What Monkeys Don’t Have Tails? The Truth About Tailless Primates

The primates best known for having absolutely no tail are apes, not monkeys, and the mix-up between those two groups is the root of this question for most people. Gibbons, orangutans, gorillas, chimpanzees, bonobos, and humans are all completely tailless, but none of them are monkeys. Among actual monkeys, though, a handful of species have tails so short they are practically invisible, with the Barbary macaque being the most famous example. The reality is messier and more interesting than a simple list, because tail length across the primate order is a spectrum shaped by genetics, locomotion, and climate.

Apes Are Not Monkeys, But That Is the Whole Problem

When most people picture a “monkey without a tail,” they are thinking of a chimpanzee or a gorilla. Both lack tails entirely. But biologically, chimpanzees and gorillas are apes, a separate branch of the primate family tree. The apes, formally called hominoids, include the so-called great apes (orangutans, gorillas, chimpanzees, and bonobos) plus the lesser apes (gibbons and siamangs) and humans. Every single one of these species is tailless. That uniform taillessness is one of the defining physical traits of the group.

Monkeys, by contrast, almost always have tails. The two main groups of monkeys, the Old World monkeys of Africa and Asia and the New World monkeys of Central and South America, diverged tens of millions of years ago, yet both retained tails. So the short answer to “what monkeys don’t have tails” is: almost none. But “almost” is doing real work in that sentence, because a few monkey species have tails that are reduced to short stubs, and understanding why takes you into some genuinely fascinating biology.

Monkeys with Tails You Would Never Notice

The Barbary macaque of North Africa and Gibraltar is the monkey that most often gets called “tailless.” It does technically have a tail, but it is a vestigial stub of roughly one to two centimeters, invisible under fur. If you see Barbary macaques at the Rock of Gibraltar, they look completely tailless to the naked eye. This is likely why many older texts refer to them as “Barbary apes,” even though they are genuine macaques and firmly belong in the monkey family.

The stump-tailed macaque of Southeast Asia is another strong contender. Its tail is short enough that “stump-tailed” became the common name. Mandrills and drills, the massive, colorful monkeys of West African forests, also sport tails that are surprisingly stubby relative to their body size. And the pig-tailed langur of the Mentawai Islands near Sumatra has a tail so short it earned that pig comparison. None of these species are completely tailless the way an ape is, but they have clearly moved down the spectrum toward tail reduction.

What these short-tailed monkeys tend to share is a lifestyle that involves a good deal of time on the ground or on large, stable branches rather than leaping through thin, flexible canopy. That pattern holds broadly across primates: species that spend more time walking on the ground or moving along stout limbs have less functional need for a long tail and, over evolutionary time, have sometimes lost much of it.

Why Apes Lost Their Tails Entirely

The complete loss of the tail in apes appears to trace back to a single genetic event in the ancestor shared by all hominoids, roughly 25 million years ago. Researchers identified an insertion of a short DNA element, called an Alu element, into an intron of the TBXT gene, which is one of the key genes controlling tail development across vertebrates. This inserted element pairs with a neighboring Alu element that sits in the reverse orientation, and together they cause a unique splicing event that shortens the protein the gene produces. When this altered version of TBXT was introduced into mice, the animals developed a range of tail defects, from shortened tails to complete tail absence, closely mirroring the tailless condition seen in apes.¹

The finding is remarkable partly because it suggests a single mutation, rather than a gradual accumulation of many changes, may have been the primary trigger for one of the most visually obvious differences between apes and monkeys. The same Alu insertion is found in every living ape species but in no monkey species, which fits the timeline of it appearing once in the common ancestor and being inherited by all descendants.

There is a trade-off worth noting. In the mouse experiments, the altered TBXT gene also slightly increased the rate of neural tube defects, a category of birth defects that still affects roughly one in a thousand human pregnancies. The researchers speculated that whatever advantage taillessness conferred, likely related to a shift toward a more upright posture and suspensory locomotion, was strong enough to outweigh this cost.¹

What Tails Actually Do for the Primates That Keep Them

If losing a tail can come with a cost, why have most primates kept theirs? The answer comes down to balance, and the effect is more dramatic than you might expect. Research on primates walking along narrow branches shows that long tails function as dynamic counterweights. When a primate begins to tip to one side, it sweeps its tail rapidly in the direction of the imbalance, generating angular momentum that pushes the body back toward center. This “tail-whip” mechanism was observed across multiple species, but it was used most heavily by larger, more terrestrial primates who were less naturally adept at branch-walking, suggesting the tail becomes especially critical when an animal’s body size or habits push it toward instability.²

Laboratory studies on squirrel monkeys confirm this picture. When these monkeys were tested on narrower and more unstable supports, they significantly increased the magnitude and amplitude of their tail movements, treating the tail almost like a tightrope walker’s pole. Wild platyrrhine monkeys observed in their natural habitat did the same thing, extending and exaggerating tail displacements on narrow and swaying branches. The researchers concluded that long, mobile tails should be considered part of the fundamental toolkit for safe arboreal movement.³

Not all tailed primates use their tails the same way, though. Comparisons between squirrel monkeys and tamarins showed distinct strategies. Tamarins have relatively longer tails and move them through larger arcs at higher speeds, using the tail dynamically to regulate momentum on the fly. Squirrel monkeys, with shorter tails relative to body size, tend to hold the tail in a more fixed, depressed position, using it as a passive counterbalance rather than an active stabilizer.⁴ The point is that even among monkeys that all have tails, the tail can serve quite different mechanical roles depending on its proportions and the animal’s habits.

Prehensile Tails and the New World Monkey Advantage

Some New World monkeys took tail evolution in a completely different direction: they turned the tail into a fifth limb. Spider monkeys, howler monkeys, woolly monkeys, and capuchins all have prehensile tails that can grip branches and support the animal’s full body weight. The underside of a spider monkey’s tail tip is even bare of fur and covered in skin ridges, like a fingerprint, for better grip.

This ability is not just a matter of behavior; it required significant changes in tail anatomy. Prehensile-tailed monkeys, especially the atelines (spider monkeys and their close relatives), have unusually well-developed flexor and intertransversarii muscles running along the tail, with long tendons that give fine control over curling and gripping motions. Spider monkeys in particular have a bulkier set of muscles on the inner side of the tail and a more forward origin point for certain muscle groups compared to other prehensile-tailed species, reflecting their extreme reliance on tail suspension.⁵

Prehensile tails evolved only in the New World monkey lineage. No Old World monkey or ape has one, and the trait appears to have evolved independently more than once within the New World group. Across primates generally, tail length tends to evolve in functional coordination with limb proportions, presumably to assist body balance, with prehensile tails and specialized foraging strategies representing the major exceptions to that pattern.⁶ A prehensile tail is no longer just a balancing rod; it has been repurposed into a grasping tool, which relaxes the usual constraints on how long a tail “should” be relative to body and limb size.

Climate and Altitude Shape Tail Length Too

Genetics and locomotion are not the only forces acting on primate tails. Temperature matters. Allen’s rule, a broad pattern in biology, predicts that animals in colder environments tend to have shorter extremities, including tails, because shorter appendages lose less body heat. This has been tested directly in toque macaques across different elevations in Sri Lanka. Populations living at higher elevations, where temperatures are lower, had significantly shorter tails relative to body size than populations at lower, warmer elevations. Elevation was the single best predictor of relative tail length, and the differences between subspecies were linked in part to their altitudinal and ecological separation.⁷

This finding is a good reminder that when you see a monkey species with a noticeably short tail, the explanation is not always about locomotion or an evolutionary march toward ape-like taillessness. Sometimes a short tail is simply the thermoregulatory optimum for a cold habitat. The Tibetan macaque, one of the most cold-adapted primates, has a conspicuously short tail, and while multiple factors probably contribute, the thermal logic fits neatly.

Every Human Briefly Has a Tail

Humans, as apes, are born without tails. But every human embryo grows one during early development. Detailed imaging of human embryos shows that the tail lengthens and narrows through the first several weeks of development, becoming a sharply pointed structure by roughly the sixth week. After that peak, the tail begins to shorten and its tip becomes increasingly translucent. By about the eighth week, only a short, curved stump remains, and the tail disappears completely soon after, its vertebrae fusing into the coccyx, the small triangular bone at the base of your spine.⁸

On very rare occasions, a baby is born with a soft, boneless tail-like appendage. These “human tails” are not true tails in the way a monkey has one; they lack vertebrae and voluntary muscle. They are thought to result from incomplete regression of the embryonic tail and are typically removed surgically without complications. Their existence, though, is a vivid reminder that the genetic instructions for building a tail have not been deleted from the human genome. They have been switched off by the altered splicing of the TBXT gene, but the underlying developmental program is still there, and occasionally it runs a little longer than it should.

Other Primates with Reduced Tails

Apes get the most attention for taillessness, and Barbary macaques get the runner-up spotlight among monkeys, but there is another group of primates with dramatically reduced tails that often goes unmentioned: the lorises and pottos. These are strepsirrhine primates, the broader group that also includes lemurs, and they live in the tropical forests of Africa and Asia. Unlike their close relatives the galagos (bushbabies), which have long, heavy tails suited to their leaping lifestyle, pottos and lorises have markedly reduced tails and roughly equal fore- and hindlimb proportions.⁹

Lorises and pottos are extremely slow, deliberate climbers. They creep along branches with a vise-like grip, rarely leaping or making quick movements. A long tail would serve no balancing function for an animal that never moves fast enough to lose its balance, and it would be wasted weight to carry around. Their tail reduction appears to be an independent evolutionary event from what happened in apes, arrived at for different functional reasons but following the same underlying logic: when a tail stops paying for itself biomechanically, natural selection allows it to shrink.

The Fossil Record and Timing

Pinning down exactly when tail loss occurred in the ape lineage has been a long-running puzzle in paleontology. The genetic evidence points to around 25 million years ago, but fossil evidence is harder to come by, because tails are made of small, delicate vertebrae that rarely preserve well. One key specimen is Proconsul, an early ape from the Miocene epoch, roughly 18 to 20 million years ago. Studies of Proconsul’s skeleton have been interpreted as evidence that it lacked a tail, making it the earliest ape in the fossil record for which taillessness can be inferred. If that interpretation holds, it means apes had already lost their tails by the time Proconsul was alive, consistent with the genetic dating of the Alu insertion event several million years earlier.

The gap between the estimated genetic event and the first fossil evidence is not unusual for this kind of evolutionary question. Soft-tissue traits rarely leave direct fossil traces, and paleontologists often have to infer tail presence or absence from the shape of the sacrum and the number of caudal vertebrae preserved, both of which can be ambiguous in fragmentary fossils. The convergence of genetic and fossil evidence on a Miocene-era tail loss is about as strong a case as this type of evolutionary reconstruction gets.

Why the Language Confusion Persists

In everyday English, “monkey” is used as a catch-all term for any non-human primate, and no amount of zoological correction seems to change that. Children’s books show chimpanzees labeled as monkeys. News stories about great apes regularly use “monkey” in the headline. The phrase “monkeying around” applies to humans imitating ape behavior. This linguistic slippage is why the question “what monkeys don’t have tails” gets asked so often: people are genuinely trying to make sense of a group of animals they have been taught to call monkeys but that clearly look different from the long-tailed capuchins and macaques they also see.

The distinction matters for understanding primate biology, but it is worth being generous about why people get it wrong. In many languages, there is no separate word for “ape” at all. German uses “Affe” for both monkeys and apes. Japanese uses “saru” broadly. The English distinction between “monkey” and “ape” is somewhat unusual in giving separate common names to groups that are, after all, closely related branches of the same family tree. If you remember one thing from this topic, it is that all the completely tailless primates are apes, and apes are not monkeys, even though your brain has been trained since childhood to call them that.