Traditionally, only a single night monkey species (Aotus trivirgatus) was recognized, but chromosomal evidence revealed considerable diversity. This is hardly surprising, given the extensive geographical range covered by Aotus. It is now widely accepted that there are between five and nine night monkey species, and eight species can be recognized as a basic minimum. Night monkeys provide a graphic example of a persistent failure to recognize cryptic species among nocturnal primates, because they differ relatively little in visually obvious characters. Molecular evidence indicates that some individual night monkey species diverged at a very early stage and also suggests that there is no close relationship between the night monkey lineage and any other group of New World monkeys. It is hence appropriate to recognize a separate family Aotidae for the night monkeys, rather than just a subfamily (Aotinae). It has been customary to include the titi monkeys (genus Callicebus) with the night monkeys in the subfamily Aotinae, but molecular evidence does not indicate any phylo-genetic association between Aotus and Callicebus, so there is no justification for classifying these two genera together.
The night monkeys can be divided into a gray-neck group of four species occurring essentially north of the River Amazon (Aotus hershkovitzi, Aotus lemurinus, Aotus trivirgatus and Aotus vociferans) and a red-necked group of four species occurring almost exclusively south of the Amazon (Aotus azarai, Aotus miconax, Aotus nancymaae and Aotus nigriceps).
As is the case for New World monkeys, generally, there is very little fossil evidence to document the evolution of night monkeys. However, some fragmentary remains from the early Miocene of the La Venta site in Colombia have been allocated to a species in the modern genus Aotus (Aotus dindinen-sis). The lower jaw and teeth closely resemble those of the modern night monkey and a fragment of the skull indicates that large eyes were present, suggesting that nocturnal habits were already present as in living species.
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