Mammal

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Mammals
Temporal range: Late Triassic – Recent; 225 or 167–0 Ma See discussion of dates in text
Common vampire batTasmanian devilFox squirrelPlatypusHumpback whaleGiant armadilloVirginia opossumHumanTree pangolinColugoStar nosed molePlains zebraEastern grey kangarooNorthern elephant sealAfrican elephantElkGiant pandaBlack and rufous elephant shrew
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Amniota
Clade: Synapsida
Clade: Mammaliaformes
Class: Mammalia
Linnaeus, 1758
Living subgroups

A mammal (from

extant species of mammals have been described and divided into 29 orders
.

The largest orders of mammals, by number of species, are the

even-toed ungulates (including pigs, camels, and whales), and the Carnivora (including cats, dogs, and seals
).

Mammals are the only living members of

therapsids during the beginning of the Middle Permian period. Mammals originated from cynodonts, an advanced group of therapsids, during the Late Triassic-Early Jurassic. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic 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

placentals, have a placenta, which enables the feeding of the fetus during gestation
.

Most mammals are

polyandrous
.

Domestication of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in farming replacing hunting and gathering 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 civilizations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (meat and dairy products), fur, and leather. Mammals are also hunted and raced for sport, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation.

Classification

Main article: Mammal classification
See also:
List of mammal species
Monotremata
  Sirenia
  Proboscidea

Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums.[2] Simpson (1945)[3] provides systematics 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

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 fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.[4]

Most mammals, including the six most species-rich

even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals, and allies.[5] According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders.[5] 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.[6] According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct.[7]

Definitions

The word "

placentals) and all descendants of that ancestor.[8] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.[9] 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. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.[10]

dentary–squamosal 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 Sinoconodon and living mammals.[11] The earliest known synapsid satisfying Kemp's definitions is Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date.[12][13]

McKenna/Bell classification

See also: Mammal classification § McKenna/Bell classification

In 1997, the mammals were comprehensively revised by

paleontologists 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.[4] Their 1997 book, Classification of Mammals above the Species Level,[14]
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,

dagger
(†).

Class Mammalia

Molecular classification of placentals

Genus-level molecular phylogeny of 116 extant mammals inferred from the gene tree information of 14,509 coding DNA sequences.[16] The major clades are colored: Marsupials (magenta), Xenarthrans (orange), afrotherians (red), laurasiatherians (green), and euarchontoglires (blue).

As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data.[17] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. 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 Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria).[18] Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.

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

paleogeographic data.[18]

Mammalia

Monotremata Ornithorhynchus anatinus

Theria

Marsupialia Macropodidæ

Placentalia
Atlantogenata

Afrotheria Elephas maximus Trichechus

Xenarthra Dasypus novemcinctus Myrmecophaga tridactyla

Boreoeutheria
Euarchontoglires

Euarchonta Cebus olivaceus Homo sapiens

Glires Rattus Lepus

Laurasiatheria

Eulipotyphla Talpidae

Scrotifera

Chiroptera Desmodontinae

Euungulata

Artiodactyla Capra walie Eubalaena glacialis

Perissodactyla Equus quagga Diceros bicornis

Ferae

Pholidota Manidae

Carnivora Acinonyx jubatus Zalophus californianus

The cladogram above is based on Tarver et al.. (2016)[20]

Evolution

Main article: Evolution of mammals

Origins

Synapsida, 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 mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.[21]

Mammaliaformes

Morganucodontidae

Docodonta

Haldanodon

Mammalia

Monotremata)

Fruitafossor

Haramiyavia

Multituberculata

Tinodon

Gobiconodonta)

Trechnotheria (incl. Theria)

Evolution from older amniotes

orbitals
, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.

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

The first amniotes apparently arose in the Pennsylvanian subperiod of the

pelycosaurs; we now know they were neither reptiles nor part of reptile lineage.[25][26]

Therapsids, a group of synapsids, evolved in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates.[25] They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs.[25] 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 probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:[27]

  • The gradual development of a bony secondary palate.
  • Abrupt acquisition of
    endothermy among Mammaliamorpha, thus prior to the origin of mammals by 30–50 millions of years [28]
    .
  • 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.[29] In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs[30] which, over an extended period (35 million years), came to include the crocodylomorphs,[31] the pterosaurs and the dinosaurs;[32] however, large cynodonts like Trucidocynodon and traversodontids 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.[33]

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

Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.[38]

Restoration of Juramaia sinensis, the oldest known Eutherian (160 M.Y.A.)[39]

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic.[39] A later eutherian relative, Eomaia, 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.[40] In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and Ukhaatherium, an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates.[41] 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 fetus 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.[42]

One of the earliest known monotremes was

eggs which are leathery and uncalcified.[45]

Earliest appearances of features

Hadrocodium, whose fossils date from approximately 195 million years ago, in the early Jurassic, 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.[46]

Fossil of Thrinaxodon at the National Museum of Natural History

The earliest clear evidence of hair or fur is in fossils of

synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts.[51]

When

therapsids.[51][52] Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,[53] 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.[54] Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.[55]

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.

mammaliformes. They are omnipresent in non-placental mammaliformes, though Megazostrodon and Erythrotherium appear to have lacked them.[57]

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, the egg-laying mammals.[58][59] In human females, mammary glands become fully developed during puberty, regardless of pregnancy.[60]

Rise of the mammals

Hyaenodon horridus, a North American species of hypercarnivore within the now-extinct order Hyaenodonta, at the Royal Ontario Museum. The genus Hyaenodon was amongst the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia.[61]

Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic, 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,[62] and underwent an exponential increase in body size (megafauna).[63] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.[62] 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.[64]

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

Protungulatum donnae as one of the first placental mammals,[67] but it has since been reclassified as a non-placental eutherian.)[68] Recalibrations of genetic and morphological diversity rates have suggested a Late Cretaceous origin for placentals, and a Paleocene origin for most modern clades.[69]

The earliest known ancestor of primates is

Archicebus achilles[70] from around 55 million years ago.[70] This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.[70]

Anatomy

Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young.[71] In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.[72]

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

  • articular (a small bone at the back of the lower jaw) and quadrate (a small bone at the back of the upper jaw).[46]
  • Middle ear – In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.[73]
  • Tooth replacement – Teeth can be replaced once (
    diphyodonty) or (as in toothed whales and murid rodents) not at all (monophyodonty).[74] Elephants, manatees, and kangaroos continually grow new teeth throughout their life (polyphyodonty).[75]
  • Prismatic enamel – The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.[76]
  • Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob.[77]

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.[78] Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.[79]

Sexual dimorphism

Sexual dimorphism in aurochs, the extinct wild ancestor of cattle

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

polygynous 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.[80]

Biological systems

Main article: Biological system

The majority of mammals have seven cervical vertebrae (bones in the neck). The exceptions are the manatee and the two-toed sloth, which have six, and the three-toed sloth which has nine.[81] All mammalian brains possess a neocortex, a brain region unique to mammals.[82] Placental brains have a corpus callosum, unlike monotremes and marsupials.[83]

Didactic models of a mammalian heart

Circulatory systems

The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.[84] The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. 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 ventricular relaxation period, 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.[85]

Respiratory systems

Raccoon lungs being inflated manually
Main article: Respiratory_system § Mammals

The

bronchi, 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 rib cage 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.[86][87] This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.[87]

Integumentary systems

adipocytes

The

setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class, though some mammals have very little.[88]
: 61 

Digestive systems

gray wolf
(right) which consumes large vertebrates

Herbivores have developed a diverse range of physical structures to facilitate the

silica-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.[90] Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.[91]

The stomach of

sacculated or much wider than the small intestine.[94]

Excretory and genitourinary systems

Bovine kidney
Genitourinary system of a male and female rabbit

The mammalian

penile urethra.[99] However, the tenrecs, golden moles, and some shrews retain a cloaca as adults.[100] In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.[99] Monotremes, which translates from Greek into "single hole", have a true cloaca.[101]

Sound production

A diagram of ultrasonic signals emitted by a bat, and the echo from a nearby object

As in all other tetrapods, mammals have a

vocal folds. The movement or tenseness of the vocal folds can result in many sounds such as purring and screaming. 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 nasal sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.[102]

Beluga whale echolocation sounds

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 thoracic cavity while pushing the lungs, heart, and trachea into the abdomen.[103] Large vocal pads can also lower the pitch, as in the low-pitched roars of big cats.[104] The production of infrasound is possible in some mammals such as the African elephant (Loxodonta spp.) and baleen whales.[105][106] Small mammals with small larynxes have the ability to produce ultrasound, which can be detected by modifications to the middle ear and cochlea. 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.[102]

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 vagus nerve. The vocal tract is supplied by the hypoglossal nerve and facial nerves. Electrical stimulation of the periaqueductal gray (PEG) region of the mammalian midbrain 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 motor cortex, which controls movement, and the motor neurons in the spinal cord.[102]

Fur

Main article: Fur
Porcupines use their spines for defense.

The primary function of the fur of mammals is thermoregulation. Others include protection, sensory purposes, waterproofing, and camouflage.[107] Different types of fur serve different purposes:[88]: 99 

  • Definitive – which may be shed after reaching a certain length
  • Vibrissae – sensory hairs, most commonly
    whiskers
  • Pelage – guard hairs, under-fur, and awn hair
  • Spines – stiff guard hair used for defense (such as in porcupines)
  • mane
    )
  • Velli – often called "down fur" which insulates newborn mammals
  • Wool – 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

musk ox has guard hairs measuring 30 cm (12 in) as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of −40 °C (−40 °F).[88]: 162–163  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 70 °C (158 °F) in the summer, but the skin stays at 40 °C (104 °F).[88]: 188  Aquatic mammals, conversely, trap air in their fur to conserve heat by keeping the skin dry.[88]
: 162–163 

A leopard's disruptively colored coat provides camouflage for this ambush predator.

Coloration

Mammalian coats are colored for a variety of reasons, the major selective pressures including

pheomelanin for a range of yellowish to reddish colors, giving mammals an earth tone.[108][109] Some mammals have more vibrant colors; certain monkeys such mandrills and vervet monkeys, and opossums such as the Mexican mouse opossums and Derby's woolly opossums, have blue skin due to light diffraction in collagen fibers.[110] Many sloths appear green because their fur hosts green algae; this may be a symbiotic relation that affords camouflage to the sloths.[111]

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.

arboreal habitat due to convergent evolution.[110]

Aposematism, 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 skunk and the powerful and aggressive honey badger.[114] Coat color is sometimes sexually dimorphic, as in many primate species.[115] Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.[116] 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 voles, 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.[88]: 166–167 [107] The dazzling black-and-white striping of zebras appear to provide some protection from biting flies.[117]

Reproductive system

Main article: Mammalian reproduction
Goat kids stay with their mother until they are weaned.

Mammals are solely

bicornuate, 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.[122][123][88]
: 220–221, 247 

Matschie's tree-kangaroo with young in pouch

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped, either through direct

epipubic bones. The oldest demonstration of this reproductive style is with Kayentatherium, which produced undeveloped perinates, but at much higher litter sizes than any modern mammal, 38 specimens.[124] Most modern mammals are viviparous, 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.[125] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.[126]

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short

plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy.[79] Even non-placental eutherians probably reproduced this way.[41] The placentals give birth to relatively complete and developed young, usually after long gestation periods.[127] They get their name from the placenta, which connects the developing fetus to the uterine wall to allow nutrient uptake.[128] 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.[124]

The mammary glands 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 nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.[129] 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.[130] Lactose is the main sugar in placental mammal milk while monotreme and marsupial milk is dominated by oligosaccharides.[131] Weaning is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.[132]

Endothermy

Nearly all mammals are

endothermic ("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 ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.[133] Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm.[134] Birds are also endothermic, so endothermy is not unique to mammals.[135]

Species lifespan

See also: Life expectancy and Maximum life span

Among mammals, species maximum lifespan varies significantly (for example the

poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species.[139] Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.[140][141][142]

Locomotion

Main article: Animal locomotion

Terrestrial

Main article: Terrestrial locomotion
Running gait. Photographs by Eadweard Muybridge, 1887

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are

unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed.[143] A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs. Giant anteaters[144] and platypuses[145] are also knuckle-walkers. Some mammals are bipeds, 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 kangaroo rats.[146][147]

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest

leaping gaits.[149] 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.[148]

Arboreal

Main article: Arboreal locomotion
Gibbons are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches.

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 brachiate (swing between trees).[150] Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys, and possums, use prehensile tails 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 squirrels 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.[150] Size relating to weight affects gliding animals such as the sugar glider.[151] Some species of primate, bat and all species of sloth 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.[150]

Aerial

Main article:
Aerial locomotion
Slow-motion and normal speed of Egyptian fruit bats flying

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.[152]

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.[153][154] By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.[155] 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.[156] The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, 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.[157]

Fossorial and subterranean

Semi-fossorial wombat (left) vs. fully fossorial eastern mole (right)
See also: Fossorial and Burrow

A fossorial (from Latin fossor, meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are

predators or for food storage.[158]

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;

vestigial 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 cursorial 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.[159]

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order Insectivora.[160]

Aquatic

Main articles: Aquatic locomotion, Marine mammal, and Aquatic mammal
A pod of
short-beaked common dolphins
swimming

Fully aquatic mammals, the cetaceans and sirenians, 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 dorsal fin to prevent themselves from turning upside-down in the water.[161][162] 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.[163]

specific gravity allows them to sink and move along the bottom of a river.[170]

Behavior

Communication and vocalization

Further information: Animal communication and Animal echolocation
predators.[171]

Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as

Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator.[177] Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasonic, below the hearing range of humans, and can be heard by other elephants up to 6 miles (9.7 km) away at still times near sunrise and sunset.[178]

Orca calling including occasional echolocation clicks

Mammals signal by a variety of means. Many give visual

scent-marking, sometimes possibly to help defend territory, but probably with a range of functions both within and between species.[182][183][184] Microbats and toothed whales including oceanic dolphins vocalize both socially and in echolocation.[185][186][187]

Feeding

A short-beaked echidna foraging for insects

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

gut flora in a multi-chambered stomach, like terrestrial herbivores.[188]

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 metabolic rate. Mammals that weigh less than about 18 ounces (510 g; 1.1 lb) 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).[189] Furthermore, mammals that weigh more than 18 ounces (510 g; 1.1 lb) 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).[190]

hypercarnivorous polar bear (Ursus maritimus)[191]

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.[192] They are grouped into three categories: mesocarnivory (50–70% meat), hypercarnivory (70% and greater of meat), and hypocarnivory (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 hyenas, allowing them to consume bones; some extinct groups, notably the Machairodontinae, had saber-shaped canines.[191]

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 zoopharmacognosy. 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.[193] 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.[194] Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with hemoglobin production, and as a laxative.[195]

Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as

aestivate in times of drought or extreme heat, for example the fat-tailed dwarf lemur (Cheirogaleus medius).[199]

Intelligence

See also: Animal cognition

In intelligent mammals, such as

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 smaller than a cat, which must think to outwit its prey.[190]

A bonobo fishing for termites with a stick

Tool use by animals may indicate different levels of learning and cognition. The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools.[200] Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.[201] Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment".[202] Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.[203]

Allometric analysis indicates that mammalian brain size scales at approximately the 2⁄3 or 3⁄4 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.[204] Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 in3) and 7.8 kilograms (17 lb) in mature males.[205]

Self-awareness 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 metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition.[206] Mammals that have passed the mirror test include Asian elephants (some pass, some do not);[207] chimpanzees;[208] bonobos;[209] orangutans;[210] humans, from 18 months (mirror stage);[211] bottlenose dolphins[a][212] killer whales;[213] and false killer whales.[213]

Social structure

Main article:
Social animal
Female elephants live in stable groups, along with their offspring.

Eusociality 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 bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat[214] and the Damaraland mole-rat.[215]

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

rhesus monkeys, 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.[217]

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 spider monkeys),[218] elephants,[219] spotted hyenas,[220] lions,[221] and dolphins.[222]

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.[223] A solitary animal, while foraging, can also be less conspicuous to predators or prey.[224]

Red kangaroos "boxing" for dominance

In a

pecking order 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.[226] 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.[227] Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (Desmodus rotundus), may be more strict.[228]

Some mammals are perfectly monogamous, meaning that they mate for life and take no other partners (even after the original mate's death), as with wolves, Eurasian beavers, and otters.[229][230] 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 (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals.[231] Lek mating occurs when males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals.[232]

All

r-selected 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 Japanese macaque (M. fuscata) do.[233]

Humans and other mammals

Main article: Human uses of mammals

In human culture

Upper Paleolithic cave painting of a variety of large mammals, Lascaux, c. 17,300 years old.

Non-human mammals play a wide variety of roles in human culture. They are the most popular of pets, with tens of millions of dogs, cats and other animals including rabbits and mice kept by families around the world.[234][235][236] Mammals such as mammoths, horses and deer are among the earliest subjects of art, being found in Upper Paleolithic cave paintings such as at Lascaux.[237] Major artists such as Albrecht Dürer, George Stubbs and Edwin Landseer are known for their portraits of mammals.[238] Many species of mammals have been hunted for sport and for food; deer and wild boar are especially popular as game animals.[239][240][241] Mammals such as horses and dogs are widely raced for sport, often combined with betting on the outcome.[242][243] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.[244] Mammals further play a wide variety of roles in literature,[245][246][247] film,[248] mythology, and religion.[249][250][251]

Uses and importance

See also:
Laboratory animal, and Pack animal
Cattle have been kept for milk for thousands of years.

The domestication of mammals was instrumental in the Neolithic development of agriculture and of civilization, causing farmers to replace hunter-gatherers around the world.[b][253] This transition from hunting and gathering to herding flocks and growing crops 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".[254]

domestic pigs,[255][256] and (1985) over 700 million rabbits.[257] Working domestic animals including cattle and horses have been used for work and transport 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, pack animals continue to transport goods.[258] Mammal skins provide leather for shoes, clothing and upholstery. Wool from mammals including sheep, goats and alpacas has been used for centuries for clothing.[259][260]

Livestock make up 62% of the world's mammal biomass; humans account for 34%; and wild mammals are just 4%[261]

Mammals serve a major role in science as

experimental animals, both in fundamental biological research, such as in genetics,[262] and in the development of new medicines, which must be tested exhaustively to demonstrate their safety.[263] Millions of mammals, especially mice and rats, are used in experiments each year.[264] A knockout mouse is a genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA. They enable the study of sequenced genes whose functions are unknown.[265] A small percentage of the mammals are non-human primates, used in research for their similarity to humans.[266][267][268]

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.[269][270]

Hybrids

Main article: Hybrid (biology)
A true quagga, 1870 (left) vs. a bred-back quagga, 2014 (right)

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.[271] Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) 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.[272] Natural hybrids will occur in hybrid zones, 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 red wolf (though this is controversial).[273]

wildtype 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 gene pool of that wild type is lost forever with its extinction. As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as Heck cattle are of the aurochs.[274]

Interbreeding 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 domestic water buffalo. Such extinctions are not always apparent from a morphological standpoint. Some degree of gene flow is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species.[278][279]

Threats

See also: Holocene extinction
Biodiversity of large mammal species per continent before and after humans arrived there

The loss of species from ecological communities,

IPBES states that the total biomass of wild mammals has declined by 82 percent since the beginning of human civilization.[285][286] Wild animals make up just 4% of mammalian biomass on earth, while humans and their domesticated animals make up 96%.[270]

Various species are predicted to

sixth mass extinction event.[292][293] Hunting alone threatens hundreds of mammalian species around the world.[294][295] Scientists claim that the growing demand for meat is contributing to biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production.[296][297][298] Another influence is over-hunting and poaching, which can reduce the overall population of game animals,[299] especially those located near villages,[300] as in the case of peccaries.[301] The effects of poaching can especially be seen in the ivory trade with African elephants.[302] Marine mammals are at risk from entanglement from fishing gear, notably cetaceans, with discard mortalities ranging from 65,000 to 86,000 individuals annually.[303]

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.[304] Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental and non-governmental organizations.[305]

Recent extinctions can be directly attributed to human influences.[306][280] The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500,[307] and around 80 mammal species have gone extinct since that time and 2015.[308] Some species, such as the Père David's deer[309] are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem.[310]: 318  Other populations are only locally extinct (extirpated), still existing elsewhere, but reduced in distribution,[310]: 75–77  as with the extinction of gray whales in the Atlantic.[311]

Notes

  1. ^ Decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition since they do not have arms and cannot touch the marked areas.[212]
  2. ^ Diamond discussed this matter further in his 1997 book Guns, Germs, and Steel.[252]

See also

External resources

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