A sloth's hand ends in long, curved claws, but one of the most surprising clues to its history is hidden farther back in the wrist. A small bone called the pisiform is unusually short and rounded in both living sloth lineages.
That might sound like a tiny anatomical footnote. It is actually a useful lesson in how evolution works. A 2021 study found that two-fingered and three-fingered sloths reach this unusual wrist shape through a distinctive growth pattern. The same feature also appears in fossil sloths, which means it may be much older than the upside-down lifestyle of the species living in trees today.
First, what is a pisiform?
The pisiform is a small bone on the outer, palm-side part of the wrist. In many mammals it is elongated and develops from two centres of bone growth, with a growth plate between them. The sloth version is different. It is compact, rounded and reduced in length.
Researchers compared two-fingered sloths, three-fingered sloths and giant anteaters using photographs, X-rays and micro-CT scans. Giant anteaters retain the longer, rod-like form common in many mammals. Both living groups of sloths have the short version, even though those two groups are not each other's closest lookalike in every part of their anatomy.
A wrist bone that grows differently
The study, available through the Europe PMC research record for PMID 34779120, found that the reduced sloth pisiform appears to develop through the loss of a secondary ossification centre. In plain English, one of the usual bone-forming centres does not appear, and the associated growth plate is probably disrupted. That limits the bone's length as the animal develops.
This is not simply the same route used by every mammal with a short pisiform. Humans and orangutans have reduced versions too, but previous work indicates that they arrived there through different developmental mechanisms. Similar-looking anatomy can therefore have different construction histories.
The result is especially interesting because the two living sloth lineages show remarkably similar patterns. The authors also noted reduced pisiforms in fossil sloths. Their cautious conclusion was that the shape probably represents an ancestral sloth trait that arose early in sloth evolution, rather than a recent feature that evolved independently when today's tree sloths adopted suspensory movement.
Does the little bone explain the famous grip?
Not by itself. Sloth hands and feet work as complete systems, involving bones, joints, muscles, tendons, claws and careful coordination. It would be tempting to point at one unusual wrist bone and call it the secret of hanging, but the pisiform study does not show that the short bone alone creates a stronger grip.
What researchers can say is that the hands and feet of living sloths are highly specialised for keeping a firm connection to branches. A videoradiography study of Linnaeus's two-fingered sloths, summarised in the Europe PMC record for PMID 20637572, found that their hands and feet maintain contact with supports rather than acting as major propulsive levers. Step length and body progression come from the movement of the limbs while the hooked extremities keep the animal securely attached.
The Smithsonian National Zoo's two-toed sloth guide adds the visible part of the picture: long limbs ending in large, curved claws, deliberate movement and a life spent mostly in the rainforest canopy. The hidden wrist bone sits inside that much larger package of adaptations.
Why old anatomy can outlive an old lifestyle
Evolution does not begin each species from a blank sheet. It modifies bodies inherited from earlier ancestors. A feature can originate in one ecological setting, persist through major changes in size or movement, and later become part of a new combination of traits.
That is why the fossil evidence matters. Sloth history includes small tree dwellers, huge terrestrial species and many forms between those extremes. If the reduced pisiform appeared early across this broad family, it cannot be explained only as a tool for modern upside-down locomotion. It is part of the deeper sloth blueprint, even if its exact function may have changed as different sloths evolved.
Small bones are valuable to evolutionary biologists because development leaves evidence in their shape and internal surfaces. By comparing living species, fossils and growth patterns, researchers can distinguish an ancient inheritance from a newer adaptation. In this case, a wrist bone only a specialist might notice helps connect today's canopy sloths with a far larger lost family.
The branch still matters
Anatomy can make a sloth wonderfully secure on a branch, but it cannot replace the branch itself. Long claws and specialised limbs help only when trees form a connected route through the forest. A gap caused by clearing, roads or development can force a sloth toward the ground, where its slow, suspensory body is far less protected.
The little pisiform therefore carries two stories. One is about deep evolutionary history and the unusual way a sloth wrist develops. The other is about the living animal around it, still dependent on a continuous canopy where careful movement, firm contact and patience make sense.
Sources and image credit
- Gavazzi, Kjosness and Reno: ossification of the sloth pisiform, Europe PMC record
- Nyakatura, Petrovitch and Fischer: two-toed sloth limb kinematics, Europe PMC record
- Smithsonian National Zoo: two-toed sloth fact sheet
Featured image: Warren Garst / Colorado State University Libraries, via Wikimedia Commons, licensed under CC BY-SA 4.0.
