Many people wonder if certain monkeys lack tails. While most monkeys possess tails, the primates typically imagined as tailless are not monkeys, but rather apes. This distinction between monkeys and apes is fundamental to understanding which primate groups naturally lack tails. This article clarifies these differences, explores the evolutionary reasons for tail loss, and identifies tailless species.
The Fundamental Difference: Monkeys and Apes
Most monkey species have tails, varying greatly in length and function. Monkeys often use their tails for balance when moving through trees, running across branches, or as a prehensile “fifth limb” for grasping in some New World species like spider monkeys. Their body structures are smaller with narrower chests, built for quadrupedal movement across branches.
Apes do not possess external tails. They are larger and more robust than monkeys, with wider chests and shoulder joints that allow for a greater range of arm movement. This anatomical difference supports their varied forms of locomotion, which often involve swinging through trees, known as brachiation, or walking upright on two legs for periods. Apes also have a larger brain-to-body-size ratio compared to monkeys, indicating more complex cognitive abilities.
Evolutionary Reasons for Tail Loss in Apes
The loss of a tail in apes is an evolutionary adaptation that occurred approximately 20 to 25 million years ago, when apes diverged from Old World monkeys. This evolutionary change is linked to shifts in their primary modes of locomotion and habitat use. For primates transitioning from solely arboreal (tree-dwelling) life to more suspensory movement or increased time on the ground, a tail could become a hindrance rather than an aid.
A tailless body structure provided an advantage by simplifying movement for an upright posture. While a tail is beneficial for balance and grasping in arboreal monkeys, it could be an impediment for apes adopting suspensory locomotion, such as brachiation, or for those moving terrestrially. The genetic mechanism behind this tail loss involves a mutation in the TBXT gene, potentially triggered by the insertion of a repetitive DNA sequence called an Alu element. This genetic change led to the formation of fewer tail vertebrae, resulting in the coccyx, or tailbone, found in apes today.
Identifying Tailless Primates
Apes are the group of primates that are naturally tailless. This superfamily, Hominoidea, includes both lesser apes and great apes. Lesser apes comprise various species of gibbons, while great apes include orangutans, gorillas, chimpanzees, bonobos, and humans. None of these species possess an external tail.
While apes are tailless and monkeys have tails, a few monkey species have very short or vestigial tails. The Barbary macaque, for instance, has a tail that is either not visible or is a very small stump hidden by its fur. Similarly, stump-tailed macaques have short, hairless tails measuring only a few centimeters. These macaques are still classified as monkeys because they retain a tail structure, however minimal, and share other key characteristics that differentiate them from apes.