Mammalia

Modern members of the class Mammalia are characterized by hair, mammary glands, specialized teeth, three middle ear ossicle.

 

The name refers to the fact that, unlike other mammals, they possess a cloaca.

 

Monotremes are also distinguished form all other mammals by the fact that they are oviparous

 

The infraclass Metatheria contains the marsupial mammals.

 

They are viviparous, bur have very short gestation periods.

 

A protective pouch, called the marsupium, covers the mammary glands of the female.

 

The young crawl into the marsupium after birth, where they feed and complete development.

They are usually born at an advanced stage of development, having been nourished within the uterus.

3800 species of eutherians that are classified into 17 orders.

 

Mammals are naturally distributed on all continents except antarctica and in all oceans.

 

The skin of mammal, f other vertebrates, consists of epidermal and dermal layers.

It protects from mechanical injury, invasion by microorganisms, and the sun's ultraviolet light.

 

Skin is also important in temperature regulation, sensory perception, excretion, and water regulation.

 

Hair is keratinized derivative to the epidermis of the skin and is uniquely mammalian.

 

A coat of hair, called pelage, usually consists of two kinds of hair.

 

Long guard hairs protect a dense coat of smaller, insulating underhairs.

 

Because hair is composed largely of dead cells, it must be periodically molted.

 

In some mammals molting occurs gradually and may not be noticed.

 

In others, hair loss occurs rapidly and may result in altered pelage characteristics.

 

Hair is also important for the sense of touch.

 

Mechanical displacement of a hair stimulates nerve cells associated with the hair root.

 

Guard hairs may sometimes be modified into thick shafted hairs called vibrissae.

 

Vibrissae occur around the legs, nose, mouth, and eyes of may mammals.

 

Their roots are richly innervated and very sensitive to displacement.

 

Air spaces in the hair shaft and air trapped between hair and skin provide an effective insulating layer.

 

A band of smooth muscle, called the arrector pili muscle, runs between the hair follicle and the lower epidermis.

 

When the muscle contracts, the hairs stand upright, increasing the amount of air trapped in the pelage and improving its insulating properties.

 

Arrector pili muscles are under the control of the autonomic nervous system, which also controls a mammal's fight or flight response

 

Hair color depends on the amount of pigment deposited in it and the quantity of air in the hair shaft.

 

Pelage is reduced in large mammals from hot climates and is some aquatic mammals that often have fatty insulation.

 

Claws are present in all amniote classes.

 

They are used for locomotion and offensive and defensive behavior.

 

Claws are formed form accumulations of keratin that cover the terminal phalanx of the digits.

 

In some mammals, they are specialized to form nails or hooves.

 

Glands develop from the epidermis of the skin.

 

Sebaceous glands are associated with hair follicles, and their oily secretion lubricates and waterproofs the skin and hair.

 

Most mammals also possess sudoriferous glands.

 

Small sudoriferous glands release watery secretions used in evaporative cooling.

 

larger sudoriferous glands secrete a mixture of salt, urea, and water, which are converted to odorous products by microorganisms on the skin.

 

Sent or musk glands are located around the face, feet, or anus of many mammals.

 

These glands secrete pheromones, which may be involved with defense, species and sex recognition, and territorial behavior.

 

Mammary glands are functional in female mammals and are present, but nonfunctional in males.

 

The milk that they secrete contains water, carbohydrates, fat, protein, minerals, and antibodies.

 

Monotremes have mammary glands that lack nipples.

 

The glands discharge milk into depressions on the belly , where it is lapped up by the young.

 

In other mammals, glands open via nipples or teats, and the young suckle for their nourishment.

 

In reptiles, the jaw articulates at two small bones at the rear of the jaw.

 

In mammals, these bones have moved onto the middle ear, and along with the stapes, form the middle ear ossicle.

 

Jaw articulation in mammals is by a single bone of the lower jaw.

 

In mammals the secondary palate is extended posteriorly by a fold of skin, called the soft palate, which almost completely separates the nasal passages from the mouth cavity.

 

The more extensive secondary palate allows mammals to breathe while chewing.

 

In mammals the teeth are often specialized for different functions, a condition called heterodont.

 

In mammals, the teeth are set into sockets of the jaw.

 

Most mammals have two sets of teeth during their life.

 

The first teeth emerge before or shortly after birth and are called deciduous or milk teeth.

 

These teeth are lost and replaced by permanent teeth.

 

There are up to four kinds of teeth in the adult mammals.

 

Incisors are the most anterior teeth in the jaw.

 

They are usually chiselilike and used for gnawing or nipping.

 

Canines are often long, stout, and conical, and are usually used for catching, killing, and tearing prey.

 

Canines and incisors have single roots.

 

Premolars are positioned next to canines, have one or two roots, and truncated surfaces for chewing.

 

Molars have broad chewing surfaces and two or three roots.

 

Mammalian species have characteristic numbers of each kink of adult tooth.

 

A dental formula is an expression of the number to teeth of each kind in one half of the upper and lower jaws.

 

Incisors, canine, premolars, and molars.

 

Mammalian teeth may be specialized for particular diets.

 

Omnivorous; feed on a variety of plant and animal materials.

 

Mammals that eat plant material often have flat, grinding posterior teeth and incisors, and sometimes canines, that are modified for nipping plant matter

 

In rodents, the incisors grow throughout life.

 

Canines and incisors of predatory mammals are used for catching, killing, and tearing prey

 

The vertebral column of mammals is divided into five regions.

 

As with reptiles and birds, the first two cervical vertebrae are the atlas and axis.

 

These are usually followed by five other cervical vertebrae.

 

The trunk is divided into thoracic and lumbar regions.

 

The thoracic region contains the ribs

 

The articulation between the thoracic vertebrae provides the flexibility needed in turning, climbing, and lying on the side to suckle young.

 

Lumber vertebrae have interlocking processes that give support, but little freedom of movement.

 

The digestive tract of mammals is similar to that of other vertebrates.

 

There are, however, many specializations for different feeding habits.

 

It is difficult to make accurate generalizations regarding the feeding habits of mammals.

 

Carnivores.

 

omnivores

 

insectivores

 

herbivores

 

Specializations in the digestive tract of most herbivores reflect the difficulty of digesting food rich in cellulose.

 

Horses, rabbits, and many rodents have an enlarged cecum at the junction of large and small intestines.

 

A cecum serves as a tn pouch where microorganisms aid in the digestion of cellulose.

 

 

Sheep, cattle, and deer are called ruminants.

 

Their stomachs are modified into four chambers.

 

The first three chambers are storage and fermentation chambers and contain microorganisms that synthesize a cellulose digesting enzyme cellulase.

 

Gases produced by fermentation are periodically belched, and some plant matter (cud) is regurgitated and rechewed.

 

Other microorganisms convert nitrogenous compounds in the food into new proteins.

 

The hearts of birds and mammals are superficially similar.

 

Both are four chambered pumps that keep blood in the systemic and pulmonary circuits separate and both evolved from the hearts of ancient reptiles.

 

One of the most important adaptations in the circulatory system of eutherian mammals concerns the distribution of respiratory gases and nutrients in the fetus.

 

Exchanges between maternal and fetal blood occur across the Placenta.

 

Although there is intimate association between maternal and fetal blood vessels, no actual mixing of blood occurs.

 

Nutrients, gases, and wastes simply diffuse between fetal and maternal blood supplies.

 

Blood entering the right atrium of the fetus is returning from the placenta and is highly oxygenated.

 

Because fetal lungs are not inflated, resistance to blood flow through the pulmonary arteries is high.

 

Therefore, most of the blood entering the right atrium bypasses the right ventricle and passes instead into the left atrium through a valved opening between the atria (foramen ovale.

 

At birth, the placenta is lost, and the lungs are inflated.

 

Resistance to blood flow through the lungs is reduced, and blood flow to them increases.

 

Flow through the ductus arteriosus decreases, and the vessel is gradually reduced to a ligament.

 

Blood flow back to the left atrium from the lungs correspondingly increases, and the valve of foramen ovale closes and gradually fuses with the tissue separating the right and left atria.

 

High metabolic rates are accompanied by adaptations for efficient gas exchange.

 

the separate nasal and oral cavities and lengthening of the snout of most mammals provide an increased surface area for warming and moistening inspired air.

 

Mammalian lungs resemble a highly vascular sponge, rather than the saclike structures of amphibians and a few reptiles.

 

the lungs, like those of reptiles, are inflated using a negative pressure mechanism.

 

Unlike reptiles and birds, however, mammals possess a muscular diaphragm that separates thoracic and abdominal cavities.

 

Inspiration results from contraction of the diaphragm and expansion of the rib cage, both of which decrease the intrathoracic pressure and allow air to enter the lungs.

 

Expiration is by elastic recoil of the lungs.

 

Forceful exhalation can be accomplished by contraction of thoracic and abdominal muscles.

 

Heat producing mechanisms of mammals are divided into two categories.

 

Shivering thermogenesis is muscular activity that results in the generation of large amounts of heat, but little movement.

 

Nonshivering thermogenesis involves heat production by general cellular metabolism, and the metabolism of special fat deposits called brown fat.

 

Heat production is effective in thermoregulation because mammals are insulated by their pelage and or fat deposits.

 

Fat deposits are also sources of energy to sustain high metabolic rates.

 

Countercurrent heat exchange systems may help regulate heat loss from exposed areas.

 

Arteries passing peripherally through the core of an appendage are surrounded by veins that carry blood back toward the body.

 

When blood returns to the body through these veins, heat is transferred from arterial blood to venous blood and returned to the body rather than lost to the environment.

 

When excess heat is produced, blood is shunted away from the countercurrent veins toward peripheral vessels, and excess heat is radiated to the environment.

 

Heat can be radiated into the air from vessels near the surface of the skin or lost by evaporative cooling from either sweat glands or respiratory surfaces during panting.

 

Mammals react in various ways to environmental extremes

 

migrate - retreat to burrows under the snow where they become less active, but are still relatively alert and easily aroused - a condition called winter sleep.

 

Hibernation ia a period of winter inactivity in which the hypothalamus of the brain slows the metabolic, heart, and respiratory rates.

 

True hibernators include the monotremes, insectivora, rodentia, chiroptera.

 

In preparation for hibernation, mammals usually accumulate large quantities of body fat.

 

After retreating to a burrow or a nest, the hypothalamus sets the body's thermostat to about 2C.

 

respiratory rate of a hibernating ground squirrel falls from 100 to 200 breaths per minute to about 4 breaths per minute.

 

The heart rate falls from 200 to 300 beats per minute to about 20 beats per minute.

 

During hibernation, a mammal may lose 1/3 to 1/2 of its body weight.

 

Arousal from hibernation occurs by metabolic heating, frequently using brown fat deposits, and it takes several hours to raise body temperature to near 37 C

 

Enlargement of the cerebral hemispheres and the cerebellum of mammals.

 

The enlargement of the cerebral cortex is accompanied by most integrative functions being shifted to this region.

 

the sense of touch is well developed

 

olfaction

 

Auditory

 

More recent adaptations include an ear flap (pinna) and the auditory tube leading to the tympanum that directs sounds to the middle ear.

 

the sensory patch of the inner ear that contains receptors for sound is long and coiled and is called the cochlea.

 

this structure provides more surface area for receptor cells and allows mammals greater sensitivity to pitch and volume than is present in reptiles.

 

Vision is an important sense in may mammals, and the structure of the eye is similar to that described for other vertebrates.

 

Accommodation occurs by changing the shape of the lens.

 

Color vision os less well developed in mammals than in reptiles and birds.

 

Dominated by rods supports the hypothesis that early mammals were nocturnal.

 

Mammals like all amniotes, have a metanephric kidney.

 

Mammals excrete urea.

 

urea is less toxic than ammonia and does not require large quantities of water in its excretion.

 

Unlike uric aced, however, urea is highly water soluble and cannot be excreted in a semisolid form; thus, some water is lost.

 

Excretion in mammals is always a major route for water loss.

 

In the nephron of the kidney, fluids and small solutes are filtered from the blood through the walls of a group of capillary like vessels, called the glomerulus.

 

the remainder of the nephron consists of tubules that reabsorb water and essential solutes and secrete particular ions into the filtrate.

 

The primary adaptation of the mammalian nephron is a portion of the tubule system called the loop of the nephron.

 

The transport process in this loop and the remainder of the tubule system allow mammals to produce urine that is 2 to 22 time more concentrated than blood.

 

Water loss varies greatly depending on activity, physiological state, and environmental temperature.

 

Respiratory water loss is minimized by condensation as warm air in the respiratory passages encounters the cooler nasal passages.

 

Excretory water loss is minimized by a diet low in protein, which reduces the production of urea.

 

Mammals have complex behaviors that enhance survival.

 

Visual cues are often used in communication.

 

the bristled fur arched back and open mouth

 

tail wagging

 

Pheromones are used to recognize members of the same species, members of the opposite sex and the reproductive state of a member of the opposite sex.

 

Pheromones may also induce sexual behavior, help establish and recognize territories,a nd ward off predators.

 

Auditory and tactile communication are also important in the lives of mammals.

 

Herd animals are kept together and remain calm as long as the array of familiar sounds.

 

Vocalizations and tactile communication are important in primate social interactions.

 

In no other group of animals has viviparity developed to the extent it has in mammals.

 

It requires a large expenditure of energy on the part of the female during development and on the part of one or both parents caring for young after they are born.

 

 

Viviparity is advantageous because females are not necessarily tied to a single nest site, but can roam or migrate to find food or a proper climate.

 

Viviparity is accompanied by the evolution of a portion of the reproductive tract where the young are nourished and develop.

 

In viviparous mammals, the oviducts are modified into one or two uteri.

 

Most mammals have a definite time or times during the year in which ova mature and are capable of being fertilized.

 

Reproduction usually occurs when climatic conditions and resource characteristics favor successful development.

 

Most female mammals undergo an estrus cycle, which includes a time during which the female is behaviorally and physiologically receptive to the male.

 

During the estrus cycle, hormonal changes stimulate the maturation of ova in the ovary and induce ovulation.

 

A few mammals (rabbits, ferrets, and mink) are induce ovulators; ovulation is induced by coitus.

 

Hormones also mediate changes in the uterus and vagina.

 

As the ova are maturing, the inner lining of the uterus proliferates and becomes more vascular in preparation for receiving developing embryos.

 

proliferation of vaginal mucosa is accompanied by external swelling in the vaginal area and increased glandular discharge.

 

no bleeding or sloughing of uterine lining usually occurs.

 

Many mammals are monestrus and go through only a single yearly estrus cycle that is sharply seasonal.

 

Wild dogs, bears, and deer are monestrus; domestic dogs are diestrus.

 

Other mammals are polyestrus.

 

Rats and mice have estrus cycles that are repeated every 4 to 6 days.

 

The menstrual cycle of female humans, apes, and monkeys is similar to the estrus cycle in that it results in a periodic proliferation of the inner lining of the uterus and is correlated with the maturation of an ovum.

 

If fertilization does not occur before the end of the cycle, menses - the sloughing of the uterine lining occurs.

 

Fertilization usually occurs in the upper 1/3 of the oviduct within ours of copulation.

 

In a few mammals, fertilization may be delayed.

 

In some bats, for example, coitus occurs in autumn, but fertilization is delayed until spring.

 

Females store sperm in the uterus for periods in excess of 2 months.

Monotremes are oviparous.

 

Ova are released form the ovaries with large quantities of yolk.

 

After fertilization, shell glands in the oviduct deposit a shell around the ovum, forming an egg.

 

Female echidnas incubate eggs in a ventral pouch.

 

platypus eggs are laid in their burrows.

 

All other mammals nourish young by a placenta through at least a portion of their development.

 

nutrients are supplied from the maternal bloodstream, not yolk.

 

In marsupials, most nourishment for the fetus comes from uterine milk secreted by uterine cells.

 

The gestation period (the length of time young develop within the female reproductive tract) varies between 8 and 40 days in different species.

 

The short gestation period is a result of the inability to sustain the production of hormones that maintain the uterine lining.

 

After birth, tiny young crawl into the marsupium, and attach to a nipple, where they suckle for an additional 60 to 270 days.

 

In eutherian mammals, the embryo implants deeply into the uterine wall.

 

Embryonic and uterine tissues grow rapidly and become highly folded and vascular, forming the placenta.

 

Gestation periods of eutherian mammals vary widely between 20 days and 19 months.

 

Following birth, the placenta and other tissues that surrounded the fetus in the uterus are expelled as afterbirth.

 

The newborns of many species are helpless at birth; others can walk and run shortly after birth.