A pair of orca have been observed killing a newborn calf - and scientists believe the murder may have been sexually motivated.
The rarely seen case of infanticide happened in December off the coast of Vancouver Island in Canada. Researchers picked up "strange" whale calls on an undersea microphone, according to journal Scientific Reports.
When they went to investigate, they found a male orca and his mother chasing a young trio of orca. Blood was in the water, and it was clear a fight had taken place.
Eventually the trio met up with the rest of their family, which included a newborn.
"They all came together after 1-2 minutes and began circling vigorously," the scientists report.
The pursuing pair attacked, the male almost biting off the baby's fluke - the end of its tail.
"The fluke of the neonate could be seen in his mouth with the body intact trailing underneath his lower jaw."
The attacker's mother chipped in by fending off attacks from the baby's mother.
The pair escaped with their prey, and three hours later dragged the calf underwater, drowning it.
Infanticide is rare in the wild, but scientists say there are two leading theories as to why it happens - it stops the mother from lactating, so she's ready to mate again; and it removes potential sexual competition.
"Infanticide not only creates a mating opportunity but can also remove the progeny of a competing male from the gene pool," the researchers said.
His mother likely helped because it'll increase the chance her genes will be passed on, through her son.
"Post-reproductive female killer whales increase the survival of adult sons by sharing ecological knowledge and prey with them."
There is no evidence the killers did it for food, as the calf was left uneaten.