Washington (AFP) - Fat elephant seals prioritize hiding themselves from predators during their vast, months-long foraging expeditions in the open ocean, while skinny seals need to take more risks until they're nice and plump, a study showed Wednesday.

The paper, published in the journal Science Advances, is the first to continuously measure changes in behavior relative to body fat, proving decades-old ecological theories about how wild animals balance perils against payoffs.

First author Roxanne Beltran, an assistant professor of ecology and evolutionary biology at UC Santa Cruz, told AFP she had long been fascinated by elephant seals, which assemble on California's beaches in winter to breed, give birth, nurse their young and molt before embarking on their summer journey across the North Pacific Ocean.

"It's difficult for us to wrap our minds around what these animals endure during the seven months, 10,000-kilometer (6,000-mile) migrations because it's so fundamentally different from what we do," said Beltran.

Despite appearances, the sea mammals are tremendous athletes. 

During their journey, they dive to depths of 700 meters (2,300 feet) in search of fish and squid, averaging 23 minutes per dive with just two minutes at the surface to breathe. If needed, they can hold their breath for up to two hours.

To learn more about what drives the seals' decision making, the researchers outfitted 71 adult female northern elephant seals with satellite tags and time-depth recorders, logging vast amounts of data between 2004-2012.

The team was particularly interested in knowing how the seals split their time between day and night, because they are caught between competing priorities, a dynamic the authors dubbed "lightscapes of fear."

The sharks and killer whales that hunt them are visual predators, so elephant seals are safer resting at night and in depths where light doesn't penetrate, rather than during the day.

At the same time, the night time is a more productive feeding period for elephant seals, when their own prey is closer to the surface. Resting at night therefore means sacrificing prime foraging time.

They know how fat they are

From the tracking data, the team was able to make out several different types of dive: foraging dives, when the seals swim down and pursue prey at depth; transit dives with no feeding activity; and rest dives, when they drift passively and are thought to be sleeping.

The resting phases enabled the team to calculate the seals' percent of body fat based on their buoyancy in the water, a method developed and validated over recent years.

"It gives us a nearly continuous record of how fat they are, which is unparalleled for any wild animal other than elephant seals," said Beltran.

At the beginning of their migration, the elephant seals are skinny and sink through the water column. But as the feeding migration progresses, they gain weight and their buoyancy becomes less negative until they become positively buoyant, when their body fat reaches 30 percent.

The team found that as they fattened up, the animals were more likely to engage in low-risk rest at night rather than high-reward hunting, proving an ecological theory known as "state-dependent risk taking" while refuting a hypothesis that feeding would always be a higher priority than rest.

The seals in good body condition also started their rest drifts at greater depths while those in poor body condition started their rest drifts closer to the surface. 

This makes sense since the fat seals will drift upward towards the dangerous sunlit surface, while the skinny seals sink further down. It also suggests the seals can perceive their own buoyancy -- in other words they know how fat they are.

Intriguingly, the team found that both the upward floating fat seals and sinking skinny seals ended their rest drifts at similar depths, suggesting a mysterious mechanism at play -- perhaps they can also perceive pressure with great accuracy.

As interest grows in fishing in the ocean's "twilight zone" of 200 to 1000 meters (650 to 3,300 feet), there is increasing potential for humans to target the same fishes the elephant seals eat. 

The findings thus illuminate future conservation efforts, said Beltran, in addition to advancing our knowledge about this still mysterious ecosystem.