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Mammalian Classification Essay


The evolution of the class Mammalia has produced tremendous diversity in form and habit. Living kinds range in size from a bat weighing less than a gram and tiny shrews weighing but a few grams to the largest animal that has ever lived, the blue whale, which reaches a length of more than 30 metres (100 feet) and a weight of 180 metric tons (nearly 200 short [U.S.] tons). Every major habitat has been exploited by mammals that swim, fly, run, burrow, glide, or climb.

There are more than 5,500 species of living mammals, arranged in about 125 families and as many as 27–29 orders (familial and ordinal groupings sometimes vary among authorities). The rodents (order Rodentia) are the most numerous of existing mammals, in both number of species and number of individuals, and are one of the most diverse of living lineages. In contrast, the order Tubulidentata is represented by a single living species, the aardvark. The Uranotheria (elephants and their kin) and Perissodactyla (horses, rhinoceroses, and their kin) are examples of orders in which far greater diversity occurred in the late Paleogene and Neogene periods (about 30 million to about 3 million years ago) than today.

The greatest present-day diversity is seen in continental tropical regions, although members of the class Mammalia live on (or in seas adjacent to) all major landmasses. Mammals can also be found on many oceanic islands, which are principally, but by no means exclusively, inhabited by bats. Major regional faunas can be identified; these resulted in large part from evolution in comparative isolation of stocks of early mammals that reached these areas. South America (the Neotropics), for example, was separated from North America (the Nearctic) from about 65 million to 2.5 million years ago. Mammalian groups that had reached South America before the break between the continents, or some that “island-hopped” after the break, evolved independently from relatives that remained in North America. Some of the latter became extinct as the result of competition with more advanced groups, whereas those in South America flourished, some radiating to the extent that they have successfully competed with invaders since the rejoining of the two continents. Australia provides a parallel case of early isolation and adaptive radiation of mammals (specifically the monotremes and marsupials), although it differs in that Australia was not later connected to any other landmass. The placental mammals that reached Australia (rodents and bats) evidently did so by island-hopping long after the adaptive radiation of the mammals isolated early on.

In contrast, North America and Eurasia (the Palearctic) are separate landmasses but have closely related faunas as the result of having been connected several times during the Pleistocene Epoch (2.6 million to 11,700 years ago) and earlier across the Bering Strait. Their faunas frequently are thought of as representing not two distinct units but one, related to such a degree that a single name, Holarctic, is applied to it.

Importance to humans

Wild and domesticated mammals are so interlocked with our political and social history that it is impractical to attempt to assess the relationship in precise economic terms. Throughout our own evolution, for example, humans have depended on other mammals for food and clothing. Domestication of mammals helped to provide a source of protein for ever-increasing human populations and provided means of transportation and heavy work as well. Today, domesticated strains of the house mouse, European rabbit, guinea pig, hamster, gerbil, and other species provide much-needed laboratory subjects for the study of human-related physiology, psychology, and a variety of diseases from dental caries to cancer. The study of nonhuman primates (monkeys and apes) has opened broad new areas of research relevant to human welfare. The care of domestic and captive mammals is, of course, the basis for the practice of veterinary medicine.

Wild mammals are a major source of food in some parts of the world, and many different kinds, from fruit bats and armadillos to whales, are captured and eaten by various cultural groups. In addition, hunting, primarily for sport, of various rodents, lagomorphs, carnivores, and ungulates is a multibillion-dollar enterprise. In the United States alone, for example, it is estimated that more than two million deer are harvested annually by licensed hunters.

Geopolitically, the quest for marine mammals was responsible for the charting of a number of areas in both Arctic and Antarctic regions. The presence of terrestrial furbearers, particularly beavers and several species of mustelid carnivores (e.g., marten and fisher), was one of the principal motivations for the opening of the American West, Alaska, and the Siberian taiga. Ranch-raised animals such as the mink, fox, and chinchilla are also important to the fur industry, which directly and indirectly accounts for many millions of dollars in revenue each year in North America alone.

Aside from pelts and meat, special parts of some mammals regularly have been sought for their special attributes. Rhinoceros horn is used for concocting potions in eastern Asia; ivory from elephants and walruses is highly prized; and ambergris, a substance regurgitated by sperm whales, was once widely used as a base for perfumes.

Some mammals are directly detrimental to human activities. House rats and mice of Old World origin now occur virtually throughout the world and each year cause substantial damage and economic loss. Herbivorous mammals may eat or trample crops and compete with livestock for food, and native carnivores sometimes prey on domestic herds. Large sums are spent annually to control populations of “undesirable” wild mammals, a practice long deplored by conservationists. Not only do they have an impact on food resources, but mammals are also important reservoirs or agents of transmission of a variety of diseases that afflict man, such as plague, tularemia, yellow fever, rabies, leptospirosis, Lyme disease, hemorrhagic fevers such as Ebola, and Rocky Mountain spotted fever. The annual “economic debt” resulting from mammal-borne diseases that affect humans and domestic animals is incalculable.

Many large mammals have been extirpated entirely or exist today only in parks and zoos; others are in danger of extinction, and their plight is receiving increased attention from a number of conservation agencies. By the early 21st century, the International Union for Conservation of Nature (IUCN) reported that nearly one-quarter of all mammals are at risk of extinction. The single greatest threat to these mammals is the continued destruction of their habitat; however, many species are also aggressively hunted. The IUCN classifies each imperiled mammal into one of the following categories: near threatened, vulnerable, endangered, critically endangered, critically endangered and possibly extinct, or extinct in the wild (seeIUCN Red List of Threatened Species).

One of the most noteworthy cases of direct extirpation by man is the Steller’s sea cow (Hydrodamalis gigas). These large (up to 10 metres, or 33 feet, long), inoffensive marine mammals evidently lived only along the coasts and shallow bays of the Komandor Islands in the Bering Sea. Discovered in 1741, they were easily killed by Russian sealers and traders for food, their meat being highly prized, and the last known live individual was taken in 1768.

Of final note is the aesthetic value of wild mammals and the relatively recent expense of considerable energy and resources to study and, if possible, conserve vanishing species, to set aside natural areas where native floral and faunal elements can exist in an otherwise highly agriculturalized or industrialized society, and to establish modern zoological parks and gardens. Such outdoor “laboratories” attract millions of visitors annually and will provide means by which present and future generations of humans can appreciate and study, in small measure at least, other kinds of mammals.

Generally, most mammalian species are either polygynous (one male mates with multiple females) or promiscuous (both males and females have multiple mates in a given reproductive season). Because females incur such high costs during gestation and lactation, it is often the case that male mammals can produce many more offspring in a mating season than can females. As a consequence, the most common mating system in mammals is polygyny, with relatively few males fertilizing multiple females and many males fertilizing none. This scenario sets the stage for intense male-male competition in many species, and also the potential for females to be choosy when it comes to which males will sire her offspring. As a consequence of the choices females make and the effort males put into acquiring matings, many mammals have complex behaviors and morphologies associated with reproduction. Many mammal groups are marked by sexual dimorphism as a result of selection for males that can better compete for access to females.

About 3 percent of mammalian species are monogamous, with males only mating with a single female each season. In these cases, males provide at least some care to their offspring. Often, mating systems may vary within species depending upon local environmental conditions. For example, when resources are low, males may mate with only a single female and provide care for the young. When resources are abundant, the mother may be able to care for young on her own and males will attempt to sire offspring with multiple females.

Other mating systems such as polyandry can also be found among mammals. Some species (e.g. common marmosets and African lions) display cooperative breeding, in which groups of females, and sometimes males, share the care of young from one or more females. Naked mole rats have a unique mating system among mammals. Like social insects (Hymenoptera and Isoptera), naked mole rats are eusocial, with a queen female mating with several males and bearing all of the young in the colony. Other colony members assist in the care of her offspring and do not reproduce themselves.

Mating System: monogamous ; polyandrous ; polygynous ; polygynandrous (promiscuous) ; cooperative breeder ; eusocial

Many mammals are seasonal breeders, with environmental stimuli such as day length, resource intake and temperature dictating when mating occurs. Females of some species store sperm until conditions are favorable, after which their eggs are fertilized. In other mammals, eggs may be fertilized shortly after copulation, but implantation of the embryo into the uterine lining may be delayed (“delayed implantation”). A third form of delayed gestation is "delayed development", in which development of the embryo may be arrested for some time. Seasonal breeding and delays in fertilzation, implantation, or development are all reproductive strategies that help mammals coordinate the birth of offspring with favorable environmental conditions to increase the chances of offspring survival.

Some mammals give birth to many altricial young in each bout of reproduction. Despite being born in a relatively underdeveloped state, young of this type tend to reach maturity relatively quickly, soon producing many altricial young of their own. Mortality in these species tends to be high and average lifespans are generally short. Many species that exemplify this type of life history strategy can be found among the rodents and insectivores. At the other end of the life history spectrum, many mammals give birth to one or a few precocial young in each bout of reproduction. These species tend to live in stable environments where competition for resources is a key to survival and reproductive success. The strategy for these species is to invest energy and resources in a few, highly developed offspring that will grow to be good competitors. Cetaceans, primates and artiodactyls are examples of orders that follow this general pattern.

Among mammals, many reproductive strategies can be observed, and the patterns listed above are the extremes of a continuum encompassing this variation. Environmental factors, as well as physiological and historical constraints all contribute to the pattern of reproduction found in any population or species. Differences in these factors among species have led to the diversity of life history traits among mammals.

Key Reproductive Features: semelparous ; iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; induced ovulation ; fertilization (Internal ); viviparous ; oviparous ; sperm-storing ; delayed fertilization ; delayed implantation ; embryonic diapause ; post-partum estrous

A fundamental component of mammalian evolution, behavior, and life history is the extended care females must give to their offspring. Investment begins even before a female's eggs become fertilized. All female mammals undergo some form of estrus cycle in which eggs develop and become ready for potential fertilization. Hormones regulate changes in various aspects of female physiology throughout the cycle (e.g., the thickening of the uterine lining) and prepare the female for possible fertilization and gestation. Once fertilization occurs, females nurture their embryos in one of three ways--either by attending eggs that are laid externally (Prototheria), nursing highly altricial young (often within a pouch, or "marsupium"; Metatheria), or by nourishing the developing embryos with a placenta that is attached directly to the uterine wall for a long gestation period (Eutheria). Gestation in eutherians is metabolically expensive. The costs incurred during gestation depend upon the number of offspring in a litter and the degree of development each embryo undergoes.

Once the young are born (or hatch, in the case of monotremes) females feed their newborn young with milk, a substance rich in fats and protein. Because females must produce this high-energy substance, lactation is far more energetically expensive than gestation. Once mammals are born they must maintain their own body temperatures, no longer being able to depend on their mother for thermoregulation, as was the case during pregnancy. Lactating females must provide enough milk for their offspring to maintain their body temperatures as well as to grow and develop. In addition to feeding their young, females must protect them from predators. In some species, young remain with their mothers even beyond lactation for an extended period of behavioral development and learning.

Depending upon the species and environmental conditions, male mammals may either provide no care, or may invest some or a great deal of care to their offspring. Care by males often involves defending a territory, resources, or the offspring themselves. Males may also provision females and young with food.

Mammalian young are often born in an altricial state, needing extensive care and protection for a period after birth. Most mammals make use of a den or nest for the protection of their young. Some mammals, however, are born well-developed and are able to locomote on their own soon after birth. Most notable in this regard are artiodactyls such as wildebeest or giraffes. Cetacean young must also swim on their own shortly after birth.

Parental Investment: pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning; inherits maternal/paternal territory; maternal position in the dominance hierarchy affects status of young

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