Saturday, April 25, 2015


This month's column in The Cambrian:

The seals look like something terrible is happening to them. Their skin is peeling off! Talk about splotchy skin. But for northern elephant seals, it’s the normal, annual molt.

The adult females are on the beach in April and May, accompanied by juvenile seals of both sexes.
Elephant seals, like only one other seal (the Hawaiian monk seal) molt their skin annually in a few short weeks. All other seals molt, but most do so gradually, so it isn’t as noticeable.

A new skin layer forms beneath the old skin. Blood supply to the old skin ceases and the old skin peels off in pieces. Scraps of it skitter across the beach. Friends of the Elephant Seals docents have samples of it to show visitors. Children enjoy handling it. Adults seem to be more dubious, extending a cautious finger for a touch.

The adult males are gone, and this year’s weaners have left on their first migration. The beach is full of seals, but the drama of the breeding season has passed. There are actually more seals on the beach in May than during the breeding season.

The seals stop eating when they are on the beach at the rookery, fasting for that four to six weeks it takes for them to complete their molt.

Females gain weight
When the females were last on the beach, at the end of lactation in February, they were thin. They’d converted their blubber to milk to nourish their pups. They’ve spent the past two months eating and gaining weight.

Now, while they are molting, other invisible, but significant, changes are happening beneath the skin. Before returning to the ocean and leaving her weaned pup on the beach in February, each seal mated to conceive a pup for next season. That fertilized egg divided a few times, then stopped developing. The tiny embryo continued to float free in the uterus, dormant.

The mother-to-be spent the next two months or so feeding freely in her ocean home. When she returns to the beach in April to molt, her body makes the changes needed to have a pup next season.
After she’s finished molting and returns to the ocean, that embryo implants in the uterine wall and begins normal development. Delaying development of the embryo gives her time to gain weight before starting the demands of the next pregnancy.

Staying at sea
Gestation is 7.5 months. When they leave the beach after molting, the expectant mothers will remain at sea until they return to give birth next January.

Delayed implantation is part of a complicated life cycle. Spending most of the year solitary in the water requires different strategies for breeding. So far as their lives are understood, males and females don’t encounter each other except on the beach during breeding season. This is how the species has worked it out.

The 2015 breeding season produced about 4,900 live young-of-the-year, a 9 percent increase over 2014. Fewer females and fewer pups were actually born, but more survived, thanks to the mild winter weather.

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