The Benefits of Being Fat, But Not Obese: Elephant Seals Dive Better

It turns out that being fat may be an advantage--at least for elephant seals. With the help of tracking devices, scientists have found that adding body fat helps seals dive more efficiently by changing their buoyancy.

"The ideal is to be neutrally buoyant, and when you move away from that in either direction there's a cost," said Daniel Costa, co-author of the new study, in a news release.

Elephant seals actually fast during their time onshore, burning the fat that they store up during the months that they spend at sea feeding on fish. In fact, a female nursing a pup can lose 30 to 40 percent of her body weight. This means that she's negatively buoyant when she heads back out to sea and can sink into the deep water without swimming very hard. In fact, she can dive between 1,000 to 2,000 feet or more in order to feed. Yet coming back up to the surface takes a lot of effort.

In order to better understand the effort it takes female elephant seals to swim and dive, the scientists tagged seals with tags that had built-in accelerometers. The researchers found that when the seals put on enough fat to achieve neutral buoyancy, the number of strokes needed to complete a round-trip from bottom to surface was at its lowest.

"At neutral buoyancy, they're able to spend more time foraging at depth because they're spending less energy moving up and down," said Costa. "We've known for years that elephant seals gain a lot of fat when they're at sea, and we've always looked at it in terms of the advantages of laying down fat to deal with period of fasting. But we hadn't thought much about how it affects their diving."

The findings reveal a bit more about how these seals thrive in the wild. It may also explain why "superweaner" pups, which grow twice as big as other pups by nursing from multiple females, are rarely seen after they leave the rookery. Their brain tells them to get as fat as possible but when they head out to sea, it's difficult for them to figure out how to feed because they're so buoyant.

The findings are published in the journal Proceedings of the Royal Society B.