Three-fingered sloths have arms that are much longer than their legs, but reach is only part of the story. New motion studies reveal how all four limbs divide the work beneath a branch.
Look closely at a three-fingered sloth moving beneath a branch and its body can seem almost stretched towards the next handhold. The arms are visibly longer than the legs, while the curved claws hook around wood like living climbing equipment. That reach helps the animal bridge small gaps without making a risky leap, but it is not the whole explanation.
A 2025 study used four calibrated high-speed cameras to record brown-throated three-fingered sloths walking below a simulated branch. The results showed a carefully coordinated system in which elbows, knees, arms and thighs follow different paths. Combined with earlier force measurements, the research gives us a much clearer picture of how a sloth's long arms work with, rather than instead of, its hindlimbs.
Not every sloth has the same proportions
First, an important distinction: the long-arm pattern is especially pronounced in three-fingered sloths of the genus Bradypus. According to the open-access limb-motion study, their forelimbs are significantly longer than their hindlimbs. Two-fingered sloths in the genus Choloepus, by contrast, have front and rear limbs that are much closer to equal length.
The two groups may look like variations on one design, but they come from separate evolutionary lineages. The Sloth Conservation Foundation's Slothopedia explains that two-fingered and three-fingered sloths evolved their upside-down lifestyles separately. Their shared habit of hanging below branches is therefore a striking example of different lineages arriving at similar solutions, with important differences in the details.
Those details include the hands. All living sloths have three toes on each hind foot. The familiar “two” and “three” labels really describe the number of fingers on the forelimbs. Three-fingered sloths have three partly fused front digits, while two-fingered sloths have two unfused ones.
Long arms create useful reach
Long forelimbs let a three-fingered sloth search a wider area for a secure grip while keeping much of its body supported. In a canopy full of branches that differ in angle, thickness and distance, reach can be more useful than speed. The sloth can extend a forelimb, hook a branch and test the connection before transferring its weight.
This does not mean the arms simply pull the animal forward like a climber doing endless chin-ups. Sloth movement is a four-limbed sequence. Each foot must release, travel and reconnect while the other contact points keep the body stable beneath the support. Curved claws provide the hook, but joints and muscles control where that hook lands and how force travels through the body.
What high-speed cameras revealed
The 2025 research focused on brown-throated three-fingered sloths moving at their preferred pace. The scientists tracked three-dimensional joint positions throughout the part of each step when a limb was supporting the animal.
Both elbows and knees stayed relatively extended during support, although the knees were generally straighter than the elbows. The upper arm also moved farther away from the body than the thigh. As the sloths moved faster, their elbows became more extended. These are modest-looking changes, but together they help place the hands and feet around a branch while the body hangs below it.
When the researchers compared their results with data from Linnaeus's two-fingered sloths, they found that the three-fingered animals used more extended joint positions. The broad movement looks similar to us, yet the two sloth lineages achieve it with different limb proportions and joint strategies.
The hindlimbs do more than follow
Long arms attract attention, but the rear limbs are not passengers. A separate 2023 study of five brown-throated three-fingered sloths measured the forces they applied to an instrumented beam. It found that the hindlimbs produced large braking forces and carried a greater share of vertical body-weight support than the forelimbs.
That sounds counterintuitive only if we imagine movement as pure forward pulling. Beneath a branch, control matters as much as propulsion. The forelimbs can help move the body onwards while the hindlimbs resist unwanted motion, support weight and help steady the animal from side to side. Braking is not wasted effort. It is part of staying secure while suspended.
The study involved a small number of animals moving in a controlled setting, so it should not be treated as the last word on every sloth in every tree. It does, however, show why a simple “long arms do all the work” story misses the most interesting part.
A slow walk is an active balancing act
Sloths are famous for conserving energy, but energy efficiency does not mean doing nothing. Every deliberate step solves several problems at once: finding a reachable support, keeping enough claws attached, controlling the body's swing and sharing forces across four limbs.
Their slow pace allows those adjustments to happen without the momentum used by faster climbers. Instead of jumping across an opening, a sloth usually relies on connected vegetation, careful extension and a secure hook. This is one reason canopy continuity matters. An arm can cross a small natural gap, but it cannot solve a road, a wide clearing or a line of isolated trees.
Built for a connected canopy
The long forelimbs of a three-fingered sloth are best understood as one part of a complete upside-down system. Reach works with curved claws. Extended joints work with specialised muscles. Hindlimb braking works with forelimb propulsion. Slow movement gives the animal time to coordinate it all.
Protecting that system means protecting the environment it evolved to use. Connected native trees give sloths real branches to reach, test and cross. Wildlife bridges can help where people have split the canopy, but preserving and restoring living habitat remains the deeper solution.
So the next time a sloth stretches one remarkably long arm towards a branch, watch the rest of the body too. The quiet work happening at the knees, hips and rear claws is just as important as the reach that first catches your eye.
Sources and image credit
- Mossor and colleagues (2025): three-dimensional limb kinematics in brown-throated three-toed sloths
- McKamy and colleagues (2023): hindlimb braking and support during suspensory locomotion
- The Sloth Conservation Foundation: Slothopedia
Featured image: Ken-ichi Ueda via Wikimedia Commons, CC BY 4.0. View the image and licence details on Wikimedia Commons.
