Beachmaster at bat. A male southern elephant seal mates with a female.

Nico de Bruyn Photography

Cheating on the Beachmaster

Elephant seals are renowned for their rather brutish breeding system, in which one big, pumped-up male jealously defends a harem of females from would-be usurpers in thunderous, bloody battles. The so-called beachmaster wins exclusive mating rights to the comparatively tiny females in his domain, who can get roughed up or killed in his throes of passion. The system is often touted as a textbook example of polygyny in the animal kingdom, but a new study shows that reality is more complex. Female elephant seals often steer clear of the appointed beach during breeding season, sticking to the high seas where they mate on the sly.

Southern elephant seals haul out on sub-Antarctic beaches only twice a year, once to molt and once to mate. During the spring breeding season, the males arrive early to stake out their territories. Females, returning to the beaches where they were born, show up a few weeks later and give birth. After the pups are weaned, the males mate with dozens or even hundreds of females in their domain, duking it out with any challengers. The females depart soon after, followed by the males.

For the few beachmasters, the system's evolutionary advantage is clear: each one gets to mate with plenty of females. As for the females, researchers have presumed that the opportunity to mate with primo males keeps them returning faithfully to the breeding colony each year.

But there was one little problem with the idea. When adult females make their breeding-colony debut, at age 3 or 4, they give birth to a pup—one obviously conceived outside the harem system. For the most part, researchers have chalked this up to youthful dalliance preceding a lifetime of harem-bound breeding, says Nico de Bruyn, a marine mammal ecologist at the University of Pretoria in South Africa and lead author of the current study. "The focus has always been a very one-sided, male-focused look at polygynous mating systems."

To see whether the virgin seals' exploits might amount to more than just a chink in the harem system, de Bruyn's team turned to an extensive data set initiated by his former Ph.D. adviser and a co-author on the paper, Marthan Bester of the University of Pretoria. For 28 years, researchers have been carefully monitoring breeding elephant seals and tagging pups at Marion Island, a windswept volcanic dot between Antarctica and South Africa. Analyzing 25 years of return data for females born there between 1983 and 1997, the team found that more than half of them repeatedly failed show up for breeding seasons. Yet they often returned the year after a break to give birth, so they must have bred elsewhere, possibly with a male lower on the totem pole than the beachmasters.

The clincher came when the team looked at data from satellite tags they'd attached to 53 females. Two of the tagged females skipped a breeding season at Marion Island, then showed up pregnant the following year. Except to molt, the AWOL seals had stayed at sea for their entire gap year. They must have bred at sea as other seal species do.

"The females have got a real choice here," says de Bruyn, whose paper will appear in the September issue of Animal Behaviour. "They're employing this totally nonpolygynous alternate mating strategy, which really opens our eyes to the whole study of polygyny in mammals and in other vertebrates as well."

Work on other species, such as horses, has suggested that polygyny may indeed benefit females—not through access to the fittest males, but by providing safety in numbers against male aggression and harassment, de Bruyn says. His team thinks female elephant seals are driven to Marion Island primarily by the need to give birth on land. Once there, joining a harem may reduce their odds of injury or death at the flippers of amorous males. Of course, smaller males without much hope of breeding on land would also benefit from the at-sea mating alternative, which clarifies why up to 75% of adult males skip the breeding colony each year, as earlier studies have shown.

"This study at last confirms what we had suspected, that sometimes southern elephant seals mate at sea," writes evolutionary biologist A. Rus Hoelzel of Durham University in the United Kingdom, in an email—though exactly how much extra-harem hanky-panky goes on will require additional tracking and genetic data. Hoelzel, who has studied elephant seals extensively and is collaborating with de Bruyn on a separate project, says that despite their drawbacks, terrestrial harems remain elephant seals' predominant breeding system, and they clearly have evolutionary advantages for both sexes, or mating at sea would have replaced them entirely.