Strepsirrhini in the context of Algeripithecus

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.

Lemurs (/ˈliːmərz/ LEE-mərz; from Latin lemurēs 'ghosts, spirits of the dead') are wet-nosed primates of the superfamily Lemuroidea (/ˌlɛmjʊˈrɔɪdiə/ LEM-yuu-ROY-dee-ə), divided into eight families and consisting of 15 genera and around 100 extant species. They are endemic to the island of Madagascar. Most existing lemurs are small, with a pointed snout, large eyes, and a long tail. They usually live in trees and are active at night.

Lemurs share resemblance with other primates, but evolved independently from monkeys and apes. Due to Madagascar’s highly seasonal climate, lemur evolution has produced a level of species diversity rivaling that of any other primate group.

Haplorhini (/hæpləˈraɪnaɪ/), the haplorhines (Greek for "simple-nosed") or the "dry-nosed" primates is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids), as sister of the Strepsirrhini ("moist-nosed"). The name is sometimes spelled Haplorrhini. The simians include catarrhines (Old World monkeys and apes, including humans), and the platyrrhines (New World monkeys).

Haplorhini was proposed by Pocock in 1918 when he realized the tarsiers were actually sister to the monkeys rather than the lemurs, also following findings of Hugh Cuming 80 years earlier and Linnaeus 160 years earlier. For Linnaeus, this ensemble of primates constituted a genus "Simia". For religious reasons, Homo constituted its own genus (which has remained).

The composition of Madagascar's wildlife reflects the fact that the island has been isolated for about 88 million years. The prehistoric breakup of the supercontinent Gondwana separated the Madagascar-Antarctica-India landmass from the Africa-South America landmass around 135 million years ago. Madagascar later split from India about 88 million years ago, allowing plants and animals on the island to evolve in relative isolation.

As a result of the island's long isolation from neighboring continents, Madagascar is home to an abundance of plants and animals found nowhere else on Earth. Approximately 90 percent of all plant and animal species found in Madagascar are endemic, including the lemurs (a type of strepsirrhine primate), the carnivorous fossa and many birds. This distinctive ecology has led some ecologists to refer to Madagascar as the "eighth continent", and the island has been classified by Conservation International as a biodiversity hotspot. As recent as 2021, the "smallest reptile on earth" was also found in Madagascar, Brookesia nana, also known as the nano-chameleon.

Lemuriformes is the sole extant infraorder of primate that falls under the suborder Strepsirrhini. It includes the lemurs of Madagascar, as well as the galagos and lorisids of Africa and Asia, although a popular alternative taxonomy places the lorisoids in their own infraorder, Lorisiformes.

Lemuriform primates are characterized by a toothcomb, a specialized set of teeth in the front, lower part of the mouth mostly used for combing fur during grooming.

Loris is the common name for the wet-nosed primates of the subfamily Lorinae (sometimes spelled Lorisinae) in the family Lorisidae. Loris is one genus in this subfamily and includes the slender lorises, Nycticebus is the genus containing the slow lorises, and Xanthonycticebus is the genus name of the pygmy slow loris.

Adapiformes is a group of early primates. Adapiforms radiated throughout much of the northern continental mass (now Europe, Asia and North America), reaching as far south as northern Africa and tropical Asia. They existed from the Eocene to the Miocene epoch. Some adapiforms resembled living lemurs.

Adapiforms are known from the fossil record only, and it is unclear whether they form a monophyletic or paraphyletic group. When assumed to be a clade, they are usually grouped under the "wet-nosed" taxon Strepsirrhini, which would make them more closely related to the lemurs and less so to the "dry-nosed" Haplorhini taxon that includes monkeys and apes.

Lemurs, primates belonging to the suborder Strepsirrhini which branched off from other primates less than 63 million years ago, evolved on the island of Madagascar, for at least 40 million years. They share some traits with the most basal primates, and thus are often confused as being ancestral to modern monkeys, apes, and humans. Instead, they merely resemble ancestral primates.

Lemurs are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with lorises, pottos, and galagos (lorisoids). Fossils from Africa and some tests of nuclear DNA suggest that lemurs made their way to Madagascar between 40 and 52 mya. Other mitochondrial and nuclear DNA sequence comparisons offer an alternative date range of 62 to 65 mya. An ancestral lemur population is thought to have inadvertently rafted to the island on a floating mat of vegetation, although hypotheses for land bridges and island hopping have also been proposed. The timing and number of hypothesized colonizations has traditionally hinged on the phylogenetic affinities of the aye-aye, the most basal member of the lemur clade.

In his Systema Naturae of 1758, Carl Linnaeus divided the Order Primates within Mammalia into four genera: Homo, Simia, Lemur, and Vespertilio. His Vespertilio included all bats, and has since been moved from Primates to Chiroptera. Homo contained humans, Lemur contained four lemurs and a colugo, and Simia contained the other Primates. Linnaeus did not think that Homo should form a distinct group from Simia, classifying them separately mainly to avoid conflict with religious authorities. If this is taken into account, Simia (including Homo) would be roughly equivalent to the Suborder Haplorhini of the Primates (while Lemur would be roughly equivalent to the Suborder Strepsirrhini).

Homo, Lemur, and Vespertilio have survived as generic names, but Simia has not. All the species have since been moved to other genera, and in 1929, the International Commission on Zoological Nomenclature ruled in its Opinion 114 that Simia be suppressed. The genus Simias is distinct and remains valid, containing a single species, the pig-tailed langur (Simias concolor).

Prosimians are a group of primates that includes all living and extinct strepsirrhines (lemurs, lorisoids, and adapiforms), as well as the haplorhine tarsiers and their extinct relatives, the omomyiforms, i.e. all primates excluding the simians. They are considered to have characteristics that are more "primitive" (ancestral or plesiomorphic) than those of simians (monkeys, apes, and humans).

Simians emerged within the Prosimians as sister group of the haplorhine tarsiers, and therefore cladistically belong to this group. Simians are thus distinctly closer related to tarsiers than lemurs are. Strepsirrhines bifurcated some 20 million years earlier than the tarsier–simian bifurcation. However, simians are traditionally excluded, rendering prosimians paraphyletic. Consequently, the term "prosimian" is no longer widely used in a taxonomic sense, but is still used to illustrate the behavioral ecology of tarsiers relative to the other primates.

The ring-tailed lemur (Lemur catta) is a medium- to larger-sized strepsirrhine (wet-nosed) primate and the most internationally recognized lemur species, owing to its long, black-and-white, ringed tail. It belongs to Lemuridae, one of five lemur families, and is the only member of the Lemur genus. Like all lemurs, it is endemic to the island of Madagascar, where it is endangered. Known locally in Malagasy as maky ([makʲ] , spelled maki in French) or hira, it ranges from gallery forests to spiny scrub in the southern regions of the island. It is omnivorous, as well as the most adapted to living terrestrially of the extant lemurs.

The ring-tailed lemur is highly social, living in groups—known as "troops"—of up to 30 individuals. It is also a female-dominant species, a commonality among lemurs. To keep warm and reaffirm social bonds, groups will huddle together. Mutual grooming is another vital aspect of lemur socialization (as with all primates), reaffirming social and familial connections, while also helping rid each other of any potential insects. Ring-tailed lemurs are strictly diurnal, being active exclusively during daylight hours. Due to this lifestyle, they also sunbathe; the lemurs can be observed sitting upright on their tails, exposing their soft, white belly fur towards the sun. They will often also have their palms open and eyes gently closed. Like other lemurs, this species relies strongly on their sense of smell, and territorial marking, with scent glands, provides communication signals throughout a group's home range. The glands are located near the eyes, as well as near the anus. The males perform a unique scent-marking behavior called spur-marking and will participate in stink fights by dousing their tails with their pheromones and "wafting" them at opponents. Additionally, lemurs of both sexes will scent-mark trees, logs, rocks or other objects by simply rubbing their faces and bodies onto it, not unlike a domestic cat.

Lorisoidea is a superfamily of nocturnal primates found throughout Africa and Asia. Members include the galagos and the lorisids. As strepsirrhines, lorisoids are related to the lemurs of Madagascar and are sometimes included in the infraorder Lemuriformes, although they are also sometimes placed in their own infraorder, Lorisiformes Gregory, 1915.

Slow lorises are a group of several species of nocturnal strepsirrhine primates that make up the genus Nycticebus. Found in Southeast Asia and nearby areas, they range from Bangladesh and Northeast India in the west to the Sulu Archipelago in the Philippines in the east, and from Yunnan province in China in the north to the island of Java in the south.

Although many previous classifications recognized as few as a single all-inclusive species, there are now at least eight that are considered valid: the Sunda slow loris (N. coucang), Bengal slow loris (N. bengalensis), Javan slow loris (N. javanicus), Philippine slow loris (N. menagensis), Bangka slow loris (N. bancanus), Bornean slow loris (N. borneanus), Kayan River slow loris (N. kayan) and Sumatran slow loris (N. hilleri). A ninth species, the pygmy slow loris (X. pygmaeus), was recently moved to the new genus Xanthonycticebus. After the pygmy slow loris, the group's closest relatives are the slender lorises of southern India and Sri Lanka. Their next closest relatives are the African lorisids, the pottos, false pottos, and angwantibos. They are less closely related to the remaining lorisoids (the various types of galago), and more distantly to the lemurs of Madagascar. Their evolutionary history is uncertain since their fossil record is patchy and molecular clock studies have given inconsistent results.

The aye-aye (Daubentonia madagascariensis) is a long-fingered lemur, a strepsirrhine primate native to Madagascar with rodent-like teeth that perpetually grow and a special thin middle finger that they can use to catch grubs and larvae out of tree trunks.

It is the world's largest nocturnal primate. It is characterized by its unusual method of finding food: it taps on trees to find grubs, then gnaws holes in the wood using its forward-slanting incisors to create a small hole into which it inserts its narrow middle finger to pull the grubs out. This foraging method is called percussive foraging, and takes up 5–41% of foraging time. The only other living mammal species known to find food in this way are the striped possum and trioks (genus Dactylopsila) of northern Australia and New Guinea, which are marsupials. From an ecological perspective, the aye-aye fills the niche of a woodpecker, as it is capable of penetrating wood to extract the invertebrates within.