Here come the bulls!

Seals in transition

Females and youngsters give way to bulls

Adult female seals and male and female juveniles occupy the beach through June. Mature bulls start arriving for their summer molt.

Elephant seals are migratory, so they are always coming and going. Summer marks the conclusion of the females’ short migration. As they leave, they embark on their long migration. The females won’t return until January, when they come to the beach to have their pups. Juveniles return in the fall.

Gestation

The adult females mated last winter, after they weaned their pups. That got the next pregnancy started by fertilizing the egg, but after that, development pauses.  The fertilized ovum makes only a few cell divisions and then stops developing, before it is ready to implant in the uterus.

Thin from nursing the pups for a month without eating, the mothers need to return to the ocean to add blubber before the fetus starts requiring nourishment. Recent research shows that they feed 20 to 24 hours a day during that two-month migration. They may travel as far as 3,700 miles.

They dive continuously, down to 1,600-5,000 feet, 20 minutes or so down and back, taking a two- to three-minute breath at the surface before making the next dive.

When the “slightly pregnant” females get back to the beach, they stop eating. Their skin starts to peel off, molting. During the molt, a surge of hormones causes the egg to resume developing and to attach to the uterus wall, beginning active gestation. It’s called delayed implantation.

There’s 11 months between mating and birth, but actual gestation is only eight months.

Feeding

The seals need to spend most of the time feeding, because they are eating small fish, most less than three inches long. They take 1,000 to 2,000 bites a day, snapping up small fish. It’s difficult, but few other predators compete with them for those abundant fish in what’s called the mesopelagic layer.

Those little fish, all together, dominate total fish biomass in the ocean. Female elephant seals evolved a feeding strategy that allows them access to lots of food that few other animals hunt. It’s a unique ecological niche.

The downside is that spending so much time feeding leaves little time for anything else, such as sleeping. Sleep among elephant seals is not well understood. They rest, and may sleep, on some of their dives, rolling onto their backs after they get deep enough and drifting back and forth like falling leaves. The seals sleep or rest less than an hour and a half a day.

Climate change

That mesopelagic layer of billions of little fish is being affected by climate change due to greenhouse gases. Changes there would be reflected in the elephant seals’ well-being.

“Elephant seals can be used as sentinels to better understand how rapid climate change alters the little-known but ecologically important mesopelagic ecosystem,” researchers concluded in their study, Forced into an ecological corner: Round-the-clock deep foraging on small prey by elephant seals.