Just enough fat is good for an elephant seal

In the animal world, fatter is often better. Fat stores can help a critter survive lean times or stressful periods such as motherhood. But for marine mammals, body size has additional implications because fat affects an animal’s buoyancy: The less fat an animal has, the denser its body is. And when average density is greater than the water in which an animal swims, it tends to sink.

Studies have suggested that marine mammals can reduce the energy costs of swimming by achieving neutral buoyancy — when the body’s average density is equal to the water the animal is swimming in. A negatively buoyant creature (one that tends to sink) would have an easier time descending into the depths, but it would also spend more energy on the ascent. Overall, negatively buoyant animals should expend more energy swimming than neutrally buoyant ones, but researchers haven’t been able to prove this because measuring energy use of marine mammals at sea is difficult (you can’t stick a whale on a treadmill in the middle of the ocean).

So Taiki Adachi of the Graduate University for Advanced Studies in Tokyo and colleagues devised a device to measure the stroke flipper frequency of northern elephant seals. That frequency is tied to the animal’s oxygen and energy use. The researchers attached this “stroke logger” to 14 female elephant seals while they were out of the water at Año Nuevo State Reserve in California to breed or molt. The researchers then recorded the animals’ movements and activity over the next few months as the elephant seals swam back out into open water to forage and fatten up. The results of the tracking appear November 5 in Proceedings of the Royal Society B.

Elephant seals spend most of their lives out in open water, feeding off bottom-dwelling organisms including squid, eels and sharks. The tracked seals, the study found, started off their time negatively buoyant — their time on land had consumed fat stores. Over the next weeks, though, the mammals gained fat and became more buoyant, as indicated by the seals’ reduced movement up or down in the water column during time spent drifting. That extra fat meant that the seals needed more strokes to go deeper but fewer as they ascended toward the surface. And overall, they needed less energy to make the round trip from the deep when they were fatter.

Neutral buoyancy had an advantage: It let the seals forage for nearly twice as long when they reached the bottom of a dive, the researchers found. The results “suggest that seals gained a foraging benefit from achieving neutral buoyancy by being fat,” Adachi and colleagues write.

Achieving that optimal level of fatness could have similar benefits for other marine mammals, or even large marine vertebrates such as white sharks, that increase and decrease fat stores over the course of long migrations, the researchers note.