Structure and function

Introduction

Mammalian evolutionary history goes back 230 million years. The earliest mammals occupied a nocturnal niche and developed a suite of traits that allowed them to adapt to the cooler temperatures of the night. Mammals are endothermic (able to produce their own heat) and most are homeothermic (able to maintain their body temperature within a particular range). Many mammalian structures and their functions are involved with maintaining body temperature, which requires efficient generation and conservation of heat.

Modern mammals evolved from early mammal-like reptiles called therapsids, which had a mosaic of reptile and mammalian traits. In defining what a mammal is, we must utilize characteristics that are preserved in the fossil record, which are largely skeletal. When a transitional fossil species has a mammal trait it is usually accompanied by a reptile trait. Consequently, there is controversy as to when the actual mammal-reptile division occurred.

The key identifier in mammalian fossils is found in the dentary-squamosal articulation because the mammalian lower jaw (the mandible) is unique. It consists of two bones, the dentaries, which articulate directly with the cranium. This is the key criterion that defines mammals. And there is a relationship between the repitian jaw and the mammalian ear. The reptilian jaw consists of two bones, the articular and the quadrate. In mammals, these were modified to become the malleus and the incus bones of the ear, which, along with the stapes (called the columella in reptiles), form the auditory ossicles in the mammalian skull. Thus, mammals have three bones in their middle ears (malleus, incus, and stapes), and reptiles have only one (the columella).

Living mammals also have many soft anatomy traits that further define them as mammals. For example, at some stage of their lives, all mammals have hair. (However, it has been suggested that the reptilian flying pterosaurs had fur, so we must be cautious about saying that hair is exclusive to mammals). On the other hand, mammary glands are unique to mammals among the living vertebrates. Another structure unique to mammals is the respiratory diaphragm that separates the thoracic and abdominal cavities. Only mammals possess this structure as a muscle, whereas other vertebrates have either a membranous diaphragm or no diaphragm at all. The mature red blood cells of mammals are enucleated (without a nucleus), whereas the red blood cells of other vertebrates contain a nucleus. The mammalian heart differs from other vertebrates in that only the left aortic arch is developed in adult mammals. In mammal brains, the neopallium (neocortex) is elaborated and expanded compared to reptile brains. Each of these unique mammal structures are discussed in context in the rest of this entry.

Integumentary form and function

The integumentary system is composed of the skin and its accessory organs. The mammalian integument has many of the characteristics that we consider mammalian. Generally mammalian skin is thicker than the skin of other vertebrates because of its function in retarding heat and water loss. The integument consists of two major regions, the epidermis and dermis. Squamous cells are produced by a basal (or germinative) layer on the border of the epidermis and dermis. As cells are produced at the basal layer they push the cells above them toward the surface of the epidermis. As they move toward the surface the squamous cells fill with the protein keratin and produce the corneum, a tough waterproof layer of dead cells on the outermost layer of the epidermis. Epidermal cells are continuously shed and replaced as they serve as mechanical protection against environmental insults.

The dermis is mainly a supportive layer for the epidermis and binds it to underlying tissues. Blood vessels in the dermis pass near the basal layer of the epidermis and provide the cells of the avascular epidermis with nutrients. The dermis also contains muscle fibers, associated with hair follicles, and nervous tissue that provides assessment of the environment. A subcutaneous layer lies below the dermis and is a site of adipose (fat) deposition, which serves as both insulation and energy storage.

Mammals have a number of skin glands that are found in no other vertebrate. Mammals have two types of coiled, tubular sweat glands, apocrine (or sudoriferous) and eccrine. Apocrine sweat glands are usually associated with a hair follicle, and secrete the odorous component of sweat. Eccrine sweat glands secrete sweat onto the surface of the skin to remove heat through evaporative cooling. Most mammals have both these glands in the foot pads. They are more widely distributed on a few mammals, including humans. Those species with a limited distribution often use a supplementary method for cooling such as panting by dogs or immersion into cool mud or water by members of the pig family. Some small mammals such as insectivores

and small rodents, bats, and aquatic mammals, do not experience heat loading and therefore do not have sweat glands.

Most mammals also have sebaceous glands distributed widely throughout the integument. Sebaceous glands consist of specialized groups of epithelial cells that produce an oily substance, sebum, that keeps hair and skin pliable, waterproof, and soft. These glands are usually associated with hair follicles. In some marine mammals such as otters and sea lions sebum is especially important in waterproofing the pelage and keeping cold water from contacting the skin, thereby preventing heat loss.

Scent glands are odoriferous glands used for social inter-actions, territory marking, and defense. One type of secretion is a pheromone, which elicits a behavioral or physiological effect on a conspecific (member of the same species). During the breeding season it may advertise the sexual receptivity of the individual. Pheromones in the urine of some rodent species is even believed to induce estrus. Scent glands are used to delineate territory (i.e., marking). The distribution of scent glands is highly variable; they may be located on the wrists (carpal glands), throat region, muzzle, the chest (sternal glands), on the head, or the back, but most commonly scent glands are found in the urogenital area (anal glands). Among the mustelids (weasel family including skunks and minks), modified anal glands are able to squirt a smelly irritant several feet (or meters) when the animal is threatened.

Ceruminous glands are the wax-producing glands located in the skin of the ear canal. They help to prevent the tympanic membrane from drying out and losing its flexibility. Ceruminous glands are modified apocrine sweat glands.

Mammary glands (mammae) are also generally believed to be modified apocrine sweat glands, although it has been suggested that they could have been derived from sebaceous glands. Mammae secrete milk that is used to feed the newborn mammal. The mammary glands in most mammals consist of a system of ducts that culminate in a nipple or teat. The one exception is found in the monotremes (egg-laying mammals). In monotremes, the mammary glands secrete milk onto hair associated with the glands and the hatchlings suck milk from these hairs. The number and location of mammae is variable among mammal species and is related to the normal size of the litter. The fewest number of mammae is two, but up to 27 are found in some marsupials. Mammae are usually on the ventral surface of placental mammals; in the marsupials they may be located in the pouch.

Hair is often described as a unique mammalian characteristic that has no structural homologue in any other vertebrate. Its distribution varies from heavy, thick pelages (fur coats) on many mammals to just a few sensory bristles (e.g., on the snout of whales or seacows). Mammalian hair originates in the epidermis, although it grows out of a tubular follicle that protrudes into the dermis. Growth occurs by rapid replication of cells in the follicle. As the shaft pushes toward the surface the cells fill with keratin and die. Each hair is composed of an outer scaly layer called the cuticle, a middle layer of dense cells called the cortex, and (in most hair shafts) an inner layer of cuboidal cells called the medulla. Each hair is associated with a sebaceous gland and a muscle (called the arrector pili) that raises the hair. Raising hair serves as a threat signal in social interactions but also increases insulation properties. The evolution of hair is part of the suite of adaptations that enabled mammals to be active at night. It served to retard heat loss by insulating the body. There are two layers of hair that form the pelage. The dense and soft underfur functions primarily as insulation by trapping a layer of air. The coarse and longer guard hair serves to shelter the underfur, keeping it dry in aquatic mammals, and to provide coloration.

Although the primary purpose of hair is insulation it has assumed other roles in living mammals. Color in hair comes from the pigments melanin and phaedomelanin. The main function of coloration appears to be camouflage, which helps the animal blend in with its surroundings. Mammals tend to have pelage colors that match their environment. One example of this is countershading. The pelage tends to be darker on the top and sides of the animal and lighter below and underneath, which under normal lighting conditions functions to obscure the form of the animal. In addition, there are various patterns on the pelage. Patterns on predators such as a tiger's stripes (Panthera tigris) help to conceal the predator. Stripes found on prey tend to confuse predators. Eye spots located above the eyes (e.g., Masoala fork-marked lemurs [Phaner furcifer] or four-eyed possums) may divert attention from the eye, confusing predators. Such patterns are called disruptive coloration. Another functional pattern is the white rump patch of the tail in mule deer (Odocioleus hemionus), which may serve as a silent alarm signal to conspecifics. But, when the tail is lowered, a predator whose eyes are fixed on the white patch might lose sight of the deer. Coloration may also identify conspecifics in visually oriented species. Blue monkeys (Cercopithecus mitis) and red-tailed monkeys (Cercopithecus ascanius) are sympatric (live in the same geographic location) and closely related, but they normally do not mate. It is believed that their distingushing facial patterns are the reason. Color patterns may also differ within a species. Often sexual dimorphism is expressed in color differences between males and females. Infants and juveniles may have different pelage colors or patterns from adults. In monkeys—one of the most visually oriented species—pelage patterns on the face, rump, or tail are used to communicate with one another. Another function of pelage patterns is that of warning potential enemies, e.g., the white stripe found on skunks might be a signal that it is well armed and can defend itself. Color changes can occur over a mammal's lifetime because hair, like skin, is replaced over time. Most mammals have two annual molts, usually correlated with the seasons. Yet others, such as humans, have hairs that grow to a particular length and then are shed. Some populations of snowshoe hares undergo three seaonal molts: a brownish-gray summer coat, a gray autumn coat, and a white winter coat.

Hair has undergone other modifications in addition to color. The guard hairs of some species have been modified for specific functions. For example, spines (or quills) are enlarged stiff hairs that are used for defense. In North American porcupines these quills have barbs that work their way into the flesh of an attacker. Vibrissae (or whiskers) are another modification of hair. These are supplied with nerves to provide tactile (touch) sensory information. These hairs are commonly found on the muzzles of many mammals such as

cats and mice, but they can be found in other body locations as well. For example, tactile hairs may be located on the wrists, above the eyes, or on the back of the neck. These hairs allow mammals to sense objects around them when low-light conditions do not allow them to see well.

Claws, nails, and hooves

The distal ends of mammal digits possess keratinized sheaths or plates that are epidermal derivatives forming claws, nails, or hooves. Only the members of the whale and sirenia (seacows) families lack these structures. Claws are usually sharp, curved, and pointed. In many cases mammal claws are very similar to the claws found in other vertebrates. A claw consists of a dorsal plate called the unguis and a ventral plate called the subunguis. The unguis is curved both in length and width and encloses the subunguis, which is connected to the digital pad at the distal end of the digit. In addition to protection, claws assist predator species, such as lions and tigers, in holding their prey. They provide traction for some arboreal species (e.g., squirrels) when scampering on branches. Sloths have long curved claws that serve as hooks for hanging. Digging mammals, such as anteaters and moles, have long claws that help them dig.

Nails are modified claws found on the first digit of some arboreal mammals and on all the digits of some primates. Nails cover only the dorsal part of digits. The unguis (called a nail plate in human anatomy) is broad and flat, and the subunguis is vestigial. It has been suggested that nails evolved in primates to prevent rolling and provide flat support for the large pad of tactile sensory tissue found on the underside of the digit. Thus nails allow both increased tactile perception and enhanced manipulative abilities. The Callitrichidae (small monkeys found in South and Central America) have secondarily evolved claws, which are not true claws because they are derived from the laterally compressed nails of their ancestors. Nails and claws may be found on the same mammal (e.g., hyraxes).

Hooves are constructed of a prominent unguis that curves around the digit and encloses the subunguis. The well-developed unguis has lent its name to the group that have hooves: the ungulates (although this is not a true taxonomic group). For ungulates, the unguis is much harder than the subunguis and does not wear away as quickly, thus developing a sharp edge.

Horns and antlers

Horns and antlers are found in the order Artiodactyla (cattle, sheep, deer, giraffes, and their relatives). Several other types of mammals have similar head structures, but true horns, originating from the frontal bone of the skull and found only among the Bovidae (cattle, antelopes, buffalo), consist of a bony core enclosed by a tough keratinized epidermal covering or sheath. True horns are not branched, although they may be curved. Horns grow throughout the life of the animal and are used for defense, display, and intraspecies combat (e.g., contests between males for mates). A variation of the true horn is the pronghorn, found on the North American pronghorn (Antilocapra americana) in the artiodactyl family Antilocapridae. A pronghorn branches and its epidermal sheath is shed on an annual basis; the sheath on a true horn is not shed. Antlers are found among the Cervidae (deer, caribou, moose, and their relatives). Mature antlers are entirely made of bone and are branched. They develop from buds covered by integument that is richly innervated and vascularized, called velvet. As the antlers grow the velvet dies and the animal usually rubs it off on tree trunks. Antlers are used for combat between males for mates. After the breeding season they are shed and replaced by a larger pair the next year; this continues until they reach their full growth. The small bony horns of giraffes (Giraffa camelopardalis) originate from the anterior portion of the parietal bones. Because they do not arise from the frontal bones they are not considered true horns. Giraffe horns are covered by furred skin and persist throughout life. Another type of horn is found on the rhinoceroses of the order Perissodactyla, the only living mammals outside the artiodactyls with a horn. The rhinoceros horn is centered over elongated nasal bones, but it lacks a bony core. It is a solid mass composed of dermal cells interspersed with tough epidermal cells.

Body design and skeletal system

As endotherms, mammals require more energy than ectothermic animals. Consequently, many mammal traits evolved to conserve energy. This is particularly true of the mammal skeleton. Mammals differ as a group from other living quadrupedal vertebrates in that their limbs are positioned directly below the body, allowing more energy-efficient locomotion. The lateral placement of the limbs on reptiles and amphibians requires them to spend considerable energy keeping their bodies lifted off of the ground while they undulate

laterally (rather than moving straight forward as mammals do). The vertical limb placement in mammals also allows removal of size constraints so that mammals may become much larger than amphibians or reptiles. (The large Mesozoic reptiles actually had limbs placed under the body also.) Another difference in the mammal skeleton is that it ceases growth in the adult, saving metabolic energy. Mammal long bones grow from bands of cartilage positioned between the diaphysis (shaft) and epiphyses (the ends). This allows permanent articulations between bones and forms well-established joints. The mammal skeleton has been simplified in that many bones have fused, decreasing the number of growth surfaces overall, and saving metabolic energy that would be used for maintenance. An example is the mammal skull, which is more ossified and simpler than those of other vertebrates. Bones that are fused in mammal skulls are separated by cartilage in reptiles. Ossification provides more surface area on bones with larger sites available for muscle attachments. Exceptions to many of these mammalian characteristics are found among the living monotremes. Some mammalogists actually believe that monotremes should be classified as therapsid reptiles because they have laterally placed forelimbs and their skeletons contain separated bones that are fused in other mammals, and, like reptiles, they retain cervical ribs. Ribs in most mammals are attached to the vertebral column only in the thoracic region; reptiles have ribs attached to cervical, thoracic, and lumbar vertebrae. This rib arrangement allows mammals to lie on their sides for resting or to suckle their young.

The bones that make up the limbs in mammals are part of the appendicular skeleton. The forelimbs of mammals are attached to the pectoral girdle, consisting of a scapula and a clavicle (collar bone). The scapula is held in place by musculature, which provides the high mobility important in many locomotor modifications. The pectoral girdle articulates with the axial skeleton (skull, vertebral column, thoracic cage) only through sternal bones, which allows a large range of motion in the shoulder. The pelvic girdle supports the hind limbs and consists of a coxal bone (the fusion of three different elements from the reptile condition) that attaches to the axial skeleton at the sacrum.

Locomotor adaptations

The musculoskeletal design of the mammalian body has accomodated many diverse means of locomotion, not only in terrestrial environments but also in aquatic and aerial niches. Many mammal species are capable of using several different means of locomotion, but much of the body configuration is determined by the dominant mode of locomotion used by a particular species.

Most mammals that are ambulatory (walking) do so on all four limbs, i.e., they are quadrupedal. Most ambulators are pentadactyl (possessing five digits) and plantigrade (walking on the soles, or plantar surface, of their feet). Pentadactyly is the primitive condition in mammals, although many lineages have reduced this number. Ambulators include bears and baboons. Some ambulators are large. As they approach a ton (0.9 tonne) in weight, adaptations for their large size are a necessity. Such animals are said to be graviportal. They have a rigid backbone and their limbs take on the appearance of a column with each element directly above the one below it. They retain all five digits in a pad that provides cushioning. Elephants are an example of a graviportal mammal. Elephants are not able to run, instead they trot, increasing their speed by walking quickly.

Cursorial locomotion (running) is accomplished in diverse ways. Among mammals there is a range of adaptations and abilities for this way of moving. Many cursors are digitigrade, i.e., their metacarpals and metatarsals are permanently raised above the substrate with only the phalanges in contact with the ground. Often the metacarpals and metatarsals are elongated and the number of digits reduced. For example, in equids (horses), the leg is supported on a single central digit of their mesaxonic foot. Other mammals, such as deer and hyenas, have legs with a paraxonic foot with two toes contacting the ground. Some mammals have one set of limbs that are paraxonic and another set that are mesaxonic.

A number of characteristics allow the generation of high speed in cursory locomotion. Reduction or loss of the clavicle that would impede forelimb movement is one adaptation. In addition, most of the musculature has shifted to the upper limb, and the lower limb has become thinner and elongated. In many cases it is the metacarpals and metatarsals that are the most elongated. In hoofed mammals the number of digits is reduced. The horse is the most extreme example with only a single digit. The elongation of the leg relative to body length produces a longer stride, thereby increasing the speed, which is equal to the length of the stride multiplied by stride rate. Another trait that increases speed is a pliant vertebral column that enables the mammal to place the hind feet in front of the fore feet when running at full clip. At very high speed, all four feet may be simultaneously off the ground. This is seen in horses, greyhounds, gazelles, and cheetahs. Cheetahs can cover a fixed distance faster than any other mammal. They are able to sprint at over 60 mph (97 kph).

Saltatory mammals use hopping as a means of locomotion. Some are quadrupedal, such as hares, using all four limbs to make their leaps. Others use another form of hopping called ricochet saltation. This is a bipedal locomotion in which only the two hind legs are involved in propulsion. Ricochet saltators have a long tail (often tufted at the end) that is used for counterbalance. In the case of the macropods (kangaroos) it is also used as a support during rest. Other ricochet saltators include kangaroo rats, jerboas, springhares, and tarsiers. In most cases ricochet saltators have reduced forelegs, long hind legs, and elongated feet. Generally it is the metatarsals have that been the most elongated. One exception is the tarsier, a small primate that retains longer forelimbs. It is the tarsal bones in the hind foot that are elongated, rather than the metatarsals. This leaves the tarsier with grasping ability in its hind paws. It is a ricochet saltator on a horizontal substrate, but in the shrub layer it employs vertical clinging and leaping, i.e., it pushes off vertical supports with its powerful hind legs and grasps a branch with its forelimbs and the upper part of its hind legs.

Arboreal mammals have many adaptations to life in the trees. One is stereoscopic vision (depth perception) in leapers and gliders. Grasping paws and opposable thumbs are often found, but they are not required. Squirrels scamper about on branches using sharp claws that provide traction. Sloths have elongated curved claws from which they hang under branches. Gibbons (small apes) have long fingers that serve as hooks, a reduced thumb, and a more dorsal scapula that allow them to swing underneath branches in the manner that children do on "monkey bars" at playgrounds. Prehensile tails are found in many arboreal mammals ranging from marsupials to primates. These tails are capable of wrapping around branches and serving as a "fifth limb." An arboreal adaptation among gliders (e.g., flying squirrels and colugos) is a ventral membrane, called the patagium, that can be spread out to generate lift for gliding. Except in the colugo, the tail is free of the patagium and is used for maneuvering. Gliders cannot ascend in flight, so they must climb before they launch. Despite that limitation, gliding can be very efficient. Colugos are able to cover over 400 ft (122 m) with a loss of only 40 ft (12 m) in altitude.

Among mammals, powered flight has evolved only in the bats, which are nocturnal fliers. In the order Chiroptera (literally "handwing") the mammalian forelimb has been modified

to form a wing with the main portion composed of the elongated bones of the hand. Bat hind limbs have been modified to help control the rear portion of the flight membrane. In addition, the feet have curved claws that enable these animals to hang in an upside down position. Flight requires a whole suite of characteristics including circulatory and thermoregulatory adaptations, and echolocation for flight in a dark environment. Most of the Old World fruit bats do not have echolocation and do not fly in complete darkness.

Some mammals have successfully adapted to aquatic environments. Semiaquatic mammals spend some time on land (e.g., yapoks [or water opossums], beavers, otters, and duck-billed platypuses). Their locomotor adaptations include a body approaching a fusiform (torpedo) shape, webbed feet (at least on the hind leg), and valvular openings to the nose and ear that can be closed to keep out water. Seals, sea lions, and walruses are more aquatic and have evolved flippers that provide efficient propulsion in water, although they still retain some locomotive ability on land. Seals propel themselves mainly by undulation of the flexible vertebral column assisted by the hind limbs. Completely aquatic mammals include the Cetacea (whales, dolphins, and porpoises) and the seacows (dugongs and manatees). The body is more fusiform than other marine mammals. The skeletal elements of the hind limbs have been lost, having been replaced by soft tissue forming a fluke that

propels the animal in concert with undulation of the posterior vertebral column. The forelimbs are flippers used for maneuvering. The cervical vertebrae of completely aquatic mammals are short, and they have completely lost their pinnae.

Cardiovascular and respiratory systems

In order to distribute nutrients and oxygen needed for metabolism, mammals need a highly efficient circulatory system. The main differences in circulatory structure between mammals and most other vertebrates are in the heart and in the red blood cells. The mammalian heart has four chambers (as do birds and crocodilian reptiles) compared to the three chambers found in the reptiles (except the crocodilians). The additional chamber is the result of a muscular wall (or septum) that divides the ventricle (lower half of the heart) into two chambers. In reptiles there is a single ventricle in which deoxygenated blood from the right atrium mixes with oxygenated blood from the left atrium. In mammals deoxygenated blood enters the right ventricle only and is then pumped to the lungs. Oxygenated blood returns from the lungs to the left atrium and then enters the left ventricle from which it is sent to the systemic circuit and the rest of the body. Thus, mammals have separated the pulmonary and systemic circuits with the result that mammalian blood is more fully oxygenated than the blood of terrestrial vertebrates with three-chambered hearts. Additionally, mature mammalian red blood cells (erythrocytes) are enucleated, i.e., they lack a nucleus, and are concave in shape. Space saved by the lack of a nucleus leaves room for additional hemoglobin molecules, the oxygen-binding molecule. The concave shape also increases surface area and places the membrane surface closer to the hemoglobin molecules facilitating gas diffusion. Thus, mammalian blood is capable of carrying more oxygen than reptilian blood.

The mammal heart is large, as are the lungs, and together these organs occupy most of the thoracic cavity. In certain taxa, these organs may be even bigger. Bats, for example, have a heart that is three times larger than the average terrestrial mammal of the same size. The mammal lung is sponge-like and consists of branched airways that terminate in microscopic sacs called

alveoli. The alveoli walls consist of epithelial tissue through which gas exchange occurs. This structural arrangement of smaller and smaller tubes and saccules increases the surface area available for respiration. For example, the respiratory membrane of human lungs is between 750 and 860 ft² (70–80 m²), which is about 40 times the surface area of the skin.

Mammals have a layer of muscle that separates the thoracic cavity from the abdominal cavity. This is called the respiratory (or muscular) diaphragm. When this muscle is relaxed its domed shape forms the floor of the thoracic cavity. When it contracts it moves towards the abdominal cavity, increasing the volume of the thoracic cavity, which draws air in from the external environment.

Digestive system

To fuel endothermy, mammals require more calories per ounce (or gram) of tissue than do ectothermic vertebrates such as reptiles. This is accomplished by more efficient digestion of food stuffs and more efficient absorption of nutrients. This efficiency begins with specialization of the teeth. Mammals have four different kinds of teeth (heterodonty) that are ideally shaped to cut, slice, grind, and crush food. An exception is the toothed whales in which all the teeth are similar (homodonty). The four types of teeth are incisors for slicing, canines for piercing, premolars for crushing or slicing, and molars for crushing. They are commonly represented by a notation called a dental formula, e.g., I2/2 C1/1 P3/3 M2/3, the dental formula for the Egyptian fruit bat (Rousettus aegyptiacus). The first in each group of two numbers represents the teeth in the upper jaw and the second is the number of teeth in the lower jaw. Multiplying the dental formula by 2 gives the total number of teeth, 34. The Egyptian fruit bat's dental formula indicates that the full set of teeth for the upper jaw is four upper incisors, two upper canines, six upper premolars, and four molars. There is a great deal of variation in the number and type of teeth present. For example, the prosimian primate, the aye-aye (Daubentonia madagascariensis), has a dental formula of I1/1 C0/0 P1/0 M3/3, which illustrates a reduction in number of some teeth and the complete loss of others. In the case of some herbivorous mammals the upper incisors are either reduced in number or completely replaced by a hard dental or gummy pad that functions as a cutting board for the lower incisors. In some gnawing mammals, such as rodents and rabbits, the upper and lower incisors grow throughout the entire life span and the canines have been lost. Modification of teeth may be extreme, such as complete loss in most anteaters, or the formation of large tusks, derived from the second upper incisors in elephants, or from the canines in walruses (Odobenus rosmarus).

The relationship between dental structure and function is so precise that the diets of long-extinct mammals can be deduced

from their teeth. The teeth are often the only fossil remains recovered from paleontological sites. Teeth perform mechanical (or physical) digestion by breaking down a food morsel into smaller pieces, providing additional surface area for action by digestive enzymes. Premolars in herbivorous mammals usually have ridges for grinding. In some carnivores such as wolves, the last upper premolar has a blade that shears against the first lower molar. Fruit-eating mammals such as flying foxes often have flattened premolars and molars.

Other modifications for efficient digestion occur in the stomach, a portion of the gastrointestinal tract. The stomach serves as a storage receptacle in most mammals and as a site of protein breakdown. A simple stomach is found in most mammal species, including some that consume fibrous plant material. In other mammals that consume a high fiber diet the stomach has become enlarged and modified to handle more difficult digestion. These modifications comprise a foregut digestive strategy, for which the stomach contains compartments where symbiotic microbes break down cellulose and produce volatile fatty acids (VFA) that can be utilized by the mammal. Foregut fermentation has been developed to the greatest degree among the mammal order Artiodactyla, which includes pigs, peccaries, camels, llamas, giraffes, deer, cattle, goats, and sheep. Rumination, reprocessing of partially digested food, is accomplished by the four-compartment stomachs of giraffes, deer, cattle, and sheep. Less complex tubular and sacculated stomachs are found in kangaroos, colobus monkeys, and sloths. Stomachs in foregut fermenting species are neutral or only slightly acidic, around pH 6.7, to provide a favorable environment for symbionts.

Food moves from the stomach to the intestines; which consist of the small intestine, where most digestion and absorption occurs, and the large intestine. The wall of the small

intestine contains epithelial tissue with small finger-like projections called villi. In turn, each villus has smaller extensions called microvilli. The villi secrete enzymes for further carbohydrate and protein digestion. The microvilli absorb the digested nutrients. The presence of the villi and microvilli in the mammal small intestine increases the absorptive surface area by at least 600 times that of a straight smooth tube. The

villi of the human small intestine, for example, provide 3,230 ft² (300 m²) of surface area whereas the surface area of a smooth tube of the same size as the small intestine is about 5.4 ft² (0.5 m²). Nutrient absorption occurs through the membranes of the microvilli of each intestinal epithelial cell. Also distributed throughout the small intestine are glands that secrete special enzymes for further digestion of proteins, carbohydrates, and lipids.

For mammals, diet and the length of the small intestine are closely correlated. Mammals that consume a diet that is either digested in the stomach (such as animal protein consumed by faunivores) or easily absorbed (such as nectar consumed by nectarivores) have a shorter small intestine than

other mammals. Herbivores that eat very fibrous plant matter tend to have the longest small intestine. The small intestines of fruit-eaters tend to be intermediate in length.

The foregut fermentation strategy of herbivores requires a medium or large body size to accommodate the necessarily large stomach. A strategy generally used by smaller herbivores is hindgut fermentation (although there are large hindgut fermenters such as horses, elephants, and howler monkeys). The hindgut, also called the large intestine, consists of the cecum and the colon. The cecum is a blind pouch that serves as the principal fermentation chamber in the hindgut strategy. As in the stomach of foregut herbivores, colonies of symbionts in the cecum of hindgut fermenters break down cellulose and excrete products advantageous to the mammal. Nutrients appear to be absorbed through the wall of both the cecum and colon, especially in the larger mammals.

Small hindgut fermenters, such as many rodents and rabbits, have the problem that food can only be retained in the gut for a short time. As it leaves the hindgut, digestion is incomplete and many valuable nutrients may be left unabsorbed. This problem is solved by a behavioral adaptation: a soft pellet is produced in the cecum, defecated, and immediately

picked up by the animal and reingested. This reingestation of feces is called coprophagy. The soft pellet then goes through the digestive process a second time and the end product is a hard fecal pellet devoid of nutrients. Many owners of pet rabbits are familiar with the hard pellet, often called a "raisin." The softer pellet is usually consumed at night (when coprophagy goes unobserved by the pet owner) and is called the "midnight pellet." Coprophagy is efficient; voles are able to extract 67–75% of the energy contained in their food.

Nervous system and sensory organs

Mammals have relatively larger brains than other vertebrates. From monotremes to marsupials to eutherians, the mammal brain increases in size and complexity, primarily by the expansion of the neopallium. The neopallium (or neo-cortex) is a mantle of gray matter that first appeared as a small region between the olfactory bulb and the larger archipallium. The neopallium in mammals has expanded over the primitive parts of the vertebrate brain, dominating it as the cerebral cortex. The cerebral cortex is a thin laminar structure consisting of six sheets of neurons. In order to increase the number of neurons in the neocortex it must be folded to fit within the skull of a mammal. For example, the surface area of the human neocortex is about 1.5 ft² (0.14 m²). With this area, it could not be simply laid over the deeper parts of the brain; folding produces gyri and sulci (folds and grooves, respectively), which gives the eutherian brain a convoluted appearance. Small mammals do not usually have convolutions, but they are almost always found once a species has reached a particular body size. Some researchers believe that the convolutions simply serve to increase the number of neurons in the neocortex, while others propose that the primary purpose of the convolutions is to increase surface area for heat dissipation. The brain produces a large amount of metabolic heat and must be cooled. Increasing the surface area provides more area for heat transfer (radiation) to occur; i.e., the convolutions produce a "radiator" for the brain.

The neocortex may be more developed or less developed depending on the mammalian species. In echolocating bats, for example, it comprises less than 50% of the brain surface because most of the bat brain is devoted to the auditory centers. Specific regions of the neocortex are specialized for particular

functions. For example, the occipital region is a visual center, the temporal region is involved with hearing, and the parietal lobe interprets touch. A structure found only in the eutherian brain is the corpus callosum, a concentration of nerve fibers that connect the two cerebral hemispheres and serve as a communication conduit between them.

Brain structure accounts for mammals' great ability to learn from their experiences. Their brain structure, combined with other neural characteristics, also accounts for mammals' acute sensory abilities. For example, mammalian smell is very acute. In some mammals, it is the most developed sense. Mammals have an elongated palate and, consequently, the nasal cavity is elongated as well. A structure in the palate of many mammals, the vomeronasal organ, detects smells from food. The development of turbinal bones covered by sensory mucosa in the nasal cavities has allowed more efficient detection of odors. Even so, some mammals have a poorer sense of smell than others, e.g., insectivorous bats, higher primates, and whales. In fact, dolphins and porpoises completely lack the olfactory apparatus. The receptors for taste are located on the tongue. Taste, interpreted in the brain in conjunction with olfactory stimuli, helps mammals identify whether food is safe to eat or not.

Mammal hearing is highly developed. In mammals, the articular and quadrate bones of the reptilian jaw were modified to become the malleus and incus bones, which, along with the stapes, form the auditory ossicles in the mammal skull. The auditory ossicles conduct vibrations to the inner ear. Another mammal modification is the evolution of a pinna, an external flap that directs sound waves into the ear canal (external acoustic meatus). Many mammal species have mobil pinnae that enable them to pinpoint the location of the sound source. Pinnae are most elaborate in the insectivorous bats, but completely lacking in most marine and subterranean species.

The mammal eye is based on the reptilian eye. Many mammals are able to see very well in low-light conditions because of a reflective mirror-like layer (tapetum lucidum) in the choroid coat beneath the retina. The tapetum lucidum produces "eyeshine," such as seen in the eyes of deer staring into automobile headlights. Vision is improved as light is reflected back across the retina so that photoreceptors can interact with the light multiple times. Some mammals have an abundance of cones in the retina providing for color vision. This is especially true of the anthropoid primates, but this is also found in other mammals, e.g., Old World fruit bats. Aquatic species usually have a nictitating membrane that covers the eye, providing protection in the underwater environment.

Thermoregulation

Mammals produce their own body heat (endothermy) as opposed to absorbing energy from the outside environment. This metabolic heat is produced mainly in their mitochondria. Internal organs such as the heart, kidney, and brain are larger in mammals than reptiles and the corresponding increase in mitochondrial membrane surface area adds to their

heat production. Mammals also regulate their body temperature within a stable range, generally between 87 and 103°F (30–39° C). This is called homeothermy. Having a constant temperature allows mammals to maintain warm muscles, which gives them the ability to react quickly, either to secure food or to escape predation. They can also maintain the optimum operating temperature for many enzymes, providing a more effective physiology. Some mammals are heterothermic (able to alter their body temperature voluntarily). Many insectivorous bats are heterothermic. When in torpor they lower their body temperature to the ambient temperature, conserving calories that would otherwise be used for heat production.

To regulate body temperature, mammals must also have the means to retain a certain amount of the heat they produce. Small mammals lose heat more rapidly than larger mammals because they have a greater proportion of surface area to volume (or, equivalently, to their body mass). Heat is lost through surface area. The higher the surface area–to-mass ratio, the greater the rate of heat loss. Fur helps to insulate a small mammal to some degree, but often it is not enough to prevent the high rate of heat loss. Small mammals often compensate by obtaining more calories per unit of time by continuously eating foods that are quickly digested and absorbed. Larger mammals' surface area–to-mass ratio decreases as their mass increases, and they lose heat at a lower rate.

Reproductive system

There are three different modes of reproduction used by mammals. The monotremes, whose extant members are the echidnas and duck-billed platypuses, lay eggs. The therians (marsupial and placental mammals) give birth to live young. Marsupial newborns are undeveloped (some mammalogists call them embryos). After only a short gestation period they must make their way to a teat outside the mother's body (a teat that may be in a pouch in species that have pouches) to finish development. The embryos of placental mammals remain in the uterus during development, and they have a nutritive connection with the mother through the placenta. The young of placental mammals are born more mature than the young of the other two groups.

The female reproductive tract in monotremes is very much like a reptile's. A cloaca (also found in amphibians, reptiles, and birds) is a common chamber for the digestive, urinary, and reproductive system. The eggs are conveyed from the ovaries through the oviducts where fertilization occurs. After fertilization the eggs are covered with albumen and a leathery shell produced by the shell gland. In therian females the reproductive organs are separate from the urinary and digestive systems. The marsupial female has two uteri, each with its own vagina. Eutherian females may have either a single uterus or paired uteri, but always a single vagina. The placental embryo implants and develops in the uterine wall.

In all therians, the male urinary and reproductive systems share a common tract, the urethra. A problem for endothermic mammals is that their body temperature may be too high to sustain viable sperm. This is not a problem for monotreme males because their body temperature is lower than that of therians, and their testes are contained in the abdominal cavity. The testes of therian males are typically contained in a scrotum, a sac-like structure that lies outside the body cavity. The testes may descend into the scrotum from the abdominal cavity only during breeding season or they may be permanently descended. The penis differs in the three main groups of mammals. The monotreme penis is attached to the ventral wall of the cloaca. The marsupial penis is directed posteriorly, contained in a sheath, and the glan penis (tip) is bifid, which accommodates the two vaginas in the marsupial females. The eutherian penis is directed forward. It may hang freely or be contained in an external sheath. In many species, including most primates, a bone called the baculum supports the penis.

Mammary glands (see also the discussion under integument) provide nourishment for the young mammal. While milk requires energy to produce, it also conserves energy for the mother: Mammals do not have to make numerous trips to find food and return with it to feed their offspring. Observations of bird parents making trip after trip in order to feed insatiable hungry mouths at the nest illustrate this point. A mammal mother obtains her food, returns to the nest or den, and can feed her young in comparative safety.

Resources

Books

Feldhamer, G. A., ed. Mammalogy: Adaptation, Diversity, and Ecology. San Francisco: McGraw-Hill, 2003.

Hildebrand, M. Analysis of Vertebrate Structure. 4th ed. New York: John Wiley & Sons, 1994.

MacDonald, D., ed. The Encyclopedia of Mammals. New York: Facts on File, 2001.

Martin, R. E., R. H. Pine, and A. F. DeBlase. A Manual of Mammalogy. 3rd ed. San Francisco: McGraw-Hill, 2001.

Neuweiler, G. The Biology of Bats. New York: Oxford

University Press, 2000.

Novak, R. M. Walker's Mammals of the World. 6th ed. Baltimore: John Hopkins University Press, 1999.

Pough, F. H., C. M. Janis, and J. B. Heiser. Vertebrate Life. 6th ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Romer, A. S. and T. S. Parson. The Vertebrate Body. 6th ed. San Francisco: Saunders College Publishing, 1985.

Welty, J. C., L. Baptista, and C. Welty. The Life of Birds. 5th ed. New York: Saunders College Publishing, 1997.

Vauhan, T. A., James M. Ryan, and Nicholas Czaplewski. Mammalogy. 5th ed. Philadelphia: Saunders College Publishing, 1999.

Periodicals

Young Owl, M., and G. O. Batzli. "The Integrated Processing Response of Voles to Fibre Content of Natural Diets." Functional Ecology 11 (1998): 4–13.

Marcus Young Owl, PhD