Mystery behind why orcas kill porpoises without eating them finally unravelled

A decades-long mystery behind killer whales harassing and even killing porpoises without consuming them may have finally been unravelled.

Fish-eating killer whales, also known as orcas, may be killing porpoises either for fun as “social play”, as a “hunting practice” or as a mis-manifestation of their natural mothering behaviour, revealed a study, published last week in the journal Marine Mammal Science.

Researchers, including those from the University of California in Davis, assessed over six decades of records of this perplexing behaviour among Southern Resident killer whales in the Pacific Northwest that has long intrigued scientists.

This particular species of killer whale are an endangered population, numbering only 75 individuals, with their survival closely tied to the fortunes of another endangered species, the Chinook salmon.

Without enough of the salmon, scientists say these orcas are in danger of extinction. And these killer whales, that are known to kill porpoises, do not eat them as they have a “completely different” ecology and culture compared to the orcas that eat marine mammals, scientists explained.

“So we must conclude that their interactions with porpoises serve a different purpose, but this purpose has only been speculation until now,” study co-author Deborah Giles said.

The latest study comes as populations of killer whales on the Iberian Peninsula were seen sinking boats off the coast of Portugal and Spain on three occasions.

While the two populations of killer whales in the Pacific Northwest and the Iberian Peninsula are different with distinct cultures, researchers speculate their affinity for “play behaviour” may be common.

In the study, researchers analysed 78 documented incidents of porpoise harassment from 1962 to 2020.

The analysis suggests one hypothesis that suggests Southern Resident killer whales may be harassing porpoises likely as a form of social play to bond, communicate, or simply for fun among themselves – a behaviour that may benefit group coordination and teamwork.

Another hypothesis highlighted by the study said orcas may be killing porpoises to hone their salmon-hunting skills.

Researchers suspect these Southern Resident killer whales may be viewing porpoises as moving targets to practice their hunting techniques, even if they do not intend to consume them.

A third theory suggests the killer whales may be attempting to provide care for porpoises they perceive as weaker or ill – a manifestation of their natural “mothering” inclination to assist others in their group.

“Mismothering behaviour – also known as ‘displaced epimeletic behaviour’ to scientists – might be due to their limited opportunities to care for young,” Dr Giles explained.

“Our research has shown that due to malnutrition, nearly 70 per cent of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died right away after birth,” she said.

While these theories exist, scientists said the exact reason may never be fully understood.