Weddell seals (Leptonychotes weddellii) are the only year-round resident marine mammal in Antarctica, and as such must rely on precise breath-by-breath navigation as they dive below shore-fast sea ice that is up to 6 meters thick. This remarkable navigation ability occurs year-round, including during conditions of total darkness in the winter months. For this deep diving mammal, failure to locate cracks or holes in the ice would be fatal. The seals maximize the distance they can travel during a dive by employing cost-efficient modes of locomotion, but their blood and muscle oxygen stores limit their aerobic capacity for diving to < 20 min and a total distance of approximately 2000 m; based on our calculations Weddell seals can only take approximately 1440 flipper strokes before they must breathe. To understand how this polar seal is able to range long distances below the ice, we investigated the tactics and sensory cues that allow the animals to home precisely away from and toward a known breathing hole.

 We conducted rigorous analyses of data we collected during three field seasons at McMurdo Sound to examine four way-finding tactics by Weddell seals, 1) geomagnetic sensitivity, 2) path integration, 3) pilotage, and 4) hydrodynamic trail following. The project was conducted on the shore-fast ice of McMurdo Sound approximately 14 km NW of McMurdo Station. Seals instrumented with a custom-designed, archival video and data recorder were deployed individually at three study sites varying in geomagnetic profiles where they performed voluntary dives. Experiments were conducted during austral Spring in three consecutive years, with deployments occurring at times of daylight and twilight.

               Three-dimensional dive profiles were reconstructed for 4,449 dives performed by 10 seals. This included 3,758 loop dives. Five pieces of evidence from our experiments indicate that Weddell seals primarily used overhead visual cues to pilot under the ice cover during the study: 1) Many of the inbound paths of long-distance dives were very straight over their entire length. 2) Inbound paths were more linear when seals returned from shallower far-points than from deeper far-points. 3) When inbound paths were not strongly linear, seals traveled directly to a frequented route then turned toward home along a straight path. 4) There was a delay in the onset of long-distance dives (learning period) when seals were released at a new location. 5) Many (74%) of the straight inbound paths were directly below known, man-made linear surface disturbances in the snow and at least some of these linear disturbances were visible from below. Our results contribute to a growing body of literature indicating that animals can learn to use artificial and sometimes ephemeral landmarks (in other words use and remember maps) to guide their movements. 

            The results of this study have been disseminated in the proceedings volumes and at presentations associated with scientific conferences, as well as in peer-reviewed publications (i.e., Fuiman, Williams and Davis, Marine Biology 167:116, 2020), book chapters and books. In addition, this project has provided new methods in low-stress wildlife animal handling that have been incorporated into protocols recommended to researchers through the Office of Protected Species (National Marine Fisheries Service). The video and data recorders that we used on this project were refined and tested with eventual transition to availability to the scientific community. Outreach programs included primary and secondary school students, and the public through websites and the Seymour Marine Discovery Center (University of California, Santa Cruz). In general, the results and videos from our instruments showing the underwater navigation of the seals attracted an extraordinary amount of press and media coverage. This project also trained three graduate students, two animal behaviorist staff members, and over five undergraduate students that include members from underrepresented groups in the STEM disciplines. Major activities included instruction in state-of-the-art instrumentation for behavioral and physiological research. Skills obtained ranged from veterinary sciences, basic biological research, technology and instrument engineering as well as field research that promote advancement in state and federal agencies, academia, as well as conservation and wildlife management organizations. Importantly, there was extensive mentorship for encouraging women in STEM disciplines through the interactions of female members of our team at the PI, post-doctoral, technician, graduate and undergraduate student levels.

Last Modified: 11/29/2020
Modified by: Randall W Davis