Often used as models for early primate anatomy and behavior, prosimians include a rich diversity of species exhibiting a complex range of social behavior, dietary specializations, and locomotor habits. Although more geographically widespread in the past, today these generally small primates are restricted to the Old World. Five of the eight major prosimian groups are known exclusively from Madagascar, while other species are found in Africa and Asia. While prosimians are only distantly related to humans, this group has allowed scientists to identify common anatomical and behavioral traits that link all primates.
Traditionally, the order Primates is divided into two major suborders, Prosimii and Anthropoidea. The latter group includes monkeys, apes, and humans, while prosimians are comprised of eight distinct families, including the cheirogaleids, lemurids, lepilemurids (megaladapids), daubentoniids, indriids, galagids, lorisids, and tarsiids. However, this Linnaean taxonomic arrangement has proven controversial, as several recent studies have shown that Prosimii isn’t a natural, monophyletic group comprised of a single common ancestor and all of its descendants. Instead, this suborder is a basal grade of primates, linked by primitive characteristics, that includes those species more primitive than Anthropoidea. Modern primatologists have thus advocated a system that divides Primates into two large, monophyletic clades: Haplorhini, which consists of Anthropoidea and tarsiids, and Strepsirhini, which includes all of the traditional prosimian families with the exception of tarsiids. Although based on phylogenetic analyses that have found tarsiids to be closer to anthropoids than to the other traditional prosimians, this nomenclature is still not universally accepted. Thus, this entry will consider all eight of the traditional prosimian families, as well as a handful of fossil groups whose relationships are poorly resolved.
Depending on which classification scheme one follows, there exist between 24 and 31 genera of living prosimians, which are further divided into 54 to 68 species. All prosimians share with other primates several basic characteristics, including grasping hands with opposable thumbs, the primitive mammalian claw modified into a flat nail, forward facing eyes that result in binocular vision, large brains, small litter size, and slow growth. The clade Strepsirhini is united by three distinct features: an unusual dental “comb,” a grooming claw on the second digit of the foot, and a laterally flaring talus (ankle bone). The clade Haplorhini, which includes anthropoids and tarsiers, is linked by several features of the soft anatomy, mostly relating to the nose and lips. Overall, the traditional Prosimii (including tarsiers) exhibits a relatively primitive anatomy compared to later primates, including a small brain. As a result, prosimians—especially cheirogaleids and lorisids—have often been used by primatologists as models for the primitive primate condition. However, despite their retention of primitive features, prosimians are a successful group that exhibits a wide range of behavior.
One of five prosimian families found exclusively on Madagascar, cheirogaleids are small-bodied, nocturnal, nest-building primates. Their reproductive system is unique among primates, with females possessing three pairs of nipples and often birthing twins. Five extant genera are included in this group, all of which have a mass of less than 500 grams. The mouse lemurs (Microcebus) are arboreal quadrupeds that move by running along small branches and leaping between terminal twigs. Mouse lemurs are faunivorous and occupy overlapping home ranges. It has been observed that the sex ratio of individual litters is influenced by the presence or absence of females other than the mother. Coquerel’s mouse lemur (Mirza) constructs large, circular nests of leaves for daytime resting, but spends the night feeding and socializing with conspecifics. The dwarf lemurs (Cheirogaleus) hibernate for long periods during the dry season, while the fork-marked lemur (Phaner) is characterized by a suite of features related to its diet of gum, including large hands and feet with expanded digit pads, keeled fingernails, and procumbent incisors. Unlike other cheirogaleids, Phaner lives in permanent groups, many of which contain one male and one female.
Likely the most well known of the prosimians, the lemurids contain the signature Malagasy lemurs, most of which are medium-sized (1-4 kilograms) and live in groups. The ring-tailed lemur (Lemur catta), marked by a long, striped tail, is the most terrestrial of the living strepsirhines, spending approximately 65% of its traveling time on the ground. Lemur has also been observed to live in large social groups of approximately 20, containing roughly equal numbers of males and females, which are dominant. The closely related Hapalemur feeds almost exclusively on bamboo, with the three recognized species each specializing on a separate part of the plant. The mongoose lemur (Eulemer mongoz) exhibits variability in its activity pattern, diet, and social organization, with most populations consisting of monogamous family units.
Often grouped with the lemurids, the genus Lepilemur (the sportive lemur) is regarded by certain researchers as distinctive enough to warrant its own family, which also includes the extinct genus Megaladapis. Lepilemur is characterized by its small size and lack of permanent upper incisors, and is signified by one of the lowest basal metabolic rates of any primate. Sportive lemurs are solitary and extremely territorial, and males have often been observed to fight over their small home ranges.
Characterized by long hind limbs used for leaping, the three extant indriid genera share many cranial features, but differ in size and activity patterns. The woolly lemur (Avahi), the only nocturnal indriid, lives in monogamous family units and is known to huddle in groups while feeding. The familiar sifaka ( Propithecus) is a large, diurnal primate known from three species. Although the sifaka is primarily a vertical leaper while in the trees, it can hop bipedally on the ground. The sifaka normally lives in groups of three to nine, which often include multiple breeding females. Males regularly change groups, which results in strong male-male competition during the breeding season. The third indriid genus, the indri (Indri), is marked by long hands and feet, slender arms, and the absence of a tail. This genus is extremely vocal, and congregates in small family groups that defend large territories.
The single genus included in the daubentoniid family, the aye-aye (Daubentonia madagascariensis), is perhaps the most bizarre living primate. This moderate sized (3 kilogram) animal is differentiated by a suite of extreme anatomical characteristics, including a set of ever-growing incisors and a long, slender third digit of the hand. Captive studies have indicated that the aye-aye uses its unique third digit to tap logs, which can reveal the presence of insect prey. If insects are located, the aye-aye will then gnaw at the log with its incisors and use its finger as a probe. The aye-aye is also known to be a non-seasonal breeder.
Galagids and Lorisids
United by a host of cranial features, the galagids and lorisids constitute a limited radiation of mainland prosimians. The African galagids (bushbabies) are represented by four genera, including Otolemur, Galago, Euoticus, and Galagoides. Among the best-studied galagids is the South African lesser bushbaby (Galago moholi), which has been observed to form social hierarchies related to age and weight. The needle-clawed galago (Euoticus elegantulus) is a specialized gum feeder, and has evolved procumbent upper incisors and claw-like nails to assist in food procurement. The lorisids, found in Africa and Asia, are characterized by their slow locomotion. The potto ( Perodicticus) is the most widespread of the African lorisids, and specializes in olfactory foraging. The slender loris (Loris), which inhabits Sri Lanka and southern India, is a small, slim primate, while the slow loris (Nycticebus) is larger and moves by slow, quadrupedal climbing.
Although of debated phylogenetic position, the tarsi-ids constitute the eighth and final living prosimian family. This group includes one genus, Tarsius, which is known from five species, all of which inhabit Southeast Asia. Sharing many soft-tissue characters with anthropoids, tarsiids are small primates characterized by extremely large eyes, which are larger than the brain. The hands and feet of tarsiids are proportionally large, and the living species show several adaptations for leaping, including a fused tibia and fibula and a long ankle region.
Adapoids and Omomyoids
Two diverse groups of extinct primates are also often included in Prosimii. The Adapoidea, which date from the early Eocene (55 million years ago), and became extinct approximately 8 million years ago, are traditionally described as “lemur-like,” although this description has been chided for being too simplistic. Primatologists recognize at least 30 genera and 80 adapoid species, and commonly this group is placed in the Strepsirhini. The Omomyoidea also date back to the Eocene, and include some 40 genera and 90 species. Often described as “tarsier-like,” omomyoids are now classified in the Haplorhini. Based on phylogenetic analyses, it has been deduced that adapoids and omomyoids likely evolved in the Paleocene and, thus, are among the oldest known primates.
Prosimian Evolution and Significance
The question of primate origins has proven controversial, but it is widely accepted that primates are closely related to Scandentia (tree shrews), Chiroptera (bats), Dermoptera (colugos), and a diverse extinct group called Plesiadapiformes. The early fossil record of primates is patchy, but genetic and paleontological evidence indicate that the Strepsirhini-Haplorhini split occurred in the Paleocene (65-57 million years ago), and the anthropoid radiation was well under way by the late Eocene (40 million years ago). While adapoids and omomyoids are well known from fossils, the fossil record of other prosimians is poor. Tarsius eoceanus and Xanthorhysis tabrumi from the middle Eocene of China indicate that tarsiids diverged from other haplorhines before this time, but most other prosimian groups are known from only a handful of fossil specimens. Despite these drawbacks, it has been suggested that, among the strepsirhines, lorisids and galagids form a monophyletic group. Additionally, there is support for a grouping of lemurids, indriids, and lepilemurids. The phylogenetic relationships of cheirogaleids and daubentoniids are still widely debated.
Historically, it was argued that a prosimian (most likely a tarsiid) directly gave rise to the anthropoid line. The modern advent of cladistic analyses has frowned on such a notion, and now sees tarsiids and anthropoids as sister groups constituting the monophyletic Haplorhini. Regardless of the validity of a direct evolutionary connection, anthropologists can learn much by studying prosimians. Although only distantly related to humans, this diverse and successful group of generally small, primitive primates sheds light on the early evolutionary history of the human lineage, and showcases characteristics that link all primates.
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