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==Anatomy and physiology== ===Head=== [[File:Primate skull series with legend cropped.png|thumb|Primate skulls showing [[postorbital bar]], and increasing brain sizes]] The primate skull has a large, domed [[Skull|cranium]], which is particularly prominent in [[Simian|anthropoids]]. The cranium protects the large brain, a distinguishing characteristic of this group.<ref name="pough" /> The endocranial volume (the volume within the skull) is three times greater in [[human]]s than in the greatest nonhuman primate, reflecting a larger brain size.<ref name="aiello" /> The mean endocranial volume is 1,201 cubic centimeters in humans, 469 cm<sup>3</sup> in [[gorilla]]s, 400 cm<sup>3</sup> in [[chimpanzee]]s and 397 cm<sup>3</sup> in [[orangutan]]s.<ref name="aiello">{{cite book |author1=Aiello, L. |author2=Dean, C. |name-list-style=amp | year=1990 | title=An Introduction to Human Evolutionary Anatomy |url=https://archive.org/details/introductiontohu00aiel | publisher=Academic Press | pages=[https://archive.org/details/introductiontohu00aiel/page/193 193] | isbn=0-12-045590-0}}</ref> The primary evolutionary trend of primates has been the elaboration of the brain, in particular the [[neocortex]] (a part of the [[cerebral cortex]]), which is involved with [[sense|sensory perception]], generation of [[motor cortex|motor commands]], spatial reasoning, [[consciousness|conscious thought]] and, in humans, [[language]].<ref name="britannica" /> While other mammals rely heavily on their [[Olfaction|sense of smell]], the arboreal life of primates has led to a [[Somatosensory system|tactile]], [[Visual perception|visually]] dominant sensory system,<ref name="britannica" /> a reduction in the olfactory region of the brain and increasingly complex social behavior.<ref name="adw">{{cite web | url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Primates.html | title="Primates" (On-line) |publisher=Animal Diversity Web | author=Myers, P. | year=1999 | access-date=2008-06-03}}</ref> The visual acuity of [[human]]s and other [[hominid]]s is exceptional; they have the most [[Spatial resolution|acute vision]] known among all vertebrates, with the exception of certain species of [[Bird of prey|predatory birds]].<ref>{{Cite journal |last=Caves |first=Eleanor M. |date=May 2018 |title=Visual Acuity and the Evolution of Signals |journal=Trends in Ecology & Evolution |volume=33 |issue=5 |pages=358β372 |doi=10.1016/j.tree.2018.03.001 |pmid=29609907 |bibcode=2018TEcoE..33..358C |url=https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(18)30052-1 |access-date=29 July 2018}}</ref><ref>{{Citation |last1=Kirk |first1=E. Christopher |title=The Evolution of High Visual Acuity in the Anthropoidea |date=2004 |url=https://doi.org/10.1007/978-1-4419-8873-7_20 |work=Anthropoid Origins: New Visions |pages=539β602 |editor-last=Ross |editor-first=Callum F. |access-date=2023-07-30 |series=Developments in Primatology: Progress and Prospects |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4419-8873-7_20 |isbn=978-1-4419-8873-7 |last2=Kay |first2=Richard F. |editor2-last=Kay |editor2-first=Richard F.}}</ref> Primates have forward-facing eyes on the front of the skull; [[binocular vision]] allows accurate distance perception, useful for the [[Brachiation|brachiating]] ancestors of all great apes.<ref name="pough" /> A [[supraorbital ridge|bony ridge]] above the eye sockets reinforces weaker bones in the face, which are put under strain during chewing. [[Strepsirrhini|Strepsirrhines]] have a [[postorbital bar]], a bone around the eye socket, to protect their eyes; in contrast, the higher primates, [[Haplorhini|haplorhines]], have evolved fully enclosed sockets.<ref name="Campbell">{{cite book |author1=Campbell, B. G. |author2=Loy, J. D. |name-list-style=amp | year = 2000 | title = Humankind Emerging | publisher = Allyn & Bacon | pages = 85 |isbn=0-673-52364-0|edition=8th }}</ref> [[File:PrimateFeet.jpg|thumb|right|upright|An 1893 drawing of the hands and feet of various primates]] Primates show an evolutionary trend towards a reduced [[snout]].<ref name="palaeos">{{cite web |author1=White, T. |author2=Kazlev, A. |name-list-style=amp |url=http://www.palaeos.com/Vertebrates/Units/480Archonta/480.400.html#Primates | title=Archonta: Primates |publisher=[[Palaeos]]| access-date=2008-06-03 | date=2006-01-08 |archive-url=https://web.archive.org/web/20080512022901/http://www.palaeos.com/Vertebrates/Units/480Archonta/480.400.html <!--Added by H3llBot--> |archive-date=2008-05-12}}</ref> Technically, Old World monkeys are distinguished from New World monkeys by the structure of the nose, and from apes by the [[dentition|arrangement of their teeth]].<ref name="adw" /> In New World monkeys, the nostrils face sideways; in Old World monkeys, they face downwards.<ref name="adw" /> Dental pattern in primates vary considerably; although some have lost most of their [[incisor]]s, all retain at least one lower incisor.<ref name="adw" /> In most strepsirrhines, the lower incisors form a [[toothcomb]], which is used in grooming and sometimes foraging.<ref name="adw" /><ref name="EncycMammals" /> Old World monkeys have eight [[premolar]]s, compared with 12 in New World monkeys. The Old World species are divided into apes and monkeys depending on the number of [[Cusp (dentistry)|cusps]] on their [[Molar (tooth)|molars]]: monkeys have four, apes have five<ref name="adw" /> - although humans may have four or five.<ref>{{cite book |title=Wheeler's Dental Anatomy, Physiology, and Occlusion |last1=Ash |first1=M. M. |last2=Nelson |first2=S. J. |last3=Wheeler |first3=R. C. |publisher=W.B. Saunders |year=2003 |isbn=978-0-7216-9382-8 |page=[https://archive.org/details/dentalanatomyphy00majo/page/12 12] |url=https://archive.org/details/dentalanatomyphy00majo/page/12}}</ref> The main hominid molar cusp ([[hypocone]]) evolved in early primate history, while the cusp of the corresponding primitive lower molar (paraconid) was lost. Prosimians are distinguished by their immobilized upper lips, the moist tip of their noses and forward-facing lower front teeth. ===Body=== [[File:Chlorocebus pygerythrus01.jpg|thumb|Vervet hindfoot showing fingerprint ridges on the sole]] Primates generally have five digits on each limb ([[Dactyly#Pentadactyly|pentadactyly]]), with a characteristic type of keratin [[Nail (anatomy)|fingernail]] on the end of each finger and toe. The bottom sides of the hands and feet have [[tactile pad|sensitive pads]] on the [[distal phalanges|fingertips]]. Most have [[thumb|opposable thumbs]], a characteristic primate feature most developed in [[human]]s, though not limited to this order ([[opossum]]s and [[koala]]s, for example, also have them).<ref name="pough" /> Thumbs allow some species to use [[Tool use by animals|tools]]. In primates, the combination of opposing thumbs, short fingernails (rather than claws) and long, inward-closing fingers is a [[relict]] of the ancestral practice of gripping branches, and has, in part, allowed some species to develop [[brachiation]] (swinging by the arms from tree limb to tree limb) as a significant means of locomotion. [[Prosimians]] have clawlike nails on the second toe of each foot, called [[toilet-claw]]s, which they use for grooming.<ref name="pough">{{cite book |author1=Pough, F. W. |author2=Janis, C. M. |author3=Heiser, J. B. |title=Vertebrate Life |url=https://archive.org/details/vertebratelife0000poug |chapter=Characteristics of Primates |year=2005 |orig-year=1979 |edition=7th |publisher= Pearson |pages=[https://archive.org/details/vertebratelife0000poug/page/630 630] |isbn=0-13-127836-3}}</ref> The primate [[clavicle|collar bone]] is a prominent element of the [[pectoral girdle]]; this allows the [[Glenohumeral joint|shoulder joint]] broad mobility.<ref name="palaeos" /> Compared to Old World monkeys, apes have more mobile shoulder joints and arms due to the dorsal position of the [[scapula]], broad ribcages that are flatter front-to-back, a shorter, less mobile spine, and with lower [[vertebra]]e greatly reduced - resulting in tail loss in some species.<ref name="CNN-20240323" /> [[Prehensile tail]]s are found in the New World [[Atelidae|atelids]], including the [[howler monkey|howler]], [[spider monkey|spider]], [[woolly spider monkey|woolly spider]], [[woolly monkey]]s; and in [[Capuchin monkey|capuchin]]s.<ref>{{cite journal |vauthors=Garber PA, Rehg JA |title=The ecological role of the prehensile tail in white-faced capuchins (Cebus capucinus) |url=https://archive.org/details/sim_american-journal-of-physical-anthropology_1999-11_110_3/page/325 |journal=American Journal of Physical Anthropology |volume=110 |issue=3 |pages=325β39 | date=November 1999 |pmid=10516564 |doi=10.1002/(SICI)1096-8644(199911)110:3<325::AID-AJPA5>3.0.CO;2-D}}</ref><ref>{{cite journal |vauthors=Russo GA, Young JW |title=Tail growth tracks the ontogeny of prehensile tail use in capuchin monkeys (Cebus albifrons and C. apella) |url=https://archive.org/details/sim_american-journal-of-physical-anthropology_2011-11_146_3/page/465 |journal=American Journal of Physical Anthropology |volume=146 |issue=3 |pages=465β73 | date=November 2011 |pmid=21953012 |doi=10.1002/ajpa.21617}}</ref> Male primates have a [[Penis#Primates|low-hanging penis]] and testes descended into a scrotum.<ref name="Ankel-Simons2010">{{cite book|author=Friderun Ankel-Simons|title=Primate Anatomy: An Introduction|url=https://books.google.com/books?id=Mwl3M6c5KzoC|date=27 July 2010|publisher=Academic Press|isbn=978-0-08-046911-9|pages=442, 521 }}</ref><ref name="EncycMammals">{{cite encyclopedia | last = Macdonald | first = David | author-link = David W. Macdonald | title = Primates | encyclopedia = The Encyclopedia of Mammals | pages = 282β307 | publisher = The Brown Reference Group plc | year = 2006 | isbn = 0-681-45659-0}}</ref> ===Sexual dimorphism=== {{Main|Sexual dimorphism in non-human primates}} [[File:Silverback.JPG|thumb|right|Distinct sexual size dimorphism can be seen between the male and female gorilla.]] [[Sexual dimorphism]] is often exhibited in [[simian]]s, though to a greater degree in Old World species (apes and some monkeys) than New World species. Recent studies involve comparing DNA to examine both the variation in the expression of the dimorphism among primates and the fundamental causes of sexual dimorphism. Primates usually have dimorphism in [[Body weight|body mass]]<ref>{{cite journal |author=Ralls, K. |year=1976 |title=Mammals in Which Females are Larger Than Males |journal=The Quarterly Review of Biology |volume=51 |issue=2 |doi=10.1086/409310 | pages=245β76 |pmid=785524|s2cid=25927323 }}</ref><ref>{{cite journal | author=Lindstedtand & Boyce | date=July 1985 | journal=The American Naturalist | volume=125 |pages=873 |doi = 10.1086/284385 |title = Seasonality, Fasting Endurance, and Body Size in Mammals | last2=Boyce | first2=Mark S. | issue=6| bibcode=1985ANat..125..873L | s2cid=84308684 }}</ref> and canine tooth size<ref>{{cite journal |author=Frisch, J. E. |year=1963 |title=Sex-differences in the canines of the gibbon (''Hylobates lar'') |journal = Primates |volume = 4 |issue = 2 |doi=10.1007/BF01659148 |pages=1β10|s2cid=189798134 }}</ref><ref>{{cite journal |author=Kay, R. F. |year=1975 |title=The functional adaptations of primate molar teeth |journal=American Journal of Physical Anthropology |volume=43 |issue=2 |pages=195β215 |doi=10.1002/ajpa.1330430207 |pmid=810034}}</ref> along with [[pelage]] and skin color.<ref>{{cite book |author=Crook, J. H. |editor=Campbell, B. G. |year=1972 |chapter=Sexual selection, dimorphism, and social organization in the primates |title=Sexual selection and the descent of man |publisher=Aldine Transaction |pages=[https://archive.org/details/sexualselection00camp/page/246 246] |isbn=978-0-202-02005-1 |chapter-url=https://archive.org/details/sexualselection00camp/page/246}}</ref> The dimorphism can be attributed to and affected by different factors, including [[mating system]],<ref name="cheverud">{{cite journal |last1=Cheverud |first1=J. M. |last2=Dow |first2=M. M. |last3=Leutenegger |first3=W. |date=November 1985 |title=The quantitative assessment of phylogenetic constraints in comparative analyses: Sexual dimorphism in body weight among primates |url=https://archive.org/details/sim_evolution_1985-11_39_6/page/1335 |journal=Evolution |volume=39 |issue=6 |pages=1335β1351 |doi=10.2307/2408790 |pmid=28564267 |jstor=2408790}}</ref> size,<ref name="cheverud" /> habitat and diet.<ref name="leutenegger">{{cite journal |last1=Leutenegger |first1=W. |last2=Cheverud |first2=J. M. | year=1982 | title=Correlates of sexual dimorphism in primates: Ecological and size variables | journal = International Journal of Primatology | volume = 3 | issue = 4 | doi=10.1007/BF02693740 | pages=387β402|s2cid=38220186 | doi-access=free }}</ref> Comparative analyses have generated a more complete understanding of the relationship between [[sexual selection]], [[natural selection]], and mating systems in primates. Studies have shown that dimorphism is the product of changes in both male and female traits.<ref name="plavcan">{{cite journal |author=Plavcan, J. M. |year=2001 |title=Sexual dimorphism in primate evolution |journal=American Journal of Physical Anthropology |volume=33 |pages=25β53 |doi=10.1002/ajpa.10011 |pmid=11786990|s2cid=31722173 |doi-access=free }}</ref> Ontogenetic scaling, where relative extension of a common growth trajectory occurs, may give some insight into the relationship between sexual dimorphism and growth patterns.<ref name="ohiggins">{{cite journal |last1=O'Higgins |first1=P. |last2=Collard |first2=M. |year=2002 |title=Sexual dimorphism and facial growth in papionine monkeys |journal=Journal of Zoology |volume=257 |issue=2 |pages=255β72 |doi=10.1017/S0952836902000857}}</ref> Some evidence from the fossil record suggests that there was [[convergent evolution]] of dimorphism, and some extinct [[hominid]]s probably had greater dimorphism than any living primate.<ref name="plavcan" /> ===Locomotion=== [[File:Diademed ready to push off.jpg|thumb|[[Diademed sifaka]], a lemur that is a vertical clinger and leaper]] Primate species move by [[brachiation]], [[bipedalism]], [[Jumping|leaping]], arboreal and terrestrial [[quadruped]]alism, [[arboreal locomotion|climbing]], [[knuckle-walking]] or by a combination of these methods. Several prosimians are primarily vertical clingers and leapers. These include many [[Galago|bushbabies]], all [[Indriidae|indriids]] (i.e., [[sifaka]]s, [[avahi (genus)|avahis]] and [[indri]]s), [[sportive lemur]]s, and all [[tarsier]]s.<ref name="Sussman1999">{{cite book|title=Primate Ecology and Social Structure Volume 1: Lorises, Lemurs and Tarsiers|author=Sussman, R. W.|year=1999|isbn=0-536-02256-9|location=Needham Heights, MA|publisher=Pearson Custom Publishing & Prentice Hall|pages=[https://archive.org/details/primateecologyso0001suss/page/78 78, 89β90, 108, 121β123, 233]|url=https://archive.org/details/primateecologyso0001suss/page/78}}</ref> Other prosimians are arboreal quadrupeds and climbers. Some are also terrestrial quadrupeds, while some are leapers. Most monkeys are both arboreal and terrestrial quadrupeds and climbers. [[Gibbon]]s, [[muriqui]]s and [[spider monkey]]s all brachiate extensively,<ref name="Strier2007" /> with gibbons sometimes doing so in remarkably acrobatic fashion. [[Woolly monkey]]s also brachiate at times.<ref name="Sussman2003" /> [[Orangutan]]s use a similar form of locomotion called quadramanous climbing, in which they use their arms and legs to carry their heavy bodies through the trees.<ref name="Strier2007" /> [[Chimpanzee]]s and [[gorilla]]s knuckle walk,<ref name="Strier2007" /> and can move bipedally for short distances. Although numerous species, such as [[australopithecine]]s and [[Homo|early hominids]], have exhibited fully bipedal locomotion, humans are the only extant species with this trait.<ref name="GlazierFlowerday2003">{{Cite book | first1 = S. D. | last1 = Glazier | first2 = C. A. | last2 = Flowerday | title = Selected Readings in the Anthropology of Religion: Theoretical and Methodological Essays | url = https://archive.org/details/selectedreadings00glaz | publisher = Greenwood Publishing Group | year = 2003 | isbn = 9780313300905 | pages = [https://archive.org/details/selectedreadings00glaz/page/53 53]}}</ref> ===Vision=== [[File:GarnettsGalago CincinnatiZoo.jpg|thumb|The ''[[tapetum lucidum]]'' of a [[northern greater galago]], typical of prosimians, reflects the light of the photographer's flash.]] The [[evolution of color vision in primates]] is unique among most [[eutheria]]n mammals. While the remote [[vertebrate]] ancestors of the primates possessed [[trichromacy|three color vision]] (trichromaticism), the [[Nocturnality|nocturnal]], [[warm-blooded]], mammalian ancestors lost one of three cones in the [[retina]] during the [[Mesozoic]] era. Fish, [[reptile]]s and birds are therefore trichromatic or [[Tetrachromacy|tetrachromatic]], while all mammals, with the exception of some primates and [[marsupial]]s,<ref>{{cite journal | last1=Arrese | year=2005 | title=Cone topography and spectral sensitivity in two potentially trichromatic marsupials, the quokka (''Setonix brachyurus'') and quenda (''Isoodon obesulus'') | journal=Proceedings of the Royal Society B | volume=272 | issue=1565 | doi=10.1098/rspb.2004.3009 | pages=791β6 | pmid=15888411 | pmc=1599861 | first1=C. A. |last2=Oddy|first2=Alison Y. | display-authors=2 | last3=Runham | first3=P. B | last4=Hart | first4=N. S | last5=Shand | first5=J. | last6=Hunt | first6=D. M | last7=Beazley | first7=L. D}}</ref> are dichromats or [[Monochromacy|monochromats]] (totally color blind).<ref name="EncycMammals" /> Nocturnal primates, such as the [[night monkey]]s and [[Galago|bush babies]], are often monochromatic. Catarrhines are routinely trichromatic due to a [[gene duplication]] of the red-green [[opsin]] gene at the base of their lineage, 30 to 40 million years ago.<ref name="EncycMammals" /><ref name="Bowmaker1991">{{cite journal |last1 = Bowmaker |first1=J. K. |last2=Astell |first2=S. |last3=Hunt |first3=D. M. |last4 = Mollon |first4=J. D. | year = 1991 | title = Photosensitive and photostable pigments in the retinae of Old World monkeys | journal = The Journal of Experimental Biology | volume = 156 | pages = 1β19 |issn=0022-0949 |url=http://jeb.biologists.org/cgi/reprint/156/1/1.pdf |access-date=2008-06-16 | pmid = 2051127 | issue=1|doi=10.1242/jeb.156.1.1 |bibcode=1991JExpB.156....1B }}</ref> Platyrrhines, on the other hand, are trichromatic in a few cases only.<ref name="Surridge2003">{{cite journal |author1=Surridge, A. K. |author2=D. Osorio |name-list-style=amp | year = 2003 | title = Evolution and selection of trichromatic vision in primates | journal = Trends in Ecology and Evolution | volume = 18 | pages = 198β205 | doi = 10.1016/S0169-5347(03)00012-0 | issue = 4}}</ref> Specifically, individual females must be [[Zygosity#Heterozygous|heterozygous]] for two [[allele]]s of the opsin gene (red and green) located on the same [[Locus (genetics)|locus]] of the [[X chromosome]].<ref name="EncycMammals" /> Males, therefore, can only be dichromatic, while females can be either dichromatic or trichromatic. Color vision in strepsirrhines is not as well understood; however, research indicates a range of color vision similar to that found in platyrrhines.<ref name="EncycMammals" /> Like catarrhines, howler monkeys (a family of platyrrhines) show routine trichromatism that has been traced to an evolutionarily recent [[gene duplication]].<ref name="Lucas2003">{{cite journal |last1=Lucas |first1=P. W. |last2=Dominy |first2=N. J. |last3=Riba-Hernandez |first3=P. |last4=Stoner |first4=K. E. |last5=Yamashita |first5=N. |last6=LorΓa-CalderΓ³n |first6=E. |last7=Petersen-Pereira |first7=W. |last8=Rojas-DurΓ‘n |first8=Y. |last9=Salas-Pena |first9=R. |last10=Solis-Madrigal |first10=S. |last11=Osorio |first11=D. |last12=Darvell |first12=B. W. | year = 2003 | title = Evolution and function of routine trichromatic vision in primates |url=https://archive.org/details/sim_evolution_2003-11_57_11/page/2636 | journal = Evolution | volume = 57 |pages = 2636β43 | doi = 10.1554/03-168 | pmid = 14686538 | issue = 11|s2cid=739130 }}</ref> Howler monkeys are one of the most specialized leaf-eaters of the New World monkeys; fruits are not a major part of their diets,<ref name="Sussman2003">{{cite book|title=Primate Ecology and Social Structure, Volume 2: New World Monkeys|edition=Revised First|author=Sussman, R. W.|year=2003|isbn=0-536-74364-9|location = Needham Heights, MA | publisher = Pearson Custom Publishing & Prentice Hall|pages=77β80, 132β133, 141β143}}</ref> and the type of leaves they prefer to consume (young, nutritive, and digestible) are detectable only by a red-green signal. Field work exploring the dietary preferences of howler monkeys suggests that routine trichromaticism was selected by environment.<ref name="Surridge2003"/>
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