To survive, tired elephant seals hide in a different way than their tick brothers.

Every year, northern elephant seals disappear on a seven-month, 6,000-mile (10,000-kilometer) journey across the North Pacific in search of edible fish and squid. They start the journey after sitting on the beach for a month or two – replacing their fur and losing fat – and gradually gain weight over the hunting trip. On these walks, elephant seals do not just swim on the surface – they dive continuously, day and night.

To rest, they swim down hundreds of meters below the surface of the ocean and then slowly retreat and move back and forth like falling leaves. These divers last about 25 minutes and are called drift divers. The resting time for drift diving is the most dangerous time for an elephant seal: Waking up in the sharks of a white shark or a deadly whale is not a good way to start the day.

We are two biologists studying diving and sleeping behavior in marine mammals. In particular, we are fascinated by the decisions elephant seals make as they travel the open ocean and manage major changes in their environment and their own bodies. The open ocean is a dangerous place, and animals must constantly measure the pressures of predation, hunger and fatigue. Choosing when to rest and when to eat has a negative impact.

Elephant seals lie underwater where they float up or sink slowly depending on whether they are fat or tired.Danielle Dube through an Art-Science residency with UC Santa Cruz Norris Natural History Center

Elephant seals have two options: rest during the dark of night and feed during the day when it is more difficult to get food through or rest during the day when there is a much higher chance of predator feeding and feeding at night when more fish and squid are available.

So we thought, are tired, hungry seals taking more risks than healthy, plump seals? Our latest study, published directly in the journal Science Advances, reveals that elephant seals improved their dangerous behavior throughout the hunting trip.

Danger vs reward

One simple question is fundamental to ecologists’ understanding of the natural world: Do hungry animals take more risks for food? This should be true in theory, as wild animals always measure the risks of hunger and predation. For most species, it is almost impossible to quantify continuous changes in health. As a result, many theories of danger and reward in the animal kingdom have been around for decades but have yet to be tested.

The ocean is a wonderful place to study danger and reward as light levels determine life and death in three dimensions: The surface of the ocean is clear, and predators can hunt much more easily, but the light descend swiftly as you dive deeper into the ocean. . For elephant seals, light levels are directly related to danger, as their main predators live in shallow waters and use light for hunting. For elephant seals, rest is safer at night when predators cannot find them.

Seals are most at risk during the day and in shallow water, but the lowest at night and in deep water.Illustrations by Danielle Dube, Infographic by Jessica Kendall-Bar

Light levels are also directly linked to reward as most elephant predators – fish and squid – migrate up and down the water column each day. During the day, when light levels are high, fish and squid live in the depths to avoid predators. However, at night, when light levels are low, fish and squid swim up closer to the surface to feed phytoplankton. For seals, hunting is more effective at night, when prey have 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 must choose one behavior over the other. Do they prioritize resting safely or feeding effectively? And does this change over time as they get fat?

A female elephant seal carries a satellite tag (on her head) and a time depth recorder (on her back).Dan Costa

Drift dives provide the answers

Thanks to a long-term monitoring program led by our colleague Dan Costa, our team had the opportunity to dive data from 71 tagged female elephant seals with small devices that record time, depth, light, world -width and longitude along every four seconds.

Interestingly – and at the heart of this research – when seals do drift divers, fat seals float up while tired seals sink. This means we can use transfer rates from our dive data to work out the percentage of seals of body fat over time. Using data on light, depth and time, we can also measure the level of risk. In other words, we know if seals are fat or tired, and we know what risk they are taking. Even better, we know these two metrics consistently over the course of their journeys.

By measuring body fat and taking risks over time, we learned that elephant seals took more risks when they were tired and prioritized safety when they were obese. Early in the hunting trip, when the average body fat was 22% fat in the body, they rested immediately after sunrise – 80% of their resting divers happened during the high-risk day. This allowed them to make most of their meals at night when it is easier to find food.

Later on the hunting trip, when seals had absorbed up to 35% of their body fat, they rested just before sunrise. Only 30% of their resting divers happened during the high-risk day. Gradual movements in body and posture over the 220-day hunting trip accumulated a remarkable six-hour shift in average rest time by the end of the trip.

We also found that fat seals lay 300 feet (100 meters) deeper in the water – where it is also 300 times darker – than where thinner seals rested. This supports the notion that seals strategically alter their exposure to light levels – using both rest (time) and rest (place) depth chart – to reduce risk. This is what we call light fear.

Animation by Jessica Kendall-Bar, illustrated by Danielle Dube through an Art-Science Residency with UC Santa Cruz Norris Natural History Center.

Lessons from seals at sea

Our study provides a window into the real-time decisions of an elephant seal in the open ocean as it measures the impact of a nap below the ocean’s surface. Although light has previously been identified as an essential environmental barrier, no study has consistently examined an animal’s use of light compared to actual shifts in its fat and health sources.

By observing these metrics together, we were able to better understand the behavior of a wild animal trying to find food while trying to be 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 was originally published An Conversation le Roxanne Beltran and Jessica Kendall-Bar at UC-Santa Cruz. Read the original article here.

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