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The US Fish and Wildlife Service (FWS) has launched an investigation into the deaths of 25 walruses in a remote region in the northwestern coast of Alaska.

The animals, 13 adults and 12 calves, were found last week in Cape Lisburne, located on the Chukchi Sea coast near the small northwest Alaskan town of Point Hope. Many of the carcasses were missing heads and tusks, leading to suspicions of possible poaching activities and prompting the FWS to open a criminal investigation.

Killing walruses is illegal under the federal Marine Mammal Protection Act, but with certain exceptions. The law allows only Alaskan Natives to hunt these animals, as Pacific walruses make up an important part of the diet of many coastal natives, and walrus tusks, bones, and hides are used to make traditional handicrafts. However, killing these animals only for their tusks, also called "head hunting," is illegal, across the board.

While the FWS and the Alaska District Attorney's office refused to comment on the incident citing the ongoing investigation, experts working on marine mammals and their habitats said the deaths could be due to a varied number of reasons.

Lara Horstmann-Dehn, a professor of marine biology at the University of Alaska Fairbanks, said it was possible that the animals died due to natural causes, such as a mass trampling event or even infections, as was suspected during a similar incident in August 2011.

Walruses, especially those found near Alaska, have been grabbing headlines for the past few years. In 2007, dozens of similar headless carcasses were found near Norton Sound, on the western coast of Alaska. More recently though, about 35,000 of them hauled out in Point Lay — barely 100 miles from Cape Lisburne — an event that has become an annual occurrence.

"If we have one area where 35,000 of these animals are huddled together, it is not really far fetched to think that you may have a smaller aggregation of the animals in close vicinity," said Horstmann-Dehn, who studies how walruses react and adapt to a changing climate.

She added that trampling is common even in modestly sized groups. And while killing walruses just for their tusks or hides is prohibited, collecting them from already dead animals is allowed, although it must be reported to the FWS. Thus, it is possible that the tusks of the Cape Lisburne walruses were collected after they died.

"Maybe a hundred or a thousand of these animals had hauled out; and when these animals feel disturbed, they can trample each other in the process of hurriedly moving into the water... it is entirely possible that 25 of them are trampled in a group of a hundred or a thousand," Horstmann-Dehn said.

The massive walrus haul outs, once rare, have become increasingly regular in recent years due to the extreme retreat of the summer ice cover in Alaska. Pacific walruses migrate between the Chukchi and the Bering Seas in tandem with the seasonal cycle of melting and expansion of sea ice.

"Walruses use ice as a platform to feed off...it's like sitting in a bus and going from one supermarket to the next," Horstmann-Dehn said. "They can jump off the floating ice and feed and again get on it and digest, and then they float on the ice into a new area and feed again."

But climate change, which is warming the Arctic twice as fast as the rest of the world. Sea ice and glaciers in the Arctic are receding, the permafrost is melting, and sea levels are rising.

A 2012 study of Pacific walruses conducted by researchers from the US Geological Survey's Alaska Science Center and Russia's Pacific Research Fisheries Center found that the lack of sea ice in September and October caused walruses to forage in areas nearer to the shore, instead of going further offshore as they did in the past.