Monkeys, members of the infraorder Simiiformes, are a highly successful and diverse group of primates inhabiting forests and savannas across Africa, Asia, and the Americas. They exhibit a remarkable range of adaptations, from complex social hierarchies to specialized physical traits, allowing them to thrive in varied environments. Their agility, intelligence, and varied forms—from the tiny pygmy marmoset to the large mandrill—offer a window into the evolution of social behavior and cognition.
What Defines a Monkey?
Monkeys are defined by physical characteristics that distinguish them from their primate relatives, the apes (hominoids) and prosimians. The most recognized difference is the presence of a tail, which nearly all monkeys possess for balance or grasping, whereas apes lack tails entirely. Monkeys are generally smaller than apes and typically move on all four limbs, a form of locomotion known as quadrupedalism, whether on branches or the ground.
Apes, such as chimpanzees and gorillas, tend to have a broader chest, longer arms, and a more complex brain structure relative to their body size. Monkeys are intelligent but do not possess the same degree of complex brain development seen in apes. The term “monkey” is a broad grouping that excludes the tailless apes and highlights their separate evolutionary lineage within the primates.
New World Versus Old World Monkeys
The infraorder of monkeys is divided geographically and taxonomically into two distinct groups that evolved separately for millions of years. This division is based on their continents of origin: the New World (Central and South America) and the Old World (Africa and Asia). Each group developed unique physical traits tailored to their environments, particularly in their nasal structures and tail utility.
New World monkeys, or Platyrrhines, are identified by their flatter faces and nostrils that face outward and are set wide apart. Many New World species, including spider monkeys and howler monkeys, possess a prehensile tail that functions as a fifth limb, capable of grasping branches and supporting their body weight. They also possess a different dental formula, typically having 36 teeth, which includes an extra premolar in each jaw quadrant compared to Old World monkeys.
Old World monkeys, or Catarrhines, have narrow noses with downward-facing nostrils that are close together, a feature shared with apes and humans. Their tails are never prehensile; they are used for balance or signaling but cannot grip or hang. Many Old World species, such as baboons and macaques, possess ischial callosities—thickened, hairless pads of skin on their buttocks—which provide a cushion for sitting comfortably on rough surfaces for extended periods.
Complex Social Structures
Monkey species organize themselves into sophisticated social structures that dictate access to resources, mates, and protection. Most monkeys live in multi-male, multi-female groups, or troops. These groups range in size from small family units, like marmosets and tamarins, to large, multi-generational communities of dozens or hundreds of individuals, as seen in baboons and macaques. Groups are often characterized by dominance hierarchies, where individuals have a distinct rank based on age, strength, or the social rank of their mother.
The maintenance of social bonds and the reduction of conflict rely heavily on allogrooming, or social grooming. This behavior is a primary tool for reinforcing alliances, establishing trust, and calming tension between individuals, not just hygiene. In many Old World monkey groups, females are the “philopatric” sex, meaning they remain in their birth group for life, creating strong, stable maternal lineages. Group cohesion is also important for defense, as monkeys rely on coordinated alarm calls to signal danger and group mobilization to ward off predators or rival troops.
Intelligence, Communication, and Tool Use
Monkeys exhibit remarkable cognitive abilities demonstrated through observational learning and problem-solving. New World capuchin monkeys are celebrated tool users, employing stones as hammers and anvils to crack open nuts with precision. This behavior requires considerable skill and force control. Other examples include long-tailed macaques in Thailand, who use human hair or thin fibers as makeshift dental floss to remove food particles from their teeth.
Communication among monkeys extends beyond simple vocalizations, demonstrating a capacity for referential communication. Vervet monkeys, for example, produce distinct alarm calls specific to different types of predators, such as eagles, leopards, and snakes. The troop reacts differently to each call, showing they understand the specific meaning of the sound, not just a general sense of alarm. This capacity for complex communication and learned behaviors underscores the cognitive flexibility that allows monkeys to adapt and thrive in diverse ecological niches.