Lemurs were first classified in 1758 by Carl Linnaeus, and the taxonomy remains controversial today, with approximately 70 to 100 species and subspecies recognized, depending on how the term "species" is defined. Having undergone their own independent evolution on Madagascar, lemurs have diversified to fill many ecological niches normally filled by other types of mammals. They include the smallest primates in the world, and once included some of the largest. Since the arrival of humans approximately 2,000 years ago, lemurs have become restricted to 10% of the island, or approximately , and many face extinction. Concerns over lemur conservation have affected lemur taxonomy, since distinct species receive increased conservation attention compared to subspecies.

The relationship between the aye-aye and the rest of the lemurs has had the greatest impact on lemur taxonomy at the family rank and above. Genetic analysis of this relationship has also clarified lemur phylogeny and supports the hypothesis that lemurs rafted to Madagascar. Despite general agreement on phylogeny, the taxonomy is still under debate. At the genus level, the taxonomy has been relatively stable since 1931, but a number of additional genera have been recognized since then.

Since the 1990s, there has been a steep increase in the number of recognized lemur species and subspecies through the discovery of new species, the elevation of existing subspecies to full species status, and the recognition of new species among previously known populations that were not even distinct subspecies. Currently living lemur species are divided into five families and 15 genera. If the extinct subfossil lemurs are included, three families, eight genera, and 17 species would be added to the count. The recent rise in species numbers is due to both improved genetic analysis and a push in conservation to encourage the protection of isolated and distinct lemur populations. Not everyone in the scientific community supports these taxonomic changes, with some preferring instead an estimate of 50 living species.

Background

Since their arrival on Madagascar, a biogeographically isolated island with a unique mammalian fauna, lemurs have diversified both in behavior and morphology. Their diversity rivals that of the monkeys and apes found throughout the rest of the world, especially when the recently extinct subfossil lemurs are considered. Ranging in size from the Madame Berthe's mouse lemur, probably the world's smallest primate, to the extinct Archaeoindris fontoynonti, the largest known prosimian, lemurs evolved diverse forms of locomotion, varying levels of social complexity, and unique adaptations to the local climate. They went on to fill many niches normally occupied by monkeys, squirrels, woodpeckers, and large grazing ungulates. In addition to the incredible diversity between lemur families, there has also been great diversification among closely related lemurs.

The arrival of humans on the island 1,500 to 2,000 years ago has taken a significant toll, not only on the size of lemur populations, but also on their diversity. Due to habitat destruction and hunting, at least 17 species and 8 genera have gone extinct and many others have become threatened. Historically, lemurs ranged across the entire island inhabiting a wide variety of habitats, including dry deciduous forests, lowland forests, spiny thickets, subhumid forests, montane forest, and mangrove. Today, their collective range is restricted to 10% of the island, or approximately . Most of the remaining forests and lemurs are found along the periphery of the island. The center of the island, the Hauts-Plateaux, was converted by early settlers to rice paddies and grassland through slash-and-burn agriculture, known locally as tavy. As erosion depleted the soil, the cyclical forest regrowth and burning ended as the forest gradually failed to return. In 2008, 41% of all lemur taxa were threatened with extinction while 42% were classified on the IUCN Red List as "Data Deficient".

Overview of taxonomic and phylogenetic classification

In the first volume of the 10th edition of Systema Naturae (1758), Carl Linnaeus, the founder of modern binomial nomenclature, created the genus Lemur to include three species: Lemur 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). Although the term "lemur" was at first intended for lorises, it was soon applied to the endemic Malagasy primates, which have been known as "lemurs" ever since. The name derives from the Latin term lemures, which refers to the "spirits of the dead" from Roman mythology. According to Linnaeus' own explanation, the name was selected because of the nocturnal activity and slow movements of the slender loris. 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. However, it has been commonly and falsely assumed that Linnaeus was referring to the ghost-like appearance, reflective eyes, and ghostly cries of lemurs. It has also been speculated that Linnaeus may also have known that some Malagasy people have held legends that lemurs are the souls of their ancestors, but this is unlikely given that the name was selected for slender lorises from India.

Since the first taxonomic classification of lemurs, many changes have been made to lemur taxonomy. Within the primate order, treeshrews (order Scandentia) were considered basal, prosimian primates—close relatives of lemurs—until the 1980s. Colugos, also incorrectly referred to as "flying lemurs", were once considered lemur-like primates, but were reclassified as close relatives of bats, and more recently as close relatives of primates within their own order, Dermoptera. Primates, together with their closest relatives, the treeshrews, colugos, and long-extinct plesiadapiforms, form the taxonomically unranked Euarchonta clade within the Euarchontoglires. Lorisids, some of which were originally placed in the genus Lemur by Carl Linnaeus, have since been moved into either their own infraorder (Lorisiformes) or their own superfamily (Lorisoidea) within Lemuriformes.

For the Malagasy primate fauna, taxonomic nomenclature proliferated during the 1800s, with the aid of museum systematists, such as Albert Günther and John Edward Gray, as well as naturalists and explorers, such as Alfred Grandidier. This nomenclature was not sorted out until decades later, when Ernst Schwarz standardized it in 1931. It was not until the 1990s that this nomenclature started to see a new wave of taxonomic change.

Suprageneric classification Although Carl Linnaeus originally grouped the first "lemurs" he classified under the order Primates, lemurs and other non-human primates were later separated from humans by being placed in the order Quadrumana by Johann Friedrich Blumenbach in 1775. (He also placed humans in the order Bimana.) This view was upheld by other famous naturalists and zoologists of the time, including Étienne Geoffroy Saint-Hilaire (who first placed lemurs in Strepsirrhini in 1812), Georges Cuvier, and (initially) John Edward Gray. By 1862, William Henry Flower, a comparative anatomist, was arguing against moving strepsirrhines out of Quadrumana into Insectivora (a now-abandoned biological grouping), claiming that their brain had features transitional between other primates and "inferior" mammals. In 1863, Thomas Henry Huxley restored the order Primates to include humans, other apes, monkeys, lemurs, and even colugos. However, opposition continued with many specialists arguing that lemurs (or "Half-apes") should be placed in their own order. In 1873, English comparative anatomist St. George Jackson Mivart countered these arguments and proceeded to define the primates by a list of anatomical features.

Since the 19th century, the classification of lemurs above the genus level has seen many changes. Early taxonomists proposed a variety of classifications for lemurs, but generally separated indriids from other lemurs and placed the aye-aye in a major group of its own; some classified the dwarf and mouse lemurs with the galagos. In 1915, William King Gregory published a classification that remained generally accepted over the next decades. He placed all the lemurs together in a "series" Lemuriformes and recognized three families: Daubentoniidae, Indriidae, and Lemuridae (including the current Cheirogaleidae and Lepilemuridae). George Gaylord Simpson's influential 1945 classification of mammals placed the treeshrews and the fossil Anagale (both now classified outside Primates) inside Lemuriformes and classified the fossil families Plesiadapidae and Adapidae in a superfamily Lemuroidea with most of the lemurs.

Although treeshrews, plesiadapids, and the like are now no longer considered to be closely related to lemurs, disagreements persist over the classification of lemurs and related groups, resulting in competing arrangements of the infraorders and superfamilies within Strepsirrhini. In one taxonomy, infraorder Lemuriformes contains all living strepsirrhines in two superfamilies, Lemuroidea for all lemurs and Lorisoidea for the lorisoids (lorisids and galagos). Alternatively, the lorisoids are sometimes placed in their own infraorder, Lorisiformes, separate from the lemurs. Yet another classification published by Colin Groves placed the aye-aye in its own infraorder, Chiromyiformes, while the rest of the lemurs were placed in Lemuriformes and the lorisoids in Lorisiformes.

+ Competing taxonomic nomenclature for lemurs and their kin 2 infraorders 3 infraorders 4 infraorders Order Primates Suborder Strepsirrhini Infraorder Adapiformes Infraorder Lemuriformes Superfamily Lemuroidea Family Archaeolemuridae Family Cheirogaleidae Family Daubentoniidae Family Indriidae Family Lemuridae Family Lepilemuridae Family Megaladapidae Family Palaeopropithecidae Superfamily Lorisoidea Suborder Haplorrhini Order Primates Suborder Strepsirrhini Infraorder Adapiformes Infraorder Lemuriformes Superfamily Lemuroidea Family Archaeolemuridae Family Cheirogaleidae Family Daubentoniidae Family Indriidae Family Lemuridae Family Lepilemuridae Family Megaladapidae Family Palaeopropithecidae Infraorder Lorisiformes Suborder Haplorrhini Order Primates Suborder Strepsirrhini Infraorder Adapiformes Infraorder Chiromyiformes Family Daubentoniidae Infraorder Lemuriformes Superfamily Cheirogaleoidea Family Cheirogaleidae Superfamily Lemuroidea Family Archaeolemuridae Family Indriidae Family Lemuridae Family Lepilemuridae Family Megaladapidae Family Palaeopropithecidae Infraorder Lorisiformes Suborder Haplorrhini

The classification of several lemur taxa has elicited particular debate. Most significantly, the placement of the aye-aye has been controversial since its introduction to Western science in 1782, and it has been a topic of debate until very recently. Arguing against Darwin's theory of natural selection, Richard Owen claimed in 1863 that the aye-aye's distinct characteristics, including its ever-growing incisors and unique, highly flexible middle finger, are so perfectly adapted for their uses in extractive foraging that they could not have evolved gradually through natural selection. More recently, the aye-aye's placement has posed problems for the rafting hypothesis for the primate colonization of Madagascar. If this species does not form a monophyletic group with the rest of the lemurs, then multiple colonization events would have had to occur to explain the occurrence of lemurs on Madagascar.