Elephant seals rely on supermums

Motherhood is a tough gig for elephant seals, with just a few females doing most of the work.

A new study documenting the reproductive success of 7735 female northern elephant seals (Mirounga angustirostris) at the Año Nuevo rookery in California, US, reveals that only 6% gave birth to 10 or more pups over their lifetimes, and these accounted for 55% of total pup production over the past five decades.

Three-quarters of all weaned female pups die before reaching maturity, so never breed at all, and most of the survivors only breed one to three times before they die. 

The reason, says research leader Burney Le Boeuf, an ecologist at the nearby University of California, Santa Cruz, is that breeding takes a toll on the young females, who must invest a lot of energy in their pup at a time when they themselves are still growing. 

Once a female elephant seal begins breeding at the age of three or four, Le Boeuf says, she is typically either pregnant or nursing for the rest of her life.

The longest-lived female in the study – published in the Canadian Journal of Zoology –produced 17 pups in her 23 years. Another produced pups in 16 consecutive years.

Elephant seals give birth to one pup a year, in winter, then spend four weeks on the beach nursing and weaning it, relying entirely on stored reserves. They then head out to sea to feed. 

During their first trips to sea, the weaned pups are naive and may struggle to find enough food or fall prey to sharks or orcas. Those that survive get larger as they age, and bigger mothers give birth to bigger pups, which are more likely to survive to maturity and breed. 

Lifetime reproductive success is an important measure of evolutionary fitness, a central concept in natural selection, yet studies of this kind are rare, Le Boeuf says, because it is hard to track large numbers of individuals throughout their lifetimes. 

Researchers have tagged elephant seals at Año Nuevo every year since 1963, not long after the colony was established in 1961. 

“With the elephant seals at Año Nuevo, we’ve been able to follow them for two generations, more than 50 years, in part because of the proximity of this colony just up the coast from the UCSC campus.”