The clever strategy with which some of today’s animals survive times hostile to life was possibly also used by animals 250 million years ago: paleontologists have discovered the oldest known evidence of a hibernation-like state in a prehistoric Antarctic inhabitant. Growth structures in the teeth of the approximately pig-sized dinosaurs suggest that they went through periods with a reduced metabolism, in which they fell into a rigidity.
A break instead of a struggle for survival: If there are certain times of the year, cold, heat, lack of water or other hostile conditions, many animal species switch off until the situation improves. This state of persistence is scientifically referred to as torpor. All body functions are kept on the back burner in order to save energy. A particularly pronounced form of torpor is hibernation. “Animals that live on or near the poles have always had to deal with the more extreme environmental conditions there,” says Megan Whitney of Harvard University in Cambridge. “Our preliminary results now suggest that entering a hibernation-like state is not a comparatively new type of adaptation – apparently this concept is very old,” said the scientist.
Early Antarctic inhabitants in their sights
The clue comes from research that Whitney and her colleague Christian Sidor from the University of Washington in Seattle carried out on fossils from representatives of the lystrosaurs. These were mostly pig-sized herbivores, which are considered to be the distant ancestors of mammals. The different types of lystrosaurs were widespread on the supercontinent Pangea around 250 million years ago. Among other things, paleontologists have also discovered their fossils in the Antarctic, which offered life for these animals in the warm period at that time. “The fact that the lystrosaurs survived the mass extinction at the end of the Permian Age and were so widely distributed in the early Triassic made them a particularly well-studied group of animals,” says Sidor.
Lystrosaurs were stocky and had a kind of beak and a pair of tusks in the upper jaw, which were probably used for foraging in the ground vegetation and for digging for roots and tubers. It was precisely these tusks that made Whitney and Sidor’s study possible. As they explain, similar to elephants’ tusks, the Lystrosaurus tusks continued to grow throughout their lives. The cross-sections of fossilized tusks can thus provide life-history information about metabolism, growth and stress.
As part of the study, Whitney and Sidor compared cross sections of the tusks of six Antarctic lystrosaurs with those of four representatives from South Africa. As they explain, the locations of the Antarctic fossils were found in the Triassic at about 72 degrees south latitude – that is, within the circle of latitude in which strong seasonal fluctuations in environmental conditions also occurred in this warm period. The South African sites, however, were far outside the Antarctic Circle in temperate zones.
Traces of a hibernation-like state
The comparison of the analysis results of the tusks from the two regions showed: The tree-ring-like growth patterns show characteristic peculiarities in the Antarctic fossils. Some of the dentin layers were therefore very close together. “These structural features that we found in the Antarctic Lystrosaurus tusks are similar to those associated with hibernation in certain modern animals,” says Whitney. “The traits also fit a pattern of small metabolic ‘reactivation events’ during a period of stress, which is most similar to what we see today in warm-blooded hibernators,” says the scientist.
Whitney and Sidor have not yet been able to prove that the lystrosaurs went through a real hibernation – which means a specific, week-long reduction in metabolism, body temperature and activity. The effect could also have been caused by a form of torpor similar to hibernation, for example by a short-term decrease in metabolism in connection with rigidity. But in any case, some kind of persistence near the South Pole seems to have made sense, say the scientists. Although the earth was much warmer during the Triassic than it is today and parts of the Antarctic were probably forested, the plants and animals south of the Antarctic Circle had to cope with extreme annual fluctuations in solar radiation.
As Whitney and Sidor conclude, their results so far are preliminary and have yet to be fully confirmed. “We hope that this hypothesis will be tested further and that additional samples will be taken from lystrosaurs and other polar vertebrates to look for signals of hibernation or other forms of freezing,” the scientists write.
Source: University of Washington, Article: Communications Biology, doi: 10.1038 / s42003-020-01207-6