Prosimians are a type of primate that include lemurs, lorises, bushbabies, and tarsiers, but not monkeys, apes, or humans (simians). They are considered to have characteristics that are more "primitive" than those of monkeys and apes. Prosimians are the only primates native to Madagascar, but are also found throughout Africa and in Asia. With the exception of the tarsiers, all extant prosimians are in the suborder Strepsirrhini. Because tarsiers, as well as some extinct prosimians, share a more recent common ancestor with monkeys and apes than with other prosimians, the prosimian are a paraphyletic group and not a clade.
The adapiforms are an extinct grouping that were both prosimians and strepsirrhines. The omomyiforms are another extinct group of prosimians, but they are believed to be haplorhines, closely related to the tarsiers, but an outgroup to the rest of the haplorhines.
Being an evolutionary grade rather than a clade, the prosimians are united by being primates with traits otherwise found in non-primate mammals. Most prosimians are nocturnal. Their diet typically are less dominated by fruit than that of the simians, and many are active arboreal predators, hunting for insects and other small animals in the trees.
Being nocturnal, the prosimians lack the colour vision of the higher primates. Like most placental mammals, they are in effect red–green colour blind. This allows for more rod cells in the retina, which may increase vision under low-light condition. The nocturnal vision is further augmented by a reflective tapetum lucidum behind the retina, similar to that found in other nocturnal mammals. This layer reflects the light that passes through the retina, increasing the photoreceptors exposure to the light. It is however not well developed in diurnal forms like many lemurs.
All prosimians possess two laterally flattened toilet claws, used for grooming. These can be found on the second toe in lemurs and lorises, and the second and third in tarsiers. Aye-ayes have functional claws on all other digits except the hallux, including a toilet claw on the second toe. Clawlike nails are however also found in the small-bodied callitrichids, a group of New World monkeys, though non of them have a toilet claw.
The prosimians have retained the primitive mammalian condition of a bicornuate uterus, with two separate uterus chambers. In the simians, the uterus chambers have fused, an otherwise rare condition among mammals. Prosimians usually have litters rather than single offspring, which is the norm in higher pimates. They also have multiple sets of nipples, at least two pairs.
While primates are often thought of as fairly intelligent animals, the prosimians are not very large brained compared to other placental mammals. Their brain-cases are markedly smaller than that of simians of comparable sizes. In the large-eyed tarsiers, the weight of the brain is about the same as that of a single eye. Prosimians generally show lower cognitive ability and live in simpler social settings than the simians. The prosimians with the most complex social systems are the diurnal lemurs, which may live in social groups of 20 individuals. The nocturnal prosimians are mainly solitary.
|Prosimians (in green brackets) are a paraphyletic group by including the tarsiers and omomyiforms to the exclusion of the simians (in red brackets).|
The prosimians were once a group considered a suborder of the primate order (suborder Prosimii - Gr. pro, before, + Latin simius/simia, ape), which was named in 1811 by Johann Karl Wilhelm Illiger. They have been shown, however, to be paraphyletic - that is, their most recent common ancestor was a prosimian but it has some non-prosimian descendents (i.e. monkeys and apes). This relationship is shown by the ranks (prosimians in bold) in the list below of the current primate classification between the order and infraorder level. The term "prosimian" is considered taxonomically obsolete, although it is used to emphasize similarities between strepsirrhines, tarsiers, and the early primates.
- Order Primates
- Suborder Strepsirrhini: non-tarsier prosimians
- Suborder Haplorrhini: tarsiers, monkeys and apes
See also 
|Wikimedia Commons has media related to: Prosimians|
- Whitten, P. L.; Brockman, D. K. (2001). "Chapter 14: Strepsirrhine reproductive ecology". In Ellison, P. T. Reproductive Ecology and Human Evolution. Transaction Publishers. pp. 321–350. ISBN 978-0-202-30658-2.
- Ali, Mohamed Ather; Klyne, M.A. (1985). Vision in Vertebrates. New York: Plenum Press. pp. 174–175. ISBN 0-306-42065-1.
- Pariente, GF (1976). "[Different aspects of the limit of the tapetum lucidum in prosimians].". Vision research 16 (4): 387–91. PMID 821249.
- Soligo, C., Müller, A.E. (1999). "Nails and claws in primate evolution". Journal of Human Evolution 36 (1): 97–114. doi:10.1006/jhev.1998.0263. PMID 9924135.
- Mittermeier, Ronald M. Nowak ; introduction by Russell A.; Rylands,, Anthony B.; Konstant, William R. (1999). Walker's primates of the world. Baltimore: Johns Hopkins University Press. p. 25. ISBN 0801862515.
- Rosenberger, Alfred L. (16 October 2010). "The Skull of Tarsius: Functional Morphology, Eyeballs, and the Nonpursuit Predatory Lifestyle". International Journal of Primatology 31 (6): 1032–1054. doi:10.1007/s10764-010-9447-x.
- Reader, S. M.; Hager, Y.; Laland, K. N. (2011-04-12). "The evolution of primate general and cultural intelligence". Philosophical Transactions of the Royal Society B: Biological Sciences 366 (1567): 1017–1027. doi:10.1098/rstb.2010.0342. Retrieved 2011-07-04.
- Rose, K. D. (2006). The Beginning of the Age of Mammals. Johns Hopkins University Press. ISBN 978-0-8018-8472-6.