The bronchial tree of most mammalian lungs is a good example of an efficient distribution system whose geometry and dimensions of branched structures are important factors in determining the efficiency of respiration. Small and flying endothermic animals have highenergy requirements, requiring morphological and physiological adaptations to reduce energy loss. Here we show that Tadarida brasiliensis, a nocturnal small bat whose energy requirements are exacerbated by this small size and by their frequent exposure to high altitude, has a different morphology in the proximal airway, sustained by a wider trachea and better scaling factors, than other non-flying mammals. This design allows a great decrease of the volume specific resistance of the proximal airway and in consequence a very low entropy production during breathing, approximately 1/18 of that expected for a nonflying mammals of similar body size.
