Two-toed sloths are usually introduced as a tidy pair: Hoffmann's two-toed sloth and Linnaeus's two-toed sloth. New genomic research suggests the real family tree may be less tidy, and much more interesting.
Scientists studying sloths across Amazonia found several deeply distinct evolutionary lineages hiding beneath those two familiar names. Their results suggest the genus Choloepus may contain at least three species. That does not mean a new sloth species has been formally named overnight, but it does show why DNA can change how we understand and protect rainforest wildlife.
What did the scientists find?
The 2026 study used the first genome-wide dataset drawn from multiple populations of two-toed sloths. Researchers compared nuclear and mitochondrial DNA to reconstruct how populations are related and how their numbers changed through time.
Today, two living species are formally recognised in the genus Choloepus: Hoffmann's two-toed sloth, Choloepus hoffmanni, and Linnaeus's two-toed sloth, Choloepus didactylus. If those names described two neat branches of the family tree, every sampled Hoffmann's sloth would be more closely related to other Hoffmann's sloths than to a Linnaeus's sloth.
That is not what the genomic evidence showed. As the Leibniz Institute for Zoo and Wildlife Research explains, some populations currently classified as Hoffmann's two-toed sloths east of the Andes grouped more closely with Linnaeus's two-toed sloths. In scientific language, the recognised Hoffmann's species appears to be paraphyletic. Put simply, the current label does not capture the full shape of the family tree.
The researchers identified previously unknown lineages and concluded that two-toed sloths may represent at least three species. They also stressed that broader geographic sampling and a formal taxonomic revision are still needed. The responsible headline is therefore "there may be more species than we realised", not "a new species has already been declared".
What is a cryptic lineage?
A lineage is a branch of shared evolutionary history. When populations have been separated for long enough, their DNA can become distinctly different even if their outward appearance remains similar. Scientists often call this cryptic diversity because the differences are easy to miss when researchers rely on shape, size, fur and other visible features alone.
For a slow-moving animal that spends most of its life high in dense forest, those hidden differences are especially difficult to map. Sloths are hard to observe, capture and sample across huge tropical regions. The Leibniz-IZW sloth evolution project notes that this canopy lifestyle helps explain why important gaps remain in our knowledge of sloth biology, population health and evolutionary history.
Genomes do not replace field observations or museum collections. They add another layer of evidence. When DNA, geography, anatomy and ecology are studied together, researchers can test whether one familiar name is masking several separately evolving populations.
How did the Amazon shape this family tree?
The study links the separation of two-toed sloth lineages to enormous changes in the South American landscape. The rise of the Andes created geographic barriers, while repeated climatic shifts during the Pleistocene changed where suitable forest could persist. Populations that had once been connected may have become isolated as habitats moved, shrank or broke apart.
The nuclear DNA analysis estimated a split of about 3.57 million years between the main Hoffmann's and Linnaeus's groups, more recent than an earlier estimate of around eight million years based on mitochondrial DNA alone. Different parts of the genome can preserve different pieces of history, which is one reason the new genome-wide dataset is valuable.
A readable summary from Phys.org, based on the Leibniz-IZW release and peer-reviewed study, describes the newly detected diversity as a record of both ancient climatic change and the final phase of Andean uplift. The forest we see today is only the latest frame in a much longer story.
Why species labels matter for conservation
Taxonomy can sound like a naming exercise, but the consequences are practical. Conservation assessments ask how large a species' population is, where it lives, how fragmented its range has become and which threats are increasing. If one widespread "species" is actually several distinct lineages with smaller ranges, averaging them together can hide risk.
The study found particularly worrying signs in the East Amazonian lineage, including lower genetic diversity and higher inbreeding. The authors cautiously connect these patterns with the region's history of climatic instability and recent deforestation, especially around the heavily altered deforestation arc. They recommend considering the distinct lineages independently when future conservation assessments are made.
Genetic diversity is not an abstract luxury. It reflects the range of inherited variation available within a population. When habitats are cleared or split into isolated patches, already distinct populations may lose opportunities to mix. Protecting connected forest therefore helps conserve not only the sloths we can count, but evolutionary diversity that may be invisible from the ground.
Has the official sloth count changed?
Not yet. Formal species recognition requires researchers to weigh the genomic evidence alongside wider sampling and other biological information. Names, diagnostic features and geographic boundaries need careful review. Until that work is completed, Hoffmann's and Linnaeus's two-toed sloths remain the two recognised species in their genus.
The study also concerns two-toed sloths, not three-fingered sloths in the separate genus Bradypus. Both groups independently evolved remarkable adaptations for hanging beneath branches, but their family histories are separated by millions of years.
A hidden map worth protecting
The loveliest part of this discovery is that the animals did not suddenly become more diverse when scientists sequenced their DNA. The diversity was there all along, written into populations living across a vast and changing rainforest.
Now the challenge is to map it accurately and protect it before habitat loss erases branches of the sloth family tree that science has only just begun to see.
Featured image: Camila Mazzoni / Leibniz Institute for Zoo and Wildlife Research. Used from the official Leibniz-IZW press release.