An orca pod has been harassing and killing porpoises for decades — but they don't eat them. Scientists still don't know why.

• Researchers found that a certain group of orcas are killing porpoises for an unknown purpose.
• The orcas could be doing it just for fun or to teach hunting skills to younger members of the pod.
• The orcas may also be "mismothering" the porpoises given limited opportunity to raise their own young.

Researchers have been watching a group of orcas in the Pacific Northwest harass and kill porpoises for nearly 60 years. But oddly enough, the killer whales don't eat the animals.

So why spend all that time and effort?

Killer whales are the largest members of the dolphin family and highly intelligent, social mammals that behave in a lot of ways we don't fully understand.

For example, ramming boats and tearing off their rudders; hunting great white sharks only to eat their liver and leave the rest to rot; and wearing dead salmon for, what can only be described as hats.

These porpoise tortures and killings are just one more example in the collection of unexplainable orca behaviors.

Orcas are killing porpoises but not eating them

From 1962 to 2020, researchers recorded and studied 78 episodes of Southern Resident Killer Whales harassing and, in many cases, killing multiple types of porpoises. They published their results in the journal "Marine Mammal Science" in September. 

But the endangered group of about 75 orcas — resident to North Pacific waters around Washington, Oregon, and British Columbia — have a primary diet of Chinook salmon, and they don't eat the porpoises they kill. 

The researchers have a few ideas for why.

Theory 1: The orcas are just having fun

One theory the paper's authors proposed is that the orcas, which are very playful animals, are just having fun at the porpoises' expense. 

"Sometimes SRKWs would continue engaging with the porpoise well after it had died, indicating that the killing of the porpoise was not the goal," the researchers reported in the study.

In some cases, the orcas would toy with the porpoises for hours. Because the animals did this for so long without even getting the benefit of a meal, the behavior could be for entertainment, the researchers added.

It wouldn't be the first time orcas have been seen playing around, Lori Marino, a neuroscientist who studies cetacean intelligence and wasn't involved in the study, told Insider.

Orcas in particular are known to "play or abuse, I don't know how to describe it, much smaller cetaceans," she said.

Theory 2: The orcas are practicing their hunting skills

The paper's authors also considered that the killer whales might be harassing the porpoises as a way of practicing their hunting skills to teach the group's younger members how to prey on Chinook salmon.

This theory would explain why some of the porpoise targets are calves, which are about the same size as adult Chinook salmon.

Orcas are highly social animals who tend to learn by observation, Marino said. They lead the majority of their lives in a group setting, in pods of up to 20 other killer whales, according to the US National Oceanic and Atmospheric Administration. 

The orcas' choice not to eat the porpoises was consistent with what Marino had seen in her years in the field. 

"We've heard of orcas who show their young how to get food in different ways. And sometimes they show them how to do it and then they don't actually eat the animal," she said. 

Theory 3: The orcas are playing "mom"

Finally, the third possibility the researchers outlined is that the orcas might be engaging in what they call displaced epimeletic behavior or "mismothering." 

Deborah Giles, a co-author of the paper, told Newsweek that such behavior "might be due to their limited opportunities to care for young."

"Our research has shown that due to malnutrition, nearly 70% of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died right away after birth," Giles added, according to Newsweek.

The researchers found that over the years, killer whales have passed down porpoise harassing "through generations and social groupings," but the precise reasons why are still a bit of a mystery.