When it comes to elephant seals, it seems the fatter the better. You would think that more weight would prevent these deep-divers from reaching deeper water, but in fact it is beneficial - to a point.

During their time onshore, elephant seals fast and burn the fat stored up during months at sea feeding on fish. A mother seal nursing a pup, for example, can lose 30 to 40 percent of her body weight. And when these females head back out to sea their decreased body weight makes them negatively buoyant - meaning they can sink into deep water without expending much energy. This means it's easier to dive when heading back out again, but when surfacing it makes for a harder workout.

The study, published in the journal Proceedings of the Royal Society B, found that as time went on and elephant seals started to put on more fat, they are able to maximize the time spent feeding during dives. However, seals have to be careful not to tip the scales too much because there is a limit to the benefits of being fat - and the same goes for being too skinny.

"The ideal is to be neutrally buoyant, and when you move away from that in either direction there's a cost," co-author Daniel Costa explained in a press release.

Neutrally buoyant means these massive mammals neither float up nor sink down in the water, allowing them to spend less energy swimming. And given that they dive to depths of 1,000 to 2,000 feet or more, they need all the help they can get.

Using a new type of tracking device, called a Stroke Logger, on female elephant seals, researchers could count the number of strokes they took. This provided the research team with a way to measure the energy costs of swimming.

As the migration progressed and the seals put on more fat, the total number of strokes needed to complete a round-trip gradually declined, reaching its lowest point when the seals achieved neutral buoyancy. Specifically, they swam harder and with more strokes on the descent of their dives, but decreased their strokes when surfacing. Without the fat, the return trip would be a lot harder.

Costa compared having less fat to a scuba diver wearing a weight belt.

"Getting down is easier, but you have to swim hard to come back up."

As seals gain weight it's easier to get back up and harder to swim back down, since fat floats. That's why it took more strokes to swim down the fatter they were, compared to less strokes when they were negatively buoyant and skinny.

So while more fat may make elephant seals more huggable, it also makes them more effective deep-divers overall.