Erebus Bay, Antarctica, is home to the southernmost population of the world’s southernmost living mammal — the Weddell seal. These seals may look like couch potatoes when they are resting on the ice, but Weddell seals go to great lengths to make a living in the harshest and most rapidly changing environments. Weddell seals are exceptional divers that can reach depths of more than 900 meters (2,952 feet) with some dives lasting 96 minutes — well beyond their aerobic threshold.
New research from the Woods Hole Oceanographic Institution (WHOI) and partners sheds light on a novel dive foraging strategy, striking a fine balance when it comes to pushing their breath hold capacities and trying to maximize use of the seasonal Antarctic light.
According to the just published article in Communications Biology, the seals strategically conduct their deepest, longest, and most extreme dives earlier in the day, rather than during peak foraging times at midday.
Lead author Michelle Shero, an assistant scientist in Biology at WHOI, explained, “These extreme dives require longer recuperation times once the seals return to the surface. So, if the seals make extreme dives in the middle of the day when it is maximum light — there may be lots of prey around that are easy to see, but the seals would still ‘miss out’ because they’d have to spend a lot of time recuperating. Not conducting extreme dives when prey is most accessible, may actually be the best strategy.”
“We are seeing these animals through a whole year when it’s daylight 24/7, a combination of daytime and nighttime, and then dark 24/7, providing a ‘natural experiment’ to better understand how the seals’ feeding strategies change with a range of light conditions. During peak daylight hours, almost all animals including the seal’s prey will descend deep into the water column to avoid predation. We expected that the seals would follow their food and that their deepest, longest dives might occur at midday when the sun is overhead. But what we surprisingly found was that the seals appeared to actually avoid making their most extreme dives during midday,” said Shero, “allowing the seals to keep diving over and over without having to pause for long.”
“This allows them to spend almost all of their time underwater, foraging under high-light conditions, which is best for visual hunters,” continued Shero. “These animals are making very strategic decisions about when to make their long dives, so they can best interact with the daily changing environment in the Antarctic, allowing them to maximize their prey.”
The researchers outfitted the seals with Conductivity Temperature Depth-Satellite Relay Dive Loggers, which collected a total of 8,913 seal days of behavioral data from 59 adult seals throughout the western Ross Sea in the Antarctic. The data helped shed light on the tradeoffs the seals make between the conservation of their oxygen stores and the energetic gains that could be realized with capturing prey.
“For me, one of the more surprising findings of this research was our discovery that Weddell seals ‘plan’ their activities in a much more nuanced way than I had previously appreciated,” said journal article co-author Jennifer Burns, professor and chair of the Department of Biological Sciences at Texas Tech University, Lubbock. “Not only did the seals shift the average depth and duration of their dives in response to current light levels, but they also made their longest and most taxing dives at times when their recovery period was least likely to negatively impact overall foraging success. So not only were they planning when to forage, but it also seems that they were thinking ahead and planning when to rest.”
“Although it may not be surprising that a visual predator would maximize foraging when daylight is most intense, scientifically showing Weddell seals are in tune with circadian rhythms/light levels while foraging is fascinating,” said article co-author Kimberly Goetz, research fisheries biologist with the Marine Mammal Laboratory in Seattle, Washington. “Another important finding was that the circadian patterns were at odds with physiological constraints, in that exceedingly long dives did not overlap with optimal conditions for visual hunting presumably due to the time needed to recover afterward. Instead, animals likely opt to take full advantage of daylight by performing shallower dives that do not require as much recovery time before continuing to forage. The findings in this paper are likely applicable to other diving visual predators inhabiting marine environments with extreme variations in light levels throughout the year, including emperor penguins.”
Article co-author Daniel Costa, distinguished professor of ecology and evolutionary biology at the University of California, Santa Cruz, noted, “Weddell seals live in one of the most hostile environments on the planet and need to keep their internal clock running during periods when the sun never sets or when the sun never rises. Our study found that they take great advantage of the periods of short-day length to make the longest dives that push their physiological ability to the extreme to do the most hunting when there is light to enable their search for food and or breathing holes in the ice.
Funding for this research was provided by the National Science Foundation (NSF) and the Every Page Foundation, with logistical support provided by NSF’s U.S. Antarctic Program, Raytheon Polar Services, and Lockheed Martin ASC.