Across parts of South America, enormous tunnels run through rock and compacted sediment. Their walls carry long grooves that look like claw marks, and some passages open into chambers large enough for several people to stand inside. These are palaeoburrows, trace fossils left by animals that lived during the Ice Age. One of the leading suspects is wonderfully unexpected: the giant ground sloth.
Modern sloths are small, tree-dwelling specialists. Their extinct relatives were far more varied. Some walked on the ground, some were much larger than a person, and several had powerful forelimbs tipped with formidable claws. Researchers think those claws may have helped excavate some of the largest animal-made tunnels ever identified.
What exactly is a palaeoburrow?
A palaeoburrow is an ancient animal shelter preserved in the ground. Unlike an ordinary cave formed mainly by flowing water or the slow dissolution of rock, a palaeoburrow shows evidence of deliberate excavation. Shape, wall texture and scratch patterns can all help distinguish one from a natural cavity.
Brazil has become a centre for this research. Pesquisa FAPESP reported that researchers had documented more than 1,500 palaeoburrows in southern and southeastern Brazil. In northern Minas Gerais, tunnels in the so-called Valley of Giants reach chambers several metres wide and high. Their arched passages and repeated grooves do not look like the products of ordinary erosion.
The burrows themselves are a kind of fossil evidence. Scientists call preserved signs of behaviour, such as footprints, nests and burrows, trace fossils or ichnofossils. A skeleton tells us what an animal was built like. A tunnel can reveal something about what it actually did.
Why suspect giant ground sloths?
The strongest clue is written across the walls. Researchers have compared the size and shape of the grooves with fossil claws from animals that lived in the same regions. The main candidates are giant armadillo relatives and several groups of ground sloths, especially robust mylodontids rather than the very largest Megatherium-type sloths.
A 2026 overview from the University of Saskatchewan's Ichnoplanet research group describes large South American burrows classified as Megaichnus minor and Megaichnus major. It notes proposed links to ground sloths such as Scelidotherium and Glossotherium. The grooves, tunnel dimensions and known anatomy all strengthen the case for an animal architect with large, curved claws.
That does not mean every big tunnel was made by a sloth. Giant armadillos were capable diggers too, and different animals may have enlarged or reused the same shelter. No ground-sloth bones have been found inside many of the Brazilian tunnels. The case therefore rests largely on trace evidence, anatomical comparisons and the absence of better candidates.
What would make an enormous sloth start digging?
That question is still open. Shelter from cold or dry conditions is one possibility. A burrow could have provided a more stable temperature than the landscape outside, which may have mattered to animals whose body-temperature control was limited. Resting, raising young or escaping danger have also been suggested.
Some chambers contain smooth hollows that researchers have interpreted as possible resting places. Several hollows in one chamber might mean that more than one animal used it, but this is not proof of a social colony. Different individuals could have occupied the same burrow years or generations apart.
The scale creates another puzzle. Some passages are so extensive that one animal may not have completed them in a single lifetime. The BBC's account of Brazilian palaeoburrow research explains that scientists are using three-dimensional mapping and searching for traces such as ancient fur to understand how these structures were made. A tunnel may have begun as a natural crack, then been widened and reused repeatedly.
A very different kind of sloth story
The image of a powerful ground sloth scraping through rock feels almost incompatible with today's slow canopy animals. Yet both belong to the same remarkable evolutionary story. Sloths were never a single fixed design. Over millions of years they occupied trees, open ground and, in the case of the extinct aquatic genus Thalassocnus, even coastal waters.
Lestodon, shown in the reconstruction accompanying this article, was one of the large South American ground sloths. It is a useful picture of the sort of heavy-bodied herbivore that once shared these landscapes, although the illustration does not prove that Lestodon made a particular tunnel. The Wikimedia Commons file record identifies the artwork as a life reconstruction linked to scientific work on this genus.
Why the tunnels matter now
Palaeoburrows preserve behaviour that bones alone cannot. Their scratches, polished surfaces, sediments and later human markings can hold evidence from several chapters of South American history. Damage from treasure hunting, construction or careless visits can scramble those layers before scientists have a chance to study them.
The honest answer to the title question is therefore: probably in some cases, but not beyond doubt in every case. Giant ground sloths are compelling candidates for many of the largest tunnels, while giant armadillos and repeated occupation remain part of the explanation. That uncertainty is not a weakness. It is what makes the tunnels such exciting evidence. They let us watch palaeontologists reconstruct behaviour from marks left in stone, one scratch at a time.
Sources
- Pesquisa FAPESP: Shelter for giants
- Ichnoplanet: Giant ground sloths made massive palaeoburrows in Brazil
- BBC Travel: Brazil's mysterious tunnels made by giant sloths
- Wikimedia Commons: Lestodon life reconstruction and licence
Image credit: Pedro Cuaranta, from Tomassini et al., via Wikimedia Commons, licensed CC BY 4.0.