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.
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