Given a choice between giving birth on land or sea ice, Pacific walrus mothers most often choose ice. Likewise, they prefer sea ice for molting, mating, nursing and resting between dives for food. Trouble is, as the century progresses, there's going to be far less ice around.
How well walruses cope with less sea ice is at the heart of a legal fight over whether walruses should be listed as a threatened species, giving them an added protection against human encroachments.
The federal government in 2008 listed polar bears as a threatened species because of diminished sea ice brought on by climate warming. That year the Center for Biological Diversity petitioned to do the same for walruses.
However, the U.S. Fish and Wildlife Service concluded in October 2017 that walruses are adapting and no one has proven that they "need" sea ice. "It is unknown whether Pacific walruses can give birth, conduct their nursing during immediate post-natal care period, or complete courtship on land," said Justice Department lawyers in defending the decision.
Pacific walrus males grow to 12 feet (3.7 meters) long and up to 4,000 pounds (1,815 kilograms) – more than an average midsize sedan. Females reach half that weight. Walruses dive and use sensitive whiskers to find clams and snails in dim light on the sea floor.
Historically hunted for ivory tusks, meat and blubber, walruses since 1972 have been shielded by the Marine Mammal Protection Act. Only Alaska Native subsistence hunters may legally kill them.
The array of stresses and uncertainty about the walruses' future are enough evidence for listing them as threatened, the Center for Biological Diversity argues.
In the last decade, walruses that gathered on shores have suffered hundreds of stampede deaths, and the loss of ice floes has pushed them away from feeding areas, said Shaye Wolf, climate science director for the nonprofit conservation group. "They're not adapting. They're suffering," Wolf said.
Scientists advising the Fish and Wildlife Service say the answer is not so clear cut, and much is unknown about how sea ice loss will affect walruses.
