MOBILITY OF THE AXIAL REGIONS IN A CAPTIVE AMAZON RIVER DOLPHIN (INIA GEOFFRENSIS) - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 72

I NTRODUCTION
Cetacean evolution has emphasized morphological adaptations to a fully aquatic lifestyle
(e.g., Thewissen, 1998; Reidenberg, 2007; Uhen, 2007). In particular, studies of odontocete
axial anatomy emphasize features that create a stable, relatively rigid vertebral column and
associated connective tissues for rapid swimming (Pabst, 1996; Fish, 1997; Long et al.,
1997). The vertebral morphology of many toothed whales (odontocetes) have some fusion of
vertebrae in the cervical region and rostrocaudally short vertebral bodies (or centra),
characteristics which limit mobility (Buchholtz, 2001). Some axial flexibility is clearly
required for propulsion alone, as in dorsoventral fluke movements (Figure, 1a; mobility in the
sagittal plane). Axial mobility also affects swimming posture (e.g., in banking), which is
important for maneuverability (Fish, 1997). Some lateral flexion (that is, bending in the
horizontal plane; Figure 1b) is used by all cetaceans. However, since morphology constrains
lateral mobility in odontocetes (see below), lateral flexibility has not been subject to much
scrutiny.
Figure 1. Mobility of a cetacean in the sagittal (a) and horizontal planes (b). Lateral view of a river
dolphin showing dorsoventral bending of the body during propulsive movements of the flukes (a) and
dorsal view of a river dolphin showing lateral bending of the torso (b).
Some dolphins possess relatively greater vertebral mobility than others. Certain extant
river dolphins, such as those of the genera Inia , Lipotes , and Platanista , have unfused cervical
vertebrae and other morphologic characteristics that increase mobility (Klima et al., 1980;
Fish, 1997; Buchholtz, 2001). Captive or wild Inia geoffrensis (Amazon River dolphin) are
well known for extreme mobility during locomotion (Schreib et al., 1994; Fish, 1997). For
example, the ability to touch the rostrum to the tail by lateral bending of the neck and
peduncle has been observed in a captive Inia (Schreib et al., 1994). The same animal was
observed by Fish (1997), who noted turns could be accomplished by lateral flexion rather
than by rotating (banking) the body away from the horizontal plane. A great mobility of the
pectoral limbs has been observed in captive Inia (Layne and Caldwell, 1964; Klima et al.,
1980), in which the forelimbs are frequently used in "oar-like" movements to assist in turns
(Klima et al., 1980; Smith et al., 1994) and a captive Inia used such movements to maintain a
stationary location in an enclosure (T.D. Smith, personal observations).
Numerous studies have utilized video imaging to determine critical aspects of locomotory
behavior in aquatic or semiaquatic mammals (e.g., Smith et al., 1976; Fish, 1994; Buchholtz,