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Found: The Ideal Fatness for Elephant Seals

By Elizabeth Preston
Nov 11, 2014 8:24 PMNov 20, 2019 1:06 AM


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Like many new mothers, a female elephant seal puts herself on a strict diet after giving birth. She dives into the Pacific and spends two months eating everything she can find. It's only by working hard at building up her blubber stores that she can get back her ideal body. Northern elephant seals (Mirounga angustirostris) spend 9 to 10 months of the year at sea. Twice annually, the animals haul their enormous bodies ashore. In the winter, they gather on beaches in Mexico and Southern California for breeding and mating. Females deliver their pups and nurse them; males defend "harems" of dozens of mates and work on impregnating them again. While on land, the seals fast. Then they go back to the ocean, abandoning the babies to their own devices. In the spring, the seals return to the same beaches to molt, shedding their fur and even some skin before spending the rest of the year in the ocean. During their travels, northern elephant seals may migrate as far as Alaska. They make dives almost half a mile deep, pursuing squid, fish, and other animals unfortunate enough to be in their paths. But to regain the body mass that they lost while fasting on land, they have to bank their calories. Energy that they save while swimming can be spent on longer dives. Energy gained from a stomach full of squid can be used to hunt some more. Taiki Adachi, a graduate student in the polar science department at Tokyo's Graduate University for Advanced Studies, wanted to learn how a migrating seal's increasing blubberiness affects its swimming. Does a fatter, more buoyant seal need to spend less energy on swimming and diving? And is this beneficial overall? He and his colleagues developed a new type of accelerometer to find out. When worn by an elephant seal, the device can monitor cyclic patterns in speed and count each surge forward as one stroke of the flippers. By also tracking depth and swimming angle, the device can constantly measure the seal's rate of strokes per distance traveled. Seals that make more strokes are working harder. The researchers captured 14 female Mirounga angustirostris and affixed the accelerometers to their backs. They also outfitted each seal with radio and GPS transmitters. Half the seals were monitored during their "short migration," the two months following breeding. The rest were tracked during the seven-month "long migration" that follows molting. Although the scientists were limited by the battery life of their instruments, they were able to collect data over the entire short migration, as well as the first 140 days or so of the long migration. The GPS transmitters announced when the elephant seals had returned to their home beaches. There, scientists used radio signals and plain old binoculars to pick out tagged seals from the rest of the colony. After removing the loggers, they sent the seals back on their way. For any point in time, the scientists could estimate a seal's fatness by seeing how much it drifted down in the water when it wasn't actively swimming. At the beginning of each migration, the starved seals had "negative buoyancy." In other words, they tended to sink. But as their roving fish binge progressed, the seals became more and more buoyant. As the blubbery seals gained buoyancy, swimming became easier. They needed slightly more flipper strokes to make their deep dives, but many fewer strokes to ascend. This meant that overall, fatter seals used fewer strokes to cover the same distance. The scientists had predicted that saving energy in swimming would allow the seals to spend more energy elsewhere, and this seemed to be true. As the seals got fatter, they doubled the amount of time they spent at the bottom of their dives

, from about 10 minutes to 20. (The bottom of the dive is where they find the most food.) After two months at sea, all the seals were still negatively buoyant, though their blubber had notably increased their buoyancy. After about five months, when the loggers stopped gathering data for the long migration, 5 out of 7 seals had become "neutrally buoyant"—when drifting in the ocean, they didn't sink or rise. Fatter seals can spend less energy swimming and more time eating, which gives them even more energy. So do they keep gaining blubber indefinitely? "Yes, I think ​they get fatter to become positively buoyant," Adachi says. If he could have monitored the seals all the way to the end of their long migration, he thinks he would have seen them gain so much blubber that they tended to float. Other research has found that elephant seals become positively buoyant, he adds. Adachi thinks the best state for elephant seals—the body type that keeps them swimming most efficiently—is neutral buoyancy. Yet the hungry animals, gearing up for their next fast, keep eating beyond that. Adachi says that when elephant seals come to shore after their long migration, 40 percent of their body mass is fat. For them, it's the perfect beach body.

Image: by Allie_Caulfield (via Flickr)

Adachi, T., Maresh, J., Robinson, P., Peterson, S., Costa, D., Naito, Y., Watanabe, Y., & Takahashi, A. (2014). The foraging benefits of being fat in a highly migratory marine mammal. Proceedings of the Royal Society B: Biological Sciences, 281 (1797), 20142120-20142120 DOI: 10.1098/rspb.2014.2120

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