The clade Strepsirrhini is one of the two suborders of primates. One of the most distinguishing characteristic of these 118 species is their wet noses, and it is this feature for which the grouping is named. The Greek name means having a curved or bent nose (a terpsimbrotos compound of strepho "bend" and rhis "nose"). Madagascar's only primates (apart from humans) are strepsirrhines, although others can be found in southeast Asia and Africa.
Strepsirrhines are considered to have more primitive features and adaptations than their haplorrhine ("dry-nose", in Greek "simple nose") cousins. Their moist nose is connected to the upper lip, which is connected to the gum, giving them a limit to the facial expressions they can manage. Their brain to body ratio tends to be smaller, indicating a lower intelligence. Their brain's olfactory lobes are larger, lending to the notion that they have a stronger reliance on smell. Their snouts are generally elongated giving them a dog-like appearance, although this is true of some monkeys, too. Strepsirrhines also have a post-orbital bar, adding to the primitive nature when compared to the suborder Haplorrhini. The strepsirrhines have also retained the ability to enzymatically manufacture vitamin C, which has been lost by all the haplorrhines, including the Tarsiidae.
With the exception of the Aye-aye, all strepsirrhines have a toothcomb—tightly clustered incisors and canine teeth—that is used for grooming. Another grooming adaptation is a claw on the second toe of all strepsirrhines, while the big toe is widely separated from the others allowing a vise-like grip for locomotion.
About 75% of species are nocturnal and all of these have a tapetum, a shiny, reflective layer in the back of their eyes, although several diurnal species like the Ring-tailed Lemur have it as well. Many of the nocturnal species also have very sensitive hearing and ears they can move independently to capture sounds even better.
Strepsirrhine reproduction differs greatly from haplorrhine reproduction. Instead of an individual cycle, strepsirrhines have a breeding season. They also have a litter of offspring and the females have a Y-shaped (bicornate) uterus and multiple sets of nipples.
Early classification schemes broke the Primate order into the suborders Prosimii (prosimians) and Anthropoidea (simians - monkeys and apes). However the prosimian tarsiers have been shown to be more closely related to the simians, and so it has been moved into the Anthropoidea, which is now renamed as Haplorrhini and Prosimii renamed as Strepsirrhini. Other classifications split Strepsirrhini directly into four superfamilies: Daubentonioidea, Lemuroidea, Loroidea (including Cheirogaleidae) and Indroidea. However, significant evidence suggests that Cheirogaleidae is not related to the lorises, and that Indridae is sister-group to Lemuridae.
If the Aye-aye represents a group that is ancestral to all the rest of Strepsirrhini, then it evolved away from the strepsirrhine line between 63 million years ago (mya) (when the strepsirrhines split from the primitive primate line) and 50 mya (the lemur/loris split). If Chiromyiformes is to be considered as the sister only to the lemurs, then it must have evolved after the lemur/loris split 50 mya.
The adapids are an extinct polyphyletic grouping that were most certainly prosimians and closely related to the strepsirrhines. The omomyids are another extinct group of prosmians but they are believed to be haplorrhines, closely related to the tarsiers, but an outgroup to the rest of the haplorrhines.