For us landlubbers, a few extra pounds can be energetically and aesthetically costly. But the opposite is true for northern elephant seals (Mirounga angustirostris, pictured). Adding to their girth makes them more buoyant and more efficient swimmers, marine biologists report online today in the Proceedings of the Royal Society B. Diving for marine mammals takes a lot of energy, and the more energy it takes to move, the less time they can spend underwater chasing down prey. Early research had indicated that it was good to be denser than water, as that made diving easier. But what about coming back up? Biologists made a special logger that counted flipper strokes per meter and measured depth and acceleration for months and attached them to 14 female northern elephant seals as they traveled as far as 4000 kilometers away from their breeding grounds in California. During breeding season, the females fast, becoming quite lean; they put on the pounds because they hunt as they migrate. Once they plump up, they need to take half as many strokes per meter and have an easier time ascending after a dive, the researchers say. As a result, the seals stayed underwater longer and hunted more, they said.