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A mammal () is a animal of the class Mammalia (). Mammals are characterized by the presence of milk-producing for feeding their young, a region of the brain, or , and three middle ear bones. These characteristics distinguish them from and , from which their ancestors diverged in the Period over 300 million years ago. Around 6,400 of mammals have been described and divided into 29 orders.

The largest orders of mammals, by number of species, are the , , and (including , moles and ). The next three are the (including , and ), the even-toed ungulates (including , , and ), and the (including , , and ).

Mammals are the only living members of ; this , together with (reptiles and birds), constitutes the larger clade. The early synapsids were , a group that included the famous . The synapsids split into several diverse groups of non-mammalian synapsids—traditionally and incorrectly referred to as mammal-like reptiles or by the term , and now known as stem mammals or protomammals—before giving rise to during the beginning of the period. Mammals originated from , an advanced group of therapsids, during the Late -Early . The modern mammalian orders arose in the and periods of the era, after the extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present.

The basic mammalian body type is , and most mammals use their four extremities for terrestrial locomotion; but in some, the extremities are adapted for , in the air, in trees, , or . Mammals range in size from the to the —possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the . All modern mammals give birth to live young, except the five species of , which are egg-laying mammals. The most species-rich group of mammals, the cohort called , have a , which enables the feeding of the fetus during .

Most mammals are , with some possessing large brains, , and tool use. Mammals can communicate and vocalize in several ways, including the production of , , , singing, echolocation; and, in the case of humans, complex . Mammals can organize themselves into fission-fusion societies, harems, and —but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.

of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in replacing as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first . Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food ( and ), , and . Mammals are also and raced for sport, and are used as in science. Mammals have been depicted in since times, and appear in literature, film, mythology, and religion. Decline in numbers and of many mammals is primarily driven by human and habitat destruction, primarily .

Classification
, rodents (blue); , bats (red); and , shrews (yellow).

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Mammal classification has been through several revisions since initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums.

(2023). 9781284032093, Jones and Bartlett Learning.
Simpson (1945) provides of mammal origins and relationships that had been taught universally until the end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found: The 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 . Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are : , , , , , and other gnawing mammals; : bats; and : , moles, and . The next three biggest orders, depending on the biological classification scheme used, are the : , , and ; the : and even-toed ungulates; and the which includes , , , , , and allies. According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 , 153 families and 29 orders. In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct.

Definitions
The word "" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia as the of mammals, the consisting of the most recent common ancestor of living ( and ) and mammals ( and ) and all descendants of that ancestor. Since this ancestor lived in the period, Rowe's definition excludes all animals from the earlier , despite the fact that Triassic fossils in the have been referred to the Mammalia since the mid-19th century.
(1871). 9781345182484, John Murray.
If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. is more closely related to monotremes than to therian mammals while and are more closely related to the therians; as fossils of all three genera are dated about in the , this is a reasonable estimate for the appearance of the crown group.

T. S. Kemp has provided a more traditional definition: " that possess a – jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of and living mammals.

(2023). 9780198507604, Oxford University Press. .
The earliest known synapsid satisfying Kemp's definitions is , dated , so the appearance of mammals in this broader sense can be given this date.

McKenna/Bell classification
In 1997, the mammals were comprehensively revised by and Susan K. Bell, which has resulted in the McKenna/Bell classification. The authors worked together as at the American Museum of Natural History. 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. Their 1997 book, Classification of Mammals above the Species Level,
(1997). 9780231110136, Columbia University Press.
is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the groupings.

In the following list, groups are labelled with a dagger (†).

Class Mammalia

Molecular classification of placentals
As of the early 21st century, molecular studies based on analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by presence/absence data. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—, and —which in the . The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are (basal Boreoeutheria), (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). Boreoeutheria in turn contains two major lineages— and .

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as or mitochondrial) and varying interpretations of data.

The above is based on Tarver et al.. (2016)

Evolution
Origins
, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from the reptile lineage. Crown group mammals evolved from earlier during the . The cladogram takes Mammalia to be the crown group.

Evolution from older amniotes
The first fully terrestrial were . Like their amphibious early predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while generally need to lay their eggs in water.

The first amniotes apparently arose in the Pennsylvanian subperiod of the . They descended from earlier amphibious tetrapods, which lived on land that was already inhabited by and other invertebrates as well as , and other plants. Within a few million years, two important amniote lineages became distinct: the , which would later include the common ancestor of the mammals; and the , which now include , , , and (including ). Synapsids have a single hole (temporal fenestra) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early such as . Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" or ; we now know they were neither reptiles nor part of reptile lineage.

, a group of synapsids, evolved in the , about 265 million years ago, and became the dominant land vertebrates. They differ from basal in several features of the skull and jaws, including: larger skulls and which are equal in size in therapsids, but not for eupelycosaurs. The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with , some of which could easily be mistaken for mammals. Those stages were characterized by:

(1984). 9780709915348, Croom Helm.
  • The gradual development of a bony secondary .
  • Abrupt acquisition of among , thus prior to the origin of mammals by 30–50 millions of years .
  • Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
  • The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).

First mammals
The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids. In the early Triassic, most medium to large land carnivore niches were taken over by which, over an extended period (35 million years), came to include the , the and the dinosaurs; however, large cynodonts like and still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal niche from the mid-Jurassic onwards;

(2023). 9781461443544, Springer.
The Jurassic , for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish. Most, if not all, are thought to have remained nocturnal (the nocturnal bottleneck), accounting for much of the typical mammalian traits. The majority of the mammal species that existed in the were multituberculates, eutriconodonts and . The earliest known is , found in 125 million-year-old in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues. The oldest known fossil among the ("true beasts") is the small shrewlike , or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic. A later eutherian relative, , dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. In particular, the extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and , an early Cretaceous animal in the eutherian order . This also applies to the multituberculates. They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
(2023). 9781421406435, Johns Hopkins University Press.

One of the earliest known monotremes was , which lived about 120 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce ()—as lizards and birds also do—

(1995). 9780868401430, University of New South Wales.
and they lay eggs which are leathery and uncalcified.

Earliest appearances of features
, whose fossils date from approximately 195 million years ago, in the early , provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.
(2023). 9780801884726, Johns Hopkins University Press.

The earliest clear evidence of hair or fur is in fossils of and , from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the and (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae () and so were evidence of hair or fur;

(1982). 9780124041202, Academic Press.
it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard has foramina that are almost identical to those found in the nonmammalian cynodont .
(1986). 9780874745245, Smithsonian Institution Press.
Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon. Studies on Permian suggest that non-mammalian of the epoch already had fur, setting the evolution of hairs possibly as far back as .

When first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian . Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,

(1988). 9780671619466, Simon and Schuster. .
but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians. Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.

The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago. bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in , suggesting that it is a between them and . They are omnipresent in non-placental mammaliformes, though and appear to have lacked them.

(1979). 9780520039513, University of California Press.

It has been suggested that the original function of ( production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals. In human females, mammary glands become fully developed during puberty, regardless of pregnancy.

Rise of the mammals
Therian mammals took over the medium- to large-sized ecological niches in the , after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna). Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.

Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late through the . However, no placental fossils have been found from before the end of the Cretaceous. The earliest undisputed fossils of placentals come from the early , after the extinction of the non-avian dinosaurs. (Scientists identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals, but it has since been reclassified as a non-placental eutherian.) Recalibrations of genetic and morphological diversity rates have suggested a origin for placentals, and a Paleocene origin for most modern clades.

The earliest known ancestor of primates is Archicebus achilles from around 55 million years ago. This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.

Anatomy
Distinguishing features
Living mammal species can be identified by the presence of , including to nourish their young.
(1977). 9780039102845, Holt-Saunders International.
In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.
(2023). 9780716738732, Sinauer Associates, Inc..

Many traits shared by all living mammals appeared among the earliest members of the group:

  • – The (the lower jaw bone, which carries the teeth) and the (a small bone) meet to form the joint. In most , including early , the joint consists of the (a small bone at the back of the lower jaw) and (a small bone at the back of the upper jaw).
  • – In crown-group mammals, sound is carried from the by a chain of three bones, the , the and the . Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.
  • Tooth replacement – Teeth can be replaced once () or (as in toothed whales and rodents) not at all (). Elephants, manatees, and kangaroos continually grow new teeth throughout their life ().
  • Prismatic enamel – The coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the to the tooth's surface.
  • Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra; most other , in contrast, have only one such knob.

For the most part, these characteristics were not present in the Triassic ancestors of the mammals. Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.

Sexual dimorphism
On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Most mammalian orders also exhibit male-biased sexual dimorphism, although some orders do not show any bias or are significantly female-biased (). Sexual size dimorphism increases with body size across mammals (Rensch's rule), suggesting that there are parallel selection pressures on both male and female size. Male-biased dimorphism relates to sexual selection on males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated by , and the degree of male-biased size dimorphism. The degree of sexual selection is also positively correlated with male and female size across mammals. Further, parallel selection pressure on female mass is identified in that age at weaning is significantly higher in more species, even when correcting for body mass. Also, the reproductive rate is lower for larger females, indicating that fecundity selection selects for smaller females in mammals. Although these patterns hold across mammals as a whole, there is considerable variation across orders.
(2023). 9780199208784, Oxford University Press. .

Biological systems
The majority of mammals have seven cervical vertebrae (bones in the neck). The exceptions are the and the , which have six, and the which has nine.
(2023). 9781420041637, CRC Press.
All mammalian brains possess a , a brain region unique to mammals. Placental brains have a , unlike monotremes and marsupials.

Circulatory systems
The mammalian has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.
(2023). 9780808923718, Churchill Livingstone.
The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four . Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the , the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.
(2023). 9781938168130, Rice University Press. .

Respiratory systems
The of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and , and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.
(2023). 9780071793131, McGraw-Hill Medical.
This type of lung is known as a bellows lung due to its resemblance to blacksmith .
(2023). 9789380704746, Jaypee Brothers Medical Publishing.

Integumentary systems
The (skin) is made up of three layers: the outermost epidermis, the and the . The epidermis is typically 10 to 30 cells thick; its main function is 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 15 to 40 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 , which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species; require a thick hypodermis () for insulation, and have the thickest blubber at .
(1988). 9780935848472, Brill Archive.
Although other animals have features such as whiskers, , , or cilia that superficially resemble it, no animals other than mammals have . It is a definitive characteristic of the class, though some mammals have very little.
(2023). 9780801886959, Johns Hopkins University Press.

Digestive systems
Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed structures that reflect their feeding preferences. For instance, (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and . animals that tend to eat hard, -rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth.
(2023). 9780226724904, University of Chicago Press. .
Most carnivorous mammals have teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.

The stomach of even-toed ungulates (Artiodactyla) is divided into four sections: the , the reticulum, the and the (only have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus (or ), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulolytic (, and ) produce , which is needed to break down the in plants. Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged , where it is fermented by bacteria.

(2023). 9780763762995, Jones and Bartlett.
Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The is either absent or short and simple, and the large intestine is not or much wider than the small intestine.
(2023). 9781110768578, Adam and Charles Black.
Excretory and genitourinary systems
The mammalian involves many components. Like most other land animals, mammals are , and convert into , which is done by the as part of the .
(2023). 9788120341074, PHI Learning Pvt. Ltd..
, a waste product derived from , is passed through and with the help of enzymes excreted by the liver.
(2023). 9780199830121, Oxford University Press, US.
The passing of bilirubin via bile through the gives mammalian a distinctive brown coloration.
(2023). 9783540768388, Springer.
Distinctive features of the include the presence of the and , and of a clearly distinguishable and , which is due to the presence of elongated loops of Henle. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidneys of pinnipeds, and bears. Most adult placental mammals have no remaining trace of the . In the embryo, the divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the and , while in males it forms the entirety of the . However, the , , and some retain a cloaca as adults. Biological Reviews – Cambridge Journals In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.
(1977). 9780039102845, Holt-Saunders International.
Monotremes, which translates from into "single hole", have a true cloaca.
(2023). 9780544859937, Mariner Books.

Sound production
As in all other tetrapods, mammals have a that can quickly open and close to produce sounds, and a supralaryngeal which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to . The larynx controls the pitch and of sound, but the strength the lungs exert to also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using . The movement or tenseness of the vocal folds can result in many sounds such as and . Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.

Some mammals have a large larynx and thus a low-pitched voice, namely the hammer-headed bat ( Hypsignathus monstrosus) where the larynx can take up the entirety of the while pushing the lungs, heart, and trachea into the . Large vocal pads can also lower the pitch, as in the low-pitched roars of . The production of is possible in some mammals such as the ( Loxodonta spp.) and . Small mammals with small larynxes have the ability to produce , which can be detected by modifications to the and . Ultrasound is inaudible to birds and reptiles, which might have been important during the Mesozoic, when birds and reptiles were the dominant predators. This private channel is used by some rodents in, for example, mother-to-pup communication, and by bats when echolocating. Toothed whales also use echolocation, but, as opposed to the vocal membrane that extends upward from the vocal folds, they have a melon to manipulate sounds. Some mammals, namely the primates, have air sacs attached to the larynx, which may function to lower the resonances or increase the volume of sound.

The vocal production system is controlled by the cranial nerve nuclei in the brain, and supplied by the recurrent laryngeal nerve and the superior laryngeal nerve, branches of the . The vocal tract is supplied by the hypoglossal nerve and . Electrical stimulation of the periaqueductal gray (PEG) region of the mammalian elicit vocalizations. The ability to learn new vocalizations is only exemplified in humans, seals, cetaceans, elephants and possibly bats; in humans, this is the result of a direct connection between the , which controls movement, and the in the spinal cord.

Fur
The primary function of the fur of mammals is . Others include protection, sensory purposes, waterproofing, and camouflage. Different types of fur serve different purposes:
  • Definitive – which may be after reaching a certain length
  • Vibrissae – sensory hairs, most commonly
  • Pelage – guard hairs, under-fur, and
  • Spines – stiff guard hair used for defense (such as in )
  • – long hairs usually used in visual signals. (such as a lion's mane)
  • – often called "down fur" which insulates newborn mammals
  • – long, soft and often curly

Thermoregulation
Hair length is not a factor in thermoregulation: for example, some tropical mammals such as sloths have the same length of fur length as some arctic mammals but with less insulation; and, conversely, other tropical mammals with short hair have the same insulating value as arctic mammals. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the has guard hairs measuring as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of . Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool; a camel's fur may reach in the summer, but the skin stays at . , conversely, trap air in their fur to conserve heat by keeping the skin dry.

Coloration
Mammalian coats are colored for a variety of reasons, the major selective pressures including , , communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of ; for brown and black colors and for a range of yellowish to reddish colors, giving mammals an . Some mammals have more vibrant colors; certain monkeys such and , and opossums such as the Mexican mouse opossums and Derby's woolly opossums, have blue skin due to light diffraction in fibers. Many sloths appear green because their fur hosts green ; this may be a relation that affords to the sloths.

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey. In arctic and subarctic mammals such as the ( Alopex lagopus), ( Dicrostonyx groenlandicus), ( Mustela erminea), and ( Lepus americanus), seasonal color change between brown in summer and white in winter is driven largely by camouflage. Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely habitat due to convergent evolution.

, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling and the powerful and aggressive . Coat color is sometimes sexually dimorphic, as in many primate species. Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection. Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker colored coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as , have darker fur in the winter. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin. The dazzling black-and-white striping of appear to provide some protection from biting flies.

Reproductive system
Mammals are solely (an animal is born with either male or female genitalia, as opposed to where there is no such schism).
(2023). 9784431566090, Springer. .
In male placentals, the is used both for urination and copulation. Depending on the species, an may be fueled by blood flow into vascular, spongy tissue or by muscular action. A penis may be contained in a when not erect, and some placentals also have a penis bone ().
(1998). 9780792383369, Springer Science & Business Media. .
Marsupials typically have forked penises,
(1987). 9780521337922, Cambridge University Press. .
while the penis generally has four heads with only two functioning. The of most mammals descend into the which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a , and on the outside, while the internal system contains paired , 1–2 , 1–2 and a . Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placental mammals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two have a single cervix that connects to the vagina, a , which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.
(2023). 9781489959881, Springer. .
(2023). 9780030250347, Jones & Bartlett Publishers.

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped, either through direct or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of . The oldest demonstration of this reproductive style is with , which produced undeveloped , but at much higher litter sizes than any modern mammal, 38 specimens. Most modern mammals are , giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex-determination system different from that of most other mammals. In particular, the of a platypus are more like those of a chicken than those of a therian mammal.

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short period, typically shorter than its and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's . This is the condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy.

(2023). 9781421406435, Johns Hopkins University Press.
Even non-placental eutherians probably reproduced this way. The placentals give birth to relatively complete and developed young, usually after long gestation periods.
(2023). 9781410910509, Raintree.
They get their name from the , which connects the developing fetus to the uterine wall to allow nutrient uptake.
(2023). 9788121925570, S. Chand and Company.
In placental mammals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.

The of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly. Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages.

(2023). 9781139457422, Cambridge University Press.
is the main sugar in placental mammal milk while monotreme and marsupial milk is dominated by .
(2023). 9781421420424, Johns Hopkins University Press.
is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.
(2023). 9780226440118, University of Chicago Press.

Endothermy
Nearly all mammals are ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.
(2023). 9780805366242, Benjamin Cummings. .
Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the produces little metabolic heat, so it is considered an operational . Birds are also endothermic, so endothermy is not unique to mammals.
(1997). 9780521570985, Cambridge University Press.

Species lifespan
Among mammals, species maximum lifespan varies significantly (for example the has a lifespan of two years, whereas the oldest is recorded to be 211 years). Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow, it was found that DNA excision repair capability increased systematically with species lifespan among seven mammalian species. Species lifespan was observed to be robustly correlated with the capacity to recognize DNA double-strand breaks as well as the level of the DNA repair protein Ku80. In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be up-regulated in the longer-lived species. The cellular level of the DNA repair enzyme poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species. Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.

Locomotion
Terrestrial
Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are , walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are , walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement. Some animals such as are , walking on the tips of their toes. This even further increases their stride length and thus their speed.
(1998). 9780716726371, W. H. Freeman and Company.
A few mammals, namely the great apes, are also known to , at least for their front legs. and platypuses are also knuckle-walkers. Some mammals are , using only two limbs for locomotion, which can be seen in, for example, humans and the great apes. Bipedal species have a larger field of vision than quadrupeds, conserve more energy and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and .

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal and , but they employ many other gaits occasionally, including a four-legged crawl in tight spaces. Mammals show a vast range of , the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and .

(1995). 9781565934160, Nelson Thornes.
Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.

Arboreal
Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to (swing between trees).
(1985). 9780674327757, Belknap Press.
Many arboreal species, such as tree porcupines, , spider monkeys, and , use to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and reorient the direction of forces the animal applies. This is what allows to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat. Size relating to weight affects gliding animals such as the . Some species of primate, bat and all species of achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.

Aerial
Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.

The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to maneuver more accurately and fly with more lift and less drag. By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds. The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly.

(2023). 9780826344038, University of New Mexico Press.
The surface of their wings is equipped with touch-sensitive receptors on small bumps called , also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.

Fossorial and subterranean
A fossorial (from Latin fossor, meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are , and . Many species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in temperature regulation while others use the underground habitat for protection from or for .Damiani, R, 2003, Earliest evidence of cynodont burrowing, The Royal Society Publishing, Volume 270, Issue 1525

Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; , for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial the eyes are degenerated and useless, talpa moles have eyes and the cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order .

Aquatic
Fully aquatic mammals, the cetaceans and , have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a to prevent themselves from turning upside-down in the water. The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.
(1989). 9780644060561, Australian Government Publications.

mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.

(2023). 9780520270572, University of California Press.
Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of in their skin that may increase and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim. They rely on their fore-flippers for locomotion in a wing-like manner similar to and . Fore-flipper movement is not continuous, and the animal glides between each stroke. Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage; the hind-flippers serve as stabilizers.
(1990). 9780520064973, University of California Press. .
Other semi-aquatic mammals include beavers, , and platypuses. Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have skeletal structures,
(1999). 9780856611315, T & AD Poyser Ltd.
adapted to carrying their enormous weight, and their allows them to sink and move along the bottom of a river.

Behavior
Communication and vocalization
Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as , to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roaring of and . The of the humpback whale may be signals to females;
(2023). 9780123735539, Academic Press.
they have different dialects in different regions of the ocean. Social vocalizations include the territorial calls of , and the use of frequency in greater spear-nosed bats to distinguish between groups. The gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground. similarly have complex calls that signal the type, size, and speed of an approaching predator. Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are , below the hearing range of humans, and can be heard by other elephants up to away at still times near sunrise and sunset.

Mammals signal by a variety of means. Many give visual anti-predator signals, as when deer and , their fit condition and their ability to escape,

(2023). 9780198526841, Oxford University Press.
or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected. Many mammals make use of , sometimes possibly to help defend territory, but probably with a range of functions both within and between species. and including vocalize both socially and in echolocation.
(2023). 9780226795997, Chicago University Press.
(1993). 9783540978350, Springer-Verlag.

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 other animals—this is a diet (and includes insectivorous diets). Other mammals, called , eat plants, which contain complex carbohydrates such as cellulose. An herbivorous diet includes subtypes such as (seed eating), (leaf eating), (fruit eating), (nectar eating), (gum eating) and (fungus eating). The digestive tract of an herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichambered or in a large cecum. Some mammals are , consuming to absorb the nutrients not digested when the food was first ingested. An eats both prey and plants. Carnivorous mammals have a simple because the , and found in meat require little in the way of specialized digestion. Exceptions to this include who also house in a multi-chambered stomach, like terrestrial herbivores.

The size of an animal is also a factor in determining diet type (Allen's rule). 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 . Mammals that weigh less than about are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an 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 (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 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 ( or ).

Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favor of one more than the other. Since plants and meat are digested differently, there is a preference for one over the other, as in bears where some species may be mostly carnivorous and others mostly herbivorous. They are grouped into three categories: (50–70% meat), (70% and greater of meat), and (50% or less of meat). The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food. Hypercarnivores, however, have conical teeth and sharp carnassials meant for slashing, and in some cases strong jaws for bone-crushing, as in the case of , allowing them to consume bones; some extinct groups, notably the , had saber-shaped .

Some physiological carnivores consume plant matter and some physiological herbivores consume meat. From a behavioral aspect, this would make them omnivores, but from the physiological standpoint, this may be due to . Physiologically, animals must be able to obtain both energy and nutrients from plant and animal materials to be considered omnivorous. Thus, such animals are still able to be classified as carnivores and herbivores when they are just obtaining nutrients from materials originating from sources that do not seemingly complement their classification. For example, it is well documented that some ungulates such as giraffes, camels, and cattle, will gnaw on bones to consume particular minerals and nutrients. Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as ), aid with hemoglobin production, and as a laxative.

Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as . In the period preceding hibernation, larger mammals, such as bears, become to increase fat stores, whereas smaller mammals prefer to collect and stash food. The slowing of the metabolism is accompanied by a decreased heart and respiratory rate, as well as a drop in internal temperatures, which can be around ambient temperature in some cases. For example, the internal temperatures of hibernating arctic ground squirrels can drop to , however the head and neck always stay above . A few mammals in hot environments in times of drought or extreme heat, for example the fat-tailed dwarf lemur ( Cheirogaleus medius).

(2023). 9783642024207, Springer-Verlag.

Intelligence
In intelligent mammals, such as , the is larger relative to the rest of the brain. itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. , 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 . In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.
(2023). 9780789477644, DK Publishing.

Tool use by animals may indicate different levels of and . The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing from rocks or breaking open shells), with some populations spending 21% of their time making tools. Other tool use, such as using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.

(2023). 9780061671845, Harper.
Tools may even be used in solving puzzles in which the animal appears to experience a .
(2023). 9780871401083, Liveright.
Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.

was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. analysis indicates that mammalian brain size scales at approximately the or exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence. have the largest brain mass of any animal on earth, averaging and in mature males.

appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as . The traditional method for measuring this is the , which determines if an animal possesses the ability of self-recognition. Mammals that have passed the mirror test include Asian elephants (some pass, some do not); chimpanzees; ; ;

(1994). 9780465053131, BasicBooks.
, from 18 months ();
(1992). 9780389209966, Rowman & Littlefield.
bottlenose dolphins
(1995). 9780521441087, Cambridge University Press.
killer whales; and false killer whales.

Social structure
is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as , ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat and the Damaraland mole-rat.

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of ( and ) are unique among primates in their degree of cooperative care of young. set up an experiment with , presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.

A fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and ), elephants, , lions, and dolphins.

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food.

(1991). 9783034877268, Springer.
A solitary animal, while foraging, can also be less conspicuous to predators or prey.

In a hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs, and a is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas. Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems where one or a few males (resident males) have exclusive breeding rights to females in a group. Non-resident males can also be accepted in harems, but some species, such as the common vampire bat ( Desmodus rotundus), may be more strict.

Some mammals are perfectly monogamous, meaning that they and take no other partners (even after the original mate's death), as with wolves, , and otters. There are three types of polygamy: either one or multiple dominant males have breeding rights (polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (). It is much more common for polygynous mating to happen, which, excluding , are estimated to occur in up to 90% of mammals. Lek mating occurs when males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals.

All (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of . They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are , and invest more time and energy into their young than do animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque ( Macaca nemestrina) do not participate in child care, whereas the males of the ( M. fuscata) do.

(1981). 9781461331506, Plenum Press.

Humans and other mammals
In human culture
Non-human mammals play a wide variety of roles in human culture. They are the most popular of , with tens of millions of dogs, cats and other animals including and mice kept by families around the world. Mammals such as , horses and deer are among the earliest subjects of art, being found in Upper Paleolithic such as at . Major artists such as Albrecht Dürer, and are known for their portraits of mammals. Many species of mammals have been for sport and for food; deer and are especially popular as game animals.
(2023). 9781440338564, F+W Media.
Chapters on hunting deer, wild hog (boar), rabbit, and squirrel. Mammals such as and are widely raced for sport, often combined with .
(1981). 9780720711066, Pelham Books.
There is a tension between the role of animals as companions to humans, and their existence as individuals with . Mammals further play a wide variety of roles in literature,
(2023). 9781412930130, Sage.
film, mythology, and religion.
(2023). 9780714150840, The British Museum Press.

Uses and importance
The domestication of mammals was instrumental in the Neolithic development of agriculture and of , causing farmers to replace around the world. This transition from hunting and gathering to and was a major step in human history. The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes".

mammals form a large part of the raised for across the world. They include (2009) around 1.4 billion , 1 billion , 1 billion , and (1985) over 700 million rabbits. including cattle and horses have been used for work and from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery. In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas. In mountainous regions unsuitable for wheeled vehicles, continue to transport goods.

(2023). 9780824754969, CRC Press.
Mammal skins provide for , and . from mammals including sheep, goats and has been used for centuries for clothing.
(2023). 9780684313948, .
Mammals serve a major role in science as , both in fundamental biological research, such as in genetics, and in the development of new medicines, which must be tested exhaustively to demonstrate their safety. Millions of mammals, especially mice and rats, are used in each year. A is a genetically modified mouse with an inactivated , replaced or disrupted with an artificial piece of DNA. They enable the study of genes whose functions are unknown. A small percentage of the mammals are non-human primates, used in research for their similarity to humans.

Despite the benefits domesticated mammals had for human development, humans have an increasingly detrimental effect on wild mammals across the world. It has been estimated that the mass of all wild mammals has declined to only 4% of all mammals, with 96% of mammals being humans and their livestock now (see figure). In fact, terrestrial wild mammals make up only 2% of all mammals.

Hybrids
Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes.
(2023). 9781845933982, Cambridge University Press.
Hybrids between different subspecies within a species (such as between the and ) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.
(2023). 9783540938699, Springer.
Natural hybrids will occur in , where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the (though this is controversial).

Artificial selection, the deliberate selective breeding of domestic animals, is being used to recently extinct animals in an attempt to achieve an animal breed with a that resembles that extinct ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial of that wild type is lost forever with its . As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as are of the .

(2023). 9789546422354, Pensoft Publishers.

wild species evolved to a specific ecology can be threatened with extinction through the process of genetic pollution, the uncontrolled hybridization, genetic swamping which leads to homogenization or out-competition from the hybrid species. When new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact, extinction in some species, especially rare varieties, is possible. can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the . Such extinctions are not always apparent from a morphological standpoint. Some degree of is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species.

(2023). 9780642583369, Rural Industrial Research and Development Corporation of Australia.

Threats
The loss of species from ecological communities, , is primarily driven by human activity. This has resulted in , ecological communities depleted of large vertebrates.
(2023). 9781605352893, Sinauer Associates, Inc. Publishers.
In the Quaternary extinction event, the mass die-off of variety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as the , into extinction.
(1984). 9780816511006, University of Arizona Press.
The 2019 Global Assessment Report on Biodiversity and Ecosystem Services by states that the total biomass of wild mammals has declined by 82 percent since the beginning of human civilization. Wild animals make up just 4% of mammalian biomass on earth, while humans and their domesticated animals make up 96%.

Various species are predicted to become extinct in the near future, among them the , , and species of and . According to the WWF's 2020 Living Planet Report, vertebrate populations have declined by 68% since 1970 as a result of human activities, particularly , population growth and intensive farming, which is evidence that humans have triggered a sixth mass extinction event. Hunting alone threatens hundreds of mammalian species around the world. Scientists claim that the growing demand for is contributing to biodiversity loss as this is a significant driver of and habitat destruction; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production. Another influence is over-hunting and poaching, which can reduce the overall population of game animals, especially those located near villages,

(2023). 9781402052835
as in the case of . The effects of poaching can especially be seen in the with African elephants. Marine mammals are at risk from entanglement from fishing gear, notably , with discard mortalities ranging from 65,000 to 86,000 individuals annually.
(1996). 9789251035559, Food and Agriculture Organization of the United Nations.

Attention is being given to endangered species globally, notably through the Convention on Biological Diversity, otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats.

(1994). 9782831702223, International Union for Conservation of Nature.
Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental and non-governmental organizations.

Recent extinctions can be directly attributed to human influences. The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500, and around 80 mammal species have gone extinct since that time and 2015.

(2023). 9781421417189, Johns Hopkins University Press.
Some species, such as the Père David's deer are extinct in the wild, and survive solely in captive populations. Other species, such as the , are ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem.
(2023). 9781449661397, Jones & Bartlett Learning.
Other populations are only (extirpated), still existing elsewhere, but reduced in distribution, as with the extinction of in the .
(2023). 9780123735539, Academic Press.

Notes
See also
  • List of mammal genera – living mammals
  • List of mammalogists
  • List of monotremes and marsupials
  • List of placental mammals
  • List of prehistoric mammals
  • List of threatened mammals of the United States
  • Lists of mammals by population size
  • Lists of mammals by region
  • Mammals described in the 2000s
  • Mammals in culture

External resources

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