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 77

certain variables, such as intervertebral disc thickness, which could differentially affect
proportions of the axial regions. That said, two deviations from the thoracic line (the cervical
and torso angles) provide valid assessments of atlantooccipital/cervical and overall torso
mobility. A second caveat is that there is no means to verify that the upper ends of these
ranges represent true limits in axial mobility. Nonetheless, the acute lateral flexion measured
in these regions in the captive Amazon River dolphin support qualitative descriptions of
mobility in captive Inia (Schreib et al., 1994; Fish, 1997).
D OLPHIN A XIAL M OBILITY AND N AVIGATION
Since no comparable data exist for other cetaceans, it cannot unequivocally be said that
Inia are unique in lateral mobility. Certainly, lateral mobility in other riverine dolphins would
provide advantages in relatively complex, contained environments, in comparison to oceanic
environments. Published accounts of swimming behavior in other riverine species are scarce,
however. Dolphins of the genus Platanista habitually ―side-swim,‖ thus favoring dorsoventral
bending for navigating turns. In captive Lipotes vexillifer, banking during high speed turns
has been described (―body bent inward and turned to one side‖) by Zhou & Zhang (1991).
These authors provide little description of lateral mobility, except to indicate the head may be
moved ―side to side‖.
Pelagic species have been subject to far more scrutiny (e.g., Smith et al., 1976; Fish,
1997; Buchholtz, 2001). Fish (1997) notes that lateral flexion of the neck is used to initiate
turns in pelagic odontocetes. In unpowered turns (i.e., gliding turns without propulsive fluke
movements), the caudal vertebrae in the peduncle may be laterally flexed. Thus, lateral
mobility is of some importance to other odontocetes, at least at slow speeds. In high speed
swimming, however, lateral movements are minimized in oceanic dolphins. Instead, body
posture (i.e., banking) and flipper position are used to navigate turns (Fish, 1997).
Fish (1997) observed that Inia have no reliance on banking for turning. Aside from some
side-swimming trained behaviors, this was borne out by our observations (albeit on the same
individual dolphin as observed by Fish). This relates in part to swimming speed as Inia have
slow speed and a small turning radius. In most marine dolphins, turns occur at a faster speed
at a high radius (Fish, 2002). The lateral mobility in Inia also may be of very important to
navigation in extremely complex environments, specifically one that includes seasonally
flooded forests (Best & da Silva, 1989). Data on lateral mobility in other dolphins inhabiting
environments that differ in complexity would be of great interest.
A XIAL A NATOMY AND D OLPHIN M OBILITY
Generally, cetaceans differ anatomically from other mammals in a manner that restricts
vertebral mobility. Cetaceans have an extreme length reduction of the vertebral centrum (or
body; Figure 6) in the cervical region (see vertebral regions in Figure 3) compared to
semiaquatic mammals (e.g., pinnepeds and otters). This decreases mobility while enhancing
axial stability (Buchholtz, 2001). Fully aquatic mammals living in the ―most extreme aquatic
habits‖ (Buchholtz, 2001 - p 179) have the shortest relative centrum lengths of unfused