Each year, northern elephant seals set out on a seven-month, 10,000-kilometer journey across the North Pacific in search of fish and squid to eat. They begin the journey after spending a few months on the beach – changing their coats and losing fat – and gradually gain weight during the foraging trip. During these excursions, elephant seals don’t just swim at the surface – they dive continuously, day and night.
© Dan Costa
Female elephant seals go on seven-month feeding rides balancing danger, hunger and exhaustion.
To rest, they swim down hundreds of feet below the ocean surface, then roll slowly onto their backs and drift like falling leaves. These dives last approximately 25 minutes and are called drift dives. Drift diving dormancy is the most dangerous time for an elephant seal: waking up in the jaws of a white shark or killer whale is not a good way to start the day.
Charging error
We are two biologists studying diving behavior and sleep in marine mammals. We are particularly fascinated by the decisions that elephant seals make as they roam the open ocean and navigate extreme changes in their environment and their own bodies. The open ocean is a dangerous place and animals must constantly weigh the risks of predation, hunger and exhaustion. Choosing when to rest and when to eat has serious consequences.
Elephant seals have two options: rest during the safety of the dark night and feed during the day when food is more difficult, or rest during the day when they are much more likely to be eaten by a predator and feed during the night when there is more. fish and squid are available.
So we asked ourselves, do lean, hungry seals take more risks than fat, healthy seals? Our latest study, just published in the journal Science Advances, reveals that elephant seals have refined their at-risk behavior during the foraging journey.
Risk versus reward
A simple question is fundamental to ecologists’ understanding of the natural world: Do hungry animals take more risks to find food? This should be true in theory, as wild animals are constantly weighing the risks of starvation and predation. For most species it is nearly impossible to measure continuous changes in health. As a result, many theories of risk and reward have existed in the animal kingdom for decades, but these have yet to be tested.
The ocean is a fascinating place to study risk and reward, because light levels determine life and death in three dimensions: the ocean’s surface is clear and predators can hunt much more easily; but the light quickly fades as you dive deeper into the ocean. For elephant seals, light levels are directly related to risk, as their main predators live in shallow waters and use light to hunt. For elephant seals, resting at night is safer when predators cannot find them.
© Illustrations by Danielle Dube, Infographic by Jessica Kendall-Bar
Sleeping seals are most at risk during the day and in shallow water, while the risk is lowest at night and in deep water.
Light levels are also directly related to reward, as most elephant seals – fish and squid – migrate up and down the water column every day. During the day, when the light is high, fish and squid stay in the depths to avoid predators. But at night, when light levels are low, fish and squid swim closer to the surface to feed on phytoplankton. For seals, foraging at night is more efficient, when prey has emerged from the depths to find their own food.
This means that the best time to eat is also the safest time to rest, and seals should choose one behavior over another. Do they prioritize safe rest or eating efficiently? And does this change over time as they get fat?
© Dan Costa
A female elephant seal wears a satellite label (on her head) and a time-depth recorder (on her back).
Drift dives provide the answers
Thanks to a long-term monitoring program led by our colleague Dan Costa, our team had access to dive data from 71 adult elephant seal females tagged with small devices that record time, depth, light, latitude and longitude every four seconds.
Interesting – and central to this research – is when seals drift dives, fat seals float up while skinny seals sink. This means that we can use drift speeds from our dive data to calculate the body fat percentage of the seals over time. Using data on light, depth and time, we can also approximate the risk level. In other words, we know whether seals are fat or skinny, and we know how much risk they are taking. In fact, we know both statistics continuously during their foraging journeys.
By simultaneously measuring body fat and risk-taking over time, we learned that elephant seals took more risks when they were lean and prioritized safety when they were fatter. Early in the foraging trip, when seals averaged only 22% body fat, they rested just after sunrise – 80% of their rest dives occurred during the risky day. This allowed them to do most of their foraging work at night when food is easier to find.
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Later in the foraging trip, when the seals had pumped up to 35% body fat, they rested just before dawn. Only 30% of their rest dives took place during the high-risk day. Gradual shifts in body condition and behavior over the 220-day foraging trip amounted to an impressive six-hour shift in mean rest time at the end of the trip.
We also found that thicker seals lay 300 feet (100 meters) deeper in the water – where it is also 300 times darker – than where thinner seals rested. This further supports the idea that seals strategically adjust their exposure to light levels – using both a rest schedule (time) and depth of rest (space) – to minimize risk. We call this the light landscape of fear.
Lessons from seals at sea
Our study provides a glimpse into the real-time decision-making of an elephant seal in the open ocean, while weighing the consequences of a nap beneath the ocean’s surface. While light has previously been identified as a critical environmental constraint, no study has continuously tracked an animal’s use of the light landscape against extreme shifts in its fat storage and health.
By tracking these statistics together, we were able to better understand the behavior of a wild animal trying to find food, while trying to prevent it from becoming food. Using elephant seals as a model, we can begin to understand how these rules apply to other species – from birds to bats to bears – and scale up to affect entire ecosystems.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.
Roxanne Beltran receives funding from the National Science Foundation and National Geographic.
Jessica Kendall-Bar receives funding from the National Science Foundation and the Office of Naval Research.
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