25-Million-Year-Old Fossil Thought To Be Earliest Known Example Of Hibernation

An artistic interpretation of the pig-sized Lystrosaurus. Crystal Shin

Hibernation has a long history within the animal kingdom, a new study has found, as evidence has emerged of a hibernation-like state in an animal that lived in the Early Triassic around 250 million years ago. The discovery, published in the journal Communications Biology, shows the fossil remains of a Lystrosaurus, a distant relative of mammals that lived in Antarctica.

Hibernation is a common feature among animals that live near to or in polar regions, as food is scarce and the energetic cost of keeping warm is great in the harshest of winter months. In hibernation, animals go into a state of dormancy where they are completely inactive for a certain amount of time. In torpor, a hibernation-like state, an animal’s metabolic rate slows, lowering their body temperature and energetic needs.

This new fossil evidence suggests some kind of torpor state emerged in vertebrates long before mammals and dinosaurs evolved. The Lystrosaurus lived just before Earth’s largest extinction event at the end of the Permian Period, which saw the disappearance of 70 percent of the planet’s vertebrate species. A small, stocky forager, it spread across the single continent Pangea, which was made up of all of the continents on Earth including Antarctica.

This thin-section of the fossilized tusk from an Antarctic Lystrosaurus shows layers of dentine deposited in rings of growth. At the top right is a close-up view of the layers, with a white bar highlighting a zone indicative of a hibernation-like state. Scale bar is 1 millimeter. Megan Whitney/Christian Sidor

Their remains have been found in India, China, Russia, parts of Africa, and Antarctica. The fossil evidence reveals they were mostly around the size of a pig, though some were as long as 1.8 to 2.4 meters (6 to 8 feet). They didn’t have teeth but instead a pair of large tusks, which the researchers suspect was used to forage and burrow for vegetation, roots, and tubers in the ground.

The tusks were an integral part of the fossil’s discovery, as they act like a time capsule of the animal’s life history, giving details about its metabolism, growth, illnesses, and strains. They grew continuously throughout the Lystrosaurus’s life, essentially acting like a bony journal of the animal’s life.

The researchers were able to take cross-sections of the fossilized tusks of six Lystrosaurus from Antarctica and four from South Africa. The tusks from both regions grew similarly, with dentine layers that grew in concentric circles much like the rings of a tree. The tusks from Antarctica, however, were unique as they had closely spaced, thick rings that the researchers say indicated the animal was laying down less dentine due to stress, which matched the stress marks seen in the teeth of modern animals that hibernate. The evidence isn’t sufficient to know for certain if Lystrosaurus hibernated or went into a state of torpor, but it shows they exhibited some kind of "winter slowing" in cold regions.

"Cold-blooded animals often shut down their metabolism entirely during a tough season, but many endothermic or 'warm-blooded' animals that hibernate frequently reactivate their metabolism during the hibernation period," said lead author Megan Whitney, a postdoctoral researcher at Harvard University, in a statement. "What we observed in the Antarctic Lystrosaurus tusks fits a pattern of small metabolic 'reactivation events' during a period of stress, which is most similar to what we see in warm-blooded hibernators today.”

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