As humans, it is not always easy to understand the behavior of other animals. For decades, researchers have observed killer whales in the Pacific Northwest attacking and even killing dolphins. But surprisingly, these expert hunters do not eat their victims. This reveals the mystery of exactly why they behave like this.
These killer whales, also known as Southern Native Orcas, are a genetically and culturally diverse population of orcas living in the Pacific Northwest. They are an endangered group of about 75 members spread across three herds. The lives of these orcas are closely linked to the lives of chinook salmon, which are also endangered. As you can imagine, if salmon disappear, these orcas will also disappear. But despite their preference for salmon, these whales have been known to attack and kill other small marine mammals before eating them.
To better understand this unusual behavior, an international team of researchers examined interactions recorded over 60 years between southern orcas and harbor porpoises in the Salish Sea, a coastal sea located in British Columbia and the US state of Washington. These attacks have been observed for a long time, but this behavior appears to be becoming increasingly frequent and spreading among the orca population.
“Killer whales are incredibly complex and intelligent animals,” says Sarah Teman of the SeaDoc Society, part of the UC Davis School of Veterinary Medicine. “We found that behaviors that harass harbor dolphins are transmitted across generations and across social groups. This is a spectacular example of killer whale culture. However, we don’t expect Southern Native killer whales to start eating porpoises. The culture of eating salmon is deeply ingrained in Southern Native society. “These whales need healthy salmon populations to survive.”
“I’m often asked why Southerners don’t eat seals or harbor dolphins instead,” says Wild Orca’s Deborah Giles. “This is because fish-eating killer whales are fish-eating killer whales, cetacean-eating whales, even though the two populations live in the same waters.” It has a completely different ecology and culture from orcas. “We must therefore conclude that their interactions with harbor dolphins serve a different purpose, but this purpose has so far been merely speculation.”
Some kind of social game?
According to Giles and his colleagues’ study, orcas may be attacking the dolphin as a form of social play. As with other intelligent creatures, play is an important bonding behavior. It may be possible for these larger animals to attack smaller mammals as a way to foster social cohesion.
Similarly, this behavior may represent hunting practice in which orcas hone their teamwork and coordination skills to hunt for salmon. In this case, dolphins may be serving as “moving targets” to which predatory techniques can be applied. Unfortunately, it is not easy to tell the difference between game and hunting in these species.
Finally, this behavior may be an example of “false mothering,” in which orcas view small harbor dolphins as weaker or sicker and therefore in need of care. Female orcas are known to carry the bodies of their deceased babies in similar ways.
“Inappropriate maternal behavior, also known to scientists as ‘displaced epimeletic behavior,’ may be due to limited opportunities to care for offspring,” explains Giles. “Our research has shown that approximately 70% of Southern Resident killer whale pregnancies are due to malnutrition.” showed that it resulted in miscarriage or the death of the offspring immediately after birth.”
This research highlights the need to protect salmon populations in the region, as well as the orcas’ entire hunting range. The threat to the future of salmon threatens not just a single species, but the entire ecosystem.
The research was published in the journal Marine Mammal Science.
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