial glands are open at the lips which do not play any role in digestion.
Salivary glands which open into the buccal cavity are lingual, mandibular, maxillary, cricoary tenoid, palatinal and sphenopalatinal glands. The multi cellular salaivary glands are four pairs. They are Infra orbital, parotid, sublingual and sub-maxillary glands. Pharynx is marked off. On the roof of pharynx near the junction of two jaws a pair of openings is called Eustachian apertures. Pharynx is not sharply demarcated from the buccal cavity. It receives the openings of esophagus and the glottis.
Esophagus is a narrow tube and straight extends through the neck. Mucous glands are present. Oesophagus is a bng and narrow tube. It has thick walls. Mucous glands are absent. Oesophagus is a long thin walled tube. It is clearly marked off from the pharynx as well as stomach. The oesophagus is dilated into a thin-walled sac the crop. It secrete pigeon milk in both sexes and used to feed the young birds.
The wall of oesophagus is produced into the cardiac stomach to form cardiac valve. Stomach is a sac- like structure. Its anterior part is cardiac stomach and posterior part is pyloric stomach. At the end of pyloric stomach a small constriction is present. It possesses a pyloric sphincter. Stomach is divided into a glandular proventriculus and posterior muscular gizzard.
Gizzard acts like grinding apparatus. Stomach is divisible into cardiac, fundic and pyloric parts. Pyloric stomach contains pyloric valve. Intestine is differentiated into duodenum and ileum. Intestine very long and very much coiled because is a herbivorous animal