Primate

Primates Temporal range: 65.9–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N Early Paleocene to Present
Ring-tailed lemur Red slender loris Northern greater galago Philippine tarsier Black spider monkey Hamadryas baboon Lar gibbon Humans
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Mirorder:	Primatomorpha
Order:	Primates Linnaeus, 1758[1]
Suborders
Strepsirrhini Haplorhini †Altiatlasius
Range and density of non-human primates.
Synonyms
Plesiadapiformes (cladistically including crown primates[2])

Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians (monkeys and apes). Primates arose 74–63 million years ago first from small terrestrial mammals, which adapted for life in tropical forests: many primate characteristics represent adaptations to the challenging environment among tree tops, including large brain sizes, binocular vision, color vision, vocalizations, shoulder girdles allowing a large degree of movement in the upper limbs, and opposable thumbs (in most but not all) that enable better grasping and dexterity. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g (1 oz), to the eastern gorilla, weighing over 200 kg (440 lb). There are 376–524 species of living primates, depending on which classification is used. New primate species continue to be discovered: over 25 species were described in the 2000s, 36 in the 2010s, and six in the 2020s.

Primates have large brains (relative to body size) compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, which is the dominant sensory system in most mammals. These features are more developed in monkeys and apes, and noticeably less so in lorises and lemurs. Some primates, including gorillas, humans and baboons, are primarily ground-dwelling rather than arboreal, but all species have adaptations for climbing trees. Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees (brachiation); terrestrial locomotion techniques include walking on two hindlimbs (bipedalism) and modified walking on four limbs (quadrupedalism) via knuckle-walking.

Primates are among the most social of all animals, forming pairs or family groups, uni-male harems, and multi-male/multi-female groups. Non-human primates have at least four types of social systems, many defined by the amount of movement by adolescent females between groups. Primates have slower rates of development than other similarly sized mammals, reach maturity later, and have longer lifespans. Primates are also the most cognitively advanced animals, with humans (genus Homo) capable of creating complex languages and sophisticated civilizations, while non-human primates have been recorded using tools. They may communicate using facial and hand gestures, smells and vocalizations.

Close interactions between humans and non-human primates (NHPs) can create opportunities for the transmission of zoonotic diseases, especially virus diseases including herpes, measles, ebola, rabies and hepatitis. Thousands of non-human primates are used in research around the world because of their psychological and physiological similarity to humans. About 60% of primate species are threatened with extinction. Common threats include deforestation, forest fragmentation, monkey drives, and primate hunting for use in medicines, as pets, and for food. Large-scale tropical forest clearing for agriculture most threatens primates.

The English name primates is derived from Old French or French primat, from a noun use of Latin primat-, from primus ('prime, first rank').^[3] The name was given by Carl Linnaeus because he thought this the "highest" order of animals.^[4] The relationships among the different groups of primates were not clearly understood until relatively recently, so the commonly used terms are somewhat confused. For example, ape has been used either as an alternative for monkey or for any tailless, relatively human-like primate.^[5][6]

Sir Wilfrid Le Gros Clark was one of the primatologists who developed the idea of trends in primate evolution and the methodology of arranging the living members of an order into an "ascending series" leading to humans.^[7] Commonly used names for groups of primates such as prosimians, monkeys, lesser apes, and great apes reflect this methodology. According to our current understanding of the evolutionary history of the primates, several of these groups are paraphyletic, or rather they do not include all the descendants of a common ancestor.^[8]

In contrast with Clark's methodology, modern classifications typically identify (or name) only those groupings that are monophyletic; that is, such a named group includes all the descendants of the group's common ancestor.^[9]

The cladogram below shows one possible classification sequence of the living primates, with common (traditional) name groupings on the right:^[10][11]

Primatomorpha Dermoptera Primates Strepsirrhini Lemuriformes[a] lemurs (superfamily Lemuroidea) lorises and allies (superfamily Lorisoidea) Haplorhini Tarsiiformes tarsiers (superfamily Tarsioidea) Simiiformes New World monkeys (parvorder Platyrrhini) Catarrhini Old World monkeys (superfamily Cercopithecoidea) Hominoidea gibbons (family Hylobatidae) Hominidae orangutans (subfamily Ponginae) Homininae gorillas (tribe Gorillini) Hominini humans (g. Homo) chimpanzees, bonobos (g. Pan)

prosimians monkeys lesser apes great apes

• Prosimians contain two monophyletic groups (the suborder Strepsirrhini, or lemurs, lorises and allies, as well as the tarsiers of the suborder Haplorhini); it is a paraphyletic grouping because it excludes the Simiiformes, which also are descendants of the common ancestor Primates.
• Monkeys comprise two monophyletic groups, New World monkeys and Old World monkeys, but is paraphyletic because it excludes hominoids, superfamily Hominoidea, also descendants of the common ancestor Simiiformes.
• Apes as a whole, and the great apes, are paraphyletic if the terms are used such that they exclude humans.

Thus, the members of the two sets of groups, and hence names, do not match, which causes problems in relating scientific names to common (usually traditional) names. Consider the superfamily Hominoidea: In terms of the common names on the right, this group consists of apes and humans and there is no single common name for all the members of the group. One remedy is to create a new common name, in this case hominoids. Another possibility is to expand the use of one of the traditional names. For example, in his 2005 book, the vertebrate palaeontologist Benton wrote, "The apes, Hominoidea, today include the gibbons and orangutan ... the gorilla and chimpanzee ... and humans";^[15] thereby Benton was using apes to mean hominoids. In that case, the group heretofore called apes must now be identified as the non-human apes.

As of 2021^[update], there is no consensus as to whether to accept traditional (that is, common), but paraphyletic, names or to use monophyletic names only; or to use 'new' common names or adaptations of old ones. Both competing approaches can be found in biological sources, often in the same work, and sometimes by the same author. Thus, Benton defines apes to include humans, then he repeatedly uses ape-like to mean 'like an ape rather than a human'; and when discussing the reaction of others to a new fossil he writes of "claims that Orrorin ... was an ape rather than a human".^[16]

Order Primates was established by Carl Linnaeus in 1758, in the tenth edition of his book Systema Naturae,^[17] for the genera Homo (humans), Simia (other apes and monkeys), Lemur (prosimians) and Vespertilio (bats). In the first edition of the same book (1735), he had used the name Anthropomorpha for Homo, Simia and Bradypus (sloths).^[18] In 1839, Henri Marie Ducrotay de Blainville, following Linnaeus and aping his nomenclature, established the orders Secundates (including the suborders Chiroptera, Insectivora and Carnivora), Tertiates (or Glires) and Quaternates (including Gravigrada, Pachydermata and Ruminantia),^[19] but these new taxa were not accepted.

Before Anderson and Jones introduced the classification of Strepsirrhini and Haplorhini in 1984,^[20] (followed by McKenna and Bell's 1997 work Classification of Mammals: Above the species level),^[21] Primates was divided into two superfamilies: Prosimii and Anthropoidea.^[22] Prosimii included all of the prosimians: Strepsirrhini plus the tarsiers. Anthropoidea contained all of the simians.

Euarchontoglires Glires Rodentia (rodents) Lagomorpha (rabbits, hares, pikas) Euarchonta Scandentia (treeshrews) Primatomorpha Dermoptera (colugos) Primates †Plesiadapiformes crown primates

Order Primates is part of the clade Euarchontoglires, which is nested within the clade Eutheria of Class Mammalia. Recent molecular genetic research on primates, colugos, and treeshrews has shown that the two species of colugos are more closely related to primates than to treeshrews,^[23] even though treeshrews were at one time considered primates.^[24] These three orders make up the clade Euarchonta. The combination of this clade with the clade Glires (composed of Rodentia and Lagomorpha) forms the clade Euarchontoglires. Variously, both Euarchonta and Euarchontoglires are ranked as superorders. Some scientists consider Dermoptera to be a suborder of Primates and use the suborder Euprimates for the "true" primates.^[25]

The primate lineage is thought to go back at least near the Cretaceous–Paleogene boundary or around 74–63 (mya).^{[26][27][28][29][30]} The earliest possible primate/proto-primate may be Purgatorius, which dates back to Early Paleocene of North America ~66mya.^[31][32] The oldest known primates from the fossil record date to the Late Paleocene of Africa, c.57 mya (Altiatlasius)^[33] or the Paleocene-Eocene transition in the northern continents, c. 55 mya (Cantius, Donrussellia, Altanius, Plesiadapis and Teilhardina).^[34][35][31] Other studies, including molecular clock studies, have estimated the origin of the primate branch to have been in the mid-Cretaceous period, around 85 mya.^[36][37][38]

By modern cladistic reckoning, the order Primates is monophyletic. The suborder Strepsirrhini, the "wet-nosed" primates, is generally thought to have split off from the primitive primate line about 63 mya,^[39] although earlier dates are also supported.^[40] The seven strepsirrhine families are the five related lemur families and the two remaining families that include the lorisids and the galagos.^[1][41] Older classification schemes wrap Lepilemuridae into Lemuridae and Galagidae into Lorisidae, yielding a four-one family distribution instead of five-two as presented here.^[1] During the Eocene, most of the northern continents were dominated by two groups, the adapiforms and the omomyids.^[42][43] The former are considered members of Strepsirrhini, but did not have a toothcomb like modern lemurs; recent analysis has demonstrated that Darwinius masillae fits into this grouping.^[44] The latter was closely related to tarsiers, monkeys, and apes. How these two groups relate to extant primates is unclear. Omomyids perished about 30 mya,^[43] while adapiforms survived until about 10 mya.^[45]

According to genetic studies, the lemurs of Madagascar diverged from the lorisoids approximately 75 mya.^[40] These studies, as well as chromosomal and molecular evidence, also show that lemurs are more closely related to each other than to other strepsirrhine primates.^[40][46] However, Madagascar split from Africa 160 mya and from India 90 mya.^[47] To account for these facts, a founding lemur population of a few individuals is thought to have reached Madagascar from Africa via a single rafting event between 50 and 80 mya.^[40][46][47] Other colonization options have been suggested, such as multiple colonizations from Africa and India,^[42] but none are supported by the genetic and molecular evidence.^[40]

Until recently, the aye-aye has been difficult to place within Strepsirrhini.^[1] Theories had been proposed that its family, Daubentoniidae, was either a lemuriform primate (meaning its ancestors split from the lemur line more recently than lemurs and lorises split) or a sister group to all the other strepsirrhines. In 2008, the aye-aye family was confirmed to be most closely related to the other Malagasy lemurs, likely having descended from the same ancestral population that colonized the island.^[40]

Suborder Haplorhini, the simple-nosed or "dry-nosed" primates, is composed of two sister clades.^[1] Prosimian tarsiers in the family Tarsiidae (monotypic in its own infraorder Tarsiiformes), represent the most basal division, originating about 58 mya.^[48][49] The earliest known haplorhine skeleton, that of 55 MA old tarsier-like Archicebus, was found in central China,^[50] supporting an already suspected Asian origin for the group.^[51] The infraorder Simiiformes (simian primates, consisting of monkeys and apes) emerged about 40 mya,^[43] possibly also in Asia; if so, they dispersed across the Tethys Sea from Asia to Africa soon afterwards.^[52] There are two simian clades, both parvorders: Catarrhini, which developed in Africa, consisting of Old World monkeys, humans and the other apes, and Platyrrhini, which developed in South America, consisting of New World monkeys.^[1] A third clade, which included the eosimiids, developed in Asia, but became extinct millions of years ago.^[53]

As in the case of lemurs, the origin of New World monkeys is unclear. Molecular studies of concatenated nuclear sequences have yielded a widely varying estimated date of divergence between platyrrhines and catarrhines, ranging from 33 to 70 mya, while studies based on mitochondrial sequences produce a narrower range of 35 to 43 mya.^[35][54] The anthropoid primates possibly traversed the Atlantic Ocean from Africa to South America during the Eocene by island hopping, facilitated by Atlantic Ocean ridges and a lowered sea level.^[42] Alternatively, a single rafting event may explain this transoceanic colonization. Due to continental drift, the Atlantic Ocean was not nearly as wide at the time as it is today.^[42] Research suggests that a small 1 kg (2.2 lb) primate could have survived 13 days on a raft of vegetation.^[55] Given estimated current and wind speeds, this would have provided enough time to make the voyage between the continents.

Apes and monkeys spread from Africa into Europe and Asia starting in the Miocene.^[56] Soon after, the lorises and tarsiers made the same journey. The first hominin fossils were discovered in northern Africa and date back 5–8 mya.^[43] Old World monkeys disappeared from Europe about 1.8 mya.^[57] Molecular and fossil studies generally show that modern humans originated in Africa 100,000–200,000 years ago.^[58]

Although primates are well studied in comparison to other animal groups, several new species have been discovered recently, and genetic tests have revealed previously unrecognised species in known populations. Primate Taxonomy listed about 350 species of primates in 2001;^[11] the author, Colin Groves, increased that number to 376 for his contribution to the third edition of Mammal Species of the World (MSW3).^[1] However, publications since the taxonomy in MSW3 was compiled in 2003 have pushed the number to 522 species, or 708 including subspecies.^[59]

Primate hybrids usually arise in captivity,^[60] but there have also been examples in the wild.^[61][62] Hybridization occurs where two species' range overlap to form hybrid zones; hybrids may be created by humans when animals are placed in zoos or due to environmental pressures such as predation.^[61] Intergeneric hybridizations, hybrids of different genera, have also been found in the wild. Although they belong to genera that have been distinct for several million years, interbreeding still occurs between the gelada and the hamadryas baboon.^[63]

On 24 January 2018, scientists in China reported in the journal Cell the creation of two crab-eating macaque clones, named Zhong Zhong and Hua Hua, using the complex DNA transfer method that produced Dolly the sheep, for the first time.^{[64][65][66][67][68]}

The primate skull has a large, domed cranium, which is particularly prominent in anthropoids. The cranium protects the large brain, a distinguishing characteristic of this group.^[69] The endocranial volume (the volume within the skull) is three times greater in humans than in the greatest nonhuman primate, reflecting a larger brain size.^[70] The mean endocranial volume is 1,201 cubic centimeters in humans, 469 cm³ in gorillas, 400 cm³ in chimpanzees and 397 cm³ in orangutans.^[70] 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 sensory perception, generation of motor commands, spatial reasoning, conscious thought and, in humans, language.^[71] While other mammals rely heavily on their sense of smell, the arboreal life of primates has led to a tactile, visually dominant sensory system,^[71] a reduction in the olfactory region of the brain and increasingly complex social behavior.^[72] The visual acuity of humans and other hominids is exceptional; they have the most acute vision known among all vertebrates, with the exception of certain species of predatory birds.^[73][74]

Primates have forward-facing eyes on the front of the skull; binocular vision allows accurate distance perception, useful for the brachiating ancestors of all great apes.^[69] A bony ridge above the eye sockets reinforces weaker bones in the face, which are put under strain during chewing. Strepsirrhines have a postorbital bar, a bone around the eye socket, to protect their eyes; in contrast, the higher primates, haplorhines, have evolved fully enclosed sockets.^[75]

Primates show an evolutionary trend towards a reduced snout.^[76] Technically, Old World monkeys are distinguished from New World monkeys by the structure of the nose, and from apes by the arrangement of their teeth.^[72] In New World monkeys, the nostrils face sideways; in Old World monkeys, they face downwards.^[72] Dental pattern in primates vary considerably; although some have lost most of their incisors, all retain at least one lower incisor.^[72] In most strepsirrhines, the lower incisors form a toothcomb, which is used in grooming and sometimes foraging.^[72][77] Old World monkeys have eight premolars, compared with 12 in New World monkeys. The Old World species are divided into apes and monkeys depending on the number of cusps on their molars: monkeys have four, apes have five^[72] - although humans may have four or five.^[78] 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.

Primates generally have five digits on each limb (pentadactyly), with a characteristic type of keratin fingernail on the end of each finger and toe. The bottom sides of the hands and feet have sensitive pads on the fingertips. Most have opposable thumbs, a characteristic primate feature most developed in humans, though not limited to this order (opossums and koalas, for example, also have them).^[69] Thumbs allow some species to use 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-claws, which they use for grooming.^[69]

The primate collar bone is a prominent element of the pectoral girdle; this allows the shoulder joint broad mobility.^[76] 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 vertebrae greatly reduced - resulting in tail loss in some species.^[6] Prehensile tails are found in the New World atelids, including the howler, spider, woolly spider, woolly monkeys; and in capuchins.^[79][80] Male primates have a low-hanging penis and testes descended into a scrotum.^[81][77]

Sexual dimorphism is often exhibited in simians, 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 mass^[82][83] and canine tooth size^[84][85] along with pelage and skin color.^[86] The dimorphism can be attributed to and affected by different factors, including mating system,^[87] size,^[87] habitat and diet.^[88]

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.^[89] Ontogenetic scaling, where relative extension of a common growth trajectory occurs, may give some insight into the relationship between sexual dimorphism and growth patterns.^[90] Some evidence from the fossil record suggests that there was convergent evolution of dimorphism, and some extinct hominids probably had greater dimorphism than any living primate.^[89]

Primate species move by brachiation, bipedalism, leaping, arboreal and terrestrial quadrupedalism, climbing, knuckle-walking or by a combination of these methods. Several prosimians are primarily vertical clingers and leapers. These include many bushbabies, all indriids (i.e., sifakas, avahis and indris), sportive lemurs, and all tarsiers.^[91] 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. Gibbons, muriquis and spider monkeys all brachiate extensively,^[57] with gibbons sometimes doing so in remarkably acrobatic fashion. Woolly monkeys also brachiate at times.^[92] Orangutans 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.^[57] Chimpanzees and gorillas knuckle walk,^[57] and can move bipedally for short distances. Although numerous species, such as australopithecines and early hominids, have exhibited fully bipedal locomotion, humans are the only extant species with this trait.^[93]

The evolution of color vision in primates is unique among most eutherian mammals. While the remote vertebrate ancestors of the primates possessed three color vision (trichromaticism), the nocturnal, warm-blooded, mammalian ancestors lost one of three cones in the retina during the Mesozoic era. Fish, reptiles and birds are therefore trichromatic or tetrachromatic, while all mammals, with the exception of some primates and marsupials,^[94] are dichromats or monochromats (totally color blind).^[77] Nocturnal primates, such as the night monkeys and 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.^[77][95] Platyrrhines, on the other hand, are trichromatic in a few cases only.^[96] Specifically, individual females must be heterozygous for two alleles of the opsin gene (red and green) located on the same locus of the X chromosome.^[77] 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.^[77]

Like catarrhines, howler monkeys (a family of platyrrhines) show routine trichromatism that has been traced to an evolutionarily recent gene duplication.^[97] Howler monkeys are one of the most specialized leaf-eaters of the New World monkeys; fruits are not a major part of their diets,^[92] 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.^[96]

Richard Wrangham stated that social systems of primates are best classified by the amount of movement by females occurring between groups.^[98] He proposed four categories:

• Female transfer systems – females move away from the group in which they were born. Females of a group will not be closely related whereas males will have remained with their natal groups, and this close association may be influential in social behavior. The groups formed are generally quite small.^[98] This organization can be seen in chimpanzees, where the males, who are typically related, will cooperate in defense of the group's territory.^[99] Evidence of this social system (called patrilocal residence when used by the Anthropology) has also been found among Neanderthal remains in Spain^[100] and in remains of Australopithecus and Paranthropus robustus groups in southern Africa.^[101][102] Among New World Monkeys, spider monkeys and muriquis use this system.^[103]

• Male transfer systems – while the females remain in their natal groups, the males will emigrate as adolescents. Group sizes are usually larger.^[98] This system is common among the ring-tailed lemur, capuchin monkeys and cercopithecine monkeys.^[57]
• Monogamous species – a male–female bond, sometimes accompanied by a juvenile offspring. There is shared responsibility of parental care and territorial defense. The offspring leaves the parents' territory during adolescence.^[98] Indri, lariang tarsiers, Callitrichidae monkeys and gibbons use this system, although "monogamy" in this context does not necessarily mean absolute sexual fidelity.^[104][105] These species do not live in larger groups.
• Solitary species – males and females live in overlapping home ranges.^[98][105] This type of organization is found in lorises, galagos, mouse lemurs, aye-ayes and orangutans.^[105]

Other systems are known to occur as well. For example, with howler monkeys and gorillas both the males and females typically transfer from their natal group on reaching sexual maturity, resulting in groups in which neither the males nor females are typically related.^[92][106] Some prosimians, colobine monkeys and callitrichid monkeys also use this system.^[57]

The transfer of females or males from their native group is likely an adaptation for avoiding inbreeding.^[107] An analysis of breeding records of captive primate colonies representing numerous different species indicates that the infant mortality of inbred young is generally higher than that of non-inbred young.^[107][108] This effect of inbreeding on infant mortality is probably largely a result of increased expression of deleterious recessive alleles (see Inbreeding depression).

Primatologist Jane Goodall, who studied in the Gombe Stream National Park, noted fission-fusion societies in chimpanzees.^[109] There is fission when the main group splits up to forage during the day, then fusion when the group returns at night to sleep as a group. This social structure can also be observed in the hamadryas baboon,^[110] spider monkeys^[92] and the bonobo.^[110] The gelada has a similar social structure in which many smaller groups come together to form temporary herds of up to 600 monkeys.^[110] Humans also form fission-fusion societies. In hunter-gatherer societies, humans form groups which are made up of several individuals that may split up to obtain different resources.^[111]

These social systems are affected by three main ecological factors: distribution of resources, group size, and predation.^[112] Within a social group there is a balance between cooperation and competition. Cooperative behaviors in many primates species include social grooming (removing skin parasites and cleaning wounds), food sharing, and collective defense against predators or of a territory. Aggressive behaviors often signal competition for food, sleeping sites or mates. Aggression is also used in establishing dominance hierarchies.^[112][113]

In November 2023, scientists reported, for the first time, evidence that groups of primates, particularly bonobos, are capable of cooperating with each other.^[114][115]

Several species of primates are known to associate in the wild. Some of these associations have been extensively studied. In the Tai Forest of Africa, several species coordinate anti-predator behavior. These include the Diana monkey, Campbell's mona monkey, lesser spot-nosed monkey, western red colobus, king colobus (western black and white colobus), and sooty mangabey, which coordinate anti-predator alarm calls.^[116] Among the predators of these monkeys is the common chimpanzee.^[117]

The red-tailed monkey associates with several species, including the western red colobus, blue monkey, Wolf's mona monkey, mantled guereza, black crested mangabey and Allen's swamp monkey.^[110] Several of these species are preyed upon by the common chimpanzee.^[118]

In South America, squirrel monkeys associate with capuchin monkeys.^[119] This may have more to do with foraging benefits to the squirrel monkeys than anti-predation benefits.^[119]

The mating systems of primates vary between monogamy, polyandry, polygyny and polygynandry. In monogamous species, adult males and females form long-lasting pair bonds. Compared to other systems, there is little competition for mating rights and males and females tend to be similar in size. Polyandry, which involves groups consisting of single females mating with multiple males, may arise as a secondary mating system in monogamous species. In the brown-mantled tamarin, a female may breeding with one or two males. Polyandry may have developed due to the high frequency of twin births, which require more help in raising.^[105]

Polygynous species include gorillas, Hanuman langurs, geladas, hamadryas baboons, proboscis monkeys, and golden snub-nosed monkeys, and consists of one male mating with multiple females within a harem or one-male unit. Sexual dimorphism tends to be higher in these species and males may also develop prominent secondary sex characteristics. In the patriarchal hamadryas baboon, the males aggressively herd females into their groups and violently discipline those that wander. By contrast, in gelada society, which is based on female kinship, a male is dependent on the support of the females in his unit and cannot impose on them. Polygynous males must defend their harems from rivals, who may try to take over.^[105]

In some species, such as ring-tailed lemurs, sifakas, macaques, most baboons, mangabeys, squirrel monkeys, woolly monkeys, spider monkeys, woolly spider monkeys, chimpanzees and bonobos, both males and females mate with multiple partners. Polygynandry occurs in multimale-multifemale groups, and since females mate many times before conception, males have large testicles for sperm competition. Males may exist in a dominance hierarchy and those at the top will try to monopolize access to the females. Consortships may occur in some species but these are short-term. In solitary-living species, males and females mate with partners whose home ranges they overlap with. This is known as a 'dispersed' mating system.^[105]

Genetic evidence indicates that humans were predominantly polygynous for most of their existence as a species, but that this began to shift during the Neolithic, when monogamy started becoming widespread concomitantly with the transition from nomadic to sedentary societies.^[120] Most modern human societies consist of monogamous marriages, but allow for polygyny, particularly for those of a high status.^[105]

Female primates may signal to the male their receptiveness though various displays including eye-contact, tongue-clicking and presenting of the rump. Female lemurs, lorises and galagos will position themselves in the lordosis pose while female chimpanzees, bonobos and some Old World monkeys develop sexual swellings on the rump. Copulation in primates typically involves the males mounting the females from behind, as with most mammals. Belly-to-belly copulation has been recorded in apes, both gibbons and the great apes. Human sex positions are modifications of these two positions.^[121]

Primates may engage in sexual activity as part of social bonding; including homosexual behaviour.^[121] Such behavior play an important role in bonobo society in particular. female bonobos engage in mutual genital-rubbing behavior, possibly to bond socially with each other, thus forming a female nucleus of bonobo society. The bonding among females enables them to dominate most of the males.^[122]

Primates have slower rates of development than other mammals. All primate infants are breastfed by their mothers (with the exception of some human cultures and various zoo raised primates which are fed formula) and rely on them for grooming and transportation. In some species, infants are protected and transported by males in the group, particularly males who may be their fathers. Other relatives of the infant, such as siblings and aunts, may participate in its care as well. Most primate mothers cease ovulation while breastfeeding an infant; once the infant is weaned the mother can reproduce again. This often leads to weaning conflict with infants who attempt to continue breastfeeding.^[57] Primates have a longer juvenile period between weaning and sexual maturity than other mammals of similar size.^[57]

Infanticide is common in polygynous species such as gray langurs and gorillas. Adult males may kill dependent offspring that are not theirs so the female will return to estrus and thus they can sire offspring of their own. Social monogamy in some species may have evolved to combat this behavior.^[123] Polygynandry may also lessen the risk of infanticide since paternity becomes uncertain.^[124]

Some primates such as galagos and New World monkeys use tree-holes for nesting, and park juveniles in leafy patches while foraging. Other primates follow a strategy of "riding", i.e. carrying individuals on the body while feeding. Adults may construct or use nesting sites, sometimes accompanied by juveniles, for the purpose of resting, a behavior which has developed secondarily in the great apes.^[125][126] During the juvenile period, primates are more susceptible than adults to predation and starvation; they gain experience in feeding and avoiding predators during this time.^[57] They learn social and fighting skills, often through playing.^[57] Primates, especially females, have longer lifespans than other similarly sized mammals,^[57] this may be partially due to their slower metabolisms.^[127] Late in life, female catarrhine primates appear to undergo a cessation of reproductive function known as menopause; other groups are less studied.^[128]

Primates exploit a variety of food sources. It has been said that many characteristics of modern primates, including humans, derive from an early ancestor's practice of taking most of its food from the tropical canopy.^[129] Most primates include fruit in their diets to obtain easily digested nutrients including carbohydrates and lipids for energy.^[57] Primates in the suborder Strepsirrhini (non-tarsier prosimians) are able to synthesize vitamin C, like most other mammals, while primates of the suborder Haplorhini (tarsiers, monkeys and apes) have lost this ability, and require the vitamin in their diet.^[130]

Many primates have anatomical specializations that enable them to exploit particular foods, such as fruit, leaves, gum or insects.^[57] For example, leaf eaters such as howler monkeys, black-and-white colobuses and sportive lemurs have extended digestive tracts which enable them to absorb nutrients from leaves that can be difficult to digest.^[57] Marmosets, which are gum eaters, have strong incisor teeth, enabling them to open tree bark to get to the gum, and claws rather than nails, enabling them to cling to trees while feeding.^[57] The aye-aye combines rodent-like teeth with a long, thin middle finger to fill the same ecological niche as a woodpecker. It taps on trees to find insect larvae, then gnaws holes in the wood and inserts its elongated middle finger to pull the larvae out.^[131] Some species have additional specializations. For example, the grey-cheeked mangabey has thick enamel on its teeth, enabling it to open hard fruits and seeds that other monkeys cannot.^[57] The gelada is the only primate species that feeds primarily on grass.^[132]

Tarsiers are the only extant obligate carnivorous primates, exclusively eating insects, crustaceans, small vertebrates and snakes (including venomous species).^[133] Capuchin monkeys can exploit many different types of plant matter, including fruit, leaves, flowers, buds, nectar and seeds, but also eat insects and other invertebrates, bird eggs, and small vertebrates such as birds, lizards, squirrels and bats.^[92]

The common chimpanzee eats an omnivorous frugivorous diet. It prefers fruit above all other food items and even seeks out and eats them when they are not abundant. It also eats leaves and leaf buds, seeds, blossoms, stems, pith, bark and resin. Insects and meat make up a small proportion of their diet, estimated as 2%.^[134][135] The meat consumption includes predation on other primate species, such as the western red colobus monkey.^[117] The bonobo is an omnivorous frugivore – the majority of its diet is fruit, but it supplements this with leaves, meat from small vertebrates, such as anomalures, flying squirrels and duikers,^[136] and invertebrates.^[137] In some instances, bonobos have been shown to consume lower-order primates.^[138][139]

Until the development of agriculture approximately 10,000 years ago, Homo sapiens employed a hunter-gatherer method as their sole means of food collection. This involved combining stationary food sources (such as fruits, grains, tubers, and mushrooms, insect larvae and aquatic mollusks) with wild game, which must be hunted and killed in order to be consumed.^[140] It has been proposed that humans have used fire to prepare and cook food since the time of Homo erectus.^[141] Around ten thousand years ago, humans developed agriculture,^[142] which substantially altered their diet. This change in diet may also have altered human biology; with the spread of dairy farming providing a new and rich source of food, leading to the evolution of the ability to digest lactose in some adults.^[143][144]

Predators of primates include various species of carnivorans, birds of prey, reptiles, and other primates. Even gorillas have been recorded as prey. Predators of primates have diverse hunting strategies and as such, primates have evolved several different antipredator adaptations including crypsis, alarm calls and mobbing. Several species have separate alarm calls for different predators such as air-borne or ground-dwelling predators. Predation may have shaped group size in primates as species exposed to higher predation pressures appear to live in larger groups.^[145]