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More than 3,000 northern elephant seals return to Ano Nuevo each year to give birth, breed and molt during mating season, which lasts from mid-December through the end of March. (Shmuel Thaler/Sentinel)

PESCADERO -- Atop a dune at Año Nuevo State Park a woody croak sails over the salty ocean air.

He growls at the offending party, warning him to leave her alone. Knowing he cannot win the fight, the smaller male backs off, biding his time until he can try again.

Northern elephant seal breeding season is in full swing at Año Nuevo, 25 miles north of Santa Cruz. It's a fascinating and gruesome sight. During the season, visitors to the park can go on guided tours to see the seals up close and learn about their mating habits.

Every year, from December to March, adult elephant seals return to their birthplace to birth their pups, nurse and mate. In mid-January, the females are in the middle of the birthing process. A few small wrinkled pups lie next to their moms as they nurse. Other females are still waiting to give birth.

Whether they're nursing or still pregnant, the females have to endure constant male attention. They have no hope of fending them off on their own. At the start of the breeding season, females weigh about 1,500 pounds, but the hefty males can weigh as much as 4,000 pounds -- comparable to an SUV. When a female finds herself trapped beneath an unwanted suitor, the dominant bull steps in. He is responsible for the safety and comfort of all the females in his harem, even though none of their pups are his. He's playing the long game -- by protecting the females as they nurse now, he is ensuring that those females will want to mate with him later, after their pups have weaned.

When they're born, elephant seal pups weigh about 75 pounds and are covered in a black coat to keep warm, but they

will quadruple their bulk after four weeks of nursing.

"In order to do that, the milk must be very high in fat," docent Michael Braude said.

Elephant seal milk contains about 55 percent fat, several times the fat content of ice cream.

Once the pups are weaned, the mothers mate again and return to the sea. Meanwhile, "that pup is only 4 weeks old, it weighs 300 pounds, but there's a problem: It doesn't know how to swim and it doesn't know what it's supposed to eat. So it's going to spend the next eight to 12 weeks just living off its body fat teaching itself how to swim before it goes off to search for food," Braude said.

Unfortunately many pups die during the breeding season. Some starve, others get washed out to sea and drown. Some pups meet their demise under the weight of two brawling bulls.

BRUTAL BULLS

"Even if you're not the most dominant bull, you still want to dominate over other bulls," Braude said as two bulls fought on the beach. Their heads were thrown back and their mouths were open, displaying their formidable teeth. They're hoping for the chance to mate with stray females. "Eventually some females will leave the harem," Braude said, "and then they're fair game," so the less dominant males wait on the fringes for their chance.

The bulls' necks are covered in jagged lines -- literally battle scars -- from previous fights. As a seal collects more scars, the tissue forms a protective layer around the neck, allowing him to continue fighting. Researchers call the ring a chest shield and in older dominant bulls, it can encircle the neck.

Recently, researchers working at Año Nuevo discovered that males use and recognize unique calls.

"The males are able to remember each other's vocalizations. They won't challenge other males that they've already lost to," Braude said.

IN THE FUTURE

Año Nuevo provides a unique habitat for breeding elephant seals.

"It's like this huge off-ramp because it's kind of level," interpreter Mike Merritt said.

Other habitats such as Piedras Blancas and Point Reyes are rockier, forcing the seals to give birth much closer to the ocean where the pups can get washed away.

But the environment is changing. Twenty years ago, most of the park was covered in sandy dunes. But as farmers irrigated the nearby land, they raised the water table high enough for plants' roots to reach. Now the hills are covered in shrubs and grass. It's possible that Año Nuevo will see fewer seals in the coming years if the vegetation creeps closer to the shore.

For the time being, though, midwinter in the park provides a family friendly activity.

Sacramento resident Avery Quinton, 7, said she had fun on the tour.

"I liked listening to them flop around," she said. "I'm going to tell my friends about how I saw some little baby seals."

Follow Sentinel reporter Cynthia McKelvey at Twitter.com/NotesofRanvier

IF YOU GO

Elephant Seal Guided Walk

WHEN: Several tours per day, through March 31. Call for reservations.

WHERE: Año Nuevo State Park, 1 New Years Creek Road, Pescadero

COST: $10 for parking $7 per ticket. Free admission for children younger than 3.

Information: Visit http://www.parks.ca.gov/?page_id=27613 or call 800-444-4445.

Elephant Seal Breeding Season

Elephant seals return in December and begin birthing. They nurse their pups for four weeks. By February, most of the pups are weaned and the females enter heat to mate with the males. The adults then return to the sea, leaving the pups on their own. By March, all the surviving pups have taught themselves to swim and enter the ocean in search for their first meal.