tiger

The first tigers were members of the Felidae family of big cats that lived in northern Asia during the Late Pleistocene epoch. They migrated south and east and evolved into specific types of larger tigers according to their habitats. The ancient saber-tooth tigers were not ancestors of modern tigers. Three of the eight tiger subspecies, Bali, Caspian, and Javan, became extinct during the twentieth century.

Anatomy
Tigers range in weight according to their subspecies and gender. Sumatran male tigers are the lightest, weighing 110 kilograms (250 pounds). Females weigh approximately twenty kilograms less. Siberian male tigers weigh as much as 225 kilograms (500 pounds). The heaviest known tiger weighed 465 kilograms (1,025 pounds). From head to the base of the tail, tigers measure from1.4 to 2.8 meters (4.5 to 9 feet) long, and their tails are from 90 to 120 centimeters (3 to 4 feet). They use their tails to balance and to communicate. Tiger skulls have a big saggital crest which anchors a large jaw muscle. Tigers' vertebra and joints are flexible. Their hind legs are longer than their front legs, providing impulsion and assisting leaping when they are chasing game. They have five toes on their front paws and four toes on their hind paws. Each toe has a retractable claw which is 80 to 100 millimeters (3 to 4 inches) long and helps them restrain prey and climb trees.Mature tigers have thirty teeth. The canine teeth are 75 to 90 millimeters (2.5 to 3 inches) long. Tigers have triangular, erect ears set atop broad skulls, with their eyes positioned on the front of their face. Tigers' eyes have reflecting retinas which enable excellent night vision. Tigers' sense of smell is also acute; they can distinguish different animals by smell and exhibit the flemen response. Tigers have nineteen pairs of chromosomes, which determine genetic patterns. Their coats are colored shades of orange, with black or brown stripes of varying widths and lengths and white accents around the eyes, ruffs, and other body parts. Rarely, Bengal tigers with the two necessary alleles are born with a white foundation coat and blue eyes. They are not albinos or a separate subspecies. Tigers' stripes vary according to subspecies, with Sumatran tigers having the most and Siberian tigers having the fewest. Each tiger's stripes are unique and function as camouflage. Fur thickness varies with seasonal changes and geography. Siberian tigers have almost twice the number of hairs per square centimeter than Sumatran tigers.

Behavior
Tigers are solitary, preferring to hunt alone.Amale tiger's territory averages twenty-six to seventyeight square kilometers (ten to thirty square miles), depending on the availability of prey. Some Siberian tigers roam territories of 1,036 square kilometers (400 square miles). Tigers' territories often overlap, with several females sharing territorial space with one male. Tigers scratch on trees, leave fecal droppings, and spray urine to mark their territory. Male tigers occasionally fight. Tigers have several vocalizations to communicate aggression and receptiveness to other tigers. Females attain sexual maturity at age three and males at age four. After a four-month gestation, females have litters of two to five cubs which are born blind and are vulnerable to predators such as pythons. The cubs drink their mother's milk for two months, then feed at her kills until they are about two to three years old and capable of hunting alone. Tigers can catch and kill prey as large as 160 to 900 kilograms (440 to 2,000 pounds). They stalk and ambush ungulates, knocking prey to the ground and biting the neck or throat to sever the spinal cordor suffocate the animal.Tigers can consume twenty to twenty-five kilograms (sixty to seventy pounds) of meat daily. They drag carcasses into vegetated areas and gorge on a kill, then fast. Tigers also eat termites and snakes. Some tigers, especially in the Sunderbans river delta of India and Bangladesh, have attacked and killed humans.

Conservation
Adult tigers are hunted by poachers for their hides, bones, teeth, and body parts or for sale to exotic pet traders. Much of their jungle habitat has been destroyed during wars or for agricultural use. As a result, only about five thousand to seven thousand tigers are alive in the wild. Authorities estimate that an equivalent number are kept as exotic pets in North America and in zoos, sanctuaries, and circuses. Tiger censuses have been taken by counting pugmarks in known tiger habitats. In captivity, tiger hybrids include ligers, the hybrid of lion fathers and tiger mothers, and tigons, produced by tiger fathers and lioness mothers. Conservation breeding programs are aspiring to preserve and increase the tiger population.

Tasmanian devil

The Tasmanian devil belongs to a primitive group of mammals, the marsupials, which are found today primarily in Australia, Tasmania, New Guinea, southern Mexico, Central America, and South America. The name "marsupials" refers to the animal's marsupium, the pouchlike structure to which the immature young move after they are born, becoming attached for several months as they complete their development. The well-known pouch of kangaroos is the classic example of a marsupium, although it should be noted that a few marsupials do not have pouches and in some, including the Tasmanian devil, the pouch opens downward. During their long geological separation from the rest of the world, marsupials in Australia and South America underwent adaptive radiation, which produced an enormous diversity of forms as species became adapted to various habitats. As a result, many marsupials resemble placental mammals although they are not closely related. Thus, there are marsupials that resemble flying squirrels, moles, woodchucks, cats, and dogs. The Tasmanian devil, although a marsupial, has many similarities in structure and behavior to a dog.

Physical Characteristics
The devil belongs to a family of marsupials known as the Dasyuridae, which are found in Tasmania, Australia, New Guinea, and some nearby islands. The group includes the Tasmanian wolf or tiger, the numbat or banded anteater, and shrewlike, catlike, and ratlike forms. The Tasmanian devil is distinctive even within this distinctive group of marsupials. It received its name, "The Devil", from the early European settlers to Tasmania, who were awed by its fierce behavior and loud vocalizations. The devil actually is a stocky but short-limbed animal, doglike in shape, and only weighs between 4.5 and 12 kilograms, standing only about 30 centimeters tall. Its dark, mostly black color also contributes to its "evil" reputation. The animal does have a voracious appetite. It has a large head and very powerful jaws with long canine teeth. The devil is the largest carnivorous marsupial, extremely strong for its size; it can feed on animals larger than itself, including small kangaroos. It usually rests during the day and seeks food at night. Its food is varied and may include amphibians, lizards, rodents, insects, and poultry. The devilmayprey on live animals or may feed on carrion. Its nightly movementsmaycover distances up to ten miles or more. The legs are short and the animal usually moves along slowly, although it is capable of running quickly for short distances.

Early Life
As is true of all marsupials, the young are born alive after a relatively short gestation period- about three weeks. Breeding usually takes place in early spring. The newborns travel from the birth canal to the pouch, which is backward- opening in this species, and attach themselves to a nipple,where they remain secure for about four months. For the next several months, the young will venture outside of the pouch at times and, finally, become weaned and independent by the end of the year. Breeding usually commences between the second and third year. Reproduction is somewhat limited, as the mother has only four nipples and usually more than four young are born. It is not unusual for only two young to survive to weaning. The Tasmanian devil has had a persecuted history, especially with the advent of European settlers. Its fierce appearance, loud screeches, and occasional predation on domestic livestock and poultry caused it to be hunted, trapped, and poisoned to the point of near extinction. Its value in cleaning up carcasses was not appreciated. The Tasmanian devil finally became protected by law in 1941. Going from outlaw to icon, the Tasmanian devil was selected as the symbol of the Tasmanian National Parks and Wildlife Service.

Salamanders

Salamanders and newts comprise one of three groups of amphibians living today. The other two, the caecilians (Gymnophiona) and the frogs and toads (Anura) can be readily distinguished by their body forms. Like other amphibians, salamanders and newts have glandular skin that lacks scales, feathers, or hair. Considering only species living today, salamanders and newts are a small group compared to the number of species of frogs and toads. Whereas frogs and toads are represented by about four thousand species, only about four hundred species of salamanders and newts are living today. Systematists, biologists who study the classification of plants and animals, recognize ten families of salamanders. Newts are simply salamanders that are classified in the family Salamandridae; they can be distinguished fromother salamanders by many osteological (bony) features and by their generally rough skin, compared to the smooth skin of other salamanders.

Anatomy of Salamanders and Newts
Salamanders have long, lizardlike bodies with long tails and four small legs. Many species have costal grooves along the sides of the body; the number of these grooves varies among species and can help with identification. Olfaction (sense of smell) is used during courtship, and males of many species have specialized glands on the body. Males of many species of lungless salamanders have a gland on the chin that is used to deliver hormones to the female during courtship. Salamanders have a larval stage, but unlike frogs and toads, in which the tadpole is very different from the adult frog, larval salamanders are similar in body form to adults. Larval forms are frequently found in water and retain external gills, which are lost at metamorphosis (transformation to the adult stage). Species that breed in ponds, where oxygen levels are low, have large, bushy gills for added surface area to increase the intake of oxygen. In contrast, species that breed in streams, which have high oxygen levels, have larvae with short gills. One of the most successful groups of salamanders are the Plethodontidae, the lungless salamanders. Most species of these salamanders live and breed on land, never entering water. They have no lungs, and oxygen uptake occurs primarily through the thin, porous skin. One requirement for this gaseous exchange is moisture, and these salamanders live primarily in damp, cool forests.

Life History of Salamanders and Newts
Primitive families of salamanders deposit eggs in water and have aquatic larvae. Other families of salamanders are unique among amphibians in producing a spermatophore for the transfer of sperm from the male to the female. The spermatophore is a gelatinous structure with a sperm cap resembling a tiny mushroom. The spermatophore is transferred from the male to the female in an elaborate courtship ritual. In some species, the male rubs secretions from a gland under his chin over the body of the female and entices her to follow him about. He then deposits a spermatophore on the substrate of the pond, which the female straddles and picks up with her cloaca. Fertilization is therefore internal in those species that produce a spermatophore. Lungless salamanders are active on the forest floor during moist or humid periods. Males are antagonistic to one another and appear to establish small territories that they defend from other males. The territorial encounters include biting and chasing and can result in injuries, including loss of part of the tail.

Defense Against Predators
Like other amphibians, salamanders and newts have toxic skin secretions produced by skin glands that are used in various ways as defense mechanisms to repel predators. In some species of newts, glands are concentrated on the dorsum, and when disturbed by a predator, the salamander displays an "unken reflex." This display includes bending the body in a U-shape and showing bright coloration of the underbelly. At the same time, the animal becomes immobile, thus decreasing the chance that a predator will attack. Other species have glands concentrated on the tail and engage in tail lashing or tail undulation. In tail lashing, the salamander violently whips its tail toward the predator, which attacks the tail and tastes the noxious secretions. In tail undulation, the body of the salamander remains immobile while the tail is waved in a sinuous fashion above the body. The tail in these species can be autotomized, or broken from the body by the salamander. Thus, if a predator attacks the waving tail, the salamander loses the tail but escapes with its life. Other species actively defend themselves if attacked by a predator. Amphiumas are large, powerful salamanders that live in ponds, swamps, or marshes in the southeast United States. Adults reach one meter in length and can inflict a painful bite with their sharp teeth. In a unique manner of defense, slender salamanders can secrete copious amounts of an adhesive substance from their glands. When attacked by a garter snake, the salamander's secretions glue the snake's body to itself, and it is unable to swallow the salamander.

Ratfish

The other major subgroup of cartilaginous fishes, the subclass Holocephali, comprises about twenty-five or thirty living marine species, most or all of which are placed in a single family, the Chimaeridae. They have a single gill opening on each side, like the bony fishes, but unlike them have a soft (rather than bony) gill cover. These fishes, commonly called chimaeras or ratfish (because of their long, slender tails), live and feed on the ocean bottom, usually in deep water. They have pavementlike teeth for crushing their mollusk and crustacean food, and they have a venomous spine on the leading edge of the first dorsal fin (the forwardmost of the unpaired fins on the upper surface of the body) for defensive purposes. Male ratfishes have a fingerlike barbed clasper, of unknown function, on the top of the head, and two pairs of claspers on the ventral (belly) side of the body.At least one pair of these ventral claspers is involved in mating. The female lays eggs in leathery capsules somewhat like skate egg cases.

Ichthyology
The study of sharks, rays, and related species is part of the larger discipline known as fish biology, or ichthyology. This science has its origins in the writings of Aristotle more than twenty-three centuries ago. He was the first to report, for example, that the sex of sharks can be determined by the structure of the pelvic fins, that is, by the presence of claspers in the male. Aristotle also contrived some rather fanciful interpretations of shark anatomy and behavior, as in his explanation for the fact that the shark mouth is on the under side of the head, far back from the tip of the snout, unlike the mouths of most other fish. In his view, this made it difficult for the shark to feed on its prey, requiring it to turn on its back, and thus nature allowed some chance for the poor animals to escape the jaws of this ravenous predator. Modern study of cartilaginous fishes, like fish biology in general, involves several disciplines. Ichthyology, or systematic ichthyology, is particularly concerned with the naming and classifying of species and higher taxa (taxonomic categories) and determining their interrelationships. Living cartilaginous fishes are probably better known (that is, more of the extant species have been discovered) than living bony fishes, simply because they tend to be larger, more conspicuous, and less secretive.Yet it was not until 1976 that one of the largest shark species, a deep-water filterfeeding species called the megamouth shark (Megachasma pelagios), was discovered near Hawaii. There may exist many additional Chondrichthyes species in deep ocean waters and remote coral reef areas. Chondrichthyes systematics (that is, the classification of the fishes) has undergone many changes and revisions as more has become known about fossil representatives and about the characteristics of the anatomy, biochemistry, and the like, of the living species. Studies of fossil cartilaginous fishes are limited almost entirely to samples of teeth, since these are virtually the only body parts durable enough to be preserved in the fossil record. Nevertheless, there is enough information in the characteristics of the teeth so that knowledge of the interrelationships of fossil species, both among one another and with living species, is quite advanced. It is known, for example, that the enormous hand-sized fossil teeth of Carcharodon megalodon, which lived about twenty million years ago, are so similar to the smaller teeth of the white shark (Carcharodon carcharias) that they both belong in the genus Carcharodon. Systematic study of modern species requires collection of specimens, generally by means of nets, traps, hooks, and lines, or spearing. Specimens are then preserved in some way and maintained in a museum collection. Entire specimens, if they are relatively small, can be maintained in diluted alcohol after fixation in formalin (formaldehyde solution). Other specimens, especially large ones, are dissected, and only certain parts are preserved, particularly the head skeleton with jaws and teeth. Certain new techniques provide taxonomic information from samples of living tissue. Karyotyping (analysis of the chromosomes), protein analysis (determination of the amino acid sequence), and DNA hybridization (estimation of genetic similarities) are all techniques that can elucidate interrelationships among the Chondrichthyes. Other disciplines concerned with the study of cartilaginous fishes include fisheries biology (the science of management and exploitation of commercially important fish species) and comparative physiology.

Reindeer

Reindeer are large deer, native to subarctic and arctic regions of northern Europe and Asia. They are related to North American caribou, as both are variants of the species Rangifer tarandus. Reindeer can be domesticated and have long been valuable possessions of humans in those regions of the world. They yield meat, cheese, butter, clothes, and draft animals able to carry heavy burdens. Many Eurasian reindeer still run wild and are trapped for domestication. Whether wild or domesticated, reindeer are herbivores, eating only plants. Their diet is grass, moss, leaves, twigs, and lichens. They often obtain food by scraping snow cover with their antlers and hooves. Reindeer are diurnal, meaning that they are active only during the day. They spend most of their time seeking food. Their preferred habitats are barren, open plains (tundra), forests, grasslands,andmountains.

Physical Characteristics of Reindeer
Reindeer differ from most deer in having large, deeply cleft hooves, hairy muzzles to help to keep them warm, and antlers on both males and females. Reindeer have long bodies and legs. Their hooves are broad, to provide footing on snow and ice. Male reindeer are four feet tall at shoulder height and weigh up to six hundred pounds. Females are shorter but reach similar maximum weights. Both genders grow up to seven feet long. Their thick, waterproof fur is brown in summer and gray-brown in winter. White fur covers their rumps, tails, and the lower portions of their legs. Males have white neck manes during mating season. Reindeer do not see well, but they have an excellent sense of smell. Reindeer antlers have pointed branches (points). In females, they grow to two-foot lengths, while males' antlers reach five-foot lengths. Very large male antlers have forty points. Those of females only have a few points. As in other deer, reindeer antlers are shed and regrown each year. Males lose their antlers in winter and females lose their antlers in late spring. The antlers that grow back are larger than those replaced. Antlers are important during mating season, when males fight for mates. Fights can damage antlers, so if they were not shed and regrown each year, many males would be unable to fight well, lose fights, and be unable to mate. Reindeer are also ruminants, animals that chew and swallow their food more than once. After a little while, food that was swallowed reenters the ruminant mouth from the stomach. Reindeer and other ruminants chew the food, swallow it again, and the food enters a different stomach for additional digestion. The process, also called cud chewing, helps reindeer to get maximum amounts of nutrients and vitamins from their difficult-to-digest food. Reindeer are social animals. They live in groups of about 20 most of the year. The groups consist of a male, his mates, and their young. Reindeer migrate great distances each fall and spring to feeding and mating grounds, travelling in herds of up to 100,000 and migrating about twenty miles per day. Reindeer mate mostly in October. Gestation is about eight months long. The female leaves the herd to give birth to one calf, in May or June. The calf weighs up to twenty pounds. Mother and calf then rejoin the herd and the calf nurses for six months.Acalf can mate when three years old. The life span of reindeer is up to fifteen years.

North American Reindeer Imports
Reindeer are excellent sources of food, clothes, and draft animals, as the Laplanders of Finland food source for the Inuit of Alaska, who live in a comparable environment with a similar social structure, the U.S. Office of Education imported thirteen hundred reindeer from Siberia near the end of the nineteenth century. Several million reindeer are now found throughout Alaska. In 1935, the Canadian government set up a herd of reindeer in the Yukon Territory to benefit Native Americans and Inuit. This herd also flourished and Native Americans and Inuitnowownall reindeer herds in North America. The deer satisfy many of their basic needs, becoming a valuable North American resource.

Rhino

Rhinoceroses (rhinos), which are among the world's largest land animals, belong to the ungulate family Rhinocerotidae. There are three Asian and two African species existing today; the fossil record shows several dozen extinct species as well. The name of the animal comes fromGreek rhino + ceros, meaning "nose-horned."

Physical Characteristics of Rhinoceroses
Rhinos weigh up to four tons and have short, thick, supportive legs. Rhino skin is thick, gray to brown in color, hangs loosely on the body, and is almost hairless. In the Asian species, skin folds at the junctures of the neck and limbs make them look armored. The Asian species also have have incisors and canine teeth, which are missing in the African species. Rhinos have long, prehensile upper lips, for grasping branches and removing leaves, which they eat. Depending on the species, rhinos have one or two nose horns. In two-horned species, the horn closest to the end of the snout is longer. The horns are made of keratin, a fibrous substance that also composes hair. The horns are used for digging food, for defense, and in mating combats. Rhinos are ungulates with three toes per foot, each of which ends in hooflike nails. Each front foot has a vestigial fourth toe. Rhinos, which are ruminants related to horses, eat grass, bulbs, leafy twigs, and shrubs. Although they look clumsy, rhinos can run as fast as horses. They have sharp vision, very good smell, and excellent hearing. Their keen hearing is due in part to their funnelshaped ears, that swivel in different directions.

Rhino Life Cycles
Most rhinos are both diurnal and nocturnal, active in daylight hours as well as after dark. They eat during the cool mornings and evenings, staying in mud wallows during hot afternoons. Rhinos have few enemies because of their size and their dangerous horns-an angry rhino charges its attackers. Humans are rhinos' great enemies, killing them for their horns, which are used in jewelry or medicinally. Most rhinos, especially males, live alone except during mating. There are some exceptions to solitary living: mothers live with their offspring, and young males or females may form same-sex groups. Males have territories, which are marked and defended. They fight each other for mates. While rhinosmaynot live with others of theirown species, they almost always have symbiotic birds, called oxpeckers, living on and around them. The birds eat insects from the rhinos' skins. This gives the birdfood, and frees the rhino fromthe insects. Mating takes place year round, and gestation lasts up to fifteen months. The female gives birth to a baby that weighs between 100 and 150 pounds. The young rhino stays with mother for 2.5 years, though it can feed itself in 2.5 months. Rhinos mate at seven to ten years of age. Females wait for approximately three years between gestations, only becoming pregnant after the previous offspring has left them. Rhinos live for up to fortyfive years.

Rhinoceros Species
There are five rhino species: three in Asia and Malaya, and two in Africa. African rhinos are twohorned and classified as "black" or "white", though all are bluish-gray. Black rhinos live in habitats from mountain forests to scrub lands. Their maximum body length is 10 feet, their height is 5 feet at the shoulder, and they weigh 1.75 tons. Each has a front horn up to 3.5 feet long. The rear horn is shorter. Black rhinos are mostly nocturnal, eating in the cool morning and evening hours and wallowing in river mud during the hot daylight hours. They eat grass, leaves, herbs, fruit, branches, and twigs. In the wild, a male has a marked territory, which he defends. When the territories of several males overlap, they form groups that share resources and defend the combined territory from strangers. White rhinos, similar to black rhinos, are the largest land mammals except elephants. Their maximum length is 13 feet, shoulder height is 6.75 feet, and they weigh 4 tons. Females use their horns for digging, defense, and guiding their offspring. Nearly extinct, white rhinos exist only in preserves. Indian rhinos-the largest Asian rhino species- average 10 feet in length and 5.5 feet in shoulder height, and weigh 2.75 tons. They have one thick, foot-long horn; their skin is sprinkled with knobs, and folds at the limb joints make them look as though they have armor. Females, although 75 percent the weight of males, have similar body heights and lengths. These rhinos live in marshy jungles and eat reeds, grass, twigs, and plant shoots. There are 1,500 Indian rhinos, all living in preserves and protected by legislation. Javan rhinos are similar to the Indian species, but smaller. They occur only inWestern Java, though they once lived in forests of Bengal, Burma, Borneo, Java, and Sumatra. Sumatran rhinos, the smallest rhinos, have two horns. They are approximately 4 feet tall, and weigh about 1 ton. Unlike the smooth-skinned African rhinos, they are hairy, especially on the tail and ears. The few living Sumatran rhinos are in Sumatra's forested hills. Rhinoceroses are reputedly dangerous. However, they are usually peaceful and timid, except when threatened. Legally protected rhinos suffer from the market for rhino horn, reputed to be a medicine and aphrodisiac in traditional Asian medicinal practice. This market has been a major factor in driving four of the five rhino species into endangerment.

Rodents

Rodents, comprising about two thousand species, form the largest, most abundant mammal order. They are found almost everywhere on the earth. Most are ground dwellers and many rodent species dwell underground in burrows or tunnel networks of varying complexity and size. However, rodents also dwell in tree nests (squirrels) or lodges in ponds and streams (beavers), or simply run in herds (capybaras). Judging from fossil remains, rodents were widespread and plentiful fifty million years ago. It is believed that they evolved from small, insect-eating mammals, and did not develop into large species until a million years ago. The largest ancient rodents were giant, bear-sized beavers. Contemporary rodents are usually small. However, the largest modern rodents are herbivorous capybaras, which grow to approximately 100 pounds as adults. Rodents also show remarkable diversity in their diets. These range from the vegetarian capybaras to the all-encompassing diet of omnivorous rats, which will eat meat. Rodents have many roles relative to humans. Hamsters and other small rodents are pets, capybaras are eaten as food, chinchillas are fur sources, and a few, such as rats and mice, are pests that compete with humans for their food crop supplies. The tremendous adaptability of rodents, especially rats, explains their wide geographical distribution in areas differing hugely in climate.

Physical Characteristics of Rodents
Among the two thousand known rodent species, size varies widely. Some small adult mouse species weigh about a fifteenth of a pound. At the other extreme, capybaras, largest of contemporary rodents, are the size of pigs. Regardless of size, all rodents possess pairs of large, chisel-like front teeth in both the upper and lower jaws. The roots of these incisor teeth are located far back in rodent jawbones and grow continuously. Rodents lack the tearing teeth (cuspids) of carnivores as well as several premolars. Therefore, a large space exists between their incisors and molars. This allows the incisors to operate well in gnawing. The design of rodent dentition also allows the gnawed food to be transferred easily to the molars for efficient grinding. In addition, the muscles of the rodent lower jaw are arranged so as to enable its easy movement backward, forward, and laterally. This optimizes grinding of gnawed food. Rodent incisors are different from those in other animals. Their continued growth from the root is valuable, especially because only the front surfaces of these teeth are protected by enamel, the hardest material in teeth. Thus, gnawing food causes the rear surfaces of the teeth to wear down faster than their front surfaces. This wear pattern is the basis for development of the chisel-like incisor edges. It continues as long as a rodent eats regularly, keeping the incisors sharp. Another interesting aspect of rodent mouths is that cheek fur grows inside the mouth and fills up the space between incisors and molars. This hair acts as padding and filters out food chunks too large to be swallowed comfortably. Other than the special development of "gnawing machinery" of the mouth and teeth, rodents are anatomically unspecialized, with no other ubiquitous anatomic features. Where any special characteristic has developed in some rodents, it appears to be due to environmental need. For example, claws and front paws of burrowing rodents, such as woodchucks and moles, make them efficient diggers. In addition, gliding adaptations in some squirrels allow them to "fly" (or actually glide) from tree to tree. Furthermore, leaping rodents such as the kangaroo rat use both hind feet together to enhance leaping capability. Yet another such adaptation is the webbed feet seen in beavers.

The Lives of Rodents
Rodents, like all other mammals, are warmblooded. They carry offspring to term in a uterus where each fetus is connected to the mother via the placenta, give birth to them, and nurse them. Depending on the rodent, the sequence of events between fertilization and the end of the nursing period takes between 5.5 weeks for a small mouse, to well over a year for large rodents. The process is easiest to describe for rats, although it is quite similar for mice and hamsters. After fertilization, rat eggs make their way into a complex uterus which can hold eight to sixteen fetuses. There, each attaches to the uterine wall and develops, over three weeks, into a rat pup. The pups are born pink, hairless, blind, and incompletely developed. They are then nurtured by their mothers, who have the instinct of all mammals to care for their offspring. Rats breast-feed their pups for three weeks. At the end of this time, they are fully covered in hair, have full vision, and have begun to eat foods other than milk. In another month the pups are sexually mature and can breed. This makes it clear why omnivorous wild rats pose a threat to humans.Anypair of rats can produce up to eighty offspring per year. Furthermore, within six weeks after birth, any two offspring can, and do, reproduce. Inbred laboratory rats live for two to three years, depending on the strain. Males are much larger than females (often twice their size) and may attain body weights up to two pounds. In the wild the life expectancy of rats varies greatly. However, reports of animals living for over five years occur. Some males have been reported to be as large as small cats or dogs. Wild rats live in complex tunnels as colonies of a hundred or more animals. Other rodents live different versions of the life of rats. Litter size, gestation time, group organization all vary. For example, the larger rodents have only a few offspring per litter, and some rodents live in tree nests (squirrels) or lodges in ponds (beavers). Life expectancies may be ten years or more, assuming death by natural causes.

Destructive and Beneficial Rodents
Rats and mice interact extensively with humans in a destructive fashion. The problems involved are competition for food, and disease transmission fromrodents to humans. Rats and mice, viewed as pests, are known to eat 10 to 25 percent of grain crops grown, harvested, and stored worldwide. This percentage varies depending upon the extent of use of rodenticides, such as warfarin, in various nations and the extent of agricultural technology. Very careful use of rodenticides is important because they are quite toxic to humans. Rodents are disease vectors, historically causing outbreaks of serious epidemics of the bubonic plague and tularemia. This was especially serious during the Middle Ages, when rats were responsible for the transmission of the Black Death. Currently, most sporadic outbreaks of rodent-derived infectious disease are handled by use of rodenticides to kill carriers and antibiotics to destroy rodent-borne microorganisms that infect humans. Most often it is not the rodents themselves that cause disease outbreaks. Rather, infection occurs as contaminated fleas and ticks move from rodents to humans. Rats are seen as the main disease vectors because they abound near and in human habitations. However, mice and any other infected rodents can be disease vectors. Concerning beneficial use of rodents, one can point to the myriad rats, mice, hamsters, and guinea pigs utilized as laboratory animals in testing and developing pharmaceuticals, the identification of toxic cosmetic, paint, and food components, isolation of disease cures, and so on. This aspect of research is likely to become less common because a large segment of the population deems it morally inappropriate to submit animals to these testing procedures. Another benefit of rodents that is becoming morally unacceptable is harvesting rodent fur. Beaver fur was once hugely important to the world fur trade. Presently, as beaver are nearly extinct, the use of rodents to provide fur for human use has shifted to muskrats, nutria, and chinchillas, which are valued for their attractive, luxuriant coats.