Thursday, April 11, 2013

The Beachmaster and the Harem

"Blorpitude" may not be a real word outside of Cute Overload, but it's the first term to come to my mind when I think of the rotund elephant seal. Enormous sacks of rubbery, blubbery mammal mellowing on the beach or inchworming their way across the sand? Blorp, blorp blorp.

Northern elephant seals (Mirounga angustirostris) are sufficiently distinct from their southern brethren (M. leonine) to be considered a separate species, but their appearance and behavior are still similar in many respects. Thus, this classic David Attenborough clip provides a compelling introduction to today's creature feature, even though the subjects are of the southern variety. (I can't embed the link here, but you have to go watch it before you continue reading. I'm serious. Sir Attenborough uses his walking stick to fend off an aggressive bull but still manages to keep narrating. It's truly one of the greatest moments in nature documentary history.)

Male and female elephant seals are physically dimorphous, with bulls boasting an outlandish, elephantine proboscis and thick neck skin. They average 14-16 feet long and may tip the scales at as much as three tons. The comparatively dainty females are just 10-12 feet in length and weigh 1200-2000 lbs.

For a sense of scale, here I am by a, erm, spectacularly tame, battle-scarred bull.

A female elephant seal lounging in the sun

Blorp-tastic elephant seals at Año Nuevo State Park

You can be forgiven for thinking they all look alike, though. Northern elephant seals underwent a severe population bottleneck in the mid 1800s as a result of sealing (the slaughter of seals for their oily blubber). Fewer than 100—possibly as few as 20—individuals remained by the 1890s. The sausage-like pinnipeds have since rebounded in number, with current population estimates ranging from 150,00 to 175,000 seals. 

One of the best places to observe northern elephant seals is Año Nuevo State Park in central California. The seals first came to these beaches in 1955, and the park today hosts a population of over 5,000 animals.

Año Nuevo Island and beaches. The two blobs at the shoreline are elephant seals. The lighthouse station visible in the distance was shut down in 1948, and the island is closed to the public.
I'm not sure why females so drastically outnumber males.  Maybe males stay away from breeding beaches until they are sexually mature? (Females may mate as early as age 2, but males usually don't until around age 8.) Is male mortality that much higher? Females do live about six years longer, on average.

Males arrive in early December and immediately begin duking it out to establish a rigid social hierarchy. Only the highest ranking males are allowed the privilege of breeding, so fights between lustful bulls are intensely violent. The enormous beasts rear up, bellow like jet engines, and lunge at one another, bashing their chests together and viciously gouging their opponent's face and neck with their sharp canines.

A dominant male eventually emerges on the blood-spattered battlefield: the beachmaster. He has won the right to exclusively mate with all the females in his harem—as many as 100 on some beaches. Lower-ranking males must defer to him. 

A dominant bull (left) rears up and barks a threat call at a subordinate male (right) that dared approach. The threatened seal wisely backed down and moved away.

This hard-won honor does not come without a price, however. The beachmaster must constantly guard his harem against rivals desirous of his position and subordinate males that attempt to sneak in and mate whilst he is otherwise engaged. He cannot venture out to sea to feed for up to three months. He rarely sleeps. Exhausted by the demands of this rank and the unsurprisingly strenuous mating schedule—alpha males may sire 100 pups in a single breeding season—most beachmasters die a year or two after their reign ends. A few supremely macho males rank number one for three consecutive years, but they are the exception.

Females begin arriving in mid- to late December and establish the harems that the beachmaster so jealously guards. About six days after hauling out, they give birth to a single pup. Junior grows from around 75 lbs. at birth to a whopping 250-350 lbs. in less than a month, courtesy of the mother's extremely fat-rich milk. A few ambitious pups—mostly males—manage to con other females into nursing them as well. These "super weaners" (ha) can reach a massive 600 lbs!

Females, nursing pups, and weaners (pups whose mothers have returned to sea) at Año Nuevo. Note the small, wrinkled pup to the right; it was likely orphaned and unable to find a foster mother—only around a quarter of orphans are able to find a female willing to adopt them—and will not survive.

During their last four days on shore, females become sexually receptive and may mate several times (usually with the beachmaster or another high-ranking male that manages to get past his defenses). If a male attempts to mate with her when she's not in the mood, though—when she's not in estrus, or because he is an undesirable weakling of low social status, or when she's giving birth (no, really)—she will bellow, flail around to try to get away, and flip sand into his face. 

Such efforts are irrelevant, however, when the female in question is dead. Yes, male elephant seals possess such an implacable libido that they will even attempt necrophilia. Sometimes multiple times with the same corpse. Seriously. You can't make this stuff up.

Males are equally heedless of the presence of any pups between them and the object of their attraction. Because pups stay close to their mother during the 25-28 day nursing period, they are smooshed with alarming frequency by sex-crazed bulls.

A male elephant seal attempts to mate (foreground). The female was not having any of that.

The female rebuffs his advances and rejoins her pup, which luckily was on the other side of its mother during the encounter.

Even when they make their way across the beach to the sea—abruptly weaning their pups by desertion—females must run a gauntlet of males desperate for a shot at fulfilling their genetic imperative. These low-ranking bulls lurk at the fringes of the harem and lunge en masse at females leaving the protection of the herd, sometimes injuring or even killing their targets.

Fewer than a third of males get the chance to mate each breeding season. In fact, the five highest-ranking Romeos are the busiest, performing 50-90% of the matings. Many males die before they even reach sexual maturity (age 5, though they don't usually rank high enough to mate before they're 8 years old). With only a relative few bulls succeeding in passing on their DNA each year, the mating habits of the northern elephant seal have likely exacerbated the species' low genetic diversity, a legacy of the 19th century bottleneck. 

Strangely, even though elephant seals mate about 11 months before they give birth, their gestation period is only 7 months long. How does that work? In fact, this discrepancy is explained by an extraordinary adaptation called "delayed implantation." The fertilized egg develops into a blastocyst (a small ball of developing cells) but then puts itself on hold, floating freely in the uterus until the female is ready to return to sea after coming ashore for the summer molt. Weird, I know.

Elephant seal biology is unquestionably bizarre, but the successful recovery of these marine mammals gives us hope for conservation programs worldwide. The California population is growing at some 20-30% each year, and scientists estimate certain breeding beaches are actually approaching capacity. Blorp on!

(Information on visiting Año Nuevo after the cut)
Northern elephant seals breed along the west coast of North America from Baja California in Mexico to the Farallon Islands, 30 miles west of San Francisco. Año Nuevo State Park (23 miles north of Santa Cruz, 55 miles south of S.F.) is one of the best places to see these animals.

The park is open year-round, but you'll need to sign up for a guided tour to visit the seal viewing areas during breeding season (Dec. 15 to Mar. 31 this year). Book at least a month in advance if you plan to visit during the peak mating period—early to mid-February—especially for a weekend tour. In fact, one source pegs the "peak day of copulation frequency" as February 14. Happy Valentine's Day!

Elephant seals are in residence throughout the year, though the beaches are the most packed during the winter breeding period and spring/summer molt. See the photo below for a diagram of seal seasonality at Año Nuevo:

Elephant Seal Links
Año Nuevo Island elephant seal webcam
Southern elephant seal photographs by National Geographic photographer Paul Nicklen (see the last 7 or so photos)

A turkey vulture patrols the sky above Año Nuevo in search of elephant seal entrées


Crump, Marty. Headless Males Make Great Lovers. The University of Chicago Press, 2005. (Possibly one of my all-time favorite book titles :-) )

"Elephant Seals." Año Nuevo State Park: California Department of Parks and Recreation.

Le Boeuf, Burney J. Male-male competition and reproductive success in elephant seals. American Zoologist 1974, 14:163-176.

Le Boeuf, Burney J. and Mesnick, Sarah. Sexual behavior of male northern elephant seals: I. Lethal injuries to adult females. Behaviour 1990, 116:143-162.

Le Boeuf, Burney J. and Laws, Richard M. Elephant Seals: An Introduction to the Genus. In Le Boeuf, Burney J. and Laws, Richard M. [Ed.] Elephant Seals: Population Ecology, Behavior, and Physiology. University of California Press, 1994.

Nicklen, Paul. Polar Obsession. National Geographic Books, 2009.

"Northern Elephant Seals, Mirounga angustirostris." MarineBio.

Reiter, Joanne et al. Female competition and reproductive success in northern elephant seals. Animal Behavior 1981, 29:670-687.

Weber, Diana S. et al. An empirical genetic assessment of the severity of the northern elephant seal population bottleneck. Current Biology 2000, 10:1287-1290.

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