Lung morphology of cursorial and non-cursorial mammals: lagomorphs as a case study for a pneumatic stabilization hypothesis.

Gross lung morphology is examined in representative species from four genera within the order Lagomorpha (Lepus californicus, Sylvilagus nuttalli, Oryctolagus cuniculus, Ochotona princeps), and compared with a representative rodent out-group (Spermophilus richardonsii). Examination of pulmonary morphology reveals several correlations between the thoracic morphology and locomotor behavior. Lepus, the most cursorial species, exhibits a distinct suite of characteristics: 1) tissue of the right cranial lobe interposed between the heart and sternum; 2) well-defined grooves in the lung tissue for both the aorta and ribs; 3) a fibrous pericardial attachment to the sternum; 4) relatively large heart and lung mass. Sylvilagus, a sprinter, exhibits these features to a lesser degree, whereas Oryctolagus and Ochotona, non-cursorial species, lack most of these features. This same suite of pulmonary features is also observed in a wide range of unrelated cursorial taxa (including selected Artiodactlya, Perissodactyla, Carnivora). Corrosion casts of the internal airways demonstrate that the cursorial and non-cursorial taxa examined here have similar branching patterns despite their variable external morphologies. The juxtaposition of pulmonary lobes, heart, and ribs leads to the hypothesis that the lungs themselves provide mechanical support of the heart and visceral mass during locomotion. Analyses of cineradiographic and pneumotachographic data obtained from Oryctolagus tend to support a pneumatic stabilization hypothesis: the lungs themselves, intimately associated with the chest walls and positively pressurized during landing, may provide some mechanical support to the viscera. This mechanism may be important in stabilizing the relatively large hearts of the most cursorial species during running.