The mammalian order Primates includes roughly 500 species, most of which are found in the tropics. As a group, primates are characterized by a constellation of traits that reflect adaptations to arboreal habitats, dietary flexibility, and extensive parental care. Not every trait is present in every species, and when they are present, most traits are expressed to varying degrees in different groups. Nevertheless, all primates possess most or all of the following:

The order is divided into two suborders, the Strepsirrhini and the Haplorrhini (Figure 1.1). The strepsirrhines include the more “primitive” species,meaning those that appear to resemblemost closely the earliest primate ancestors in body plan and, presumably, behavior. Strepsirrhines have a well-developed sense of smell, eyes that are not completely forward facing and that lack a protective bony socket, and a characteristic form of locomotion called “vertical clinging and leaping.” A large percentage of strepsirrhines are small and nocturnal and do not form permanent social groups. Like many other mammals, most strepsirrhines are seasonal breeders, with females remaining sexually receptive for narrowly limited periods of time each year.

The haplorrhines include the monkeys, the apes, and humans. With the exception of the several owl monkey species (genus Aotus) in Central and South America, haplorrhines are diurnal. They rely greatly on sight, which is reflected in several morphological traits: Their eyes are more forward facing, enhancing depth perception (stereopsis), and are housed in bony sockets. Additionally, their retinas are more sensitive, and they have a comparatively large area of brain cortex associated with visual processing.