Scientists say whales are among the world's most intelligent animals, exhibiting complex social behavior including self-awareness and suffering.

The Washington state-based Center for Whale Research said the endangered orca named Tahlequah, also known as J35, was spotted carrying her deceased calf in Puget Sound off Seattle on New Year's Day.

"J35 has been seen carrying the body of the deceased calf," the center said in an Instagram post Thursday.

"This behavior was seen previously by J35 in 2018 when she carried the body of her deceased calf for 17 days," it said.

When Tahlequah was carrying her previous deceased newborn seven years ago she was seen sometimes nudging its body with her nose and sometimes gripping it with her mouth, US media reported.

"It's a very tragic tour of grief," Center for Whale Research founder Ken Balcomb told public broadcaster NPR at the time.

The center said the loss of the latest female newborn was "particularly devastating" because Tahlequah has now lost two of her four documented calves.

"We hope to have more information on the situation through further observation," the post said.

The center also said Tahlequah's pod had been joined by another newborn. "The calf's sex is not yet known but the team reports that the calf appeared physically and behaviorally normal," the center said.

Tahlequah and her pod mates are Southern Resident Killer Whales, a population listed as endangered in the United States.

There are only three pods in the population, numbering around 70 whales. They spend several weeks of each spring and fall in the waters of Puget Sound.

Their numbers are dwindling owing to a combination of factors, including a reduction in their prey and the noise and disturbance caused by ships and boats, according to the National Marine Fisheries Service.