A recent study published in the journal *Nature* suggests that warm-bloodedness in mammals may have originated in the Late Triassic period, approximately 233 million years ago. This research, based on fossils of the inner ear of mammalian ancestors, enhances our understanding of mammalian evolutionary history.
Warm-bloodedness (or endothermicity) is a key characteristic of mammals and birds, allowing them to maintain a nearly constant core body temperature and thus live in a wide variety of environments. Endothermic animals are more active, can migrate farther, and move faster than cold-blooded ectothermic animals; while cold-blooded ectothermic animals are generally slower, less active, and have weaker aerobic capacity. However, the exact time when endothermicity first appeared in mammalian evolutionary history remains difficult to determine because most fossil evidence is incomplete.
A joint team of scientists from the University of Lisbon in Portugal, the Natural History Museum in the UK, and the Field Museum of Natural History in the US believes that the structure of the semicircular canals in the inner ear of mammalian ancestors may help understand when the transition to endothermicity in mammals began. These semicircular canals contain a fluid called endolymph, the viscosity of which varies with the animal's body temperature.
The team studied the structural changes of the semicircular canals in 56 extinct species that are ancestors of mammals, and discovered alterations related to endothermicity, such as narrowing of the semicircular canals. They found that the semicircular canal structure evolved abruptly in mammalian ancestors during a period of climate instability in the Late Triassic, suggesting that endothermicity evolved roughly during this period. These findings are associated with an increase in body temperature of 5–9 degrees Celsius and enhanced aerobic and anaerobic capacity.
