Why Are Sloths So Slow? Scientists May Have Found the Answer
Sloths are the slowest mammals on Earth, but their life in dense forests makes them difficult to study. Now, for the first time, the genome of a two-toed sloth has been sequenced and analyzed, shedding new light on the genetics behind its exceptionally slow metabolism. By tracing its evolution, a study published in the journal BMC Biology identified sloth-specific “jumping genes” (transposons) that have been preserved for millions of years and are linked to metabolism. The findings not only reveal the genetics behind the sloth’s unique biology, but could also lead to research that sheds light on metabolism and aging in other mammals, including humans, and even on long-duration space missions.
Along with armadillos and anteaters, sloths belong to Xenarthra, the only group of placental mammals that originated in South America. The animals in this group have existed for 65.5 million years, and the extinct ancestors of sloths included giant ground sloths the size of elephants. By contrast, modern sloths are arboreal and belong to two groups, two-toed sloths, the Linnaeus’s two-toed sloth and Hoffmann’s two-toed sloth, and three-toed sloths, the brown-throated sloth, pale-throated sloth, maned sloth and pygmy sloth.
Sloths have the lowest metabolic rate among mammals, often less than half of what would be expected for their body size. To conserve energy, they can switch between regulating their own body temperature and adapting to the environment, maintaining a temperature of around 5 degrees Celsius. Although they are slow, sloths are highly capable long-distance swimmers in search of a mate.
To better understand the sloths’ extraordinary biology, the study’s authors extracted DNA from tissue samples taken from captive sloths, which was sequenced at the Max Planck Institute for Molecular Cell Biology and Genetics in Germany. Researchers from the Sanger Institute in Cambridge and the Leibniz Institute for Zoo and Wildlife Research and Veterinary Medicine in Berlin then analyzed the sloth genome and compared the sequence with the genomes of other mammals, including anteaters and armadillos, using comparative genomics in order to understand what makes sloths unique.
The researchers found that sloth genomes contain DNA segments capable of copying themselves and moving around within the cell’s genome. When such a segment jumps into a functioning gene, it can disrupt it and cause a mutation. At other times, the segment brings genetic information that gives the cell new abilities, driving evolutionary processes. Using genomics to look back in time and map the evolution of sloths, the researchers found that these “jumping genes” emerged in the last common ancestor of all living sloth species about 30 million years ago. They have since been preserved over time, making them unique, deeply rooted genetic sequences in sloths.
The team was surprised to find that many of these genes are linked to mitochondria, the cells’ “power stations” that produce their energy, and to metabolic pathways. Because sloths have one of the most unusual metabolisms among mammals, the researchers believe these sloth-specific genes are connected to their remarkable adaptations to the environment and to the evolution of their slowest metabolism.
“Many experiments in the field of evolution have been carried out so far, but by studying unusual animals like sloths, we sometimes discover biological solutions that humans never developed,” said Dr. Marcela Uliano-Silva, senior bioinformatician at the Sanger Institute. “By using genomics to look back in time, we found ‘jumping genes’ that sloths have preserved for millions of years. These sloth-specific genes are linked to mitochondria and metabolic pathways, suggesting they may be related to the evolution of their slowest metabolism.”
According to Dr. Camila Mazzoni, from the Department of Evolutionary Genetics at the Leibniz Institute, although sloths have the slowest metabolism of any mammal, they remain healthy. Understanding how they achieve this could provide new insights into how cells manage energy efficiently. “The study’s findings suggest that sloths may develop genetic ‘backup systems’ that help compensate for their ‘calm mitochondria’ and support their unique lifestyle.”
