Shape change of the thoracic cavity in bottlenose dolphins (Tursiops truncatus) with implications for respiratory mechanics

ABSTRACT

Bottlenose dolphins (Tursiops truncatus) exhibit explosive respiration, which suggests that the thoracic cavity must undergo rapid changes in volume. This study investigated how shape change may occur during the rapid increase (inspiration) and decrease (expiration) in thoracic volume, using excised thoracic units from two stranded bottlenose dolphins. To mimic inspiration and expiration, thoracic skeletal elements (vertebral ribs, sternal ribs, and sternum) and associated musculature were manipulated to extreme cranial and caudal respiratory postures, respectively. At each of these extreme postures the (1) length of the thorax, (2) height and width of the thoracic cranial opening, and (3) thoracic circumference, height, and width at the level of each rib, were measured and corresponding cross-sectional areas and volumes were computed. Rib angles associated with each thoracic posture were measured on digital, scaled photographs using EasyCAD software. The shape changes observed during manipulations replicated an increase (i.e. extreme cranial posture) and decrease (i.e. extreme caudal posture) in volume within the thoracic cavity, consistent with inspiration and expiration, respectively. Along the length of the thoracic unit, thoracic height under went larger dimensional changes than did thoracic width and length between cranial and caudal respiratory postures. The shape change across the two most extreme postures achieved in this study (cranial suspended versus caudal supine) yielded a maximal estimated thoracic cavity volume change of 56%. Rib angle measurements suggest that the cranial ribs are the primary respiratory ribs that allow for maximal shape change, and, thus, volume change required during explosive respiration.