Monkeys are primates (excluding apes and humans) with anatomy adapted for an arboreal lifestyle. The number of fingers a monkey has highlights the shared evolutionary history between these animals and people. Their limbs are highly adapted for movement and manipulation in complex forest environments. Understanding the structure of their digits provides insight into how they move and handle food.
The Standard Count: Five Digits Per Limb
Most monkeys have five digits on each of their four limbs. This results in a total of 20 fingers and toes across their two hands and two feet, a count shared with nearly all primates, including humans. These digits are composed of bones called phalanges.
In the standard primate structure, the fingers and smaller toes generally contain three phalanges each. The thumb and the big toe are exceptions, typically possessing only two phalanges. This five-digit pattern, known as pentadactyly, is a feature retained from ancient mammalian ancestors and is conserved across the primate order.
Defining the Monkey Hand and Foot
The primary function of a monkey’s digits is prehension, the ability to grasp and manipulate objects or surfaces. This is possible due to the presence of an opposable digit. In most monkeys, the hands possess an opposable thumb, or pollex, which facilitates fine motor control for tasks like picking fruit or grooming.
A key distinction from human anatomy is that a monkey’s feet are often as capable of grasping as its hands. The big toe, or hallux, is typically opposable, allowing the foot to function like a second pair of hands for climbing and securing a grip on branches. This dual grasping capability in all four limbs means monkeys are often described as being quadrupedal, moving on all fours, even in the trees.
Specialized Digits in Old World and New World Monkeys
While the five-digit count is the rule, certain species exhibit specialized adaptations that modify this standard structure. Some New World monkeys, particularly the Spider Monkey genus (Ateles), have a significantly reduced or absent thumb. This adaptation results in a hand that acts like a hook, which is efficient for brachiation, or swinging, through the forest canopy.
The genus name Ateles derives from a Greek word meaning “imperfect,” referencing this lack of a functional thumb. This specialization means some spider monkeys effectively have only four functional fingers on their hands.
Additionally, the smallest New World monkeys, such as marmosets and tamarins, possess sharp, down-turning claws on all digits except for their big toes. These claw-like nails aid them in clinging to tree bark and accessing tree sap, a significant part of their diet.