Seal Olympians
Three seals’ long migrations featured in new exhibit
The world’s attention may be on human athletes at the
Olympics in Paris, but elephant seal observers are celebrating seal athletes. A
new exhibit at the Friends of the Elephant Seal Visitor Center in San Simeon
puts three seals on the metaphorical platform, for migrations of more than 6,000
miles over seven months.
The exhibit characterizes them as three human swim
champions: Diana after Diana Nyad, Katie after Katie Ledecky, and
Trudy after Gertrude Ederle. Diana Nyad is a long-distance open water
swimmer who swam from Cuba to Florida in 2013, through stinging jellyfish and sharks.
Katie Ledecky is a competitive swimmer who has won seven Olympic gold medals
(2012, 2016, and 2020) and 21 world championships. Gertrude Ederle was the
first woman to swim the English Channel in 1926.
 |
| Kathy Curtis photo |
Two annual migrations
Female elephant seals have the advantage over human
swimmers, spending 10 months of the year in the ocean. They swim two annual
migrations, a short one and a long one. The short migration, about ten weeks,
comes after they wean their pups in the early spring. Elephant seal mothers
don’t eat while they are nursing their pups, so they lose about a third of
their body weight. It’s physiologically draining.
By the time the pup is big enough to wean, the mother
is thin. She needs to regain blubber. She leaves the beach in February or
March, returning in April or May. At that time, she stays on the beach for four
or five weeks to molt her skin. When she returns to the ocean, it’s for her
long migration, seven to eight months. The next time she comes out of the
water, it will be to have her pup in January.
What are they doing out there?
All elephant seal females spend those months feeding in
the open waters of the North Pacific. But some go far west, 3,200 miles west,
to feed. Roxanne Beltran’s research team at University of California Santa Cruz
documented the long migrations by tagging female seals. One, nicknamed Diana,
was tagged twice, and made the same journey twice.
Female elephant seals have to forage almost constantly
to gain weight and support the developing pup. Most forage on small fish in the
mesopelagic layer, although about 15 percent dive down to forage on the ocean
floor along the continental shelf or at seamounts.
They have to gain enough weight to support them in
their travels, and to develop a pup and then carry them through nursing the pup
for a month. That’s a lot of blubber.
They share the mesopelagic layer with a few other
large species: sperm whales, beaked whales, blue sharks, and salmon sharks. Scientists
don’t know much about how they all share this ocean resource.
The New York Times recently published an OpEd on the Ocean Twilight Zone.
Season statistics
The exhibit gives these athletes’ stats on their
migrations: How far they went, how long they were at sea, how much weight they
gained. With one exception, they each did it while pregnant, having a pup every
year.
Trudy and Diana spent 223 days at sea. Katie made her
trip, to a point 3,436 miles west, and back in only 185 days.
What’s out there?
They all went to the same general location, but
probably didn’t interact with each other. Seals are solitary.
“Although there are a lot
of elephant seals, maybe 240,000 total, they are foraging in a HUGE volume of
the ocean,” Beltran wrote in an email. “so they likely don't encounter each other
often.”
Perfect timing
The tricky part is heading back to the
beach in time to give birth to the pup. They have to find their way around the
open ocean, catch and eat enough fish to gain weight and support the developing
pup, and then turn around and find their way to the beach to have that pup.
They give birth within five days of returning to the beach.
Beltran’s research team found that the
seals have a “map sense” that allows them to know how far they are from that
home beach, and how to manage their time, at about 100 miles a day, to get
back.
“We found that the seals’ abilities to adjust the
timing of their return migration is based on the perception of space and time,
which further elucidates the mechanisms behind their astonishing navigational
feats,” the academic paper’s summary states.
No one yet knows how they do it. Geomagnetic,
celestial, acoustic, or olfactory senses may be involved.
For now, observers use electronic tagging to learn
what the seals are doing, and welcome them back to the beach when they arrive
in January, in time to have their pups.
August seal viewing
The bulls, fully mature ones and younger ones, are on
the beach at Piedras Blancas. They are molting their skin, the annual shedding.
FES docents have samples of shed skin for visitors to handle.
Bulls are generally quiet at this time. With no
females to fight over, they spend their days resting on the sand.
No comments:
Post a Comment