The nasal cavity is a vital component of the respiratory system that heats
and humidifies inhaled air in all vertebrates. Despite this common function,
the shapes of nasal cavities vary widely across animals. To understand this
variability, we here connect nasal geometry to its function by theoretically
studying the airflow and the associated scalar exchange that describes heating
and humidification. We find that optimal geometries, which have minimal
resistance for a given exchange efficiency, have a constant gap width between
their side walls, but their overall shape is restricted only by the geometry of
the head. Our theory explains the geometric variations of natural nasal
cavities quantitatively and we hypothesize that the trade-off between high
exchange efficiency and low resistance to airflow is the main driving force
shaping the nasal cavity. Our model further explains why humans, whose nasal
cavities evolved to be smaller than expected for their size, become obligate
oral breathers in aerobically challenging situations.Comment: 7 pages, 4 figure
