Sloths are masters at saving energy. However, it was still unclear which genetic tricks enable their extremely slow metabolism. Complete genome sequencing of the two-toed sloth now provides clues. According to this, massive changes took place in the sloth genome in the course of evolution: large numbers of genes were repeatedly copied and inserted into other parts of the genome. Many of these so-called jumping genes are connected to the metabolism and the power plants of the cells.
Sloths have adapted perfectly to a leisurely life in the trees of the South American rainforest. Since they feed mainly on low-energy leaves, the animals avoid any unnecessary expenditure of energy. Most of the time they hang motionless from branches with their specialized claws. When sloths do move, they do so in slow motion. Their body temperature is low and dependent on the ambient temperature, their muscle mass is reduced to the bare minimum, and their metabolic rate is often less than half of what would be expected for animals of their size.
“These metabolic adaptations, in conjunction with the characteristic morphology of their forelimbs, enable them to live a finely tuned and maximally sluggish life,” explains a team led by Marcela Uliano-Silva from the Wellcome Sanger Institute in Cambridge. However, how these extraordinary adaptations could develop in the course of evolution was previously unclear.
Gene copies enable evolutionary innovations
Now Uliano-Silva and her colleagues have sequenced the entire genome of a two-toed sloth and have discovered a possible explanation. As the researchers discovered, the sloths’ genome is rich in so-called jumping genes, also known as transposons. These are genes that can copy themselves and insert them into other places in the genome. Depending on where the copies end up, they can cause serious problems such as cancer or have positive effects and drive evolution through the newfound genetic diversity.
For the sloths, the evolutionary benefit apparently outweighed the differences: no other known mammalian genome contains as many transposons as theirs, as the analyzes revealed. In addition, almost half of the inserted copies were genetically active – an exceptionally high value. According to Uliano-Silva and her team, many of these go back to the last common ancestor of today’s sloths around 30 million years ago and have been firmly anchored in the sloth genome ever since. But the team also found signs of younger insertions.
Saving energy like a sloth?
But what function do the inserted genes have? The researchers found an indication of this when they examined which genes the copies came from: “A significant proportion of these retrocopies have precursor genes that are functionally linked to mitochondrial processes and cell metabolism,” reports the team. Accordingly, these jumping genes could influence the function of the cell power plants and enable the sloths to live their leisurely, frugal lifestyle.
This finding could possibly also be relevant for human medicine. “Many human diseases – including diabetes, age-related disorders, neurodegeneration and muscle loss – are associated with problems in energy production and mitochondrial function,” says co-author Pedro Galante from Sírio Libanês Hospital in São Paulo. “Although further research is needed, sloth cell lines could provide a natural model for understanding how organisms cope with energy poverty and what goes awry in disease.”
Source: Marcela Uliano-Silva (Wellcome Sanger Institute, Cambridge, UK) et al., BMC Biology, doi: 10.1186/s12915-026-02632-5