In Erebus Bay, Antarctica, live Weddell seals, the southernmost population of the world’s southernmost mammals. While they may appear lazy as they lie on the ice, these seals are remarkable divers, capable of plunging to depths exceeding 900 meters (2,952 feet) and holding their breath for up to 96 minutes—far beyond what is typically sustainable.
A recent study by the Woods Hole Oceanographic Institution (WHOI) and its collaborators has revealed an innovative approach that these seals use when hunting for food in the challenging and ever-changing Antarctic landscape.
As detailed in a new article in Communications Biology, the seals tend to make their deepest and longest dives earlier in the day instead of during the prime feeding hours at noon.
Lead researcher Michelle Shero, an assistant scientist in Biology at WHOI, stated, “Because extreme dives require longer rest periods once the seals ascend, making these dives during peak daylight when prey is abundant might not be beneficial, as they would have to recuperate for a long time. By opting not to perform these intense dives when prey is most visible, they actually optimize their feeding strategy.”
“Our year-long observations capture a unique cycle that includes continuous daylight, a mix of day and night, and complete darkness. This provides a rare opportunity to study how the seals adjust their feeding strategies according to varying light levels. Contrary to what we expected, we found that the seals tend to avoid their most taxing dives at midday, instead diving frequently without long recovery times,” Shero explained.
“This strategy enables the seals to maximize their underwater activity, allowing them to hunt effectively in bright light, which is advantageous for visual predators,” Shero added. “They show remarkable judgment in scheduling their deep dives to align with the daily fluctuations of their environment, enhancing their prey capture.”
To gather data, researchers equipped the seals with Conductivity Temperature Depth-Satellite Relay Dive Loggers, recording 8,913 seal days of behavior from 59 adults in the western Ross Sea. This information illuminated the seals’ trade-offs between preserving their oxygen supplies and seizing energy opportunities by catching prey.
Co-author Jennifer Burns, a professor and chair of the Department of Biological Sciences at Texas Tech University in Lubbock, expressed, “One surprising discovery was that Weddell seals exhibit far more intricate planning in their activities than I initially recognized. They not only adapted their dive depth and duration based on light conditions but also timed their longest and most strenuous dives to minimize the impact of recovery on their overall foraging success. It appears they’re not just deciding when to hunt but also strategically planning their rest periods.”
Co-author Kimberly Goetz, a research fisheries biologist at the Marine Mammal Laboratory in Seattle, mentioned, “While it seems logical for a visual hunter to maximize foraging in bright conditions, it’s fascinating to see that Weddell seals synchronize their hunting with light levels. Another key finding was that their natural rhythms conflicted with physiological realities, as prolonged dives did not coincide with optimal hunting conditions due to the recovery time required afterward. Thus, they likely prioritize shorter dives that allow quicker recovery to make the most of daylight.” The insights from this research could also apply to other visual predators in variable light marine environments, including emperor penguins.
Daniel Costa, a distinguished professor of ecology and evolutionary biology at the University of California, Santa Cruz, noted, “Weddell seals navigate one of the planet’s most extreme habitats, needing to regulate their internal clocks amid constant light or darkness. Our findings show that they strategically utilize shorter daylight periods to perform their longest dives, testing their physical limits to hunt effectively when light is available for locating food and breathing openings in the ice.”
This research was funded by the National Science Foundation (NSF) and the Every Page Foundation, with logistical assistance from NSF’s U.S. Antarctic Program, Raytheon Polar Services, and Lockheed Martin ASC.
