We all know that the early bird catches the worm, but how many of us have stopped to apply the same thought process to the Weddell seal? Swap ‘worm’ for bottom-dwelling fish, squid, octopus, or prawns and you’ve stumbled into the territory of fascinating new findings into the strategic deep-diving habits of the planet’s southernmost living mammal. 

New research from the team at Woods Hole Oceanographic Institution (WHOI) has revealed that the Weddell seal is taking a far more nuanced approach to its daily diving and feeding routine than we initially thought – choosing to perform its most extreme dives in the early Antarctic mornings.

It’s well-documented that Weddell seals, the southernmost population of which inhabit the waters of Erebus Bay in Antarctica, go to great lengths to live in one of the planet’s harshest and most rapidly changing environments, reaching depths of more than 900 metres with some dives lasting up to 96 minutes.

But new research has now emerged that sheds a little more light on the strategic thinking, planning, and execution of these canny divers’ approach to their deepest dives.

According to an article now published in Communications Biology, Weddell seals strategically conduct their deepest, longest, and most extreme dives earlier in the day, rather than at peak foraging times at midday.

The thinking behind this approach? The paper’s lead author of the paper, Michelle Shero, an assistant scientist in Biology at WHOI, positions that it’s to allow the seals to maximise their recuperation time without encroaching on crucial mid-depth foraging time when under the midday sun.

“These extreme dives require longer recuperation times once the seal returns to the surface. So, if the seal makes extreme dives in the middle of the day when it is maximum light – there may be a lot of prey around that are easy to see, but the seals would still ‘miss out’ as they spent this time recuperating,” she said. 

“So, not conducting extreme dives when prey is most accessible may actually be the best strategy,” she added.

Conducting this research in the waters of Erebus Bay in Antarctica offered the researchers a natural experiment environment, enabling them to observe the animals’ diving habits through a whole year. Owing to its position, Antarctica is subject to daylight 24 hours a day in the summer and 24 hours of darkness during the winter.

It’s this that gave the researchers the chance to better understand how the seals’ feeding habits 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,” said Shero. “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 found was that the seals appeared to actually avoid making their most extreme dives during midday, allowing them to keep diving over and over without having to pause for long. This allows them to spend almost all their time underwater, foraging under high-light conditions, which is best for visual hunters.”

Shero and the team have, therefore, concluded that the Weddell seal is “making very strategic decisions” about when to perform their deepest and longest dives, allowing them to “best interact with the daily changing environment in the Antarctic.”

The team conducted their research by outfitting a select number of seals with Conductivity Temperature Depth-Satellite Relay Dive Loggers (antennas strapped to their heads) which collected a total of 8,913 seal days of behavioural data from 59 adult seals throughout the western Ross Sea in the Antarctic. The data helped shed this new light on the tradeoffs seals are making each day between the conservation of their oxygen stores and the energetic gains that could be realised 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 the journal article’s 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. Not only were they planning when to forage, but it also seems they were thinking ahead and planning when to rest.”

The team has since suggested that these findings are “likely applicable” to other visual predators inhabiting marine environments with extreme variations in light levels throughout the year, including the emperor penguin.

“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,” said co-author Daniel Costa. “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 or breathing holes in the ice.”

This research was funded by the National Science Foundation (NSF) and the Every Page Foundation, with support provided by NSF’s US Antarctic Programme.

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