In January 2007, colleagues from Sweden joined Dudzinski during data collection at Dolphin Encounters for an exiting new research collaboration. Drs. Mats Amundin and Christer Blomqvist, and two of Mats’ graduate students, joined DCP at Dolphin Encounters to implement use of the MOSART tag (housed in pec packs worn by dolphins) and their newly designed and built ELVIS system. Data collected with these devices could provide information for new interpretations of how dolphins use echolocation for foraging and communication.
Pec Packs and the MOSART tag
The combination of the Pec Pack, a silicone envelope that fits over a dolphin’s right pectoral fin, and the MOSART (MObile Submersible Acoustic Recording of Transients) tag, a recording device that fits inside the Pec Pack, is designed to record the possible exchange of pulsed signals between two dolphins engaged in a mutual social interaction. Combining this technology with video data collected using the MVA allows collaborating researchers from DCP and Sweden to compare audible signals collected from the senders using the MVA with the signals received by a dolphin wearing the Pec Pack.
In January 2007, this combination was used at Dolphin Encounters. Our goal for this data collection effort was to gather recordings of click sounds from any dolphin in the proximity of the tag wearer. These recordings are valuable because they add to the database of sounds collected from dolphins in captivity, and allow for a comparison between animals living in pools with those in natural enclosures, previously scarcely recorded with the MOSART tag. Studies like these are important; data collection is dependent on the rare and occasional social interactions during which the tag carrier may be involved.
The MOSART study at DE was successful in several ways. Eleven recording sessions were obtained, with 2 dolphins wearing the tag for a total of 130 minutes. This resulted in a total of 45 min of recordings, as the MOSART tag does not record continuously, but only after being triggered by a pulsed sound above a set trig level being aimed at the tag. In 6 of these sessions, the animal carrying the tag was filmed by Kathleen using the MVA. In one session, one of the calves approached the female wearing the tag, and apparently made a sonar exploration of the tag, which was recorded on video. In another session, Jake carried the tag while engaged in the coral sand (ELVIS) experiment. He behaved roughly the same as without the tag, further demonstrating that habituation to the pec pack was successful.
The data files are being analyzed for number of clicks, pulse repetition rate (pulses per second), click power spectrum, and received sound pressure level. Together with previous datasets, these parameters will make it possible to find out whether specific sound types are correlated with certain behaviors or social situations. Also, it is important to find out to what extent the animals use their pulsed sound directionality (as indicated by higher frequency peaks in the click power spectrum) to address social signals to a specific animal, i.e. the tag carrier.
The objective of this study is to shed new light on a feeding method first observed in wild dolphins on the Little Bahamas banks known as “crater feeding” or benthic feeding. During this foraging activity, dolphins apparently detect fish buried deep in the coral sand, and then plunge head first into the sand to catch it. The ELVIS (Echo-Location Visualization Integration System) system, including a 4-by-4 hydrophone matrix mounted on fabric, is well suited to study the possible use of sonar in this behavior. Custom-made software makes it possible t trace the sonar beam axis of a dolphin exploring the area covered by the hydrophone matrix. A controlled, simulated foraging arena was set up in one of the DE pools. The ELVIS system was buried under a layer of about 5 cm of fine coral sand, and three identical objects (targets) were made from PVC tubing. Each target was filled with 3 iron nails to enhance their sonar target strength (the target strength of PVC is very weak). When buried, the PVC tubes filled with water, making a hollow in the sand. This allowed two hypotheses concerning what cues the dolphins might use to find the buried fish in the wild to be tested: the echo of the fish/object itself and the echo of the hollow in the sand created by the fish’s
In total, 25 sessions were carried out with the three animals (all male) allocated to participate: Jake, Stormy, and Shawn. Although no thorough analysis has yet been done, the preliminary conclusions are that all three dolphins used their sonar intensively when they tried to find the objects, whether they were partly or completely buried in the sand, and that they had obvious problems finding them even under only 5-10 mm of sand. We cannot exclude the possibility that this was due to these dolphins being inexperienced with sonar seabed exploration. They had never been asked to perform such a task before; instead they have been trained to pick up objects on the surface of the seabed, which is an easy task compared to locating buried objects. There was an evident difference in the body posture of these dolphins and the wild dolphins engaged in crater feeding; the body of the latter pointed at an angle in reference to the seabed, whereas the former were almost vertical. This means that the wild dolphins avoid the strong seabed surface echo that may mask the much weaker echo from the buried object. The acoustic attenuation in coral sand seems to be considerable, so any echo returning from an object buried in the sand will be severely weakened.