SUMMARYThe production of venous gas emboli (VGE) resulting from altered dive behavior is postulated as contributing to the stranding of beaked whales exposed to mid-frequency active sonar. To test whether nitrogen gas uptake during repetitive breath-hold diving is sufficient for asymptomatic VGE formation in odontocetes, a bottlenose dolphin (Tursiops truncatus Montagu) was trained to perform 10-12 serial dives with 60s surface intervals to depths of 30, 50, 70 or 100m. The dolphin remained at the bottom depth for 90s on each dive. Doppler and/or two-dimensional imaging ultrasound did not detect VGE in the portal and brachiocephalic veins following a dive series. Van Slyke analyses of serial, post-dive blood samples drawn from the fluke yielded blood nitrogen partial pressure (P N2 ) values that were negligibly different from control samples. Mean heart rate (HR; ±1s.d.) recorded during diving was 50±3beatsmin -1 and was not significantly different between the 50, 70 and 100m dive sessions. The absence of VGE and elevated blood P N2 during post-dive periods do not support the hypothesis that N 2 supersaturation during repetitive dives contributes to VGE formation in the dolphin. The diving HR pattern and the presumed rapid N 2 washout during the surface-interval tachycardia probably minimized N 2 accumulation in the blood during dive sessions.

Moore and others (2008) previously showed that bottlenose dolphins are capable of beam steering and controlling the vertical and horizontal widths of the echolocation beam. A follow-on study was performed using the same methods as the previous study, but with a younger animal and a higher resolution diamond-shaped hydrophone array for characterizing the beam. The dolphin performed a target detection task while stationed on a biteplate with targets placed up to 34 deg to either the left or right of the dolphin’s longitudinal axis. The dolphin was capable of beam steering more than 28 deg to either side, which is a greater capability than previously reported and which exceeded the geometric coverage of the array. Frequency band-limited beam patterns suggested the presence of two beams, spatially separate from one another and which corresponded to higher and lower frequency energies. The finding is consistent with prior anatomical and acoustic evidence of two echolocation click sound sources in the delphinids. In addition, at low frequencies, a local minimum was observed at the center of the beam. The functional significance of the local minimum, if any, is unknown.