Northern fur seals have experienced major changes in their behavior, ecology, and geographic range of over the past 1,000 years, according to a new study appearing in the May 21 early online edition of The Proceedings of the National Academy of Sciences.
Using techniques from archaeology, biochemistry, and ecology, a team of researchers* has reconstructed the species? prehistoric geographical range. Among other results, the scientists found that the northern fur seal?s reproductive behavior was very different in the past than it is today.
“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 who performed much of the work as a graduate student at the University of California, Santa Cruz.
Today, the main breeding colony of northern fur seals is on the Pribilof Islands in the Bering Sea. But the species once had major breeding colonies at more hospitable latitudes in California, the Pacific Northwest, and the eastern Aleutian Islands. 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. Prehistoric northern fur seals also nursed their young for much longer than modern fur seals, which wean their pups just four months after birth.
Newsome said the study is important because fur seals and many other species were decimated by commercial harvesting long before scientists were able to study them first hand. “What we consider natural for a species may not have been its natural state prior to human disturbance,” he added.
The disappearance of northern fur seals from some temperate regions coincided with the arrival of Russian and European fur traders about 200 years ago. But in central and northern California, fur seal populations collapsed about 800 years ago, long before European contact. The cause of the earlier collapse remains unclear, but it may have been climate change, human hunting, or a combination of factors.
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.
“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.
Groundwork for this study began in the 1990s, when co-authors Diane Gifford-Gonzalez and Paul Koch, both of UC-Santa Cruz, teamed up to investigate the abrupt disappearance of northern fur seals from the archaeological record along California’s central coast.
“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.”