A new study examines the inexplicable and violent behavior of killer whales toward porpoises. Researchers now present three possible explanations for why the animals attack their smaller relatives without eating them.
Orcas have been observed for years seemingly harassing and sometimes killing porpoises (a smaller toothed whale similar to dolphins) without eating the animal, and scientists have long wondered what causes this behavior.
In a new study published in Marine Mammal Science, researchers from Wild Oceans and the University of California Davis’ Karen C. Drayer Wildlife Health Center in Washington, D.C., and others have identified the cause. The Drayer Wildlife Health Center in Washington studied the behavior to try to explain it.
The researchers examined 78 interactions between killer whales and harbor porpoises in the Salish Sea between 1962 and 2020 and came up with three possible explanations. The first is that it is a social interaction, or play, for the orcas. Play is common among social animals and can help animals communicate better.
Another explanation could be that it’s just hunting. Killer whales often hunt and eat salmon, which could mean they are “practicing” on porpoises.
A third possibility is “mismothering behavior” where the theory is that killer whales perceive porpoises as weaker or sick, so they want to “help” them. This is based on the fact that females have been observed carrying around their dead calves and doing the same with dead porpoises. Helping others in their group is a common behavior for killer whales.
Killer whales in the Northeast Pacific are a dying species, researchers have found that about 70% of pregnancies end in miscarriage or stillbirth, and their diet consists mostly of king salmon, which is also a dying species. The reason these killer whales do not eat porpoises is because their ecology is different from that of killer whales that eat marine mammals.
“Complex and intelligent”
– I am frequently asked, why don’t the Southern Residents just eat seals or porpoises instead. It’s because fish-eating killer whales have a completely different ecology and culture from orcas that eat marine mammals—even though the two populations live in the same waters, says Deborah Giles of Wild Ocra.
– So we have to conclude that their interactions with porpoises have a different purpose, but until now that purpose has only been speculation, she says.
The researchers say we may never know why killer whales harass and kill porpoises, but we can see that the behavior has been passed down through generations, which in itself shows the complex culture of the animals.
– Killer whales are incredibly complex and intelligent animals. We found that porpoise-harassing behavior has been passed on through generations and across social groupings. It’s an amazing example of killer whale culture, said Sarah Teman of UC Davis.
Facts: Killer Whale (Orcinus orca)
The largest toothed whale in the family Delphinidae, which also includes dolphins and porpoises. There are a number of different types of killer whales that differ in their behavior, what they eat, and how they look. Different killer whales can also make different sounds.
A female can be up to seven meters long and weigh four tons; a male can be up to 9.5 meters long and weigh eight tons.
Killer whales are found in all major oceans, although it is unclear how many there are in total, and in some places they are considered endangered.