Why do killer whales harass porpoises without eating them? New study explores

Fish-eating killer whales in the Pacific Northwest have puzzled scientists for decades with their behavior of harassing and killing porpoises without consuming them. A recent study led by Deborah Giles of Wild Orca and Sarah Teman of the SeaDoc Society, a program of the UC Davis School of Veterinary Medicine, delved into over 60 years of recorded interactions between Southern Resident killer whales and porpoises in the Salish Sea. The aim was to shed light on the reasons behind this perplexing behavior.

The Southern Resident killer whales, an endangered group with just 75 individuals, depend heavily on chinook salmon for their survival, which are also endangered. While some may wonder why these orcas don’t simply eat seals or porpoises, the answer lies in their distinct ecology and culture compared to marine mammal-eating orcas, even though they share the same waters. It’s become clear that the interactions between these killer whales and porpoises serve a different purpose, although the exact nature of this purpose has remained speculative until now.

Three plausible explanations

The enigma of Southern Resident killer whales engaging in porpoise harassment, observed as far back as 1962, has stumped scientists for years. Deborah Giles, Sarah Teman, and their collaborative team meticulously analyzed 78 documented incidents of porpoise harassment spanning from 1962 to 2020. Their investigation presents three compelling explanations:

Social Play: Killer whales might be using porpoise harassment as a form of social play. Like many intelligent species, these whales engage in playful activities, possibly fostering group cohesion, communication, or simply having fun. This behavior could contribute to improved group coordination and teamwork.

Hunting Practice: Another theory posits that porpoise harassment serves as a training ground for honing salmon-hunting skills. Southern Resident killer whales might view porpoises as agile practice targets to sharpen their hunting techniques, regardless of their intention to consume them.

Mismothering Behavior: An intriguing idea suggests that these whales might be attempting to care for porpoises they perceive as weaker or ill, reflecting their innate inclination to assist fellow group members. Females have been observed carrying their deceased calves, and similarly, they’ve been seen carrying porpoises.

Deborah Giles explained, “Mismothering behavior, also known as ‘displaced epimeletic behavior’ to scientists, could be a result of their limited opportunities to care for their young. Our research has uncovered the harsh reality that nearly 70% of Southern Resident killer whale pregnancies have resulted in miscarriages or calves that died shortly after birth due to malnutrition.”

Is it harassment, play or something else? Killer whales interact with a porpoise. Credit: Wild Orca

Salmon specialists

While the study by Giles, Teman, and their collaborators provides fascinating insights, it acknowledges that the precise reason for porpoise harassment may forever elude us. What remains crystal clear, however, is that porpoises do not factor into the Southern Resident killer whale diet. These killer whales have finely tuned diets specialized for salmon, rendering the notion of them feasting on porpoises highly improbable.

Deborah Giles notes, “Killer whales are incredibly intricate and intelligent creatures. We’ve discovered that the behavior of porpoise harassment has been passed down through generations and across different social groups, showcasing the remarkable complexity of killer whale culture. Nevertheless, we don’t anticipate a sudden shift in the Southern Resident killer whales’ dietary preferences. The tradition of feasting on salmon is deeply ingrained in their society. These whales depend on healthy salmon populations for their survival.”

This research underscores the critical importance of safeguarding salmon populations in the Salish Sea and across the entire habitat range of these whales. Ensuring an ample supply of salmon is not only crucial for the Southern Resident killer whales’ survival and well-being but also for maintaining the overall health of the Salish Sea ecosystem.

Affinity for play

This research coincides with a separate population of killer whales making international headlines on the Iberian Peninsula. These Iberian orcas have garnered attention for their interactions with boats, notably sinking vessels on three occasions along the coasts of Portugal and Spain. It’s important to note that the Southern Resident killer whales and the Iberian Peninsula orcas are distinct populations with their own unique cultures. However, one intriguing commonality they share is a penchant for playful behavior.

The study involved collaboration with various partners, including the University of Exeter, Fisheries and Oceans Canada, Orca Behavior Institute, National Oceanic and Atmospheric Administration, Cascadia Research, The Whale Museum, Center for Whale Research, Ocean Research College Academy (ORCA) at Everett Community College, Bay Cetology, North Gulf Oceanic Society, George Mason University, and Marine-Med.

Source: UC Davis

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