Ancient DNA reveals genetic resilience of Pacific walruses

DNA extracted from ancient bones has helped scientists at the University of Alaska Fairbanks shed light on the surprising resilience of Pacific walruses in the face of historical commercial overhunting.

The research, published in the journal Scientific Reports, has important implications for the future of the iconic Arctic species amid modern environmental challenges, including the disappearance of sea ice due to the effects of climate change.

Pacific walruses inhabit the cold northern waters of the Bering and Chukchi seas and depend on sea ice to feed, molt and calve. Floating ice gives them a place to haul out of the water to rest and allows them to roam freely over a large territory. 

When the U.S. acquired Alaska from Russia in 1867, Yankee whalers began hunting walruses intensively for their hides, blubber and ivory tusks. Within 20 years, overhunting had cut the population of 200,000 animals in the region by at least half. The decline triggered a famine among the Indigenous inhabitants of the Bering Strait region, who relied on walruses for subsistence. 

Two more waves of overhunting followed. Each time, hunting paused when the walruses became too scarce to be hunted profitably. That allowed the population to bounce back, triggering another period of intensive hunting. The last wave of overhunting ended in about 1960. 

Researchers wanted to learn how the sudden population decreases affected Pacific walruses’ genetic diversity. To find out, they analyzed DNA from archaeological samples – some as much as 3,300 years old – and compared it to data from modern specimens. 

“We expected genetic diversity would be lost as a result of the population contraction,” said lead author Kendall Mills, who recently earned her Ph.D. from UAF. 

“The big surprise was that the overhunting actually didn't reduce their genetic diversity much at all,” she said. “We found no difference between ancient and modern Pacific walruses.”

Researchers suspect walruses maintained genetic diversity because of their wide-ranging behavior, aided by the floating sea ice. Because they weren’t constrained by land, they were less prone to localized inbreeding, which can lead to an “extinction vortex.”

An extinction vortex occurs when a population becomes so small and genetically homogeneous that individuals become more susceptible to genetic disorders, which contributes to additional population decline. Without intervention, species caught in an extinction vortex may reach a point where they cannot recover, ultimately facing extinction.

“Once a species drops below a certain threshold, there’s a ratcheting effect,” Mills said. 

The term “ratcheting effect” describes a situation where progress or change becomes locked in a certain direction, making it difficult to reverse course. 

The study suggests that although Pacific walruses will almost certainly continue to decline in number as the planet warms, they may be less susceptible to the ratcheting effects of inbreeding, at least initially. However, ongoing habitat loss of sea ice from climate change poses new challenges.

According to Lara Horstmann, chair of UAF’s Department of Marine Biology, the loss of Pacific walruses would affect the culture and subsistence lifestyle of Indigenous people. The animals also play a vital role in the ocean ecosystem.

Walruses feed by suctioning clams, mollusks, worms and other creatures from the seabed in shallow waters. As they forage, their tusks mix oxygen into the sediment and release nutrients into the water. 

“There’s a lot of life in the ocean, and much of it requires oxygen,” Horstmann said. “Without the walruses stirring things up, you’d lose a lot of prey for other animals. You’d lose the worms that live in the sediment, the nematodes, the mussels.”

This latest study adds to a body of knowledge gleaned from more than a decade of research funded by a $1.7-million grant from the National Science Foundation, with additional support from the Jay Pritzker Foundation. The ancient DNA specimens were obtained from the National Museum of Natural History at the Smithsonian Institution and the UA Museum of the North, home to the world’s largest collection of marine mammal specimens, according to the study’s senior author Link Olson, curator of mammals. They were processed at UAMN’s Ancient DNA Laboratory.

The researchers hope understanding the species’ genetic responses to changes in population size can help guide conservation efforts.

“What surprises me is how resilient they are, given all of the changes that they’ve gone through,” Horstmann said. “It is really incredible that they can yo-yo back. They are adapting by changing their feeding habits and their calving intervals as necessary. They are just incredibly adaptable. And for an animal that seems so unique and specialized, that, to me, is surprising and hopeful.”

ADDITIONAL CONTACTS: Kendall Mills, kkmills@alaska.edu; Link Olson, leolson@alaska.edu, 907-474-5998; Lara Horstmann, lara.horstmann@alaska.edu, 907-474-7724

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