Abstract
Most mammals possess stamina because their locomotor and respiratory (i.e., ventilatory) systems are mechanically coupled. These systems are decoupled, however, in bottlenose dolphins (Tursiops truncatus) as they swim on a breath hold. Locomotion and ventilation are coupled only during their brief surfacing event, when they respire explosively (up to 90% of total lung volume in approximately 0.3 s) (Ridgway et al. 1969 Science 166:1651โ1654). The predominantly slowโtwitch fiber profile of their diaphragm (Dearolf 2003 J Morphol 256:79โ88) suggests that this muscle does not likely power their rapid ventilatory event. Based on Bramble's (1989 Amer Zool 29:171โ186) biomechanical model of locomotorโrespiratory coupling in galloping mammals, it was hypothesized that locomotor muscles function to power ventilation in bottlenose dolphins. It was further hypothesized that these muscles would be composed predominantly of fastโtwitch fibers to facilitate the bottlenose dolphin's rapid ventilation. The gross morphology of craniocervical (scalenus, sternocephalicus, sternohyoid), thoracic (intercostals, transverse thoracis), and lumbopelvic (hypaxialis, rectus abdominis, abdominal obliques) muscles (n = 7) and the fiberโtype profiles (n = 6) of selected muscles (scalenus, sternocephalicus, sternohyoid, rectus abdominis) of bottlenose dolphins were investigated. Physical manipulations of excised thoracic units were carried out to investigate potential actions of these muscles. Results suggest that the craniocervical muscles act to draw the sternum and associated ribs craniodorsally, which flares the ribs laterally, and increases the thoracic cavity volume required for inspiration. The lumbopelvic muscles act to draw the sternum and caudal ribs caudally, which decreases the volumes of the thoracic and abdominal cavities required for expiration. All muscles investigated were composed predominantly of fastโtwitch fibers (range 61โ88% by area) and appear histochemically poised for rapid contraction. These combined results suggest that dolphins utilize muscles, similar to those used by galloping mammals, to power their explosive ventilation. J. Morphol., 2008. ยฉ 2008 WileyโLiss, Inc.