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 germina-tive) 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
Mammals have different tooth shapes for different functions. Herbivores typically have large, flattened teeth for chewing plants. Rodents' evergrowing incisors are used for gnawing. Carnivores have teeth for holding and efficiently dismembering their prey. (Illustration by Jacqueline Mahannah)
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 interactions, 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 exam ple 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 asca-nius) 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 calcaneum
Plantigrade m metapodial
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metapodial single Unguligrade
Foot structure of mammals: plantigrade (bear); digitigrade (dog); un-guligrade (horse). (Illustration by Jacqueline Mahannah)
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.
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