Primates are a diverse order of mammals, encompassing a wide range of species from small, tree-dwelling creatures to large, ground-dwelling apes. This group includes lemurs, monkeys, and humans, all sharing common evolutionary traits such as flexible hands and feet, and forward-facing eyes that provide depth perception.
The Primate Family Tree’s Main Branches
The order Primates is scientifically divided into two primary suborders: Strepsirrhini and Haplorrhini. Strepsirrhini are often referred to as “wet-nosed” primates, while Haplorrhini are known as “dry-nosed” primates, a distinction reflecting a key physical difference between the groups.
Strepsirrhini: The “Wet-Nosed” Primates
The Strepsirrhini suborder includes lemurs, lorises, and galagos, found primarily in Madagascar, Africa, and Southeast Asia. A defining characteristic of strepsirrhines is the presence of a rhinarium, a moist, naked skin surrounding the nostrils, which enhances their sense of smell. This reliance on olfaction is also reflected in their relatively large olfactory lobes in the brain. Most strepsirrhines are nocturnal, possessing large eyes with a reflective layer called a tapetum lucidum to improve night vision.
Strepsirrhines also exhibit several specialized dental and grooming features. Many have a tooth comb, a unique arrangement of lower incisors and canines that project forward, primarily used for grooming fur. They typically have a grooming claw on the second digit of their foot. Their brains are generally smaller in proportion to their body size compared to haplorrhines.
Haplorrhini: The “Dry-Nosed” Primates
The Haplorrhini suborder encompasses tarsiers, monkeys (both Old World and New World), apes, and humans. Unlike strepsirrhines, haplorrhines lack a rhinarium. This group shows a reduced reliance on the sense of smell and a heightened dependence on vision, including advanced color vision. Their eyes are typically forward-facing, and they possess a postorbital plate that fully encloses the back of the eye socket.
Haplorrhines generally have larger brains relative to their body size compared to strepsirrhines. They do not possess a tooth comb or a grooming claw, although tarsiers are an exception, retaining a grooming claw on some digits. Most haplorrhine species are diurnal, meaning they are active during the day. Examples include the diverse range of New World monkeys like howler and spider monkeys, Old World monkeys such as baboons and macaques, and all apes, including chimpanzees, gorillas, and humans.
Key Differences Between the Suborders
The fundamental distinction between Strepsirrhini and Haplorrhini lies in their sensory adaptations and associated physical traits. Strepsirrhines are characterized by their “wet nose” (rhinarium), which signifies a reliance on olfaction, while haplorrhines have a “dry nose” and prioritize vision. This manifests in strepsirrhines having a longer snout and larger olfactory centers in their brains, contrasting with the flattened faces and forward-facing eyes of haplorrhines that enhance visual acuity.
Difference is the presence of specialized grooming tools. Strepsirrhines typically possess a tooth comb and a grooming claw, features absent in most haplorrhines. Furthermore, strepsirrhines generally have smaller brain-to-body ratios than haplorrhines. While many strepsirrhines are nocturnal, with adaptations for low-light vision, most haplorrhines are diurnal. These distinctions highlight the divergent evolutionary paths that have shaped the two major suborders of primates.