A research team led by Seishiro Tada and Takanobu Tsuihiji of the University of Tokyo showed that the warm-blooded descendants of warm-blooded dinosaurs, aided by larger nasal cavities, developed a better nasal cooling system than cold-blooded animals. The study published in the journal Royal Society for Open Scienceprovides evidence for the evolution of nasal cooling in warm-blooded animals from their theropod dinosaur ancestors.
Warm-blooded animals maintain a high body temperature through internal heat sources. Birds, humans, and other mammals are endothermic. But ectotherms, or cold-blooded animals like reptiles, use external heat sources to keep warm. Humans seek air conditioning or a cold drink on a hot summer day to cool off. How do other animals prevent their heat-sensitive brain from overheating or frying? The answer may be in their noses.
In endothermic states like birds, mammals, and you, the nose not only smells odors and spasms, but also aids in heat exchange thanks to small, undulating, coil-shaped structures called respiratory turbinates made of bone tissue and cartilage. They may help moisten the inhaled air, and exchange heat from the circulatory system, which can cool the brain.
A previous study showed that nasal cavity size in ectotherms and endotherms correlates with body size. “But its physiological role is controversial,” said Seichiro Tada, a graduate student at the University of Tokyo. Does the large nasal cavity help maintain whole body temperature through the circulatory system or simply cool the large brains from absorbing heat? If the goal is to cool their brains, large-brained ones may have larger nasal cavities and efficient turbine-assisted cooling. To test this, the researchers sought to elucidate the essential role of the respiratory tracts and the physiological function of the nasal cavity of non-avian dinosaurs and their living descendants.
The team studied the head samples of 51 current endotherms and ectotherms, and the skull of a theropod dinosaur called Velociraptor mongoliensis using computed tomography or computed tomography and reconstructed the structures of their nose. They found that compared to exothermics, endotherms have well-developed turbinates and a nasal cavity that is proportionally larger in relation to their head size—not body size as previously thought. This means that the large nasal cavity helps cool their brain.
The researchers also found that the non-flying theropod dinosaur Velociraptor mongolensis had a smaller nasal cavity and therefore could not regulate temperature as efficiently as its current descendants, the birds, indicating a comparatively less developed brain that did not require efficient cooling. They may have used other methods to cool their hot brains; remains a mystery. The nasal cooling system may have evolved in parallel with the changes in the structure of their skull that we now see in modern heat absorbents.
Tada was pleasantly surprised by “the possible scenario of cranial evolution seen in a previously unnoticed lineage of dinosaurs and birds.” “Our findings highlight the importance of the nose in deducing the physiology of fossil forms such as dinosaurs and enable us to study the evolution of the radical modification of the skull from non-flying theropod dinosaurs to modern birds from a new angle,” he noted.
Evolutionary process towards the bird-like cephalic thermoregulatory system in Theropoda shown on the basis of nasal structures, Royal Society for Open Science (2023). DOI: 10.1098/rsos.220997. royalsocietypublishing.org/doi/10.1098/rsos.220997
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