Resting seals molt their skin
The spring months bring more seals to Piedras Blancas
than even the winter breeding season. All 5,000 or so adult females return,
along with five thousand or so non-breeding juveniles. It’s the busiest beach
season for these migratory seals.
The seals look ragged, like something is wrong with
them. Their skin is peeling off. For
northern elephant seals, it’s normal, the annual molt. They will be on the
beach through June.
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| Raggedy old brown skin peels off. |
Berney LeBoeuf, in his book Elephant Seals: Pushing the Limits on Land and at Sea, recounts an incident from the 1960s, when elephant seals were unfamiliar to local residents. A woman called police to investigate the scruffy-looking seal on the beach near her house. The policeman who responded agreed that the seal was suffering, and shot it to relieve its misery. “Of course, the seal was fine,” he writes, “it was just molting, as residents came to learn with the increasing influx of seals in the following years.”
Molting
Elephant seals, like only one other seal (the Hawaiian
monk seal), molt their skin annually in a few weeks. All other seals molt, but
gradually, one hair at a time, like dogs, so it isn’t as noticeable.
Elephant seals form a new skin layer under the old
skin. Blood stops supplying the old skin. It dies and peels off in pieces.
Scraps of it skitter across the beach. Friends of the Elephant Seal docents
have samples of it to show visitors. Children enjoy handling it. Adults are
more dubious, extending a cautious finger for a touch.
The seals stop eating when they are on the beach at
the rookery, fasting for that four weeks it takes for them to complete their
molt. Female seals lose about a quarter of their body weight while they lie on
the beach and let their skin peel off.
The new skin and hair underneath is gray. The hairs
are short, but soon stand up and grow longer. Look for pearly gray seals next
to brown and tan seals.
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| Silvery gray among last year's brown skin and fur. |
The skin begins to molt off around eyes and ears, old
scars, and other body orifices. Then the belly and sides and back peel off.
Old scars remain, but temporary marks peel off with
the skin. That’s one of the challenges of identifying seals for research study.
Numbers bleached or dyed on the fur peel off when the skin molts. Resighting
seals with dyed numbers before their skin peels off rarely happens. It depends
on the luck of the seal being at a place and time when someone sees the mark
and reports it.
If you see a marked seal, report it to a docent or
contact elephantseal.org.
Seals can also be identified with small plastic
flipper tags, but those numbers are difficult to read from a distance.
Pup embryo development
Unseen, next year’s pups are starting to develop. The
females mated after they weaned their pups in February and March. That
fertilized egg entered a state of embryonic diapause, or delayed implantation.
Development was suspended, allowing the mother to regain blubber and condition
after the weight loss of nursing her pup. Embryonic diapause times all births
to happen around the same time.
These slightly pregnant females spent the next ten
weeks feeding freely. Now, in April and May, their bodies prepare for the
embryo to begin gestation.
Embryonic diapause happens in other mammals. In some,
it’s a response to conditions. In elephant seals, all pregnancies start this
way.
Juveniles
Juveniles, male and female seals not yet ready to
breed, have been foraging and growing at sea while the adults dominated the
beach during the winter breeding season. The beach, although crowded, is calmer
now. No bulls fighting for dominance.
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| Seals of various sizes and ages molt together, except for the largest bulls, which are migrating. |
Bulls are on their northern migration. They will
return to the beach in July and August, time for them to molt their skin.
Mercury
bioaccumulation
The
molted skin takes with it some of the mercury that the seals have accumulated
from the mercury in the ocean. The mercury got there from coal-burning plants, discharged
into the atmosphere and rained down on the oceans. Top predators such as
elephant seals bioaccumulate it as it goes up the food chain.
Shed
skin and other excretions from the elephant seals at Ano Nuevo are a major
source of methylmercury contamination there, influencing the bottom of the food
web.
Methyl
mercury is a human neurotoxin. Mercury affects the seals’ hormone levels,
immune system and the ability to reproduce successfully, which may affect the
population. Research continues on how mercury may be affecting the seals.
Their
peaceful spring rest on the beach belies the drama of their lives, within their
pregnant bodies, and in the effects of human activities on their habitat.