Amammal(fromLatinmamma'breast')[1]is avertebrateanimal of theclassMammalia(/məˈmeɪli.ə/). Mammals are characterized by the presence ofmilk-producingmammary glandsfor feeding their young, a broadneocortexregion of the brain,furorhair,and threemiddle ear bones.These characteristics distinguish them fromreptilesandbirds,from which their ancestorsdivergedin theCarboniferousPeriod over 300 million years ago. Around 6,400extantspecies of mammals have been described and divided into 27orders.[2]The study of mammals is calledmammalogy.
| Mammals Temporal range:
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Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Amniota |
| Clade: | Synapsida |
| Clade: | Mammaliaformes |
| Class: | Mammalia Linnaeus,1758 |
| Living subgroups | |
The largest orders of mammals, by number ofspecies,are therodents,bats,andeulipotyphlans(includinghedgehogs,molesandshrews). The next three are theprimates(includinghumans,monkeysandlemurs), theeven-toed ungulates(includingpigs,camels,andwhales), and theCarnivora(includingcats,dogs,andseals).
Mammals are the only living members ofSynapsida;thisclade,together withSauropsida(reptiles and birds), constitutes the largerAmniotaclade. Early synapsids are referred to as "pelycosaurs."The more advancedtherapsidsbecame dominant during theGuadalupian.Mammals originated fromcynodonts,an advanced group of therapsids, during the LateTriassicto EarlyJurassic.Mammals achieved their modern diversity in thePaleogeneandNeogeneperiods of theCenozoicera, after theextinction of non-avian dinosaurs,and have been thedominantterrestrial animal group from 66 million years ago to the present.
The basic mammalian body type isquadrupedal,with most mammals using fourlimbsforterrestrial locomotion;but in some, the limbs are adapted for lifeat sea,in the air,in treesorunderground.Thebipedshave adapted to move using only the two lower limbs, while the rear limbs ofcetaceansand thesea cowsare mere internalvestiges.Mammals range in size from the 30–40 millimetres (1.2–1.6 in)bumblebee batto the 30 metres (98 ft)blue whale—possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for thebowhead whale.All modern mammals give birth to live young, except the five species ofmonotremes,which lay eggs. The most species-rich group is theviviparousplacental mammals,so named for the temporary organ (placenta) used by offspring to draw nutrition from the mother duringgestation.
Most mammals areintelligent,with some possessing large brains,self-awareness,andtool use.Mammals can communicate and vocalize in several ways, including the production ofultrasound,scent marking,alarm signals,singing,echolocation;and, in the case of humans, complexlanguage.Mammals can organize themselves intofission–fusion societies,harems,andhierarchies—but can also be solitary andterritorial.Most mammals arepolygynous,but some can bemonogamousorpolyandrous.
Domesticationof many types of mammals by humans played a major role in theNeolithic Revolution,and resulted infarmingreplacinghunting and gatheringas 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 firstcivilizations.Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (meatanddairy products),fur,andleather.Mammals are alsohuntedand raced for sport, kept aspetsandworking animalsof various types, and are used asmodel organismsin science. Mammals have been depicted inartsincePaleolithictimes, and appear in literature, film, mythology, and religion. Decline in numbers andextinctionof many mammals is primarily driven by humanpoachingandhabitat destruction,primarilydeforestation.
Classification
Over 70% of mammal species are in the ordersRodentia,Chiroptera,andEulipotyphla.
Mammal classification has been through several revisions sinceCarl Linnaeusinitially defined the class, and at present[when?],no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums.[3]Simpson(1945)[4]providessystematicsof 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: Thepaleontological recordhas been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept ofcladistics.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.[5]
Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species areRodentia:mice,rats,porcupines,beavers,capybaras,and other gnawing mammals;Chiroptera:bats; andEulipotyphla:shrews,moles,andsolenodons.The next three biggest orders, depending on thebiological classificationscheme used, are theprimates:apes,monkeys,andlemurs;theCetartiodactyla:whalesandeven-toed ungulates;and theCarnivorawhich includescats,dogs,weasels,bears,seals,and allies.[6]According toMammal Species of the World,5,416 species were identified in 2006. These were grouped into 1,229genera,153familiesand 29 orders.[6]In 2008, theInternational Union for Conservation of Nature(IUCN) completed a five-year Global Mammal Assessment for itsIUCN Red List,which counted 5,488 species.[7]According to research published in theJournal of Mammalogyin 2018, the number of recognized mammal species is 6,495, including 96 recently extinct.[8]
Definitions
The word "mammal"is modern, from the scientific nameMammaliacoined by Carl Linnaeus in 1758, derived from theLatinmamma( "teat, pap" ). In an influential 1988 paper, Timothy Rowe defined Mammaliaphylogeneticallyas thecrown groupof mammals, thecladeconsisting of themost recent common ancestorof livingmonotremes(echidnasandplatypuses) andtherians(marsupialsandplacentals) and all descendants of that ancestor.[9]Since this ancestor lived in theJurassicperiod, Rowe's definition excludes all animals from the earlierTriassic,despite the fact that Triassic fossils in theHaramiyidahave been referred to the Mammalia since the mid-19th century.[10]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.Ambondrois more closely related to monotremes than to therian mammals whileAmphilestesandAmphitheriumare more closely related to the therians; as fossils of all three genera are dated about167million years agoin theMiddle Jurassic,this is a reasonable estimate for the appearance of the crown group.[11]
T. S. Kemphas provided a more traditional definition: "Synapsidsthat possess adentary–squamosaljaw articulation andocclusionbetween 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 ofSinoconodonand living mammals.[12]The earliest-known synapsid satisfying Kemp's definitions isTikitherium,dated225Ma,so the appearance of mammals in this broader sense can be given thisLate Triassicdate.[13][14]However, this animal may have actually evolved during the Neogene.[15]
Molecular classification of placentals
As of the early 21st century, molecular studies based onDNAanalysis have suggested new relationships among mammal families. Most of these findings have been independently validated byretrotransposonpresence/absence data.[17]Classification systems based on molecular studies reveal three major groups or lineages of placentals—Afrotheria,XenarthraandBoreoeutheria—whichdivergedin theCretaceous.The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group isbasal.These hypotheses areAtlantogenata(basal Boreoeutheria),Epitheria(basal Xenarthra) andExafroplacentalia(basal Afrotheria).[18]Boreoeutheria in turn contains two major lineages—EuarchontogliresandLaurasiatheria.
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 asnuclearormitochondrial)[19]and varying interpretations ofpaleogeographicdata.[18]
| Tarver et al. 2016[20] | Sandra Álvarez-Carretero et al. 2022[21][22] |
|---|---|
Evolution
Origins
Synapsida,a clade that contains mammals and their extinct relatives, originated during thePennsylvanian subperiod(~323 million to ~300 million years ago), when they split from the reptile lineage. Crown group mammals evolved from earliermammaliaformsduring theEarly Jurassic.The cladogram takes Mammalia to be the crown group.[23]
| Mammaliaformes |
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Evolution from older amniotes
The first fully terrestrialvertebrateswereamniotes.Like their amphibious earlytetrapodpredecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developingembryoto breathe but keep water in. Hence, amniotes can lay eggs on dry land, whileamphibiansgenerally need to lay their eggs in water.
The first amniotes apparently arose in the Pennsylvanian subperiod of theCarboniferous.They descended from earlierreptiliomorphamphibious tetrapods,[24]which lived on land that was already inhabited byinsectsand other invertebrates as well asferns,mossesand other plants. Within a few million years, two important amniote lineages became distinct: thesynapsids,which would later include the common ancestor of the mammals; and thesauropsids,which now includeturtles,lizards,snakes,crocodiliansanddinosaurs(includingbirds).[25]Synapsids have a single hole (temporal fenestra) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the earlyPermiansuch asDimetrodon.[26]Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" orpelycosaurs;we now know they were neither reptiles nor part of reptile lineage.[27][28]
Therapsids,a group of synapsids, evolved in theMiddle Permian,about 265 million years ago, and became the dominant land vertebrates.[27]They differ from basaleupelycosaursin several features of the skull and jaws, including: larger skulls andincisorswhich are equal in size in therapsids, but not for eupelycosaurs.[27]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 withprobainognathiancynodonts,some of which could easily be mistaken for mammals. Those stages were characterized by:[29]
- The gradual development of a bony secondarypalate.
- Abrupt acquisition ofendothermyamongMammaliamorpha,thus prior to the origin of mammals by 30–50 millions of years[30].
- Progression towards an erect limb posture, which would increase the animals' stamina by avoidingCarrier'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.
- Thedentarygradually 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
ThePermian–Triassic extinction eventabout 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids.[31]In the early Triassic, most medium to large land carnivore niches were taken over byarchosaurs[32]which, over an extended period (35 million years), came to include thecrocodylomorphs,[33]thepterosaursand the dinosaurs;[34]however, large cynodonts likeTrucidocynodonandtraversodontidsstill occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.[35]
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 nocturnalinsectivoreniche from the mid-Jurassic onwards;[36]the JurassicCastorocauda,for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish.[37]Most, if not all, are thought to have remained nocturnal (thenocturnal bottleneck), accounting for much of the typical mammalian traits.[38]The majority of the mammal species that existed in theMesozoic Erawere multituberculates, eutriconodonts andspalacotheriids.[39]The earliest-known fossil of theMetatheria( "changed beasts" ) isSinodelphys,found in 125-million-year-oldEarly Cretaceousshalein China's northeasternLiaoning Province.The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.[40]
The oldest-known fossil among theEutheria( "true beasts" ) is the small shrewlikeJuramaia sinensis,or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic.[41]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.[42]In particular, theepipubic bonesextend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals andUkhaatherium,an early Cretaceous animal in the eutherian orderAsioryctitheria.This also applies to the multituberculates.[43]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 theirfetusduring gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and thereforepregnancywas short, as in modern marsupials. This suggests that the placenta was a later development.[44]
One of the earliest-known monotremes wasTeinolophos,which lived about 120 million years ago in Australia.[45]Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca)—as reptiles and birds also do—[46]and they layeggswhich are leathery and uncalcified.[47]
Earliest appearances of features
Hadrocodium,whose fossils date from approximately 195 million years ago, in the earlyJurassic,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.[48]
The earliest clear evidence of hair or fur is in fossils ofCastorocaudaandMegaconus,from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in themaxillaeandpremaxillae(bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur;[49][50]it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizardTupinambishas foramina that are almost identical to those found in the nonmammalian cynodontThrinaxodon.[28][51]Popular sources, nevertheless, continue to attribute whiskers toThrinaxodon.[52]Studies on Permiancoprolitessuggest that non-mammaliansynapsidsof the epoch already had fur, setting the evolution of hairs possibly as far back asdicynodonts.[53]
Whenendothermyfirst appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammaliantherapsids.[53][54]Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,[55]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.[56]Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures.[57]
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 eutherianEomaiaand the metatherianSinodelphys,both dated to 125 million years ago.[58]Epipubicbones, a feature that strongly influenced the reproduction of most mammal clades, are first found inTritylodontidae,suggesting that it is asynapomorphybetween them andMammaliaformes.They are omnipresent in non-placental Mammaliaformes, thoughMegazostrodonandErythrotheriumappear to have lacked them.[59]
It has been suggested that the original function oflactation(milkproduction) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals.[60][61]In human females, mammary glands become fully developed during puberty, regardless of pregnancy.[62]
Rise of the mammals
Therians took over the medium- to large-sized ecological niches in theCenozoic,after theCretaceous–Paleogene extinction eventapproximately 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,[64]and underwent an exponential increase in body size (megafauna).[65]The increase in mammalian diversity was not, however, solely because of expansion into large-bodied niches.[66]Mammals diversified very quickly, displaying an exponential rise in diversity.[64]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.[67]
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 lateEocenethrough theMiocene.[68]However, no placental fossils have been found from before the end of the Cretaceous.[69]The earliest undisputed fossils of placentals come from the earlyPaleocene,after the extinction of the non-avian dinosaurs.[69](Scientists identified an early Paleocene animal namedProtungulatum donnaeas one of the first placental mammals,[70]but it has since been reclassified as a non-placental eutherian.)[71]Recalibrations of genetic and morphological diversity rates have suggested aLate Cretaceousorigin for placentals, and a Paleocene origin for most modern clades.[72]
The earliest-known ancestor of primates isArchicebus achilles[73]from around 55 million years ago.[73]This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm.[73]
Anatomy
Distinguishing features
Living mammal species can be identified by the presence ofsweat glands,includingthose that are specialized to produce milkto nourish their young.[74]In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.[75]
Many traits shared by all living mammals appeared among the earliest members of the group:
- Jaw joint– Thedentary(the lower jaw bone, which carries the teeth) and thesquamosal(a smallcranialbone) meet to form the joint. In mostgnathostomes,including earlytherapsids,the joint consists of thearticular(a small bone at the back of the lower jaw) andquadrate(a small bone at the back of the upper jaw).[48]
- Middle ear– In crown-group mammals, sound is carried from theeardrumby a chain of three bones, themalleus,theincusand thestapes.Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.[76]
- Tooth replacement– Teeth can be replaced once (diphyodonty) or (as in toothed whales andmuridrodents) not at all (monophyodonty).[77]Elephants, manatees, and kangaroos continually grow new teeth throughout their life (polyphyodonty).[78]
- Prismatic enamel– Theenamelcoating on the surface of a tooth consists of prisms, solid, rod-like structures extending from thedentinto the tooth's surface.[79]
- Occipital condyles– Two knobs at the base of the skull fit into the topmostneck vertebra;most othertetrapods,in contrast, have only one such knob.[80]
For the most part, these characteristics were not present in the Triassic ancestors of the mammals.[81]Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.[82]
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-biasedsexual dimorphism,although some orders do not show any bias or are significantly female-biased (Lagomorpha). 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 dimorphismrelates to sexual selectionon males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated bymating systems,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 morepolygynousspecies, 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.[83]
Biological systems
The majority of mammals have sevencervical vertebrae(bones in the neck). The exceptions are themanateeand thetwo-toed sloth,which have six, and thethree-toed slothwhich has nine.[84]All mammalian brains possess aneocortex,a brain region unique to mammals.[85]Placental brains have acorpus callosum,unlike monotremes and marsupials.[86]
Circulatory systems
The mammalianhearthas four chambers, two upperatria,the receiving chambers, and two lowerventricles,the discharging chambers.[87]The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). Aftergas exchangein the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the fourpulmonary 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 theventricular 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 viacoronary arteries.[88]
Respiratory systems
Thelungsof mammals are spongy and honeycombed. Breathing is mainly achieved with thediaphragm,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 andbronchi,and expands thealveoli.Rela xing 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 wallcontracts,increasing pressure on the diaphragm, which forces air out quicker and more forcefully. Therib cageis 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.[89][90]This type of lung is known as a bellows lung due to its resemblance to blacksmithbellows.[90]
Integumentary systems
Theintegumentary system(skin) is made up of three layers: the outermostepidermis,thedermisand thehypodermis.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 ofadipose tissue,which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;[91]: 97 marine mammalsrequire a thick hypodermis (blubber) for insulation, andright whaleshave the thickest blubber at 20 inches (51 cm).[92]Although other animals have features such as whiskers,feathers,setae,orciliathat superficially resemble it, no animals other than mammals havehair.It is a definitive characteristic of the class, though some mammals have very little.[91]: 61
Digestive systems
Herbivores have developed a diverse range of physical structures to facilitate theconsumption of plant material.To break up intact plant tissues, mammals have developedteethstructures that reflect their feeding preferences. For instance,frugivores(animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage andseeds.Grazinganimals that tend to eat hard,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.[93]Most carnivorous mammals havecarnassialteeth (of varying length depending on diet), long canines and similar tooth replacement patterns.[94]
The stomach ofeven-toed ungulates(Artiodactyla) is divided into four sections: therumen,thereticulum,theomasumand theabomasum(onlyruminantshave 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 abolus(orcud), 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 cellulolyticmicrobes(bacteria,protozoaandfungi) producecellulase,which is needed to break down thecellulosein plants.[95]Perissodactyls,in contrast to the ruminants, store digested food that has left the stomach in an enlargedcecum,where it is fermented by bacteria.[96]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 cecum is either absent or short and simple, and the large intestine is notsacculatedor much wider than the small intestine.[97]
Excretory and genitourinary systems
The mammalianexcretory systeminvolves many components. Like most other land animals, mammals areureotelic,and convertammoniaintourea,which is done by theliveras part of theurea cycle.[98]Bilirubin,a waste product derived fromblood cells,is passed throughbileandurinewith the help of enzymes excreted by the liver.[99]The passing of bilirubin via bile through theintestinal tractgives mammalianfecesa distinctive brown coloration.[100]Distinctive features of themammalian kidneyinclude the presence of therenal pelvisandrenal pyramids,and of a clearly distinguishablecortexandmedulla,which is due to the presence of elongatedloops of Henle.Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobedreniculate kidneysof pinnipeds,cetaceansand bears.[101][102]Most adult placental mammals have no remaining trace of thecloaca.In the embryo, theembryonic cloacadivides into a posterior region that becomes part of theanus,and an anterior region that has different fates depending on the sex of the individual: in females, it develops into thevestibuleorurogenital sinusthat receives theurethraandvagina,while in males it forms the entirety of thepenile urethra.[102][103]However, theafrosoricidsand someshrewsretain a cloaca as adults.[104]In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.[102]Monotremes, which translates fromGreekinto "single hole", have a true cloaca.[105]Urine flows from theuretersinto the cloaca in monotremes and into thebladderin placental mammals.[102]
Sound production
As in all other tetrapods, mammals have alarynxthat can quickly open and close to produce sounds, and a supralaryngealvocal tractwhich filters this sound. The lungs and surrounding musculature provide the air stream and pressure required tophonate.The larynx controls thepitchandvolumeof sound, but the strength the lungs exert toexhalealso 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 usingvocal folds.The movement or tenseness of the vocal folds can result in many sounds such aspurringandscreaming.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 andnasalsounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.[106]
Some mammals have a large larynx and thus a low-pitched voice, namely thehammer-headed bat(Hypsignathus monstrosus) where the larynx can take up the entirety of thethoracic cavitywhile pushing the lungs, heart, and trachea into theabdomen.[107]Large vocal pads can also lower the pitch, as in the low-pitched roars ofbig cats.[108]The production ofinfrasoundis possible in some mammals such as theAfrican elephant(Loxodontaspp.) andbaleen whales.[109][110]Small mammals with small larynxes have the ability to produceultrasound,which can be detected by modifications to themiddle earandcochlea.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 amelonto 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.[106]
The vocal production system is controlled by thecranial nerve nucleiin the brain, and supplied by therecurrent laryngeal nerveand thesuperior laryngeal nerve,branches of thevagus nerve.The vocal tract is supplied by thehypoglossal nerveandfacial nerves.Electrical stimulation of theperiaqueductal gray(PEG) region of the mammalianmidbrainelicit 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 themotor cortex,which controls movement, and themotor neuronsin the spinal cord.[106]
Fur
The primary function of the fur of mammals isthermoregulation.Others include protection, sensory purposes, waterproofing, and camouflage.[111]Different types of fur serve different purposes:[91]: 99
- Definitive – which may beshedafter reaching a certain length
- Vibrissae – sensory hairs, most commonlywhiskers
- Pelage – guard hairs, under-fur, andawn hair
- Spines– stiff guard hair used for defense (such as inporcupines)
- Bristles– long hairs usually used in visual signals. (such as a lion'smane)
- 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, themusk oxhas 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).[91]: 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).[91]: 188 Aquatic mammals,conversely, trap air in their fur to conserve heat by keeping the skin dry.[91]: 162–163
Coloration
Mammalian coats are colored for a variety of reasons, the major selective pressures includingcamouflage,sexual selection,communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount ofmelanin;eumelaninsfor brown and black colors andpheomelaninfor a range of yellowish to reddish colors, giving mammals anearth tone.[112][113]Some mammals have more vibrant colors; certain monkeys suchmandrillsandvervet monkeys,and opossums such as theMexican mouse opossumsandDerby's woolly opossums,have blue skin due tolight diffractionincollagenfibers.[114]Many sloths appear green because their fur hosts greenalgae;this may be asymbioticrelation that affordscamouflageto the sloths.[115]
Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.[116]In arctic and subarctic mammals such as thearctic fox(Alopex lagopus),collared lemming(Dicrostonyx groenlandicus),stoat(Mustela erminea), andsnowshoe hare(Lepus americanus),seasonal color changebetween brown in summer and white in winter is driven largely by camouflage.[117]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 largelyarborealhabitat due toconvergent evolution.[114]
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-smellingskunkand the powerful and aggressivehoney badger.[118]Coat color is sometimessexually dimorphic,as inmany primate species.[119]Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.[120]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 asvoles,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.[91]: 166–167 [111]The dazzling black-and-white striping ofzebrasappear to provide some protection from biting flies.[121]
Reproductive system
Mammals reproduce byinternal fertilization[122]and are solelygonochoric(an animal is born with either male or female genitalia, as opposed tohermaphroditeswhere there is no such schism).[123]Male mammalsinseminatefemales duringcopulationandejaculatesemeninto the female reproductive tract through apenis,which may be contained in aprepucewhen not erect. Male placentals alsourinatethrough a penis, and some placentals also have a penis bone (baculum).[124][125][122]Marsupials typically have forked penises,[126]while theechidnapenis generally has four heads with only two functioning.[127]Depending on the species, anerectionmay be fueled by blood flow into vascular, spongy tissue or by muscular action.[124]Thetesticlesof most mammals descend into thescrotumwhich is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have avulva(clitorisandlabia) on the outside, while the internal system contains pairedoviducts,one or twouteri,one or twocervicesand avagina.[128][129]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 twouterine hornshave a single cervix that connects to the vagina, a 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.[130][131][91]: 220–221, 247
The ancestral condition for mammal reproduction is the birthing of relatively undeveloped young, either through directviviparyor a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence ofepipubic bones.The oldest demonstration of this reproductive style is withKayentatherium,which produced undevelopedperinates,but at much higher litter sizes than any modern mammal, 38 specimens.[132]Most modern mammals areviviparous,giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have asex-determination systemdifferent from that of most other mammals.[133]In particular, thesex chromosomesof a platypus are more like those of a chicken than those of a therian mammal.[134]
Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a shortgestationperiod, typically shorter than itsestrous cycleand 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, themarsupium,located in the front of the mother'sabdomen.This is theplesiomorphiccondition among viviparous mammals; the presence of epipubic bones in all non-placentals prevents the expansion of the torso needed for full pregnancy.[82]Even non-placental eutherians probably reproduced this way.[43]The placentals give birth to relatively complete and developed young, usually after long gestation periods.[135]They get their name from theplacenta,which connects the developing fetus to the uterine wall to allow nutrient uptake.[136]In placentals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.[132]
Themammary glandsof mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have theteatsseen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.[137]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.[138]Lactoseis the main sugar in placental milk while monotreme and marsupial milk is dominated byoligosaccharides.[139]Weaningis the process in which a mammal becomes less dependent on their mother's milk and more on solid food.[140]
Endothermy
Nearly all mammals areendothermic( "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 forectothermic( "cold-blooded" ) reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.[141]Small insectivorous mammals eat prodigious amounts for their size. A rare exception, thenaked mole-ratproduces little metabolic heat, so it is considered an operationalpoikilotherm.[142]Birds are also endothermic, so endothermy is not unique to mammals.[143]
Species lifespan
Among mammals, species maximum lifespan varies significantly (for example theshrewhas a lifespan of two years, whereas the oldestbowhead whaleis recorded to be 211 years).[144]Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability torepair DNA damageis an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow,[145]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 proteinKu80.[144]In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to beup-regulatedin the longer-lived species.[146]The cellular level of the DNA repair enzymepoly ADP ribose polymerasewas found to correlate with species lifespan in a study of 13 mammalian species.[147]Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.[148][149][150]
Locomotion
Terrestrial
Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—areplantigrade,walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, aredigitigrade,walking on their toes, the greater stride length allowing more speed. Some animals such ashorsesareunguligrade,walking on the tips of their toes. This even further increases their stride length and thus their speed.[151]A few mammals, namely the great apes, are also known towalk on their knuckles,at least for their front legs.Giant anteaters[152]and platypuses[153]are also knuckle-walkers. Some mammals arebipeds,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 andkangaroo rats.[154][155]
Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowesthorse gaitis thewalk,then there are three faster gaits which, from slowest to fastest, are thetrot,thecanterand thegallop.Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the mainhuman gaitsare bipedalwalkingandrunning,but they employ many other gaits occasionally, including a four-leggedcrawlin tight spaces.[156]Mammals show a vast range ofgaits,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 andleaping gaits.[157]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.[156]
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, tobrachiate(swing between trees).[158]Many arboreal species, such as tree porcupines,silky anteaters,spider monkeys, andpossums,useprehensile tailsto 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 allowssquirrelsto 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.[158]Size relating to weight affects gliding animals such as thesugar glider.[159]Some species of primate, bat and all species ofslothachieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.[158]
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.[160]
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.[161][162]By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.[163]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.[164]The surface of their wings is equipped with touch-sensitive receptors on small bumps calledMerkel 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.[165]
Fossorial and subterranean
A fossorial (from Latinfossor,meaning "digger" ) is an animal adapted to digging which lives primarily, but not solely, underground. Some examples arebadgers,andnaked mole-rats.Manyrodentspecies 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 intemperature regulationwhile others use the underground habitat for protection frompredatorsor forfood storage.[166]
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;pocket gophers,for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorialmarsupial mole,the eyes are degenerated and useless,Talpamoles havevestigialeyes and theCape golden molehas 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 havecursoriallegs. 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.[167]
Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete orderInsectivora.[168]
Aquatic
Fully aquatic mammals, the cetaceans andsirenians,have lost their legs and have a tail fin to propel themselves through the water.Flippermovement 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 adorsal finto prevent themselves from turning upside-down in the water.[169][170]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.[171]
Semi-aquaticmammals, 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.[172][173]Pinnipeds have several adaptions for reducingdrag.In addition to their streamlined bodies, they have smooth networks ofmuscle bundlesin their skin that may increaselaminar flowand make it easier for them to slip through water. They also lackarrector pili,so their fur can be streamlined as they swim.[174]They rely on their fore-flippers for locomotion in a wing-like manner similar topenguinsandsea turtles.[175]Fore-flipper movement is not continuous, and the animal glides between each stroke.[173]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;[172]the hind-flippers serve as stabilizers.[174]Other semi-aquatic mammals include beavers,hippopotamuses,ottersand platypuses.[176]Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies havegraviportalskeletal structures,[177]adapted to carrying their enormous weight, and theirspecific gravityallows them to sink and move along the bottom of a river.[178]
Behavior
Communication and vocalization
Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, aswarning calls,[180]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 theroaringoflionsandred deer.[181]Thesongsof the humpback whale may be signals to females;[182]they have different dialects in different regions of the ocean.[183]Social vocalizations include theterritorialcalls ofgibbons,and the use of frequency ingreater spear-nosed batsto distinguish between groups.[184]Thevervet monkeygives 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.[179]Prairie dogssimilarly have complex calls that signal the type, size, and speed of an approaching predator.[185]Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls areinfrasonic,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.[186]
Mammals signal by a variety of means. Many give visualanti-predator signals,as when deer andgazellestot,honestly indicatingtheir fit condition and their ability to escape,[187][188]or whenwhite-tailed deerand other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected.[189]Many mammals make use ofscent-marking,sometimes possibly to help defend territory, but probably with a range of functions both within and between species.[190][191][192]Microbatsandtoothed whalesincludingoceanic dolphinsvocalize both socially and inecholocation.[193][194][195]
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 acarnivorousdiet (and includes insectivorous diets). Other mammals, calledherbivores,eat plants, which containcomplex carbohydratessuch as cellulose. A herbivorous diet includes subtypes such asgranivory(seed eating),folivory(leaf eating),frugivory(fruit eating),nectarivory(nectar eating),gummivory(gum eating) andmycophagy(fungus eating). The digestive tract of a herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichamberedstomachor in a large cecum.[95]Some mammals arecoprophagous,consumingfecesto absorb the nutrients not digested when the food was first ingested.[91]: 131–137 Anomnivoreeats both prey and plants. Carnivorous mammals have a simpledigestive tractbecause theproteins,lipidsandmineralsfound in meat require little in the way of specialized digestion. Exceptions to this includebaleen whaleswho also housegut florain a multi-chambered stomach, like terrestrial herbivores.[196]
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 highmetabolic 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 a herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores).[197]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 (antsortermites).[198]
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.[200]They are grouped into three categories:mesocarnivory(50–70% meat),hypercarnivory(70% and greater of meat), andhypocarnivory(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 ofhyenas,allowing them to consume bones; some extinct groups, notably theMachairodontinae,had saber-shapedcanines.[199]
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 tozoopharmacognosy.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.[201]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.[202]Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such ashairballs), aid with hemoglobin production, and as a laxative.[203]
Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known ashibernation.[204]In the period preceding hibernation, larger mammals, such as bears, becomepolyphagicto increase fat stores, whereas smaller mammals prefer to collect and stash food.[205]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 hibernatingArctic ground squirrelscan drop to −2.9 °C (26.8 °F); however, the head and neck always stay above 0 °C (32 °F).[206]A few mammals in hot environmentsaestivatein times of drought or extreme heat, for example thefat-tailed dwarf lemur(Cheirogaleus medius).[207]
Drinking
By necessity,terrestrial animalsin captivity become accustomed to drinking water, but most free-roaming animals stay hydrated through the fluids and moisture in fresh food,[208]and learn to actively seek foods with high fluid content.[209]When conditions impel them to drink from bodies of water, the methods and motions differ greatly among species.[210]
Cats,canines,andruminantsall lower the neck and lap in water with their powerful tongues.[210]Cats and canines lap up water with the tongue in a spoon-like shape.[211]Canines lap water by scooping it into their mouth with a tongue which has taken the shape of a ladle. However, with cats, only the tip of their tongue (which is smooth) touches the water, and then the cat quickly pulls its tongue back into its mouth which soon closes; this results in a column of liquid being pulled into the cat's mouth, which is then secured by its mouth closing.[212]Ruminants and most other herbivores partially submerge the tip of the mouth in order to draw in water by means of a plunging action with the tongue held straight.[213]Cats drink at a significantly slower pace than ruminants, who face greater natural predation hazards.[210]
Manydesert animalsdo not drink even if water becomes available, but rely on eatingsucculent plants.[210]In cold and frozen environments, some animals likehares,tree squirrels,andbighorn sheepresort to consuming snow and icicles.[214]Insavannas,the drinking method ofgiraffeshas been a source of speculation for its apparent defiance of gravity; the most recent theory contemplates the animal's long neck functions like aplunger pump.[215]Uniquely,elephantsdraw water into their trunks and squirt it into their mouths.[210]Intelligence
In intelligent mammals, such asprimates,thecerebrumis larger relative to the rest of the brain.Intelligenceitself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility.Rats,for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a freshhabitat.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.[198]
Tool use by animalsmay indicate different levels oflearningandcognition.Thesea otteruses rocks as essential and regular parts of its foraging behaviour (smashingabalonefrom rocks or breaking open shells), with some populations spending 21% of their time making tools.[216]Other tool use, such aschimpanzeesusing twigs to "fish" for termites, may be developed bywatching others use toolsand may even be a true example of animal teaching.[217]Tools may even be used in solving puzzles in which the animal appears to experience a"Eureka moment".[218]Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.[219]
Brain sizewas previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios ofbrain to body massmay increase the amount of brain mass available for more complex cognitive tasks.Allometricanalysis indicates that mammalian brain size scales at approximately the2⁄3or3⁄4exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides anencephalisation quotientthat can be used as another indication of animal intelligence.[220]Sperm whaleshave the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 cu in) and 7.8 kilograms (17 lb) in mature males.[221]
Self-awarenessappears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such asmetacognitive reasoning.The traditional method for measuring this is themirror test,which determines if an animal possesses the ability of self-recognition.[222]Mammals that have passed the mirror test includeAsian elephants(some pass, some do not);[223]chimpanzees;[224]bonobos;[225]orangutans;[226]humans, from 18 months (mirror stage);[227]common bottlenose dolphins;[a][228]orcas;[229]andfalse killer whales.[229]
Social structure
Eusocialityis 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 asbees,ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat[230]and theDamaraland mole-rat.[231]
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 ofCallitrichidae(marmosetsandtamarins) are unique among primates in their degree of cooperative care of young.[232]Harry Harlowset up an experiment withrhesus 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.[233]
Afission–fusion societyis 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 toenvironmentalor 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 andspider monkeys),[234]elephants,[235]spotted hyenas,[236]lions,[237]and dolphins.[238]
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.[239]A solitary animal, while foraging, can also be less conspicuous to predators or prey.[240]
In ahierarchy,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,[241]and apecking orderis 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.[242]Dominant individuals, or Alpha s, have a high chance of reproductive success, especially inharemswhere one or a few males (resident males) have exclusive breeding rights to females in a group.[243]Non-resident males can also be accepted in harems, but some species, such as thecommon vampire bat(Desmodus rotundus), may be more strict.[244]
Some mammals are perfectlymonogamous,meaning that theymate for lifeand take no other partners (even after the original mate's death), as with wolves,Eurasian beavers,and otters.[245][246]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, excludingleks,are estimated to occur in up to 90% of mammals.[247]Lek mating occurs when males congregate around females and try to attract them with variouscourtship displaysand vocalizations, as in harbor seals.[248]
Allhigher mammals(excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree ofparental care.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 areK-selected,and invest more time and energy into their young than dor-selectedanimals. 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 withtenrecs,however this varies species to species, even within the same genus. For example, the males of thesouthern pig-tailed macaque(Macaca nemestrina) do not participate in child care, whereas the males of theJapanese macaque(M. fuscata) do.[249]
Humans and other mammals
In human culture
Non-human mammals play a wide variety of roles in human culture. They are the most popular ofpets,with tens of millions of dogs, cats and other animals includingrabbitsand mice kept by families around the world.[250][251][252]Mammals such asmammoths,horses and deer are among the earliest subjects of art, being found inUpper Paleolithiccave paintingssuch as atLascaux.[253]Major artists such asAlbrecht Dürer,George StubbsandEdwin Landseerare known for their portraits of mammals.[254]Many species of mammals have beenhuntedfor sport and for food; deer andwild boarare especially popular asgame animals.[255][256][257]Mammals such ashorsesanddogsare widely raced for sport, often combined withbetting on the outcome.[258][259]There is a tension between the role of animals as companions to humans, and their existence as individuals withrights of their own.[260]Mammals further play a wide variety of roles in literature,[261][262][263]film,[264]mythology, and religion.[265][266][267]
Uses and importance
The domestication of mammals was instrumental in theNeolithic development of agricultureand ofcivilization,causing farmers to replacehunter-gatherersaround the world.[b][269]This transition from hunting and gathering toherding flocksandgrowing cropswas 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".[270]
Domesticmammals form a large part of thelivestockraised formeatacross the world. They include (2009) around 1.4 billioncattle,1 billionsheep,1 billiondomestic pigs,[271][272]and (1985) over 700 million rabbits.[273]Working domestic animalsincluding cattle and horses have been used for work andtransportfrom the origins of agriculture, their numbers declining with the arrival of mechanised transport andagricultural 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 animalscontinue to transport goods.[274]Mammal skins provideleatherforshoes,clothingandupholstery.Woolfrom mammals including sheep, goats andalpacashas been used for centuries for clothing.[275][276]
Mammals serve a major role in science asexperimental animals,both in fundamental biological research, such as in genetics,[278]and in the development of new medicines, which must be tested exhaustively to demonstrate theirsafety.[279]Millions of mammals, especially mice and rats, are used inexperimentseach year.[280]Aknockout mouseis agenetically modified mousewith an inactivatedgene,replaced or disrupted with an artificial piece of DNA. They enable the study ofsequencedgenes whose functions are unknown.[281]A small percentage of the mammals are non-human primates, used in research for their similarity to humans.[282][283][284]
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 allwildmammals 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.[285][286]
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.[287]Hybrids between different subspecies within a species (such as between theBengal tigerandSiberian 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.[288]Natural hybrids will occur inhybrid 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 thered wolf(though this is controversial).[289]
Artificial selection,the deliberateselective breedingof domestic animals, is being used tobreed backrecently extinctanimals in an attempt to achieve an animal breed with aphenotypethat resembles that extinctwildtypeancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initialgene poolof that wild type is lost forever with itsextinction.As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, asHeck cattleare of theaurochs.[290]
Purebredwild species evolved to a specific ecology can be threatened with extinction[291]through the process ofgenetic pollution,the uncontrolled hybridization,introgressiongenetic swamping which leads to homogenization orout-competitionfrom theheterosichybrid species.[292]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.[293]Interbreedingcan swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangeredwild water buffalois most threatened with extinction by genetic pollution from thedomestic water buffalo.Such extinctions are not always apparent from amorphologicalstandpoint. Some degree ofgene flowis a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species.[294][295]
Threats
The loss of species from ecological communities,defaunation,is primarily driven by human activity.[296]This has resulted inempty forests,ecological communities depleted of large vertebrates.[297][298]In theQuaternary extinction event,the mass die-off ofmegafaunalvariety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as thewoolly mammoth,into extinction.[299][300]The 2019Global Assessment Report on Biodiversity and Ecosystem ServicesbyIPBESstates that the totalbiomassof wild mammals has declined by 82 percent since the beginning of human civilization.[301][302]Wild animals make up just 4% of mammalianbiomasson earth, while humans and their domesticated animals make up 96%.[286]
Various species are predicted tobecome extinct in the near future,[303]among them therhinoceros,[304]giraffes,[305]and species ofprimates[306]andpangolins.[307]According to the WWF's 2020Living Planet Report,vertebratewildlifepopulations have declined by 68% since 1970 as a result of human activities, particularlyoverconsumption,population growthandintensive farming,which is evidence that humans have triggered asixth mass extinctionevent.[308][309]Hunting alone threatens hundreds of mammalian species around the world.[310][311]Scientists claim that the growing demand formeatis contributing tobiodiversity lossas this is a significant driver ofdeforestationandhabitat destruction;species-rich habitats, such as significant portions of theAmazon rainforest,are being converted to agricultural land for meat production.[312][313][314]Another influence is over-hunting andpoaching,which can reduce the overall population of game animals,[315]especially those located near villages,[316]as in the case ofpeccaries.[317]The effects of poaching can especially be seen in theivory tradewith African elephants.[318]Marine mammals are at risk from entanglement from fishing gear, notablycetaceans,with discard mortalities ranging from 65,000 to 86,000 individuals annually.[319]
Attention is being given to endangered species globally, notably through theConvention on Biological Diversity,otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats.[320]Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental andnon-governmentalorganizations.[321]
Recent extinctionscan be directly attributed to human influences.[322][296]The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500,[323]and around 80 mammal species have gone extinct since that time and 2015.[324]Some species, such as thePère David's deer[325]areextinct in the wild,and survive solely in captive populations. Other species, such as theFlorida panther,areecologically extinct,surviving in such low numbers that they essentially have no impact on the ecosystem.[326]: 318 Other populations are onlylocally extinct(extirpated), still existing elsewhere, but reduced in distribution,[326]: 75–77 as with the extinction ofgray whalesin theAtlantic.[327]
See also
Notes
- ^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.[228]
- ^Diamond discussed this matter further in his 1997 bookGuns, Germs, and Steel.[268]
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Further reading
- Brown WM (2001). "Natural selection of mammalian brain components".Trends in Ecology and Evolution.16(9): 471–473.Bibcode:2001TEcoE..16..471B.doi:10.1016/S0169-5347(01)02246-7.
- McKenna MC, Bell SK (1997).Classification of Mammals Above the Species Level.New York: Columbia University Press.ISBN978-0-231-11013-6.OCLC37345734.[permanent dead link]
- Nowak RM (1999).Walker's mammals of the world(6th ed.). Baltimore: Johns Hopkins University Press.ISBN978-0-8018-5789-8.OCLC937619124.
- Simpson GG (1945). "The principles of classification and a classification of mammals".Bulletin of the American Museum of Natural History.85:1–350.
- Murphy WJ, Eizirik E, O'Brien SJ, Madsen O, Scally M, Douady CJ, et al. (December 2001). "Resolution of the early placental mammal radiation using Bayesian phylogenetics".Science.294(5550): 2348–2351.Bibcode:2001Sci...294.2348M.doi:10.1126/science.1067179.PMID11743200.S2CID34367609.
- Springer MS, Stanhope MJ, Madsen O, de Jong WW (August 2004)."Molecules consolidate the placental mammal tree"(PDF).Trends in Ecology & Evolution.19(8): 430–438.doi:10.1016/j.tree.2004.05.006.PMID16701301.S2CID1508898.Archived fromthe original(PDF)on 29 July 2016.Retrieved21 January2005.
- Vaughan TA, Ryan JM, Capzaplewski NJ (2000).Mammalogy(4th ed.). Fort Worth, Texas: Saunders College Publishing.ISBN978-0-03-025034-7.OCLC42285340.
- Kriegs JO, Churakov G, Kiefmann M, Jordan U, Brosius J, Schmitz J (April 2006)."Retroposed elements as archives for the evolutionary history of placental mammals".PLOS Biology.4(4): e91.doi:10.1371/journal.pbio.0040091.PMC1395351.PMID16515367.
- MacDonald DW, Norris S (2006).The Encyclopedia of Mammals(3rd ed.). London: Brown Reference Group.ISBN978-0-681-45659-4.OCLC74900519.
External links
- ASM Mammal Diversity DatabaseArchived25 December 2022 at theWayback Machine
- Biodiversitymapping.org – All mammal orders in the world with distribution mapsArchived2016-09-26 at theWayback Machine
- Paleocene MammalsArchived3 February 2024 at theWayback Machine,a site covering the rise of the mammals, paleocene-mammals.de
- Evolution of MammalsArchived25 January 2024 at theWayback Machine,a brief introduction to early mammals, enchantedlearning
- European Mammal Atlas EMMAArchived25 January 2024 at theWayback Machinefrom Societas Europaea Mammalogica, European-mammals.org
- Marine Mammals of the WorldArchived8 June 2019 at theWayback Machine– An overview of all marine mammals, including descriptions, both fully aquatic and semi-aquatic, noaa.gov
- Mammalogy.orgArchived1 March 2020 at theWayback MachineThe American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library
