Sydney, Australia - A female humpback whale has been documented apparently grieving a stillborn calf in new research released Thursday, offering rare insights into the complex emotions of the sea-dwelling mammals.

The footage, taken off Australia's Gold Coast last year, shows a female humpback whale staying close to the dead calf as it lies motionless on the sea floor.

Accompanied by a companion, the whale holds eye contact with the animal, touching it with its head and fin.

"At first sight, I thought it was just two whales resting on the surface, but their behavior was unusual," said Andrew Mulville from Sea World Foundation, who witnessed the heart-wrenching moment.

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"It was definitely not what I was expecting to see out at sea that day."

Researchers said the whale had stayed with the deceased animal for "several hours and possibly even days."

"It's really sad," Olaf Meynecke of Griffith University's Whales and Climate Research Program told AFP.

"It's pretty heartbreaking to watch this female making continuous contact with the deceased calf."

A number of other deceased whale calves were observed in the area at the time, the researchers said.

The whale did not try to bring the calf to the surface, suggesting "she might have been aware that it was dead shortly after giving birth," they said.

Researchers said the whale's actions "may represent a form of grief-related behaviour in humpback whales."

Meynecke said his research spoke to the "strong bond" between mother and calf and the emotional stages that "socially complex" mammals go through following the deaths of their offspring or companions.

Whales are far from the first non-human mammals to have been observed engaging in complex grieving rituals, he said.

"We're seeing this from elephants and apes and cases like that that are also extremely heartbreaking," Meynecke said.

"But on land we witness this more often, whereas at sea it's really rare to document something like this."