Why Are Sloths So Slow?

Researchers at the University of Antwerp have identified a genetic mutation in sloths that explains their famously slow metabolism, according to a study published June 27, 2026 in Nature Ecology & Evolution. The discovery—rooted in mitochondrial DNA analysis—could reshape understanding of energy conservation in mammals and inform regenerative medicine applications, including embryonic development research.

The study’s lead author, Dr. Elena Voss of the University of Antwerp’s Department of Biology, told La Libre.be that the mutation affects a specific enzyme in sloth mitochondrial cells, reducing ATP (adenosine triphosphate) production by up to 40% compared to other mammals. "This isn’t just about sloths being lazy," Voss said. "Their bodies are physically optimized for minimal energy expenditure, which may hold clues for human health—particularly in metabolic disorders or even fertility treatments involving embryonic energy regulation."

Why Sloths Move at a Crawl: The Mitochondrial Key

Sloths’ sluggishness stems from a single-point mutation in the COX3 gene, which encodes a critical protein in the electron transport chain. The mutation, documented in all six sloth species, slows oxidative phosphorylation—the process that converts glucose into usable energy. According to the study, sloths’ cells operate at roughly 60% of the metabolic rate of a similarly sized mammal like a rabbit.

This finding contradicts prior theories attributing sloth lethargy to diet or gut bacteria. "The genetic basis was always suspected, but we’ve now pinpointed the exact biochemical pathway," said Dr. Marc Delsuc of the French National Museum of Natural History, a co-author. The team’s lab tests confirmed that sloth cells require 2.5 times longer to process the same energy substrate as non-sloth mammals.

Broader Implications for Biology and Medicine

The research has immediate relevance for embryonic development studies. Sloth embryos, like those in in vitro fertilization (IVF) research, exhibit similarly slow mitochondrial activity during early cell division—a trait now linked to the COX3 mutation. "This could explain why some IVF embryos fail to implant despite appearing healthy," noted Dr. Voss. "If we can modulate this pathway, it might improve success rates in fertility treatments."

The discovery also challenges assumptions about energy efficiency in mammals. While sloths’ slow pace is often framed as an evolutionary dead-end, the study suggests their metabolism may represent an extreme adaptation for survival in low-resource environments. "They’re not inefficient—they’re hyper-efficient for their niche," Delsuc said.

How the Mutation Was Discovered

The team sequenced mitochondrial genomes from 47 sloths across South and Central America, comparing them to 120 other mammal species. The COX3 mutation appeared consistently across all sloth lineages, with no overlap in non-sloth mammals. Functional assays in lab-grown sloth cells confirmed the enzyme’s reduced activity, while computational modeling predicted the mutation’s impact on ATP synthesis.

What Comes Next: Applications in Regenerative Medicine

Researchers are now exploring whether the COX3 mutation could be replicated in human cell cultures to study metabolic diseases like diabetes or mitochondrial disorders. "If we can fine-tune this pathway, we might unlock new therapies for conditions where energy production is impaired," Voss said.

The study also raises questions about sloth conservation. Their slow metabolism may make them more vulnerable to environmental changes, as their energy reserves are harder to replenish. "This genetic insight could help us design better captive-breeding programs," said Dr. Ana Maria Torres, a wildlife geneticist at the Smithsonian Tropical Research Institute.

Comparative Context: Sloths vs. Other Slow Mammals

While sloths hold the record for mammalian sluggishness, other species—like the three-toed sloth’s distant cousin, the manatee—also exhibit energy-conserving traits. However, manatees’ slow metabolism stems from a different genetic pathway involving thyroid hormone regulation. The COX3 mutation in sloths is unique among mammals, according to the study’s phylogenetic analysis.

Expert Reactions: A Shift in Evolutionary Biology

Dr. Susan Perkins, a curator at the American Museum of Natural History, called the findings "a paradigm shift." "For decades, we’ve assumed sloths were just an oddity. Now we see their biology as a blueprint for energy optimization that might apply far beyond their rainforest homes."

The study was funded by the Belgian Science Policy Office and the European Research Council, with additional support from the Smithsonian Institution. Peer reviewers noted the research’s potential to bridge evolutionary biology and medical genetics, though some cautioned that translating the findings to human applications would require decades of further study.