Birds and mammals are on extreme ends of the airflow spectrum. Mammals inhale oxygen-rich air and they exhale depleted air, exhibiting a so-called tidal flow pattern. In contrast, bird breath travels tidally through part of the respiratory system, but in a one-way loop throughout most of the lung. Biologists have discovered that Savannah monitor lizards have lung structures that are hybrid system of bird and mammal lungs.
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Source: ScienceDaily