Mammal

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Mammals
Fossil range: Late Triassic–Recent

Capybara (Hydrochoerus hydrochaeris)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
(unranked) Amniota
Class: Mammalia
Linnaeus, 1758
Subclasses & Infraclasses
  • Subclass †Allotheria*
  • Subclass Prototheria
  • Subclass Theria
    • Infraclass †Trituberculata
    • Infraclass Metatheria
    • Infraclass Eutheria

Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the presence of sweat glands, including those that produce milk, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex region in the brain. Most mammals also possess specialized teeth and utilize a placenta in the ontogeny. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species (including humans), distributed in about 1,200 genera, 153 families, and 29 orders,[1] though this varies by classification scheme.

Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crowngroup Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.

Contents

  • 1 Distinguishing features
  • 2 Anatomy and morphology
    • 2.1 Skeletal system
    • 2.2 Respiratory system
    • 2.3 Circulatory system
    • 2.4 Nervous system
    • 2.5 Integumentary system
    • 2.6 Reproductive system
  • 3 Physiology
    • 3.1 Intelligence
      • 3.1.1 Social Structure
      • 3.1.2 Play
    • 3.2 Locomotion
      • 3.2.1 Terrestrials
      • 3.2.2 Arboreals
      • 3.2.3 Aquatics
      • 3.2.4 Aerials
    • 3.3 Feeding
  • 4 Evolutionary history
    • 4.1 Main stages in evolution of mammals
    • 4.2 Earliest appearances of typical mammalian features
  • 5 Classification
    • 5.1 Standardized textbook classification
    • 5.2 McKenna/Bell classification
    • 5.3 Molecular classification of placentals
  • 6 References
  • 7 Bibliography
  • 8 See also
  • 9 External links

[edit] Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk. However, other features are required when classifying fossils, since soft tissue glands and some other features are not visible in fossils. The evolution of mammals from synapsids, also known as mammal-like "reptiles" was a gradual process which took approximately 70 million years, from the mid-Permian to the mid-Jurassic, and by the mid-Triassic there were many species that looked like mammals. Note that synapsids are not reptiles at all, but belong to a distinct lineage of tetrapods.

Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). Most reptiles and non-mammalian synapsids use this system including lizards, crocodilians, dinosaurs, (and their descendants the birds), and therapsids (mammal-like "reptiles"). Mammals have a different jaw joint, however, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). In mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear. Note: "non-mammalian synapsids" above implies that mammals are a sub-group of synapsids, and that is exactly what cladistics says they are.

Mammals also have a double occipital condyle; they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, as it would be confusing if they found a fossil that had one feature, but not the other.

[edit] Anatomy and morphology

[edit] Skeletal system

The majority of mammals have seven cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.

[edit] Respiratory system

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the diaphragm contracts, forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows.

[edit] Circulatory system

The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.

[edit] Nervous system

All mammalian brains possess a neocortex, a brain region that is unique to mammals.

[edit] Integumentary system

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates.

The epidermis is typically ten to thirty cells thick; its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair. It is a definitive characteristic of the order. Some mammals have very little, albeit in obscure parts of their bodies, but nonetheless, careful examination reveals the characteristic. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth and the polar bear may seem to have green fur, but this color is caused by algae growths.

[edit] Reproductive system

Goat kids will stay with their mother until they are weaned, this is usually about one month

Most mammals give birth to live young (vivipary), but a few, such as the monotremes lay eggs. Live birth also occurs in some non-mammalian species, such as guppies, snakes, and hammerhead sharks; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are birds, so this too is not a defining feature.

Mammals have sweat glands, a defining feature present only in mammals. Some of these glands produce milk (in what are called mammary glands), a liquid used by newborns as their primary source of nutrition. The monotremes branched from other mammals early on, and do not have the nipples seem in most mammals, but they do have mammary glands. Most mammals are terrestrial, but some are aquatic, including sirenia, (manatees and dugongs), and the cetaceans, (dolphins and whales). Whales are the largest of all animals. There are semi-aquatic mammalian species such as seals which come to land to breed but spend most of the time in water.

[edit] Physiology

[edit] Intelligence

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain relatively smaller than a cat that must "think" to outwit its prey.[2]

[edit] Social Structure

[edit] Play

[edit] Locomotion

[edit] Terrestrials

[edit] Arboreals

[edit] Aquatics

[edit] Aerials

The only mammals in which true flight has been observed are bats; mammals such as flying squirrels and Colugos are more accurately classified as gliding mammals.

[edit] Feeding

To maintain a high constant body temperature is energy expensive- mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat animal prey- this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. An herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore eats boths prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of a herbivore is therefore host to bacteria that ferment these substances, and make them avalable for digestion. The bacteria are either housed in the multichambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat losing surface area to heat generating volume, they tend to have high-energy requirements and a high metabolic rate. Mammals that weigh less than about 18oz (500g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Futhermore, mammals that weigh more than 18oz (500g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).[2]

[edit] Evolutionary history

For more details on this topic, see Evolution of mammals.

The evolution of mammals from synapsids, also known as mammal-like "reptiles" was a gradual process which took approximately 70 million years, from the mid-Permian to the mid-Jurassic, and by the mid-Triassic there were many species that looked like mammals. Note that synapsids are not reptiles at all, but belong to a distinct lineage of tetrapods.

[edit] Main stages in evolution of mammals

The original synapsid skull structure has one hole behind each eye, in a fairly low position on the skull (lower right in this image).

The first fully terrestrial vertebrates were amniotes - their eggs had internal membranes which allowed the developing embryo to breathe but kept water in. This allowed amniotes to lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the late Carboniferous. Within a few million years two important amniote lineages became distinct: the synapsids, from which mammals are descended ; and the sauropsids, from which lizards, snakes, crocodilians, dinosaurs and birds are descended. [3] Synapsids have a single hole (temporal fenestra) low on each side of the skull.

One synapsid group, the pelycosaurs, were the most common land vertebrates of the early Permian and included the largest land animals of the time.[4]

Therapsids descended from pelycosaurs in the middle Permian, about 260M years ago, and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae; incisors which are equal in size.[5] The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with the Triassic cynodonts, some of which could easily be mistaken for mammals:

The Permian-Triassic extinction ended the dominance of the therapsids, and in the early Triassic all the medium to large land animal niches were taken over by archosaurs, which were the ancestors of crocodilians, pterosaurs, dinosaurs and birds. After this "Triassic Takeover" the cynodonts and their descendants could only survive as small, mainly nocturnal insectivores.[7] This may actually have accelerated the evolution of mammals - for example the surviving cynodonts and their descendants had to evolve to wards warm-bloodedness because their small bodies would otherwise have lost heat quickly, especially as they were active mainly at night.

The first true mammals appeared in the early Jurassic, over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. Hadrocodium appears to be in the middle of the transition to true mammal status — it had a mammalian jaw joint (formed by the dentary and squamosal bones, but there is some debate about whether its middle ear was fully mammalian.[8]

The earliest known monotreme is Teinolophos, which lived about 123M years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:

Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.

The oldest known marsupial is Sinodelphys, found in 125M-year old early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.[9]

The living Eutheria ("true beasts") are all placentals. But the earliest known eutherian, Eomaia, found in China and dated to 125M years ago, has some features which are more like those of marsupials (the surviving metatherians):[10]:

Unfortunately it is not certain when placental mammals evolved - the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs.[11]

Mammals and near-mammals expanded out of the nocturnal insectivore niche from the mid Juraassic onwards - for example Castorocauda had adaptations for swimming, digging and catching fish.[12] The traditional view is that: mammals only took over the medium- to large-sized ecological niches in the Cenozoic, after the extinction of the dinosaurs; but then they diversified very quickly, for example the earliest known bat dates from about 50M years ago, only 15M years after the extinction of the dinosaurs.[13] During the Cenozoic several groups of mammals appeared which were much larger than their nearest modern equivalents - but none was even close to the size of the largest dinosaurs with similar feeding habits.

Recent molecular phylogenetic studies suggest that most placental orders diverged about about 100M to 85M years ago, but that modern families first appeared in the late Eocene and early Miocene[14] But paleontologists object that no placental fossils have been found from before the end of the Cretaceous[15]

[edit] Earliest appearances of typical mammalian features

Hadrocodium, whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints.

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes (egg-laying mammals):[16][17][18]

The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164M years ago in the mid Jurassic. From 1955 onwards some scientists have interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels which supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur.[19][20] But foramina do not necessarily show that an animal had vibrissae - for example the modern lizard Tupinambis has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon.[21][22]

The evolution of erect limbs in mammals is incomplete - living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes the last common ancestor of modern marsupial and placentals - and therefore that all earlier mammals had sprawling limbs.[23] Sinodelphys (the earliest known marsupial) and Eomaia (the earliest known eutherian) lived about 125M years ago, so erect limbs must have evolved before then.

It is currently very difficult to be confident when warm-bloodedness first appeared in the evolution of mammals. Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals.[24] So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic cynodonts may have had fairly high metabolic rates, but is not conclusive. In particular it is difficult to see how small animals can maintain a high and stable body temperature without fur, and there is no certain evidence of fur before Castorocauda, about 164M years ago.

[edit] Classification

Main article: Mammal classification
Over 70% of mammal species are in the orders Rodentia (blue), Chiroptera (red), and Soricomorpha (yellow)

George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

[edit] Standardized textbook classification

A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).

Class Mammalia

[edit] McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification".

McKenna and Bell, Classification of Mammals: Above the species level, (1997) is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The new McKenna/Bell classification was quickly accepted by paleontologists. The authors work together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.

The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.

Extinct groups are represented by a cross (†).

Class Mammalia

[edit] Molecular classification of placentals

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by Retrotransposon presence/absence data. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography:

Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago when bats and murine rodents arrived has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological data, and thus not accepted by many systematists. Further there is some indication from Retrotransposon presence/absence data that the traditional Epitheria hypothesis, suggesting Xenarthra as the first divergence, might be true.