Editing Mammal (section)

==Anatomy==

===Distinguishing features===
Living mammal species can be identified by the presence of [[sweat gland]]s, including [[Mammary gland|those that are specialised to produce milk]] to nourish their young.<ref>{{cite book | vauthors = Romer SA, Parsons TS |year=1977 |title=The Vertebrate Body |location=Philadelphia |publisher=Holt-Saunders International |pages=129–145 |isbn=978-0-03-910284-5 |oclc=60007175}}</ref> In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.<ref>{{cite book |url= {{Google books|plainurl=yes|id=kS-h84pMJw4C|page=593}} | vauthors = Purves WK, Sadava DE, Orians GH, Helle HC |year=2001 |title= Life: The Science of Biology|location=New York|edition=6th|publisher=Sinauer Associates, Inc. |page=593 |isbn=978-0-7167-3873-2 |oclc=874883911 }}</ref>

Many traits shared by all living mammals appeared among the earliest members of the group:
* '''[[Jaw|Jaw joint]]''' – The [[dentary]] (the lower jaw bone, which carries the teeth) and the [[squamosal]] (a small [[cranial]] bone) meet to form the joint. In most [[gnathostomes]], including early [[therapsids]], the joint consists of the [[articular]] (a small bone at the back of the lower jaw) and [[Quadrate bone|quadrate]] (a small bone at the back of the upper jaw).<ref name=jawbone2006/>
* '''[[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.<ref>{{cite journal | vauthors = Anthwal N, Joshi L, Tucker AS | title = Evolution of the mammalian middle ear and jaw: adaptations and novel structures | journal = Journal of Anatomy | volume = 222 | issue = 1 | pages = 147–160 | date = January 2013 | pmid = 22686855 | pmc = 3552421 | doi = 10.1111/j.1469-7580.2012.01526.x }}</ref>
* '''Tooth replacement''' – Teeth can be replaced once ([[diphyodonty]]) or (as in toothed whales and [[Muridae|murid]] rodents) not at all ([[Dentition|monophyodont]]y).<ref>{{cite journal | vauthors = van Nievelt AF, Smith KK |year=2005 |title=To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals |journal=Paleobiology |volume=31 |issue=2 |pages=324–346 |doi=10.1666/0094-8373(2005)031[0324:trontr]2.0.co;2|s2cid=37750062 }}</ref> Elephants, manatees, and kangaroos continually grow new teeth throughout their life ([[polyphyodont]]y).<ref>{{cite journal | vauthors = Libertini G, Ferrara N | title = Aging of perennial cells and organ parts according to the programmed aging paradigm | journal = Age | volume = 38 | issue = 2 | pages = 35 | date = April 2016 | pmid = 26957493 | pmc = 5005898 | doi = 10.1007/s11357-016-9895-0 }}</ref>
* '''Prismatic enamel''' – The [[tooth enamel|enamel]] coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the [[dentin]] to the tooth's surface.<ref>{{cite journal | vauthors = Mao F, Wang Y, Meng J | title = A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia) – Implications for Multituberculate Biology and Phylogeny | journal = PLOS ONE | volume = 10 | issue = 5 | pages = e0128243 | year = 2015 | pmid = 26020958 | pmc = 4447277 | doi = 10.1371/journal.pone.0128243 | bibcode = 2015PLoSO..1028243M | doi-access = free }}</ref>
* '''[[Occipital condyle]]s''' – Two knobs at the base of the skull fit into the topmost [[Cervical vertebrae|neck vertebra]]; most other [[tetrapod]]s, in contrast, have only one such knob.<ref>{{cite journal |jstor=2453526 | vauthors = Osborn HF |year=1900 |title=Origin of the Mammalia, III. Occipital Condyles of Reptilian Tripartite Type|journal=The American Naturalist|volume=34|number=408| pages=943–947 |doi=10.1086/277821|doi-access=free | bibcode = 1900ANat...34..943O }}</ref>

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.<ref>{{cite journal| vauthors = Crompton AW, Jenkins Jr FA |year=1973|title=Mammals from Reptiles: A Review of Mammalian Origins|journal=Annual Review of Earth and Planetary Sciences|volume=1|pages=131–155|doi=10.1146/annurev.ea.01.050173.001023|bibcode=1973AREPS...1..131C}}</ref> Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.<ref name=schulkin/>

===Sexual dimorphism===
[[File:Aurochsfeatures.jpg|thumb|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 [[sexual 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 dimorphism [[Sexual selection in mammals|relates to sexual selection]] on males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated by [[mating system]]s, and the degree of male-biased size dimorphism. The degree of sexual selection is also positively correlated with male and female size across mammals. Further, parallel selection pressure on female mass is identified in that age at weaning is significantly higher in more [[Polygyny in animals|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.<ref>{{cite book |vauthors=Lindenfors P, Gittleman JL, Jones KE |title=Sex, Size and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism |chapter=Sexual size dimorphism in mammals |date=2007 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-920878-4 |pages=16–26 |url=https://swepub.kb.se/bib/swepub:oai:DiVA.org:su-16290?tab2=abs&language=en |access-date=25 January 2024 |archive-date=25 January 2024 |archive-url=https://web.archive.org/web/20240125191352/https://swepub.kb.se/bib/swepub:oai:DiVA.org:su-16290?tab2=abs&language=en |url-status=live }}</ref>

===Biological systems===
{{Main|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.<ref>{{cite book|url={{Google books|plainurl=yes|id=FIIgDk9i_GkC|page=154}} | vauthors = Dierauf LA, Gulland FM |title=CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation|location=Boca Raton|edition=2nd|publisher=CRC Press |year= 2001|page=154 |isbn=978-1-4200-4163-7|oclc=166505919}}</ref> All mammalian brains possess a [[neocortex]], a brain region unique to mammals.<ref>{{cite journal | vauthors = Lui JH, Hansen DV, Kriegstein AR | title = Development and evolution of the human neocortex | journal = Cell | volume = 146 | issue = 1 | pages = 18–36 | date = July 2011 | pmid = 21729779 | pmc = 3610574 | doi = 10.1016/j.cell.2011.06.030 }}</ref> Placental brains have a [[corpus callosum]], unlike monotremes and marsupials.<ref>{{cite journal | vauthors = Keeler CE | title = Absence of the Corpus Callosum as a Mendelizing Character in the House Mouse | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 19 | issue = 6 | pages = 609–611 | date = June 1933 | pmid = 16587795 | pmc = 1086100 | doi = 10.1073/pnas.19.6.609 | bibcode = 1933PNAS...19..609K | jstor = 86284 | doi-access = free }}</ref>
{{multiple image
| align             = right
| footer            = [[Educational toy|Didactic models]] of a mammalian heart
| footer_align      = center
| image1            = Didactic model of a mammal heart 01-FMVZ USP-07.jpeg
| width1            = 100
| image2            = Didactic model of a mammal heart 02-FMVZ USP-08.jpeg
| width2            = 100
| image3            = Didactic model of a mammal heart 03--FMVZ USP-10.jpeg
| width3            = 200
| image4            = Didactic model of a mammal heart 04--FMVZ USP-11.jpeg
| width4            = 200
| total_width       = 600
}}

====Circulatory systems====
The mammalian [[heart]] has four chambers, two upper [[atrium (heart)|atria]], the receiving chambers, and two lower [[ventricle (heart)|ventricles]], the discharging chambers.<ref>{{cite book| vauthors = Standring S, Borley NR |year=2008|title=Gray's anatomy: the anatomical basis of clinical practice|edition=40th|location=London|publisher=Churchill Livingstone|pages=960–962|isbn=978-0-8089-2371-8|oclc=213447727}}</ref> 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 vein]]s. 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 [[diastole|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 circulation|coronary arteries]].<ref>{{cite book|vauthors=Betts JF, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, Poe B, Wise JA, Womble M, Young KA|display-authors=6|title=Anatomy & physiology|year=2013|isbn=978-1-938168-13-0|oclc=898069394|url=https://cnx.org/content/m46676/latest/?collection=col11496/latest|location=Houston|publisher=Rice University Press|pages=787–846|access-date=25 January 2024|archive-date=23 February 2022|archive-url=https://web.archive.org/web/20220223063018/https://openstax.org/books/anatomy-and-physiology/pages/19-1-heart-anatomy|url-status=live}}</ref>

====Respiratory systems====
[[File:Lung expansion simulation with Raccoon.gif|thumb|right|[[Raccoon]] lungs being inflated manually]]
{{Main|Respiratory system#Mammals}}
The [[lung]]s of mammals are spongy and honeycombed. Breathing is mainly achieved with the [[diaphragm (anatomy)|diaphragm]], which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and [[bronchi]], and expands the [[pulmonary alveolus|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 [[muscle contraction|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.<ref>{{cite book| vauthors = Levitzky MG |title=Pulmonary physiology|date=2013|publisher=McGraw-Hill Medical|location=New York|isbn=978-0-07-179313-1|chapter=Mechanics of Breathing |edition=8th|oclc=940633137}}</ref><ref name=bellows/> This type of lung is known as a bellows lung due to its resemblance to blacksmith [[bellows]].<ref name=bellows>{{cite book|chapter-url={{Google books|plainurl=yes|id=DoC48j2-LnkC|page=12}} |location=New Delhi| vauthors = Umesh KB |year=2011|title=Handbook of Mechanical Ventilation|chapter=Pulmonary Anatomy and Physiology|publisher=Jaypee Brothers Medical Publishing |page=12|isbn=978-93-80704-74-6|oclc=945076700}}</ref>
{{break}}

====Integumentary systems====
[[File:The skin of mammals.jpg|thumb|235px|Mammal skin: (1) [[hair]], (2) [[epidermis]], (3) [[sebaceous gland]], (4) [[Arrector pili muscle]], (5) [[dermis]], (6) [[hair follicle]], (7) [[sweat gland]]. Not labelled, the bottom layer: [[hypodermis]], showing round [[adipocytes]]]]
The [[skin|integumentary system]] (skin) is made up of three layers: the outermost [[epidermis (skin)|epidermis]], the [[dermis]] and the [[hypodermis]]. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of [[adipose tissue]], which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;<ref name=hair/>{{rp|97}} [[marine mammal]]s require a thick hypodermis ([[blubber]]) for insulation, and [[right whale]]s have the thickest blubber at {{convert|20|in|cm}}.<ref>{{cite book| vauthors = Tinker SW |title=Whales of the World|year=1988|publisher=Brill Archive|isbn=978-0-935848-47-2|page=51|url={{Google books|plainurl=yes|id=ASIVAAAAIAAJ|page=51}}}}</ref> Although other animals have features such as whiskers, [[feather]]s, [[setae]], or [[cilia (entomology)|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.<ref name=hair>{{cite book|url={{Google books|plainurl=yes|id=udCnKce9hfoC|page=97}}| vauthors = Feldhamer GA, Drickamer LC, Vessey SH, Merritt JF, Krajewski C |year=2007|title=Mammalogy: Adaptation, Diversity, Ecology|edition=3rd|location=Baltimore|publisher=Johns Hopkins University Press|isbn=978-0-8018-8695-9|oclc=124031907}}</ref>{{rp|61}}

====Digestive systems====
{{Multiple image
| align             = right
| image1            = Aardwolfskull.jpg
| image2            = Canis lupus 02 MWNH 358.jpg
| footer            = The [[carnassial]]s (teeth in the very back of the mouth) of the [[insectivorous]] [[aardwolf]] (left) versus that of a [[grey wolf]] (right) which consumes large vertebrates
}}

Herbivores have developed a diverse range of physical structures to facilitate the [[Herbivore adaptations to plant defense|consumption of plant material]]. To break up intact plant tissues, mammals have developed [[Mammal tooth|teeth]] structures that reflect their feeding preferences. For instance, [[frugivore]]s (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialised for grinding foliage and [[seed]]s. [[Grazing]] animals 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.<ref>{{cite book| vauthors = Romer AS |year=1959|title=The vertebrate story| url = https://archive.org/details/vertebratestory00rome | url-access = registration |publisher=University of Chicago Press|location=Chicago|isbn=978-0-226-72490-4|edition=4th}}</ref> Most carnivorous mammals have [[carnassial]] teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.<ref>{{cite journal| vauthors = de Muizon C, Lange-Badré B |year=1997 |title=Carnivorous dental adaptations in tribosphenic mammals and phylogenetic reconstruction |journal=Lethaia |volume=30 |issue=4 |pages=353–366 |doi=10.1111/j.1502-3931.1997.tb00481.x |bibcode=1997Letha..30..353D }}</ref>

The stomach of [[even-toed ungulates]] (Artiodactyla) is divided into four sections: the [[rumen]], the [[Reticulum (anatomy)|reticulum]], the [[omasum]] and the [[abomasum]] (only [[ruminant]]s have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a [[bolus (digestion)|bolus]] (or [[cud]]), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulolytic [[microbe]]s ([[bacterium|bacteria]], [[protozoa]] and [[fungus|fungi]]) produce [[cellulase]], which is needed to break down the [[cellulose]] in plants.<ref name="Comparative anatomy of the stomach">{{cite journal | vauthors = Langer P | title = Comparative anatomy of the stomach in mammalian herbivores | journal = Quarterly Journal of Experimental Physiology | volume = 69 | issue = 3 | pages = 615–625 | date = July 1984 | pmid = 6473699 | doi = 10.1113/expphysiol.1984.sp002848 | s2cid = 30816018 }}</ref> [[Perissodactyls]], in contrast to the ruminants, store digested food that has left the stomach in an enlarged [[cecum]], where it is fermented by bacteria.<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=Mqv4Lo1vpk4C|page=322}}| vauthors = Vaughan TA, Ryan JM, Czaplewski NJ |title=Mammalogy | name-list-style = vanc |edition=5th |publisher=Jones and Bartlett |year=2011 |page=322 |isbn=978-0-7637-6299-5 |chapter=Perissodactyla |oclc=437300511}}</ref> 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 fibres. The cecum is either absent or short and simple, and the large intestine is not [[sacculation|sacculated]] or much wider than the small intestine.<ref>{{cite book |url={{Google books|plainurl=yes|id=B3crAAAAYAAJ|page=496}}| vauthors = Flower WH, Lydekker R |author2-link=Richard Lydekker |year=1946 |title=An Introduction to the Study of Mammals Living and Extinct |publisher=Adam and Charles Black |location=London |page=496|isbn=978-1-110-76857-8}}</ref>

====Excretory and genitourinary systems====
[[File:Glycerination of Bovine kidney.jpg|thumb|Bovine kidney]]
[[File:Image from page 702 of "Outlines of zoology" (1895) (20732795545).jpg|thumb|left|[[Genitourinary system]] of a male and female rabbit]]
The mammalian [[excretory system]] involves many components. Like most other land animals, mammals are [[ureotelic]], and convert [[ammonia]] into [[urea]], which is done by the [[liver]] as part of the [[urea cycle]].<ref>{{cite book | vauthors = Sreekumar S |title=Basic Physiology|publisher=PHI Learning Pvt. Ltd.|year=2010|pages=180–181|isbn=978-81-203-4107-4|url ={{Google books|plainurl=yes|id=IxYR9wTeXQgC|page=180}}}}</ref> [[Bilirubin]], a waste product derived from [[blood cell]]s, is passed through [[bile]] and [[urine]] with the help of enzymes excreted by the liver.<ref>{{cite book| vauthors = Cheifetz AS |title=Oxford American Handbook of Gastroenterology and Hepatology|year=2010|publisher=Oxford University Press, US|location=Oxford|isbn=978-0-19-983012-1|page=165}}</ref> The passing of bilirubin via bile through the [[intestinal tract]] gives mammalian [[feces]] a distinctive brown coloration.<ref>{{cite book| vauthors = Kuntz E |title=Hepatology: Textbook and Atlas|year=2008|publisher=Springer|location=Germany|isbn=978-3-540-76838-8|page=38}}</ref> Distinctive features of the [[mammalian kidney]] include the presence of the [[renal pelvis]] and [[renal pyramid]]s, and of a clearly distinguishable [[renal cortex|cortex]] and [[renal medulla|medulla]], which is due to the presence of elongated [[Loop of Henle|loops of Henle]]. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed [[reniculate kidney]]s of pinnipeds, [[cetacea]]ns and bears.<ref>{{cite journal | vauthors = Ortiz RM | title = Osmoregulation in marine mammals | journal = The Journal of Experimental Biology | volume = 204 | issue = Pt 11 | pages = 1831–1844 | date = June 2001 | doi = 10.1242/jeb.204.11.1831 | pmid = 11441026 | url = https://jeb.biologists.org/cgi/content/short/204/11/1831 | doi-access = free | bibcode = 2001JExpB.204.1831O | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191351/https://jeb.biologists.org/cgi/content/short/204/11/1831 | url-status = live }}</ref><ref name=VB/> Most adult placentals have no remaining trace of the [[cloaca]]. In the embryo, the [[embryonic cloaca]] divides into a posterior region that becomes part of the [[anus]], and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the [[vulval vestibule|vestibule]] or [[Urogenital sinus#Other animals|urogenital sinus]] that receives the [[urethra]] and [[vagina]], while in males it forms the entirety of the [[penile urethra]].<ref name=VB/><ref>{{cite book|last=Linzey|first=Donald W.|title=Vertebrate Biology: Systematics, Taxonomy, Natural History, and Conservation|publisher=Johns Hopkins University Press|year=2020|page=306|isbn=978-1-42143-733-0|url=https://books.google.com/books?id=Rur4DwAAQBAJ&pg=PA306|access-date=22 January 2024|archive-date=22 January 2024|archive-url=https://web.archive.org/web/20240122190826/https://books.google.com/books?id=Rur4DwAAQBAJ&pg=PA306|url-status=live}}</ref> However, the [[Afrosoricida|afrosoricids]] and some [[shrew]]s retain a cloaca as adults.<ref>{{Cite journal |url=http://journals.cambridge.org/action//displayFulltext?type=6&fid=290402&jid=BRE&volumeId=80&issueId=01&aid=275550&fulltextType=RV&fileId=S1464793104006566 |title=Biological Reviews – Cambridge Journals |journal=Biological Reviews |date=February 2005 |volume=80 |issue=1 |pages=93–128 |doi=10.1017/S1464793104006566 |access-date=21 January 2017 |archive-date=22 November 2015 |archive-url=https://web.archive.org/web/20151122104221/http://journals.cambridge.org/action//displayFulltext?type=6&fid=290402&jid=BRE&volumeId=80&issueId=01&aid=275550&fulltextType=RV&fileId=S1464793104006566 |url-status=live |last1=Symonds |first1=Matthew R. E. |pmid=15727040 }}</ref> In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.<ref name=VB>{{cite book| vauthors = Roman AS, Parsons TS |year=1977|title=The Vertebrate Body|publisher=Holt-Saunders International|location=Philadelphia|pages=396–399|isbn=978-0-03-910284-5}}</ref> Monotremes, which translates from [[Ancient Greek|Greek]] into "single hole", have a true cloaca.<ref>{{cite book|url={{Google books|plainurl=yes|https://books.google.com/books?id=wojUDAAAQBAJ|page=281}} |vauthors = Dawkins R, Wong Y |year=2016|title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution|edition=2nd|publisher=Mariner Books|location=Boston|page=281|isbn=978-0-544-85993-7}}</ref> Urine flows from the [[ureter]]s into the cloaca in monotremes and into the [[bladder]] in placentals.<ref name=VB/>

===Sound production===
[[File:Animal echolocation.svg|thumb|upright=1.35|A diagram of ultrasonic signals emitted by a bat, and the echo from a nearby object]]
As in all other tetrapods, mammals have a [[larynx]] that can quickly open and close to produce sounds, and a supralaryngeal [[vocal tract]] which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to [[phonate]]. The larynx controls the [[pitch (music)|pitch]] and [[loudness|volume]] of sound, but the strength the lungs exert to [[exhale]] also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using [[vocal fold]]s. The movement or tenseness of the vocal folds can result in many sounds such as [[purr]]ing 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 [[nasalization|nasal]] sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.<ref name=fitchbrown2006/>

[[File:Beluga_vocalizations.ogg|left|thumb|[[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]].<ref>{{cite journal| vauthors = Langevin P, Barclay RM |year=1990 |title= ''Hypsignathus monstrosus''|journal=Mammalian Species|issue=357 |pages=1–4 |doi=10.2307/3504110|jstor=3504110 |doi-access=free }}</ref> Large vocal pads can also lower the pitch, as in the low-pitched roars of [[big cat]]s.<ref>{{cite journal | vauthors = Weissengruber GE, Forstenpointner G, Peters G, Kübber-Heiss A, Fitch WT | title = Hyoid apparatus and pharynx in the lion (Panthera leo), jaguar (Panthera onca), tiger (Panthera tigris), cheetah (Acinonyxjubatus) and domestic cat (Felis silvestris f. catus) | journal = Journal of Anatomy | volume = 201 | issue = 3 | pages = 195–209 | date = September 2002 | pmid = 12363272 | pmc = 1570911 | doi = 10.1046/j.1469-7580.2002.00088.x }}</ref> The production of [[infrasound]] is possible in some mammals such as the [[African elephant]] (''Loxodonta'' spp.) and [[baleen whale]]s.<ref>{{cite journal | vauthors = Stoeger AS, Heilmann G, Zeppelzauer M, Ganswindt A, Hensman S, Charlton BD | title = Visualizing sound emission of elephant vocalizations: evidence for two rumble production types | journal = PLOS ONE | volume = 7 | issue = 11 | pages = e48907 | year = 2012 | pmid = 23155427 | pmc = 3498347 | doi = 10.1371/journal.pone.0048907 | bibcode = 2012PLoSO...748907S | doi-access = free }}</ref><ref>{{cite journal| vauthors = Clark CW |year=2004 |title= Baleen whale infrasonic sounds: Natural variability and function|journal=Journal of the Acoustical Society of America |volume=115 |issue=5|doi=10.1121/1.4783845|page=2554|bibcode=2004ASAJ..115.2554C}}</ref> 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 (cetacean)|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.<ref name=fitchbrown2006>{{cite book |chapter-url=https://homepage.univie.ac.at/tecumseh.fitch/wp-content/uploads/2010/08/Fitch2006MammalVocalProd.pdf |vauthors=Fitch WT |chapter=Production of Vocalizations in Mammals |year=2006 |title=Encyclopedia of Language and Linguistics |veditors=Brown K |publisher=Elsevier |location=Oxford |pages=115–121 |access-date=25 January 2024 |archive-date=1 June 2024 |archive-url=https://web.archive.org/web/20240601150858/https://homepage.univie.ac.at/tecumseh.fitch/wp-content/uploads/2010/08/Fitch2006MammalVocalProd.pdf/ |url-status=live }}</ref>

The vocal production system is controlled by the [[cranial nerve nucleus|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 nerve]]s. Electrical stimulation of the [[periaqueductal grey]] (PEG) region of the mammalian [[midbrain]] elicit vocalisations. The ability to learn new vocalisations 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 neuron]]s in the spinal cord.<ref name=fitchbrown2006/>

===Fur===
{{Main|Fur}}
[[File:Stekelvarken Aiguilles Porc-épic.jpg|thumb|[[Porcupine]]s use their [[spine (zoology)|spines]] for defence.]]
<!--{{Multiple images
|align=right
|width1=128
|width2=124
|image1=Male White-Cheeked Gibbon.jpg
|image2=Female White-Cheeked Gibbon.jpg
|footer=Color can be a form of [[sexual dimorphism]] as seen in the male (left) and female (right) [[Northern white-cheeked gibbon]].
}}-->

The primary function of the fur of mammals is [[thermoregulation]]. Others include protection, sensory purposes, waterproofing, and camouflage.<ref name=dawson2014>{{cite journal | vauthors = Dawson TJ, Webster KN, Maloney SK | title = The fur of mammals in exposed environments; do crypsis and thermal needs necessarily conflict? The polar bear and marsupial koala compared | journal = Journal of Comparative Physiology B | volume = 184 | issue = 2 | pages = 273–284 | date = February 2014 | pmid = 24366474 | doi = 10.1007/s00360-013-0794-8 | s2cid = 9481486 }}</ref> Different types of fur serve different purposes:<ref name=hair/>{{rp|99}}
* Definitive – which may be [[moulting|shed]] after reaching a certain length
* Vibrissae – sensory hairs, most commonly [[whisker]]s
* Pelage – guard hairs, under-fur, and [[awn hair]]
* [[spine (zoology)|Spines]] – stiff guard hair used for defence (such as in [[porcupine]]s)
* [[Bristle]]s – long hairs usually used in visual signals. (such as a lion's [[mane (lion)|mane]])
* [[Vellus hair|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 {{cvt|30|cm}} as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of {{cvt|-40|C}}.<ref name=hair/>{{rp|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 {{cvt|70|C}} in the summer, but the skin stays at {{cvt|40|C}}.<ref name=hair/>{{rp|188}} [[Aquatic mammal]]s, conversely, trap air in their fur to conserve heat by keeping the skin dry.<ref name=hair/>{{rp|162–163}}

[[File:Great male Leopard in South Afrika-JD.JPG|thumb|A [[leopard]]'s [[disruptive coloration|disruptively coloured]] coat provides [[camouflage]] for this [[ambush predator]].]]

====Coloration====
Mammalian coats are coloured for a variety of reasons, the major selective pressures including [[camouflage]], [[sexual selection]], communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of [[melanin]]; [[eumelanin]]s for brown and black colours and [[pheomelanin]] for a range of yellowish to reddish colours, giving mammals an [[earth tone]].<ref>{{cite journal | vauthors = Slominski A, Tobin DJ, Shibahara S, Wortsman J | title = Melanin pigmentation in mammalian skin and its hormonal regulation | journal = Physiological Reviews | volume = 84 | issue = 4 | pages = 1155–1228 | date = October 2004 | pmid = 15383650 | doi = 10.1152/physrev.00044.2003 | s2cid = 21168932 }}</ref><ref name="HiltonPond">{{cite journal | url=https://www.hiltonpond.org/ArticleAnimalColorsMain.html | title=South Carolina Wildlife | publisher=Hilton Pond Center | journal=Animal Colors | year=1996 | access-date=26 November 2011 | vauthors=Hilton Jr B | pages=10–15 | volume=43 | issue=4 | archive-date=25 January 2024 | archive-url=https://web.archive.org/web/20240125191353/https://www.hiltonpond.org/ArticleAnimalColorsMain.html | url-status=live }}</ref> Some mammals have more vibrant colours; certain monkeys such [[mandrill]]s and [[vervet monkey]]s, and opossums such as the [[Mexican mouse opossum]]s and [[Derby's woolly opossum]]s, have blue skin due to [[structural coloration|light diffraction]] in [[collagen]] fibres.<ref name="Prum2004"/> Many sloths appear green because their fur hosts green [[algae]]; this may be a [[symbiosis|symbiotic]] relation that affords [[camouflage]] to the sloths.<ref>{{cite journal | vauthors = Suutari M, Majaneva M, Fewer DP, Voirin B, Aiello A, Friedl T, Chiarello AG, Blomster J | display-authors = 6 | title = Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae) | journal = BMC Evolutionary Biology | volume = 10 | issue = 86 | pages = 86 | date = March 2010 | pmid = 20353556 | pmc = 2858742 | doi = 10.1186/1471-2148-10-86 | doi-access = free | bibcode = 2010BMCEE..10...86S }}</ref>

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.<ref name="bioscience.oxfordjournals.org">{{cite journal | vauthors = Caro T |year=2005 |title= The Adaptive Significance of Coloration in Mammals |journal=BioScience |volume= 55 | issue = 2 |pages= 125–136 |doi=10.1641/0006-3568(2005)055[0125:tasoci]2.0.co;2 |doi-access=free }}</ref> In arctic and subarctic mammals such as the [[arctic fox]] (''Alopex lagopus''), [[collared lemming]] (''Dicrostonyx groenlandicus''), [[stoat]] (''Mustela erminea''), and [[snowshoe hare]] (''Lepus americanus''), [[seasonal polyphenism|seasonal color change]] between brown in summer and white in winter is driven largely by camouflage.<ref>{{cite journal | vauthors = Mills LS, Zimova M, Oyler J, Running S, Abatzoglou JT, Lukacs PM | title = Camouflage mismatch in seasonal coat color due to decreased snow duration | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 110 | issue = 18 | pages = 7360–7365 | date = April 2013 | pmid = 23589881 | pmc = 3645584 | doi = 10.1073/pnas.1222724110 | bibcode = 2013PNAS..110.7360M |bibcode-access=free | doi-access = free }}</ref> Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely [[arboreal]] habitat due to [[convergent evolution]].<ref name="Prum2004">{{cite journal | vauthors = Prum RO, Torres RH | title = Structural colouration of mammalian skin: convergent evolution of coherently scattering dermal collagen arrays | journal = The Journal of Experimental Biology | volume = 207 | issue = Pt 12 | pages = 2157–2172 | date = May 2004 | pmid = 15143148 | doi = 10.1242/jeb.00989 | bibcode = 2004JExpB.207.2157P | url = https://jeb.biologists.org/content/207/12/2157.full.pdf | hdl = 1808/1599 | s2cid = 8268610 | access-date = 25 January 2024 | archive-date = 5 June 2024 | archive-url = https://web.archive.org/web/20240605171545/https://jeb.biologists.org/content/207/12/2157.full.pdf | url-status = live }}</ref>

[[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]].<ref>{{cite journal | vauthors = Caro T | title = Contrasting coloration in terrestrial mammals | doi-access = free | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 364 | issue = 1516 | pages = 537–548 | date = February 2009 | pmid = 18990666 | pmc = 2674080 | doi = 10.1098/rstb.2008.0221 }}</ref> Coat color is sometimes [[sexual dimorphism|sexually dimorphic]], as in [[Sexual dimorphism in non-human primates#Pelage color and markings|many primate species]].<ref>{{cite journal | vauthors = Plavcan JM | title = Sexual dimorphism in primate evolution | journal = American Journal of Physical Anthropology | volume = Suppl 33 | issue = 33 | pages = 25–53 | year = 2001 | pmid = 11786990 | doi = 10.1002/ajpa.10011 | s2cid = 31722173 |s2cid-access=free | doi-access = free }}</ref> Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.<ref name="eva.mpg.de">{{cite journal | vauthors = Bradley BJ, Gerald MS, Widdig A, Mundy NI |year=2012 |title=Coat Color Variation and Pigmentation Gene Expression in Rhesus Macaques (''Macaca Mulatta'') |journal=Journal of Mammalian Evolution |volume=20 |issue=3 |pages=263–270 |doi=10.1007/s10914-012-9212-3 |s2cid=13916535 |url=https://www.eva.mpg.de/pks/staff/widdig/pdf/Bradley_et_al_2012.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150924004623/http://www.eva.mpg.de/pks/staff/widdig/pdf/Bradley_et_al_2012.pdf |archive-date=24 September 2015 }}</ref> Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker coloured coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as [[vole]]s, 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.<ref name=hair/>{{rp|166–167}}<ref name=dawson2014/> The dazzling black-and-white striping of [[zebra]]s appear to provide some protection from biting flies.<ref name=Caro>{{cite journal | vauthors = Caro T, Izzo A, Reiner RC, Walker H, Stankowich T | title = The function of zebra stripes |bibcode-access=free | journal = Nature Communications | volume = 5 | pages = 3535 | date = April 2014 | pmid = 24691390 | doi = 10.1038/ncomms4535 | author1-link = Tim Caro | bibcode = 2014NatCo...5.3535C | s2cid = 9849814 |s2cid-access=free | doi-access = free }}</ref>

===Reproductive system===
{{Main|Mammalian reproduction}}
[[File:Goat family.jpg|thumb|right|[[Goat]] kids stay with their mother until they are weaned.]]
Mammals reproduce by [[internal fertilisation]]<ref name="naguib">{{Cite book|last=Naguib|first=Marc|url=https://books.google.com/books?id=KgTeDwAAQBAJ&pg=PA65|title=Advances in the Study of Behavior|date=19 April 2020|publisher=Academic Press|isbn=978-0-12-820726-0}}</ref> and are solely [[Gonochorism|gonochoric]] (an animal is born with either male or female genitalia, as opposed to [[hermaphrodite]]s where there is no such schism).<ref>{{Cite book| vauthors = Kobayashi K, Kitano T, Iwao Y, Kondo M |url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life |date=2018|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}}</ref>  Male mammals [[Ejaculation|ejaculate]] [[semen]] during [[Copulation (zoology)|copulation]] through a [[penis]], which may be contained in a [[Penile sheath|prepuce]] when not erect. Male placentals also [[urinate]] through a penis, and some placentals also have a penis bone ([[baculum]]).<ref name="Lombardi1998">{{cite book| vauthors = Lombardi J |title=Comparative Vertebrate Reproduction|url=https://books.google.com/books?id=cqQX9RMPAegC|date= 1998|publisher=Springer Science & Business Media|isbn=978-0-7923-8336-9}}</ref><ref name="Hyman1992">{{cite book |author=Libbie Henrietta Hyman |url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA583 |title=Hyman's Comparative Vertebrate Anatomy |date=15 September 1992 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=583–}}</ref><ref name="naguib" /> Marsupials typically have forked penises,<ref name="Tyndale-BiscoeRenfree1987">{{cite book| vauthors = Tyndale-Biscoe H, Renfree M |title=Reproductive Physiology of Marsupials|url=https://books.google.com/books?id=HpjovN0vXW4C|date=1987|publisher=Cambridge University Press|isbn=978-0-521-33792-2}}</ref> while the [[echidna]] penis generally has four heads with only two functioning.<ref>{{Cite journal | doi=10.1086/522847| pmid=18171162| title=One-Sided Ejaculation of Echidna Sperm Bundles| journal=The American Naturalist| volume=170| issue=6| pages=E162–E164| year=2007| vauthors = Johnston SD, Smith B, Pyne M, Stenzel D, Holt WV | bibcode=2007ANat..170E.162J| s2cid=40632746| url=https://espace.library.uq.edu.au/view/UQ:130591/UQ130591_OA.pdf}}</ref> Depending on the species, an [[erection]] may be fuelled by blood flow into vascular, spongy tissue or by muscular action.<ref name="Lombardi1998" />  The [[testicles]] of most mammals descend into the [[scrotum]] which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a [[vulva]] ([[clitoris]] and [[labia]]) on the outside, while the internal system contains paired [[oviduct]]s, one or two [[uteri]], one or two [[Cervix|cervices]] and a [[vagina]].<ref>{{cite book|last1=Bacha Jr.|first1=William J.|last2=Bacha|first2=Linda M.|title= Color Atlas of Veterinary Histology |publisher = Wiley |year = 2012|page=308|access-date = 28 November 2023 |isbn= 978-1-11824-364-0|url = https://books.google.com/books?id=08BOg2b7zRgC&pg=PA308}}</ref><ref>{{cite book|last1=Cooke|first1=Fred|last2=Bruce|first2=Jenni|title= The Encyclopedia of Animals: A Complete Visual Guide |publisher = University of California Press |year = 2004|page=79| access-date = 28 November 2023 |isbn= 978-0-52024-406-1|url = https://books.google.com/books?id=2V1tHqi4hLEC&pg=PA79}}</ref> Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placentals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two [[uterine horn]]s have 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.<ref>{{cite book| vauthors = Maxwell KE |year=2013|title=The Sex Imperative: An Evolutionary Tale of Sexual Survival|publisher=Springer|pages=112–113|isbn=978-1-4899-5988-1|url=https://books.google.com/books?id=dnf1BwAAQBAJ}}</ref><ref>{{cite book| vauthors = Vaughan TA, Ryan JP, Czaplewski NJ |year= 2011 |title= Mammalogy|publisher=Jones & Bartlett Publishers|page=387 |isbn= 978-0-03-025034-7 }}</ref><ref name="hair"/>{{rp|220–221, 247}}
[[File:Dendrolagus matschiei 1.jpg|thumb|left|[[Matschie's tree-kangaroo]] with young in pouch]]

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped young, either through direct [[vivipary]] or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of [[epipubic bones]]. The oldest demonstration of this reproductive style is with ''[[Kayentatherium]]'', which produced undeveloped [[perinate]]s, but at much higher litter sizes than any modern mammal, 38 specimens.<ref name="Hoffman&Rowe">{{cite journal | vauthors = Hoffman EA, Rowe TB | title = Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth | journal = Nature | volume = 561 | issue = 7721 | pages = 104–108 | date = September 2018 | pmid = 30158701 | doi = 10.1038/s41586-018-0441-3 | bibcode = 2018Natur.561..104H| s2cid = 205570021 }}</ref> Most modern mammals are [[viviparity|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.<ref>{{cite journal | vauthors = Wallis MC, Waters PD, Delbridge ML, Kirby PJ, Pask AJ, Grützner F, Rens W, Ferguson-Smith MA, Graves JA | display-authors = 6 | title = Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes | journal = Chromosome Research | volume = 15 | issue = 8 | pages = 949–959 | year = 2007 | pmid = 18185981 | doi = 10.1007/s10577-007-1185-3 | s2cid = 812974 }}</ref> In particular, the [[sex chromosome]]s of a platypus are more like those of a chicken than those of a therian mammal.<ref>{{cite journal | vauthors = Marshall Graves JA | title = Weird animal genomes and the evolution of vertebrate sex and sex chromosomes | journal = Annual Review of Genetics | volume = 42 | pages = 565–586 | year = 2008 | pmid = 18983263 | doi = 10.1146/annurev.genet.42.110807.091714 | url = https://www.mnf.uni-greifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | url-status = dead | archive-url = http://webarchive.nationalarchives.gov.uk/20120904084145/http%3A//www.mnf.uni%2Dgreifswald.de/fileadmin/Zoologisches_Museum/Hildebrandt/Dokumente/graves08.pdf | archive-date = 4 September 2012 | access-date = 25 January 2024 }}</ref>

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short [[gestation]] period, typically shorter than its [[estrous cycle]] and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the [[Pouch (marsupial)|marsupium]], located in the front of the mother's [[abdomen]]. This is the [[Symplesiomorphy|plesiomorphic]] condition among viviparous mammals; the presence of epipubic bones in all non-placentals prevents the expansion of the torso needed for full pregnancy.<ref name=schulkin>{{cite book| vauthors = Power ML, Schulkin J |year=2013|title=The Evolution Of The Human Placenta|publisher=Johns Hopkins University Press|url={{Google books|plainurl=yes|id=xfffGC3hjPoC|page=1891}}|pages=1890–1891|location=Baltimore|isbn=978-1-4214-0643-5|oclc=940749490}}</ref> Even non-placental eutherians probably reproduced this way.<ref name="Epipubic bones in eutherian mammals"/> The placentals give birth to relatively complete and developed young, usually after long gestation periods.<ref>{{cite book|chapter-url={{Google books|plainurl=yes|id=dK-D6HMSOQIC|page=6}}|location=Chicago| vauthors = Sally M |title=Mammals|chapter=Mammal Behavior and Lifestyle|year=2005|publisher=Raintree|page=6|isbn=978-1-4109-1050-9|oclc=53476660}}</ref> They get their name from the [[placenta]], which connects the developing fetus to the uterine wall to allow nutrient uptake.<ref>{{cite book|url={{Google books|plainurl=yes|id=23s9DAAAQBAJ|page=288}} | vauthors = Verma PS, Pandey BP |year=2013|title=ISC Biology Book I for Class XI|publisher=S. Chand and Company|page=288|location=New Delhi|isbn=978-81-219-2557-0}}</ref> 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.<ref name="Hoffman&Rowe"/>

The [[mammary gland]]s of mammals are specialised to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the [[teat]]s seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.<ref>{{cite journal | vauthors = Oftedal OT | title = The mammary gland and its origin during synapsid evolution | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 7 | issue = 3 | pages = 225–252 | date = July 2002 | pmid = 12751889 | doi = 10.1023/a:1022896515287 | s2cid = 25806501 }}</ref> 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.<ref>{{cite book| vauthors = Krockenberger A |year=2006 |title= Marsupials | chapter = Lactation| veditors = Dickman CR, Armati PJ, Hume ID |page=109|publisher=Cambridge University Press |isbn=978-1-139-45742-2}}</ref> [[Lactose]] is the main sugar in placental milk while monotreme and marsupial milk is dominated by [[oligosaccharide]]s.<ref>{{cite book| vauthors = Schulkin J, Power ML |year=2016|title=Milk: The Biology of Lactation|publisher=Johns Hopkins University Press|page=66|isbn=978-1-4214-2042-4}}</ref> [[Weaning]] is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.<ref>{{cite book | vauthors = Thompson KV, Baker AJ, Baker AM |year=2010|title=Wild Mammals in Captivity Principles and Techniques for Zoo Management|publisher=University of Chicago Press| chapter = Paternal Care and Behavioral Development in Captive Mammals | veditors = Kleiman DG, Thompson KV, Baer CK |page=374|isbn=978-0-226-44011-8|edition=2nd}}</ref>

===Endothermy===
Nearly all mammals are [[endothermy|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 [[ectotherm]]ic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.<ref>{{cite book| vauthors = Campbell NA, Reece JB |year=2002 |title=Biology |edition=6th |publisher=Benjamin Cummings |page=[https://archive.org/details/biologyc00camp/page/845 845]|isbn=978-0-8053-6624-2|oclc=47521441|url=https://archive.org/details/biologyc00camp/page/845}}</ref> 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]].<ref>{{cite journal| vauthors = Buffenstein R, Yahav S |year=1991|title=Is the naked mole-rat ''Hererocephalus glaber'' an endothermic yet poikilothermic mammal?|journal=Journal of Thermal Biology|volume=16|issue=4|pages=227–232|doi=10.1016/0306-4565(91)90030-6|bibcode=1991JTBio..16..227B }}</ref> Birds are also endothermic, so endothermy is not unique to mammals.<ref>{{cite book|chapter-url={{Google books|plainurl=yes| id=Af7IwQWJoCMC|page=218}}|location=Cambridge| vauthors = Schmidt-Nielsen K, Duke JB |year=1997|title=Animal Physiology: Adaptation and Environment|chapter=Temperature Effects|edition=5th|page=218|isbn=978-0-521-57098-5|oclc=35744403 |publisher=Cambridge University Press }}</ref>

===Species lifespan===
{{See also|Life expectancy|Maximum life span}}
Among mammals, species maximum lifespan varies significantly (for example the [[shrew]] has a lifespan of two years, whereas the oldest [[bowhead whale]] is recorded to be 211 years).<ref name="pmid19896964">{{cite journal | vauthors = Lorenzini A, Johnson FB, Oliver A, Tresini M, Smith JS, Hdeib M, Sell C, Cristofalo VJ, Stamato TD | display-authors = 6 | title = Significant correlation of species longevity with DNA double strand break recognition but not with telomere length | journal = Mechanisms of Ageing and Development | volume = 130 | issue = 11–12 | pages = 784–792 | year = 2009 | pmid = 19896964 | pmc = 2799038 | doi = 10.1016/j.mad.2009.10.004 }}</ref> Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to [[DNA repair|repair DNA damage]] is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow,<ref name="pmid4526202">{{cite journal | vauthors = Hart RW, Setlow RB | title = Correlation between deoxyribonucleic acid excision-repair and life-span in a number of mammalian species | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 71 | issue = 6 | pages = 2169–2173 | date = June 1974 | pmid = 4526202 | pmc = 388412 | doi = 10.1073/pnas.71.6.2169 | bibcode = 1974PNAS...71.2169H | doi-access = free }}</ref> 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 recognise DNA double-strand breaks as well as the level of the DNA repair protein [[Ku80]].<ref name="pmid19896964"/> In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be [[Downregulation and upregulation|up-regulated]] in the longer-lived species.<ref name="pmid27874830">{{cite journal | vauthors = Ma S, Upneja A, Galecki A, Tsai YM, Burant CF, Raskind S, Zhang Q, Zhang ZD, Seluanov A, Gorbunova V, Clish CB, Miller RA, Gladyshev VN | display-authors = 6 | title = Cell culture-based profiling across mammals reveals DNA repair and metabolism as determinants of species longevity | journal = eLife | volume = 5 | date = November 2016 | pmid = 27874830 | pmc = 5148604 | doi = 10.7554/eLife.19130 | doi-access = free }}</ref> The cellular level of the DNA repair enzyme [[poly ADP ribose polymerase]] was found to correlate with species lifespan in a study of 13 mammalian species.<ref name="pmid1465394">{{cite journal | vauthors = Grube K, Bürkle A | title = Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 89 | issue = 24 | pages = 11759–11763 | date = December 1992 | pmid = 1465394 | pmc = 50636 | doi = 10.1073/pnas.89.24.11759 | bibcode = 1992PNAS...8911759G | doi-access = free }}</ref> Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.<ref name="pmid7266079">{{cite journal | vauthors = Francis AA, Lee WH, Regan JD | title = The relationship of DNA excision repair of ultraviolet-induced lesions to the maximum life span of mammals | journal = Mechanisms of Ageing and Development | volume = 16 | issue = 2 | pages = 181–189 | date = June 1981 | pmid = 7266079 | doi = 10.1016/0047-6374(81)90094-4 | s2cid = 19830165 }}</ref><ref name="pmid7060140">{{cite journal | vauthors = Treton JA, Courtois Y | title = Correlation between DNA excision repair and mammalian lifespan in lens epithelial cells | journal = Cell Biology International Reports | volume = 6 | issue = 3 | pages = 253–260 | date = March 1982 | pmid = 7060140 | doi = 10.1016/0309-1651(82)90077-7 | doi-broken-date = 4 April 2025 }}</ref><ref name="pmid3974310">{{cite journal | vauthors = Maslansky CJ, Williams GM | title = Ultraviolet light-induced DNA repair synthesis in hepatocytes from species of differing longevities | journal = Mechanisms of Ageing and Development | volume = 29 | issue = 2 | pages = 191–203 | date = February 1985 | pmid = 3974310 | doi = 10.1016/0047-6374(85)90018-1 | s2cid = 23988416 }}</ref>

===Locomotion===
{{Main|Animal locomotion}}

====Terrestrial====
{{Main|Terrestrial locomotion}}
[[File:Muybridge race horse animated.gif|thumb|[[running|Running gait]]. Photographs by [[Eadweard Muybridge]], 1887]]
Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are [[plantigrade]], walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are [[digitigrade]], walking on their toes, the greater stride length allowing more speed. Some animals such as [[horse]]s are [[unguligrade]], walking on the tips of their toes. This even further increases their stride length and thus their speed.<ref>{{cite book|url={{Google books |plainurl=yes |id=LaaLNfY6gB8C |page=3}}| vauthors = Walker WF, Homberger DG |author-link2= Dominique G. Homberger|year=1998|title=Anatomy and Dissection of the Fetal Pig|edition=5th|location=New York|publisher=W. H. Freeman and Company|page=3|isbn=978-0-7167-2637-1|oclc=40576267}}</ref> A few mammals, namely the great apes, are also known to [[Knuckle-walking|walk on their knuckles]], at least for their front legs. [[Giant anteater]]s<ref>{{cite journal | vauthors = Orr CM | title = Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae | journal = American Journal of Physical Anthropology | volume = 128 | issue = 3 | pages = 639–658 | date = November 2005 | pmid = 15861420 | doi = 10.1002/ajpa.20192 }}</ref> and platypuses<ref>{{cite journal | vauthors = Fish FE, Frappell PB, Baudinette RV, MacFarlane PM | title = Energetics of terrestrial locomotion of the platypus Ornithorhynchus anatinus | journal = The Journal of Experimental Biology | volume = 204 | issue = Pt 4 | pages = 797–803 | date = February 2001 | doi = 10.1242/jeb.204.4.797 | pmid = 11171362 | bibcode = 2001JExpB.204..797F | hdl = 2440/12192 | url = https://jeb.biologists.org/cgi/reprint/204/4/797.pdf | access-date = 25 January 2024 | archive-date = 14 March 2024 | archive-url = https://web.archive.org/web/20240314100116/https://jeb.biologists.org/cgi/reprint/204/4/797.pdf | url-status = live }}</ref> are also knuckle-walkers. Some mammals are [[bipedalism|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 [[Mammalian vision|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 rat]]s.<ref>{{cite journal|url=http://www.philosophistry.com/static/bipedalism.html|vauthors=Dhingra P|year=2004|title=Comparative Bipedalism – How the Rest of the Animal Kingdom Walks on two legs|journal=Anthropological Science|volume=131|issue=231|access-date=11 March 2017|archive-date=21 April 2021|archive-url=https://web.archive.org/web/20210421085055/https://philosophistry.com/static/bipedalism.html|url-status=live}}</ref><ref>{{cite journal | vauthors = Alexander RM | title = Bipedal animals, and their differences from humans | journal = Journal of Anatomy | volume = 204 | issue = 5 | pages = 321–330 | date = May 2004 | pmid = 15198697 | pmc = 1571302 | doi = 10.1111/j.0021-8782.2004.00289.x }}</ref>

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest [[horse gait]] is the [[Horse gait#Walk|walk]], then there are three faster gaits which, from slowest to fastest, are the [[trot (horse gait)|trot]], the [[canter]] and the [[Horse gait#Gallop|gallop]]. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main [[gait (human)|human gaits]] are bipedal [[walking]] and [[running]], but they employ many other gaits occasionally, including a four-legged [[crawling (human)|crawl]] in tight spaces.<ref name=dagg>{{cite journal| vauthors = Dagg AI |author-link=Anne Innis Dagg|year=1973|title=Gaits in Mammals|journal=Mammal Review|volume=3|issue=4|pages=135–154|doi=10.1111/j.1365-2907.1973.tb00179.x|bibcode=1973MamRv...3..135D }}</ref> Mammals show a vast range of [[gait]]s, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and [[leaping gaits]].<ref>{{cite book| vauthors = Roberts TD |title=Understanding Balance: The Mechanics of Posture and Locomotion|url={{Google books|plainurl=yes|id=o8RvD3X8ur8C|page=211}}|location=San Diego|year=1995|publisher= Nelson Thornes|isbn=978-1-56593-416-0|page=211|oclc=33167785}}</ref> 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.<ref name=dagg/>

====Arboreal====
{{Main|Arboreal locomotion}}
[[File:Brachiating Gibbon (Some rights reserved).jpg|thumb|left|upright|[[Gibbon]]s are very good [[brachiation|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 [[brachiation|brachiate]] (swing between trees).<ref name=Cartmill>{{cite book| vauthors = Cartmill M |year=1985|chapter=Climbing|title=Functional Vertebrate Morphology| veditors = Hildebrand M, Bramble DM, Liem KF, Wake DB |pages=73–88|location=Cambridge|publisher=Belknap Press|isbn=978-0-674-32775-7|oclc=11114191}}</ref> Many arboreal species, such as tree porcupines, [[silky anteater]]s, spider monkeys, and [[Phalangeriformes|possums]], use [[prehensile tail]]s 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 [[squirrel]]s 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.<ref name=Cartmill/> Size relating to weight affects gliding animals such as the [[sugar glider]].<ref>{{cite journal| vauthors = Vernes K |year=2001|title=Gliding Performance of the Northern Flying Squirrel (''Glaucomys sabrinus'') in Mature Mixed Forest of Eastern Canada|journal=Journal of Mammalogy|volume=82|issue=4|pages=1026–1033|doi=10.1644/1545-1542(2001)082<1026:GPOTNF>2.0.CO;2|s2cid=78090049 |doi-access=free}}</ref> 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.<ref name=Cartmill/>

====Aerial====
{{Main|Aerial locomotion}}
[[File:Israeli Bats - 26 September 2015.webm|thumb|upright=1.35|Slow-motion and normal speed of [[Egyptian fruit bat]]s 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.<ref>{{cite web|url=https://blogs.bu.edu/biolocomotion/2011/10/16/bats-the-only-flying-mammal/|title=Bats – the only flying mammals|vauthors=Barba LA|work=Bio-Aerial Locomotion|date=October 2011|access-date=20 May 2016|archive-date=14 May 2016|archive-url=https://web.archive.org/web/20160514085336/http://blogs.bu.edu/biolocomotion/2011/10/16/bats-the-only-flying-mammal/|url-status=live}}</ref>

The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to manoeuvre more accurately and fly with more lift and less drag.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2007/01/070118161402.htm|title=Bats In Flight Reveal Unexpected Aerodynamics|year=2007|website=ScienceDaily|access-date=12 July 2016|archive-date=19 December 2019|archive-url=https://web.archive.org/web/20191219210807/https://www.sciencedaily.com/releases/2007/01/070118161402.htm|url-status=live}}</ref><ref name=anders>{{cite journal | vauthors = Hedenström A, Johansson LC | title = Bat flight: aerodynamics, kinematics and flight morphology | journal = The Journal of Experimental Biology | volume = 218 | issue = Pt 5 | pages = 653–663 | date = March 2015 | pmid = 25740899 | doi = 10.1242/jeb.031203 | bibcode = 2015JExpB.218..653H | s2cid = 21295393 | url = https://jeb.biologists.org/content/jexbio/218/5/653.full.pdf | access-date = 25 January 2024 | archive-date = 25 January 2024 | archive-url = https://web.archive.org/web/20240125191350/https://jeb.biologists.org/content/jexbio/218/5/653.full.pdf | url-status = live }}</ref> By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2012/04/120411084133.htm|title=Bats save energy by drawing in wings on upstroke|website=ScienceDaily|access-date=12 July 2016|year=2012|archive-date=31 May 2021|archive-url=https://web.archive.org/web/20210531050233/https://www.sciencedaily.com/releases/2012/04/120411084133.htm|url-status=live}}</ref> 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.<ref>{{cite book|url={{Google books|plainurl=yes|id=XS9y642cjvMC|page=14}}| vauthors = Karen T |year=2008 |title=Hanging with Bats: Ecobats, Vampires, and Movie Stars|publisher=University of New Mexico Press|location=Albuquerque|page=14|isbn=978-0-8263-4403-8|oclc=191258477}}</ref> The surface of their wings is equipped with touch-sensitive receptors on small bumps called [[Merkel cell]]s, 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.<ref>{{cite journal | vauthors = Sterbing-D'Angelo S, Chadha M, Chiu C, Falk B, Xian W, Barcelo J, Zook JM, Moss CF | display-authors = 6 | title = Bat wing sensors support flight control | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 27 | pages = 11291–11296 | date = July 2011 | pmid = 21690408 | pmc = 3131348 | doi = 10.1073/pnas.1018740108 | bibcode = 2011PNAS..10811291S | doi-access = free }}</ref>

====Fossorial and subterranean====
{{Multiple image|align=right|image1=Wombat3.jpg|image2=ScalopusAquaticus.jpg|total_width=400|footer=Semi-fossorial [[Southern hairy-nosed wombat|wombat]] (left) vs. fully fossorial [[eastern mole]] (right)}}
{{See also|Fossorial|Burrow}}
A fossorial (from Latin ''fossor'', meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are [[badger]]s, and [[naked mole-rat]]s. Many [[rodent]] species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in [[temperature regulation]] while others use the underground habitat for protection from [[predator]]s or for [[food storage]].<ref name=":2">Damiani, R, 2003, Earliest evidence of cynodont burrowing, The Royal Society Publishing, Volume 270, Issue 1525</ref>

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 gopher]]s, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial [[marsupial mole]], the eyes are degenerated and useless, ''[[Talpa (genus)|Talpa]]'' moles have [[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.<ref>{{cite journal|jstor=2455381|vauthors=Shimer HW|year=1903|title=Adaptations to Aquatic, Arboreal, Fossorial and Cursorial Habits in Mammals. III. Fossorial Adaptations|journal=The American Naturalist|volume=37|number=444|pages=819–825|doi=10.1086/278368|bibcode=1903ANat...37..819S |s2cid=83519668|url=https://zenodo.org/record/1431331|access-date=23 August 2020|archive-date=9 April 2023|archive-url=https://web.archive.org/web/20230409004021/https://zenodo.org/record/1431331|url-status=live}}</ref>

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order [[Insectivora]].<ref>{{cite journal | vauthors = Stanhope MJ, Waddell VG, Madsen O, de Jong W, Hedges SB, Cleven GC, Kao D, Springer MS | display-authors = 6 | title = Molecular evidence for multiple origins of Insectivora and for a new order of endemic African insectivore mammals | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 95 | issue = 17 | pages = 9967–9972 | date = August 1998 | pmid = 9707584 | pmc = 21445 | doi = 10.1073/pnas.95.17.9967 | doi-access = free | bibcode = 1998PNAS...95.9967S }}</ref>

====Aquatic====
{{Main|Aquatic locomotion|Marine mammal|Aquatic mammal}}
[[File:Living-on-the-Edge-Settlement-Patterns-by-the-Symbiotic-Barnacle-Xenobalanus-globicipitis-on-Small-pone.0127367.s001.ogv|thumb|A pod of [[short-beaked common dolphin]]s swimming]]
Fully aquatic mammals, the cetaceans and [[sirenia]]ns, have lost their legs and have a tail fin to propel themselves through the water. [[Flipper (anatomy)|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.<ref>{{cite journal | vauthors = Perry DA |year=1949 |title=The anatomical basis of swimming in Whales |journal=Journal of Zoology |volume=119 |issue=1 |pages=49–60 |doi=10.1111/j.1096-3642.1949.tb00866.x}}</ref><ref>{{cite journal | vauthors = Fish FE, Hui CA |year=1991 |title=Dolphin swimming – a review |journal=Mammal Review |volume=21 |issue=4 |pages=181–195 |url=https://darwin.wcupa.edu/~biology/fish/pubs/pdf/1991MRDolphinswimming.pdf |archive-url=https://web.archive.org/web/20060829000617/http://darwin.wcupa.edu/~biology/fish/pubs/pdf/1991MRDolphinswimming.pdf |url-status=dead |archive-date=29 August 2006 |doi=10.1111/j.1365-2907.1991.tb00292.x }}</ref> 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.<ref>{{cite book| vauthors = Marsh H |chapter-url= https://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |chapter=Chapter 57: Dugongidae |title=Fauna of Australia |volume=1 |publisher=Australian Government Publications |isbn=978-0-644-06056-1 |location=Canberra |year=1989 |oclc=27492815 |url-status=dead |archive-url=https://web.archive.org/web/20130511221756/http://www.environment.gov.au/biodiversity/abrs/publications/fauna-of-australia/pubs/volume1b/57-ind.pdf |archive-date=11 May 2013 }}</ref>

[[Semi-aquatic]] mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.<ref name=Berta63>{{cite book | vauthors = Berta A | title = Return to the Sea: The Life and Evolutionary Times of Marine Mammals | chapter = Pinniped Diversity: Evolution and Adaptations | publisher = University of California Press | date = 2012 | isbn = 978-0-520-27057-2 | pages = 62–64}}</ref><ref name="Fish 2003">{{cite journal | vauthors = Fish FE, Hurley J, Costa DP | title = Maneuverability by the sea lion Zalophus californianus: turning performance of an unstable body design | journal = The Journal of Experimental Biology | volume = 206 | issue = Pt 4 | pages = 667–674 | date = February 2003 | pmid = 12517984 | doi = 10.1242/jeb.00144 | doi-access = free | bibcode = 2003JExpB.206..667F }}</ref> Pinnipeds have several adaptions for reducing [[Drag (physics)|drag]]. In addition to their streamlined bodies, they have smooth networks of [[Muscle fascicle|muscle bundles]] in their skin that may increase [[laminar flow]] and make it easier for them to slip through water. They also lack [[Arrector pili muscle|arrector pili]], so their fur can be streamlined as they swim.<ref name=Riedman3/> They rely on their fore-flippers for locomotion in a wing-like manner similar to [[penguin]]s and [[sea turtles]].<ref name="Fish1996">{{Cite journal | vauthors = Fish FE |title=Transitions from drag-based to lift-based propulsion in mammalian swimming |doi=10.1093/icb/36.6.628 |journal=Integrative and Comparative Biology |volume=36 |issue=6 |pages=628–641 |year=1996|doi-access=free }}</ref> Fore-flipper movement is not continuous, and the animal glides between each stroke.<ref name="Fish 2003"/> 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;<ref name=Berta63/> the hind-flippers serve as stabilizers.<ref name=Riedman3>{{cite book| vauthors = Riedman M |year=1990|title=The Pinnipeds: Seals, Sea Lions, and Walruses| url = https://archive.org/details/pinnipedssealsse0000ried | url-access = registration |publisher=University of California Press|isbn=978-0-520-06497-3|oclc=19511610}}</ref> Other semi-aquatic mammals include beavers, [[hippopotamus]]es, [[otter]]s and platypuses.<ref>{{cite journal | vauthors = Fish FE | title = Biomechanics and energetics in aquatic and semiaquatic mammals: platypus to whale | journal = Physiological and Biochemical Zoology | volume = 73 | issue = 6 | pages = 683–698 | year = 2000 | pmid = 11121343 | doi = 10.1086/318108 | url = https://darwin.wcupa.edu/~biology/fish/pubs/pdf/2000PBZ-PlatToWhale.pdf | url-status = dead | citeseerx = 10.1.1.734.1217 | s2cid = 49732160 | archive-url = https://web.archive.org/web/20160804111726/http://darwin.wcupa.edu/~biology/fish/pubs/pdf/2000PBZ-PlatToWhale.pdf | archive-date = 4 August 2016 }}</ref> Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have [[wikt:graviportal|graviportal]] skeletal structures,<ref>{{cite book | vauthors = Eltringham SK |year=1999|title=The Hippos|chapter=Anatomy and Physiology|location= London|publisher=T & AD Poyser Ltd|page=8|isbn=978-0-85661-131-5|oclc=42274422}}</ref> adapted to carrying their enormous weight, and their [[specific gravity]] allows them to sink and move along the bottom of a river.<ref>{{cite magazine|title=Hippopotamus ''Hippopotamus amphibius''|magazine=National Geographic|access-date= 30 April 2016|url= https://animals.nationalgeographic.com/animals/mammals/hippopotamus/|archive-url= https://web.archive.org/web/20141125041546/http://animals.nationalgeographic.com/animals/mammals/hippopotamus/|archive-date= 25 November 2014|url-status=dead}}</ref>