Most of the word’s northern fur seals now breed on the Pribilofs in the middle of Alaska’s Bering Sea or a few other offshore islands, with the largest rookeries centered on the beaches of St. Paul. After only four months on shore, these Far North seal moms wean their newborns and take to sea — a fraction of the time other pinniped matrons like Steller sea lions invest in their own offspring.
But Northern fur seals didn’t always live like this. A new study used evidence from ancient garbage to show that the species once bred and raised pups in rookeries ranging from California to the Aleutian Chain, almost certainly spending much longer with their young.
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By analyzing bones from coastal archaeological sites, the study’s nine authors deciphered the diets of ancient people, and then deployed that data as a way to estimate the population and distribution of the animals.
“We were able to see changes in biogeography and behavior over time scales longer than ecologists usually think about,” said lead author Seth Newsome, a postdoctoral researcher at Carnegie’s Geophysical Laboratory, in a news release about the study. Newsome, who performed much of the work as a graduate student at the University of California, Santa Cruz, had eight co-authors in the study.
The bottom line: During the past 1,000 years, the seals dramatically changed their reproductive behavior, shifted their ecology and shrank their geographic range.
The new study comes as fur seals continue a population decline that accelerated in the 1990s and shows no sign of letting up. The number of pups born to northern fur seals on Pribilof Islands plunged 9 percent between 2004 and 2006, according to counts released last fall. Sorting out the biohistory of the species might hold clues to their current decline.
“The shifting baseline of northern fur seal ecology in the northeast Pacific Ocean” — to be published in the The Proceedings of the National Academy of Sciences — reconstructs a prehistoric range for fur seals that rimmed the northeast Pacific Ocean.
Fur seal pup on St. Paul
Credit: Doug O’Harra
Although the species disappeared from some areas with the arrival of Russian and European fur traders in the 1700s and 1800s, the species apparently collapsed in central and northern California about 600 years earlier, long before aboriginal cultures got disturbed. What caused this first collapse remains unclear.
“Their bones are abundant in archaeological sites in these regions, suggesting that they were prevalent in local marine ecosystems and also that they were important to human cultures,” according to a news release about the study. “Prehistoric northern fur seals also nursed their young for much longer than modern fur seals, which wean their pups just four months after birth.”
The study offers a window on what the species might have been before commercial harvests almost wiped out the population.
“What we consider natural for a species may not have been its natural state prior to human disturbance,” Newsome added in the story.
So how can ancient garbage buried in an archaeological site offer a clue to the habits of prehistoric predators with flippers, teeth and thick fur? It’s all in the chemistry.
The relative abundance of carbon and nitrogen isotopes — variants of an atom with different atomic weights — in bones can indicate what and where the animals ate when they were alive.
Nitrogen isotopes can also be used to determine how long a mammal nursed before it was weaned. Careful measurements of the prehistoric animals’ bones can be compared to those of modern animals to determine the age at which they died.
Combining these techniques, the researchers showed that prehistoric northern fur seals were year-round residents in California, and not visitors from northern waters.
Of the 14 species in the family of eared seals, which includes sea lions and fur seals, only northern and Antarctic fur seals wean their pups at four months in order to leave their breeding colonies before winter.
Northern fur seal on St. Paul beach
Credit: Doug O’Harra
“The Bering Sea has a highly productive marine ecosystem in the summer, but it shuts down in winter,” Newsome said. “Sea ice also begins its southward advance. The seals have to wean their pups and move south, and they can be found off central and northern California throughout the winter.”
All other eared seals nurse their young for about a year, and sometimes as long as two years. Prehistoric northern fur seals in temperate latitudes apparently used this “long-term” maternal strategy. Although modern northern fur seals have recently established new rookeries in temperate latitudes, these seals still retain the reproductive behavior of Pribilof Island fur seals.
The study has its roots in the 1990s, when co-authors Diane Gifford-Gonzalez and Paul Koch from UC-Santa Cruz began investigating why fur seals suddenly disappeared from the archaeological record in California.
Using bones from archaeological middens from ancient coastal communities as a way to track the rise and fall and shifts in available food has become one of the neatest tricks in modern marine science.
“The PNAS paper is a culmination of that work,” said Gifford-Gonzalez. “It shows just how much information one can get out of ancient bone samples with this kind of coordinated, multidisciplinary approach.”