Weddell seals have a surprising survival strategy

Deep in Antarctica’s Erebus Bay lives the robust Weddell seal, the southernmost mammal in the world. You’re acquainted with the seals, but how much do you really know about them?

When at rest on the icy plains, they bear resemblance to an overfed feline sprawled languidly on a rug. Yet, the comparison stops at appearances.

Weddell seals are phenomenal divers. Their intriguing underwater exploits have marked records of over 900 meters (2,952 feet) deep dives, some extending to a mind-boggling 96 minutes.

But what’s even more fascinating is their recent diving escapade that has caught the attention of marine researchers worldwide.

Diving strategy of Weddell seals

Researchers at the Woods Hole Oceanographic Institution (WHOI) have discovered a novel dive foraging strategy used by Weddell seals.

The seals, it seems, have learned to play the tactical game of survival by striking a delicate balance between their breath-holding expeditions and maximizing the use of seasonal Antarctic light.

According to the study, the seals conduct their deepest, most prolonged dives right at the start of the day, instead of during the sun-bathed peak hours.

“These extreme dives require longer recuperation times once the seals return to the surface,” said lead author Michelle Shero, an assistant scientist in biology at WHOI.

“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.”

Foraging in different light conditions

What the research team witnessed was these animals under varying light conditions, counting daylight 24/7, a mix of day and night, and then dark 24/7. This provided a “natural experiment” to help understand how the seals’ feeding strategies change with a range of light conditions.

“We expected that the seals would follow their food and that their deepest, longest dives might occur at midday when the sun is overhead,” said Shero.

“But what we surprisingly found was that the seals appeared to actually avoid making their most extreme dives during midday, allowing the seals to keep diving over and over without having to pause for long.”

This avoidance effectively allows the seals to dive repeatedly without requiring lengthy pauses.

Strategic decisions of Weddell seals

By avoiding peak-hour extreme dives, the seals effectively spend nearly all their time underwater, foraging under high-light conditions, which is best for visual hunters.

“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,” noted Shero.

Equipped with Conductivity Temperature Depth-Satellite Relay Dive Loggers, the research team collected data from 59 adult seals from the western Ross Sea in Antarctica, accounting for 8,913 seal days of behavioral data.

Weddell seals plan their activities

Study co-author Professor Jennifer Burns is chair of the Department of Biological Sciences at Texas Tech University, Lubbock.

“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 Professor Burns.

“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.”

In tune with their surroundings

“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,” noted Daniel Costa, professor of ecology and evolutionary biology at the University of California, Santa Cruz.

“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.”

The interpretation of the data has led to the realization that these seals are in tune with circadian rhythms and light levels while foraging in Antarctica. The research provides insights into the survival strategies of Weddell seals in one of the most rapidly changing environments.

The study is published in the journal Communications Biology.

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