Tuesday, October 15, 2013


  • Lemurs   are a clade of strepsirrhine primates endemic to the island of Madagascar. The word "lemur" derives from the word lemures (ghosts or spirits) from Roman mythology and was first used to describe a slender loris due to its nocturnal habits and slow pace, but was later applied to the primates on Madagascar. HP Envy 14 Series CPU FANAlthough lemurs often are confused with ancestral primates, the anthropoid primates (monkeys, apes, and humans) did not evolve from them; instead, lemurs merely share morphological and behavioral traits with basal primates. Lemurs arrived in Madagascar around 62 to 65 mya by rafting on mats of vegetation at a time when ocean currents favored oceanic dispersal to the island. HP Envy 15 Series CPU FANSince that time, lemurs have evolved to cope with an extremely seasonal environment and their adaptations give them a level of diversity that rivals that of all other primate groups. Until shortly after humans arrived on the island around 2,000 years ago, there were lemurs as large as a male gorilla. Today, there are nearly 100 species of lemurs, and most of those species have been discovered or promoted to full species status since the 1990s; HP Envy 17 Series CPU FAN however, lemur taxonomic classification is controversial and depends on which species concept is used. Even the higher-level taxonomy is disputed, with some experts preferring to place most lemurs within the infraorder Lemuriformes, while others prefer Lemuriformes to contain all living strepsirrhines, placing all lemurs in superfamily Lemuroidea and all lorises and galagos in superfamily Lorisoidea. DELL XPS M1710 CPU FAN Ranging in size from 30 g (1.1 oz) to 9 kg (20 lb), lemurs share many common, basal primate traits, such as divergentdigits on their hands and feet and nails instead of claws (in most species). However, their brain-to-body size ratio is smaller than that of anthropoid primates, and among many other traits they share with other strepsirrhine primates, they have a "wet nose" (rhinarium). Lemurs are generally the most social of the strepsirrhine primates and communicate more with scents and vocalizations than with visual signals. HP 052907A CPU FAN Many lemur adaptations are in response to Madagascar's highly seasonal environment. Lemurs have relatively low basal metabolic rates and may exhibit seasonal breeding, dormancy (such as hibernation or torpor), or female social dominance. Most eat a wide variety of fruits and leaves, while some are specialists. Although many share similar diets, different species of lemur share the same forests by differentiating niches. HP 13.V1.B3403.GN CPU FAN Lemur research focused on taxonomy and specimen collection during the 18th and 19th centuries. Although field observations trickled in from early explorers, modern studies of lemur ecology and behavior did not begin in earnest until the 1950s and 1960s. Initially hindered by political instability and turmoil on Madagascar during the mid-1970s, field studies resumed in the 1980s and have greatly increased our understanding of these primates. HP 13.V1.BJ195.F.GN CPU FAN Research facilities like the Duke Lemur Center have provided research opportunities under more controlled settings. Lemurs are important for research because their mix of primitive characteristics and traits shared with anthropoid primates can yield insights on primate and human evolution. However, many lemur species are threatened with extinction due to habitat loss and hunting. Although local traditions generally help protect lemurs and their forests, illegal logging, HP 344872-001 CPU FAN widespread poverty, and political instability hinder and undermine conservation efforts. Because of these threats and their declining numbers, the International Union for Conservation of Nature (IUCN) considers lemurs to be the world's most endangered mammals, noting that—as of 2013—up to 90% of all lemur species face extinction within the next 20 to 25 years. HP 367795-001 CPU FAN Carl Linnaeus, the founder of modern binomial nomenclature, gave lemurs their name as early as 1758, when he used it in the 10th edition of Systema Naturae. He included three species under the genus LemurLemur tardigradus (the red slender loris, now known as Loris tardigradus), Lemur catta (the ring-tailed lemur), and Lemur volans (the Philippine colugo, now known as Cynocephalus volans).[2] HP 384622-001 CPU FAN Although the term "lemur" was first intended for slender lorises, it was soon limited to the endemic Malagasy primates, which have been known as "lemurs" ever since.[3] The name derives from the Latin term lemures,[4] which refers to specters or ghosts that were exorcised during the Lemuria festival of ancient Rome.[5]According to Linnaeus' own explanation, the name was selected because of the nocturnal activity and slow movements of the slender loris.[6] HP 3MLX6TATPH0 CPU FAN Being familiar with the works of Virgil and Ovid and seeing an analogy that fit with his naming scheme, Linnaeus adapted the term "lemur" for these nocturnal primates.[7] However, it has been commonly and falsely assumed that Linnaeus was referring to the ghost-like appearance, reflective eyes, and ghostly cries of lemurs.[6] It has also been speculated that Linnaeus may also have known that the some Malagasy people have held legends that lemurs are the souls of their ancestors,[8] HP 3MLX8TATP20 CPU FAN but this is unlikely given that the name was selected for slender lorises from India. Lemurs are prosimian primates belonging to the suborder Strepsirrhini. Like other strepsirrhine primates, such as lorises, pottos, and galagos, they share ancestral (or plesiomorphic) traits with early primates. In this regard, lemurs are popularly confused with ancestral primates; however, lemurs did not give rise to monkeys and apes (simians). HP 3MLX8TATP40 CPU FAN Instead, they evolved independently in isolation on Madagascar.[9] All modern strepsirrhines including lemurs are traditionally thought to have evolved from primitive primates known as adapiforms during the Eocene (56 to 34 mya) or Paleocene (65 to 56 mya).[9][10][11]Adapiforms, however, lack a specialized arrangement of teeth, known as a toothcomb, which nearly all living strepsirrhines possess.[ HP 3MLX9TATP70 CPU FAN A more recenthypothesis is that lemurs descended from lorisiform (loris-like) primates. This is supported by comparative studies of the cytochrome b gene and the presence of the strepsirrhine toothcomb in both groups.[14][15] Instead of being the direct ancestors of lemurs, the adapiforms may have given rise to both the lemurs and lorisiforms, a split that would be supported by molecular phylogenetic studies.[14] HP 407862-001 CPU FAN The later split between lemurs and lorises is thought to have occurred approximately 62 to 65 mya according to molecular studies,[16] although other genetic tests and the fossil record in Africa suggest more conservative estimates of 50 to 55 mya for this divergence. Once part of the supercontinent Gondwana, the island of Madagascar has been isolated since it broke away from eastern Africa(~160 mya), HP 414226-001 CPU FAN Antarctica (~80–130 mya), and India (~80–90 mya).[18][19] Since ancestral lemurs are thought to have originated in Africa around 62 to 65 mya, they would have had to have crossed the Mozambique Channel, a deep channel between Africa and Madagascar with a minimum width of about 560 km (350 mi).[14] In 1915, paleontologist William Diller Matthew noted that the mammalian biodiversity on Madagascar (including lemurs) can only be accounted for by random rafting events, HP 418409-001 CPU FAN where very small populations rafted from nearby Africa on tangled mats of vegetation, which get flushed out to sea from major rivers.[20] This form of biological dispersal can occur randomly over millions of years.[14][21] In the 1940s, American paleontologist George Gaylord Simpson coined the term "sweepstakes hypothesis" for such random events.[22] HP 431312-001 CPU FAN Rafting has since been the most accepted explanation for the lemur colonization of Madagascar,[23][24] but until recently this trip was thought to be very unlikely because strong ocean currents flow away from the island.[25] In January 2010, a report demonstrated that around 60 mya both Madagascar and Africa were 1,650 km (1,030 mi) south of their present-day positions, placing them in a different ocean gyre, producing currents that ran counter to what they are today. HP 434678-001 CPU FAN The ocean currents were shown to be even stronger than today, which would have pushed a raft along faster, shortening the trip to 30 days or less—short enough for a small mammal to survive easily. As the continental plates drifted northward, the currents gradually changed, and by 20 mya the window for oceanic dispersal had closed, effectively isolating the lemurs and the rest of the terrestrial Malagasy fauna from mainland Africa.[25] HP 441137-001 CPU FAN Isolated on Madagascar with only a limited number of mammalian competitors, the lemurs did not have to compete with other evolving arboreal mammalian groups, such as squirrels.[26] They were also spared from having to compete with monkeys, which evolved later. The intelligence, aggression, and deceptiveness of monkeys gave them an advantage over other primates in exploiting the environment. HP 448016-001 CPU FAN Lemurs have adapted to fill many open ecological niches since making their way to Madagascar.[13][26] Their diversity in both behavior and morphology (outward appearance) rivals that of the monkeys and apes found elsewhere in the world.[4] Ranging in size from the 30 g (1.1 oz) Madame Berthe's mouse lemur, the world's smallest primate,[27] to the recently extinct 160–200 kg (350–440 lb) Archaeoindris fontoynonti,[28] HP 450933-001 CPU FAN lemurs evolved diverse forms of locomotion, varying levels of social complexity, and unique adaptations to the local climate.[13][29] Lemurs lack any shared traits that make them stand out from all other primates.[30] Different types of lemurs have evolved unique combinations of unusual traits to cope with Madagascar's harsh, seasonal climate. These traits can include seasonal fat storage,hypometabolism (including torpor and hibernation), small group sizes, low encephalization (relative brain size), cathemerality(activity both day and night), and strict breeding seasons.[10][29HP 456605-001 CPU FAN ] Extreme resource limitations and seasonal breeding are also thought to have given rise to three other relatively common lemur traits: female social dominance, sexual monomorphism, and male–male competition for mates involving low levels of agonism, such as sperm competition.[31] Before the arrival of humans roughly 1500 to 2000 years ago, lemurs were found all across the island.[26] HP 480481-001 CPU FAN However, early settlers quickly converted the forests to rice paddies and grassland through slash-and-burn agriculture (known locally as tavy), restricting lemurs to approximately 10% of the island's area, ~60,000 km2 (23,000 sq mi).[32] Today, the diversity and complexity of lemur communities increases with floral diversity and precipitation and is highest in the rainforests of the east coast, where precipitation and floral diversity are also at their highest.[11] HP 486799-001 CPU FAN Despite their adaptations for weathering extreme adversity, habitat destruction and hunting have resulted in lemur populations declining sharply, and their diversity has diminished, with the recent extinction of at least 17 species in eight genera,[26][28][33] known collectively as the subfossil lemurs. Most of the approximately 100 species and subspecies of lemur are either threatened or endangered. Unless trends change, extinctions are likely to continue.[34] HP 486844-001 CPU FAN Until recently, giant lemurs existed on Madagascar. Now represented only by recent or subfossil remains, they were modern forms that were once part of the rich lemur diversity that has evolved in isolation. Some of their adaptations were unlike those seen in their living relatives.[26] All 17 extinct lemurs were larger than the extant (living) forms, some weighing as much as 200 kg (440 lb),[4] and are thought to have been active during the day.[35] HP 489126-001 CPU FAN Not only were they unlike the living lemurs in both size and appearance, they also filled ecological niches that either no longer exist or are now left unoccupied.[26] Large parts of Madagascar, which are now devoid of forests and lemurs, once hosted diverse primate communities that included more than 20 lemur species covering the full range of lemur sizes. From a taxonomic standpoint, the term "lemur" originally referred to the genus Lemur, which currently contains only the ring-tailed lemur. HP 493001-001 CPU FAN The term is now used in the colloquial sense in reference to all Malagasy primates.[39] Lemur taxonomy is controversial, and not all experts agree, particularly with the recent increase in the number of recognized species.[30][40][41] According to Russell Mittermeier, the president of Conservation International (CI), taxonomist Colin Groves, and others, there are nearly 100 recognized species or subspecies of extant (or living) lemur, divided into five families and 15 genera.[42] HP 495079-001 CPU FAN Because genetic data indicates that the recently extinct subfossil lemurs were closely related to living lemurs,[43] an additional three families, eight genera, and 17 species can be included in the total.[28][33] In contrast, other experts have labeled this as taxonomic inflation,[41] instead preferring a total closer to 50 species.[30] The classification of lemurs within the suborder Strepsirrhini is equally controversial, although the most experts agree on the same phylogenetic tree. HP 517749-001 CPU FAN In one taxonomy, infraorder Lemuriformes contains all living strepsirrhines in two superfamilies, Lemuroidea for all lemurs and Lorisoidea for the lorisoids (lorisids and galagos).[17][44] Alternatively, the lorisoids are sometimes placed in their own infraorder, Lorisiformes, separate from the lemurs.[45] In another taxonomy published by Colin Groves, the aye-ayewas placed in its own infraorder, Chiromyiformes, while the rest of the lemurs were placed in Lemuriformes and the lorisoids in Lorisiformes.[46] HP 532141-001 CPU FAN Although it is generally agreed that the aye-aye is the most basal member of the lemur clade, the relationship between the other four families is less clear since they diverged during a narrow 10 to 12 million-year window between the Late Eocene (42 mya) and into the Oligocene (30 mya).[16][23] Although all studies place Cheirogaleidae and Lepilemuridae as a sister clade to Indriidae and Lemuridae, some suggest that Cheirogaleidae and Lepilemuridae diverged first,[ HP 532613-001 CPU FAN while others suggest that Indriidae and Lemuridae were the first to branch off. Lemur taxonomy has changed significantly since the first taxonomic classification of lemurs by Carl Linnaeus in 1758. One of the greatest challenges has been the classification of the aye-aye, which has been a topic of debate up until very recently.[4] Until Richard Owen published a definitive anatomical study in 1866, early naturalists were uncertain whether the aye-aye (genus Daubentonia) was a primate, rodent, or marsupial.[ HP 532614-001 CPU FAN However, the placement of the aye-aye within the order Primates remained problematic until very recently. Based on its anatomy, researchers have found support for classifying the genus Daubentonia as a specialized indriid, a sister group to all strepsirrhines, and as an indeterminate taxon within the order Primates.[15] Molecular tests have now shown Daubentoniidae is basal to all Lemuriformes,[ HP 532617-001 CPU FAN and in 2008, Russell Mittermeier, Colin Groves, and others ignored addressing higher-level taxonomy by defining lemurs as monophyletic and containing five living families, including Daubentoniidae.[42] Relationships among lemur families have also proven to be problematic and have yet to be definitively resolved.[15] To further complicate the issue, several Paleogene fossil primates from outside Madagascar, such as BugtilemurHP 533736-001 CPU FAN have been classified as lemurs.[53] However, scientific consensus does not accept these assignments based on genetic evidence,[15][52]and therefore it is generally accepted that the Malagasy primates are monophyletic.[15][23][47] Another area of contention is the relationship between the sportive lemurs and the extinct koala lemurs (Megaladapidae). Formerly grouped in the same family due to similarities in dentition,[54] they are no longer considered to be closely related due to genetic studies.[47][55] HP 535438-001 CPU FAN More taxonomic changes have occurred at the genus level, although these revisions have proven more conclusive, often supported by genetic and molecular analysis. The most noticeable revisions included the gradual split of a broadly defined genus Lemur into separate genera for the ring-tailed lemur, ruffed lemurs, and brown lemurs due to a host of morphological differences.[56][57] Due to several taxonomic revisions by Russell Mittermeier, HP 535439-001 CPU FAN Colin Groves, and others, the number of recognized lemur species has grown from 33 species and subspecies in 1994 to approximately 100 in 2008.[30][42][58] With continuing cytogenetic andmolecular genetic research, as well as ongoing field studies, particularly with cryptic species such as mouse lemurs, the number of recognized lemur species is likely to keep growing.[30] However, the rapid increase in the number of recognized species has had its critics among taxonomists and lemur researchers. HP 535441-001 CPU FAN Since classifications ultimately depend on the species concept used, conservationists often favor definitions that result in the splitting of genetically distinct populations into separate species to gain added environmental protection. Others favor a more thorough analysis. Lemurs vary greatly in size. They include the smallest primates in the world and, until recently, also included some of the largest. HP 535442-001 CPU FAN They currently range in size from about 30 g (1.1 oz) for Madame Berthe's mouse lemur (Microcebus berthae) up to 7–9 kg (15–20 lb) for the indri (Indri indri) and diademed sifaka(Propithecus diadema).[59][60] When recently extinct species are considered, the size range extended up to that of a gorilla at 160–200 kg (350–440 lb) forArchaeoindris fontoynonti. Like all primates, lemurs have five divergent digits with nails (in most cases) on their hands and feet. HP 535766-001 CPU FAN Most lemurs possess a laterally compressed, elongated nail, called a toilet-claw, on the second toe and use it for scratching and grooming.[50][61] In addition to the toilet-claw, lemurs share a variety of other traits with other strepsirrhine primates, which include a rhinarium (or "wet nose"); a fully functional vomeronasal organ, which detects pheromones; a postorbital barand the lack of postorbital closure (a wall of thin bone behind the eye); HP 537613-001 CPU FAN orbits (bony sockets that enclose the eye) that are not fully facing forward; left and right mandible (lower jaw) bones that are not fully fused; and a small brain-to-body mass ratio.[14][62] Additional traits shared with other prosimian primates (strepsirrhine primates and tarsiers) include a bicornuate (two-horned) uterus and epitheliochorial placentation.[12][62] HP 538340-001 CPU FAN Because their thumbs are only pseudo-opposable, making their movement less independent of the other fingers,[61] their hands are less than perfect at grasping and manipulating objects.[19] On their feet, they have a widely abducted hallux (first toe) which facilitates the grasping of tree limbs.[50] A common misconception is that lemurs have a prehensile tail, a trait found only in New World monkeys, particularly atelids, among primates.[61] HP 576837-001 CPU FAN Lemurs also rely heavily on their sense of smell, a trait shared with most other mammals and primitive primates, but not with the visually oriented higher primates.[19] This sense of smell is important in terms of marking territory as well as provide an indication of whether or not another lemur is a viable breeding partner. Lemurs are a diverse group of primates in terms of morphology and physiology.[30] HP 580696-001 CPU FAN Some lemurs, such as the sportive lemurs and indriids, have longer hind limbsthan forelimbs, making them excellent leapers.[63][64][65] Indriids also have a specialized digestive system for folivory, exhibiting enlarged salivary glands, a spacious stomach, and an elongated caecum (lower gut) that facilitates fermentation.[ The hairy-eared dwarf lemur (Allocebus trichotis) reportedly has a very long tongue, allowing it to feed on nectar.[50] HP 580718-001 CPU FAN Likewise, the red-bellied lemur (Eulemur rubriventer) has a feathery brush-shaped tongue, also uniquely adapted to feed on nectar and pollen.[11] The aye-aye has evolved some traits that are unique among primates, making it stand out among the lemurs. Such traits include continuously growing, rodent-like front teeth for gnawing through wood and hard seeds; a highly mobile, filiform (filament-shaped) middle finger for extracting food from tiny holes; HP 582139-001 CPU FAN large, bat-like ears for detecting hollow spaces within trees; and use of self-generated acoustical cues to forage.[49] Lemurs are unusual since they have great variability in their social structure, yet generally lack sexual dimorphism in size and canine tooth morphology.[11][39]However, some species tend towards having larger females,[49] and two species of true lemur (genus Eulemur), the gray-headed lemur (E. albocollaris) and the red lemur (E. rufus), exhibit size differences in canine teeth.[69] HP 582141-001 CPU FAN True lemurs show sexual dichromatism (sexual differences in fur coloration),[39] but the difference between the genders varies from strikingly obvious, as in the blue-eyed black lemur (E. macaco), to nearly imperceptible in the case of the common brown lemur (E. fulvus).[69] Crypsis, or the inability of humans to visually distinguish between two or more distinct species, has recently been discovered among lemurs, particularly within the sportive lemurs (Lepilemur) and mouse lemurs (Microcebus). HP 602472-001 CPU FAN With sportive lemurs, subspecies were traditionally defined based on slight morphological differences, but new genetic evidence has supported giving full species status to these regional populations.[55] In the case of mouse lemurs, the gray mouse lemur (M. murinus), golden-brown mouse lemur (M. ravelobensis), and Goodman's mouse lemur (M. lehilahytsara) were considered the same species until recently, when genetic tests identified them as cryptic species. HP 603690-001 CPU FAN The lemur dentition is heterodont (having multiple tooth morphologies) and derives from an ancestral primate permanent dentition of Indriids, sportive lemurs, the aye-aye, and the extinctsloth lemurs, monkey lemurs, and koala lemurs have reduced dentitions, having lost incisors, canines, or premolars.[75] The ancestral deciduous dentitionis, but young indriids, aye-ayes, koala lemurs, sloth lemurs, and probably monkey lemurs have fewer deciduous teeth.[54][71] HP 603691-001 CPU FAN There are also noticeable differences in dental morphology and tooth topography between lemurs.Indri, for instance, have teeth that are perfectly adapted for shearing leaves and crushing seeds.[60] In the toothcomb of most lemurs, the bottom incisors and canine teeth are procumbent (face forward rather than up) and finely spaced, thus providing a tool for either grooming or feeding.[14][54][75] For instance, indri use their toothcomb not only for grooming, HP 6043B0034801A0​2 CPU FAN but also to pry out the large seeds from the tough exocarp of Beilschmiedia fruits,[76] while fork-marked lemurs use their relatively long toothcomb to cut through tree bark to induce the flow of tree sap.[50] The toothcomb is kept clean by the sublingua or "under-tongue", a specialized structure that acts like a toothbrush to remove hair and other debris. The sublingua extends below the tip of the tongue and is tipped with keratinized, serrated points that rake between the front teeth. HP 606013-001 CPU FAN Only the aye-aye, the extinct giant aye-aye, and the largest of the extinct giant sloth lemurs lack a functional strepsirrhine toothcomb.[75][73] In the case of the aye-aye, the morphology of the deciduous incisors, which are lost shortly after birth, indicate that its ancestors had a toothcomb. These milk teeth are lost shortly after birth[79] and are replaced by open-rooted, continually growing (hypselodont) incisors.[75] The toothcomb in lemurs normally consists of six teeth (four incisors HP 606574-001 CPU FAN and two canines), although indriids, monkey lemurs, and some sloth lemurs only have a four-tooth toothcomb due to the loss of either a canine or an incisor.[14][75] Because the lower canine is either included in the toothcomb or lost, the lower dentition can be difficult to read, especially since the first premolar (P2) is often shaped like a canine (caniniform) to fill the canine's role.[54] In folivorous (leaf-eating) lemurs, except for indriids, the upper incisors are greatly reduced or absent.[54][75] HP 606575-001 CPU FAN Used together with the toothcomb on themandible (lower jaw), this complex is reminiscent of an ungulate browsing pad.[75] Lemurs are unusual among primates for their rapid dental development, particularly among the largest species. For example, indriids have relatively slow body growth but extremely fast tooth formation and eruption.[80] By contrast, anthropoidprimates exhibit slower dental development with increased size and slower morphological development.[75] HP 606609-001 CPU FAN Lemurs are also dentally precocious at birth, and have their full permanent dentition at weaning.[29] Lemurs generally have thin tooth enamel compared to anthropoid primates. This may result in extra wear and breakage to the anterior (front) teeth due to heavy use in grooming, feeding, and fighting. Little other dental health information is available for lemurs, HP 608378-001 CPU FAN except that wild ring-tailed lemurs at Berenty Private Reserve occasionally exhibit abscessed maxillary canines (seen as open wounds on the muzzle) and tooth decay, possibly due to the consumption of non-native foods. The sense of smell, or olfaction, is highly important to lemurs and is frequently used in communication.[11][13][19] Lemurs have long snouts (compared to the short snouts of haplorrhines) that are traditionally thought to position the nose for better sifting of smells,[13] HP 608772-001 CPU FAN although long snouts do not necessarily translate into high olfactory acuity since its not the relative size of the nasal cavity that correlates with smell, but the density of olfactory receptors.[81][82] Instead, the long snouts may facilitate better chewing. The wet nose, or rhinarium, is a trait shared with other strepsirrhines and many other mammals, but not with haplorrhine primates.[50] HP 610773-001 CPU FAN Although it is claimed to enhance the sense of smell,[62] it is actually a touch-based sense organ that connects with a well-developed vomeronasal organ (VNO). Since pheromones are usually large, non-volatile molecules, the rhinarium is used to touch a scent-marked object and transfer the pheromone molecules down the philtrum (the nasal mid-line cleft) to the VNO via the nasopalatine ducts that travel through the incisive foramen of the hard palate.[12] HP 610774-001 CPU FAN To communicate with smell, which is useful at night, lemurs will scent mark with urine as well as scent glands located on the wrists, inside elbow, genital regions, or the neck.[12][62] The scrotal skin of most male lemurs has scent glands.[83] Ruffed lemurs (genus Varecia) and male sifakas have a gland at the base of their neck,[12][50] while the greater bamboo lemur(Prolemur simus) and the ring-tailed lemur have glands inside the upper arms near the axilla.[12] HP 610777-001 CPU FAN Male ring-tailed lemurs also have scent glands on the inside of their forearms, adjacent to a thorn-like spur, which they use to gouge, and simultaneously, scent-mark tree branches.[50] They will also wipe their tails between their forearms and then engage in "stink fights" by waving their tail as their opponents.[12] Lemurs (and strepsirrhines in general) are considered to be less visually oriented than the higher primates, since they rely so heavily on their sense of smell and pheromone detection. HP 622028-001 CPU FAN The fovea on the retina; which yields higher visual acuity, is not well-developed. The postorbital septum (or bony closure behind the eye) in haplorrhine primates is thought to stabilize the eye slightly, allowing for the evolution of the fovea. With only a postorbital bar, lemurs have been unable to develop a fovea.[84] Therefore, regardless of their activity pattern (nocturnal, cathemeral, or diurnal), lemurs exhibit low visual acuity and high retinal summation.[29] HP 622029-001 CPU FAN Lemurs can see a wider visual field, however, than anthropoid primates due to a slight difference in the angle between the eyes. Although they lack a fovea, some diurnal lemurs have a cone-rich, although less clustered, area centralis.[84] This area centralis has a high rod-to-cone cell ratio in many diurnal species studied thus far, whereas diurnal anthropoids have no rod cells in their fovea. Once again, this suggests lower visual acuity in lemurs than in anthropoids.[86] HP 622330-001 CPU FAN Furthermore, the rod-to-cone cell ratio can be variable even among diurnal species. For instance, Verreaux's sifaka (Propithecus verreauxi) and the indri (Indri indri) have only a few large cones scattered along their predominantly rod-dominated retina. The eyes of the ring-tailed lemur contain one cone to five rods. Nocturnal lemurs such as mouse lemurs and dwarf lemurs, on the other hand, have retinas made up entirely of rod cells.[12] HP 637607-001 CPU FAN Since cone cells make color vision possible, the high prevalence of rod cells in lemur eyes suggest they have not evolved color vision.[12] The most studied lemur, the ring-tailed lemur, has been shown to have blue-yellow vision, but lacks the ability to distinguish red and green hues.[87] Due to polymorphism inopsin genes, which code for color receptivity, trichromatic vision may rarely occur in females of a few lemur species, such as Coquerel's sifaka (Propithecus coquereli) and the red ruffed lemur (Varecia rubra). HP 646578-001 CPU FAN Most lemurs, therefore, are either monochromats or dichromats. Most lemurs have retained the tapetum lucidum, a reflective layer of tissue in the eye, which is found in many vertebrates.[39] This trait is absent in haplorrhine primates, and its presence further limits the visual acuity in lemurs.[29][86] The strepsirrhine choroidal tapetum is unique among mammals because it is made up of crystalline riboflavin, and the resulting optical scattering is what limits visual acuity.[86] HP 657529-001 CPU FAN

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