Mammals
Members of the class Mammalia possess both hair and mammary glands. Their integument is complex and has many glands used for a variety of purposes: thermoregulation and excretion (sweat glands), communication (scent glands), care of the hair and skin (sebaceous oil glands), and for feeding of the young (mammary glands). They are thermic and have relatively high rates of metabolism. In keeping with their higher metabolic rates, adaptations for efficient feeding include heterodont teeth in most species and a secondarypalate to separate the respiratory and food passages (so they can breathe and chew at the same time). The circulatory systems are efficient, and they have a four-chambered heart with separate pulmonary and systemic circulations. Their brains are highly developed, fertilization is internal, and most have placental attachment of the young.
Most living mammals belong to the subclass Theria and are thought to have developed from mammal-like reptiles called therapsids during the Mesozoic (about 180 million years ago). The placentals are extremely diverse, occupying all habitable environments on earth. Members of the subclass Prototheria are so different from placentals that they may have developed from a different theriapsid species. They are represented today by only a single order, Monotre (egg-laying mammals). Today's monotremes are found in Australia, Tasmania, and New Guinea. The only living monotremes are the duckbill platypus and echidna (spiny anteater). Monotremes have several primitive characteristics: They lack teeth as adults, the braincase and other skeletal elements are reptilian in structure, they have a single ventral orifice (connected to a cloaca), and they are oviparous. Although they are homothermous, their body temperature is maintained only a few degrees above the ambient temperature. In these ways they are reptilian in their structure, reproduction, and physiology. Nonetheless, monotremes do possess hair and feed their young milk, so they do qualify as mammals.
Subclass Theria, Infraclass Metatheria. Metatherians are the marsupial mammals (kangaroos, koalas, opossums, Tasmanian wolves, and wombats). Like the placental mammals (superclass Eutheria), marsupials start their lives attached by a placenta to the maternal circulation. Although the embryo and maternal circulation do not mix, the developing young receive nutrients and discharge metabolic wastes through the placenta. Marsupial placental development, however is short-lived; and following their birth the embryos attach to a nipple within a skin pouch (marsupium) where they continue their development. The most common American opossum is Didelphis virginiana. They are nocturnal in their habits and are often found in association with humans.
Subclass Theria, Infraclass Eutheria. Eutherian mammals, including yourself, are placental beasts and belong to the infraclass Eutheria. Among the lower vertebrates the teeth are simple in structure and usually similar in shape. Mammals, however, have teeth adapted to different food-processing functions. These teeth are classified from front to back as incisors, canines, premolars, and molars. Incisors are usually small spindle-shaped teeth used for cutting, chopping, and picking up food (although in rodents they are greatly enlarged). Canines are sharp conical teeth useful for seizing prey and tearing flesh. The development of canine teeth varies with the diet of the animal. They are usually poorly developed or absent in herbivores and are especially well developed in carnivorous mammals. Sometimes there is variation according to sex (males having larger canines), while in other species they grow to become premolars and molars are collectively known as cheek teeth. Premolars are often used to puncture food while the broader molars are involved in crushing and grinding food. Depending on the diet, the premolar teeth may broaden to look much like the molars. The occlusion between the upper and lower cheek teeth forms efficient surfaces between which food can be cut, crushed, or torn according to their structure.
The number and arrangement of the teeth are important in the classification of mammals. A convenient way of expressing this arrangement is through a dental formula that depicts the number of teeth in each half jaw. For the primitive condition, the upper jaw has three incisors, one canine (fang), four premolars, and three molars. The lower jaw has the same arrangement. Because of this ancestral arrangement, mammals usually have 44 teeth at most. However, because of secondary modifications some species have no teeth (monotremes) while others have several hundred (dolphins). This primitive arrangement is modified to one degree or another amomg and within the various orders.
Among the carnivores, all teeth are represented. Using dogs as an example, they differ from the primitive condition only in a missing molar on the upper jaw. For carnivores, the important aspect of these dental formulae is that all four types of teeth are represented. The structure of the teeth is also important in classification of mammals, but this aspect of taxonomy is more complicated and will not be covered here. It may seem terribly superficial to classify mammalian orders based on tooth structure, but since the teeth are related to feeding, they are representative of other differences in anatomy and physiology.
- Order Carnivora. Carnivores are predatory mammals with clawed feet and teeth adapted for cutting flesh (although some carnivores have secondarily taken on an omnivorous habit). Carnivores are among the brightest and strongest of animals. Examples of carnivores include badgers, bears, foxes, otters, raccoons, skunks, weasels, and wolves.
- Order Primates. Members of this order are mainly arboreal (living in trees), although some, like ourselves, have moved to the ground. They have five digits on their front and hind legs, usually adorned with nails, rather than claws. They include such beasts as apes, gibbons, lemurs, monkeys, tree shrews, and humans.
- Order Rodentia. This is a very successful order since nearly half of living mammalian species are classified as rodents. All rodents have large chisellike incisors and lack canines and premolars. Examples of rodents include beavers, lemmings, marmots, mice, prairie dogs, rats, squirrels, and voles. The dental structure of all rodents is similar. They have large incisors, and the major differences are in the grinding surfaces of the molars. The lower incisors continuously grow and must be trimmed by gnawing.
- Order Lagomorpha. Rabbits, hares, and pikas, with their chisel-like teeth, look superficially like rodents (with which they were once classified). Rabbits and hares differ only slightly in their anatomy, but while rabbits are born blind, naked, and helpless, hares are furred and active. Rabbits tend to live in burrows, while hares spend most of their time above ground. While Peter is most certainly a rabbit, Bugs appears to be a hare (not a bunny).
- Order Insectivora. These are the insect-eating mammals such as shrews, moles, and hedgehogs (they also will feed on any small beast including crustaceans and worms). They have long snouts and primitive dentition. Most are terrestrial and spend much of their lives underground, although a few are semi-aquatic. Insectivores are thought to be the most primitive of the present-day mammalian orders.
- Order Chiroptera. Chiropterans (bats) are the only true flying mammals (flying squirrels, lemurs, and phalangers only glide). Their forelimbs are modified with elongated second to fifth digits to support a web of skin that extends over the forearm to the body, hindlegs, and tail. The thumb is short and armed with a claw that they can use to grasp and climb on various surfaces. Most are nocturnal insectivores, but some feed on fruit or flower nectar ("flying foxes"). All navigate by echolocation, but few are "blind as a bat" (their vision is usually as good as a rat's). And yes, Virginia, there are vampire bats. The upper incisors of vampire bats are large and sharp. These are used along with the canines to bite a triangular patch of skin from their prey so they can suck out the blood.
- Order Artiodactyla. This order contains medium- to large-sized even-toed mammals (most have two toes). Many have horns, and their toes are protected by a hoof. Most are ruminants (they chew the cud to aid in digestion of cellulose). The order includes beasts such as antelopes, camels, deer, elk, goats, hippos, llamas, oxen, sheep, and swine.
- Order Perissodactyla. This order is composed of the odd-toed mammals such as horses, zebras, and rhinos. They have one to three toes covered with a hoof. Their teeth are well adapted for chewing.
- Order Edentata. Members of this order include anteaters, sloths and armadillos. Armadillos are armor-plated on their head and dorsal body surfaces and when threatened, most can roll into a tight ball to protect their soft belly. They are burrowers that hunt by night for small vertebrates, earthworms, and other soil organisms.
Other Mammalian Orders (Subclass Theria, Infraclass Eutheria). Other eutherian include the order Pholidota (scaly anteaters or pangolins found in tropical Africa and Asia). Pangolins have horny scales covering their bodies and forelimbs armed with powerful claws, which they use for digging into termite mounds. Sea lions, seals, and walruses belong to the order Pinnipedia. All pinnipeds are marine creatures feeding mainly on fish. Other marine mammals include the order Cetacea. Among the cetaceans, toothed whales (dolphins, porpoises, and sperm whales) survive by feeding on fish, squid, and the like, while the larger baleen whales (right, blue, and gray whales) filter plankton from the water. Another order of large mammals is the Probosci (elephants). Some smaller land mammals belong to the orders, Hyracoidea (hyraxes), Tubulidentata (Varks). Last, but not least, we have the flying lemurs (order Dermoptera). They do not fly (they glide), nor are they lemurs (lemurs are primates).
Mammalian skin is built on much the same plan of other vertebrates: a dermis overlain with a multilayered epidermis which is much thicker than that of other vertebrates is another distinctive feature of mammalian skin. Hair is used for skin protection, provides sculpted surfaces used in visual communication (manes, hoods, and the like), and, by trapping air, serves as an insulating layer. The dermal and subcutaneous layers also have many more multicellular glands when compared to the other vertebrates. Finally, other structures are commonly associated with the skin, including hooves, footpads, horns, and antlers.
The outermost layer of the skin is the epidermis lying above the thick dermal layer. The stratum germinati (sitting just above the dermis) continuously undergoes mitotic divisions and is responsible for producing the upper layers. As the cells are pushed toward the outside, they undergo a process of cornification, during which time the cell's cytoplasm is replaced by a fibrous protein (keratin) and the nucleus is extruded. This, of course, kills the cells, so that they are dead by the time they reach the outermost layer (stratum corneum). The stratum corneum is especially thick on the soles of the feet and toes and forms the bulk of the footpads of many mammals. Cells of the stratum corneum are continuously sloughed off and abraded away as they perform their protective function. A darkly stained stratum granulosum may be seen as a thin band between the stratum germinativum and corneum on some preparations. Sweat glands and collections of fat cells are also found in the skin.
A sebaceous gland is associated with each hair (these produce oils to keep the hair and skin soft and pliable). An arrector pili muscle attaches to one side of the hair follicle. When these smooth muscles contract the hair is stood on end. This increases the thickness of the trapped air in the coat and provides additional insulation when temperatures fall. They are also responsible for goose bumps in humans. The main portion of the hair is composed of dead cornified cells. New hair cells are produced by hair papillae at the base of the follicle wall. Most hairs have a spongy inner medulla surounded by a dense cortex that contains the pigment. Cuticular scales cover the cortex. The scale pattern varies from species to species and can be used to identify mammalian species.
Most mammals have several types of hair. Long guard hairs often protect a dense inner coat of soft underhair. Many mammals also have tactile whiskers (vibrissae) on the snouts and patches of hairs modified to waft pheromones into the surrounding air. The stiff hairs on the "cheeks" of cats and the axillary and pubic hairs of humans are examples of these hair types.
Other Integument Derivatives. Claws or claws modified as nails or hooves are present in most mammals. Some also possess that they use for courtship and defense (sheep, cattle). Horns consist of an inner bony core surrounded by an epidermal layer that secretes an outer horny layer (no pun intended). Horns are usually permanent, do not branch, and are found in both sexes. Antlers, on the other hand, usually fall off after the mating season, usually branch, and are mostly seen on males. Antlers are bony outgrowths of the skull covered with a skin called velvet. When the antlers mature the velvet sloughs off or is scraped off by the animal. A third type of horn is composed of compressed hairlike keratin fibers (as in rhinos).