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Prosimians are a group of that includes all living and extinct Strepsirrhini (, Lorisoidea, and Adapiformes), as well as the Haplorhini and their extinct relatives, the Omomyidae, i.e. all primates excluding the simians. They are considered to have characteristics that are more "primitive" (ancestral or plesiomorphic) than those of (monkeys, apes, and humans). (2025). 9780202306582, Transaction Publishers. ISBN 9780202306582
emerged within the Prosimians as sister group of the Haplorhini , and therefore Cladistics 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.
Prosimians are the only primates native to Madagascar, but are also found throughout Africa and in Asia.
Related to their frequently nocturnal lifestyle, prosimians lack the colour vision of higher primates. Like most Placentalia, they are in effect red–green colour blind. This allows for more in the retina, which may enhance vision under low-light conditions. (1985). 9780306420658, Plenum Press. ISBN 9780306420658 Except in tarsiers, 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 , used for grooming. These are found on the second toe in lemurs and lorises, and the second and third in tarsiers. have functional 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 none of them have a toilet claw.
Male strepsirrhine prosimians have relatively large Baculum. Male tarsiers do not have baculum. 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 primates.
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 those 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.
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