Cooperative Foraging Between Killer Whales and Dolphins

Interactions between different species of marine mammals are frequently observed in the wild, yet the underlying purposes of these encounters often remain uncertain. Along the coast of British Columbia, Canada, northern resident killer whales and Pacific white-sided dolphins are known to interact closely, and individuals of these two species are often observed swimming within only a few meters of one another.

Pacific white-sided dolphin（圖片來源：NMFS Southwest Fisheries Science Center (NOAA) ，公有領域。）

Northern resident killer whales are highly social animals that live in matrilineal groups and exhibit a specialized diet. They feed almost exclusively on adult Chinook salmon. Pacific white-sided dolphins, in contrast, have a far broader diet that includes herring, squid, and smaller salmon species. Although their prey preferences partially overlap, field observations indicate that resident killer whales generally tolerate the presence of dolphins in their vicinity.

Killer whale（圖片來源：Mlewan ，公有領域。）

Several hypotheses have been proposed to explain why Pacific white-sided dolphins associate with killer whales. One possibility is kleptoparasitism, in which dolphins attempt to steal prey captured by killer whales. Another hypothesis suggests that dolphins remain near resident killer whales to reduce the risk of predation by mammal-eating transient killer whales, or possibly to avoid large sharks. This idea arises because resident and transient ecotypes of killer whales typically avoid each other. A third possibility is that the two species cooperate during hunting, thereby increasing the overall efficiency of prey capture.

Cooperative hunting within a species has been well documented among many marine mammals. In contrast, evidence of cooperative hunting between different species remains relatively rare. To address this gap and evaluate whether dolphins and killer whales might engage in cooperative foraging, researchers conducted a detailed study using aerial drone observations, acoustic recordings, and measurements of diving behavior. These combined methods allowed the researchers to analyze the interactions between resident killer whales and Pacific white-sided dolphins and determine whether their relationship involved competition, cooperation, or other strategic forms of association.

The study was conducted between August 15 and August 30, 2020. A sixteen-meter wooden research vessel, the M/V Gikumi, served as the primary platform for fieldwork, while a smaller six-meter aluminum boat was used for tagging and close tracking of individual whales. Killer whales were identified in the field using distinctive features such as saddle patch patterns and dorsal fin shapes. Suction-cup tracking tags were attached to selected individuals—specifically males and non-lactating females older than four years. After tagging, researchers closely followed the whales while drones operated within a one-kilometer radius of the research vessel, recording behavioral data. The tags were designed to detach automatically after approximately 6 to 24 hours depending on environmental conditions.

Movement tracks of tagged killer whales: green = A113, brown = I145, purple = D26, yellow = I107（圖片來源：Fortune, S. M. E et al. (2025). ，採用 CC BY 4.0 授權。）

Over four days of observation, four of the nine tagged killer whales—identified as A113, I107, I145, and D26—were found interacting with numerous Pacific white-sided dolphins. In total, 258 encounters between these whales and dolphins were recorded. Individual interaction counts varied widely: A113 had 14 encounters totaling about 13 minutes, I107 had 7 encounters totaling roughly 110 minutes, I145 had 149 encounters lasting about 63 minutes, and D26 had 88 encounters lasting approximately 50 minutes.

All observed interactions occurred during feeding activities, including prey searching, hunting, successful captures, prey handling, and food sharing. Drone footage captured 15 separate interaction sequences totaling 84 minutes. Analysis of these recordings revealed that killer whales often followed dolphins for extended periods—sometimes for as long as 25 minutes—changing direction and diving in response to dolphin movements. These observations indicate that killer whales exhibit a tendency to track dolphins during foraging.

The videos also documented six events in which dolphins chased and captured small salmon near the water surface while killer whales remained nearby. In several cases, killer whales followed dolphins underwater and performed rolling movements, suggesting that they also consumed some of the prey. After killer whales finished feeding, dolphins were occasionally observed scavenging leftover fragments of fish. Importantly, whether or not hunting attempts were successful did not appear to influence the whales' tendency to follow dolphins.

Cooperative foraging between dolphins and killer whales（圖片來源：Fortune, S. M. E et al. (2025) . ，採用 CC BY 4.0 授權。）

Because underwater hunting cannot always be observed directly, researchers also relied on acoustic recordings. These data indicated that cooperative hunting between dolphins and killer whales primarily occurred in relatively shallow waters near the ocean surface. When dolphins produced echolocation signals, killer whales often reduced or temporarily ceased their own echolocation clicks, suggesting that the whales may listen to dolphin signals to help detect prey.

Throughout the study, no aggressive or avoidance behaviors were observed between the two species. Instead, the interactions appeared compatible with cooperative behavior. Killer whales were consistently observed following dolphins, whereas dolphins rarely followed killer whales. This pattern suggests that dolphins may function as scouts, detecting potential prey and providing cues that killer whales can exploit during hunting.

The hypothesis that dolphins associate with resident killer whales to reduce predation risk from transient killer whales remains plausible and may explain why the two species sometimes remain together even when no immediate feeding advantage is apparent.

Overall, the study indicates that northern resident killer whales and Pacific white-sided dolphins may form a functional interspecific foraging partnership. Dolphins have difficulty capturing large adult Chinook salmon, but they can benefit indirectly by participating in hunts or feeding on leftover prey fragments. For killer whales, the presence of dolphins may improve prey detection and increase hunting efficiency while reducing the energy required to locate prey. No clear signs of competition or hostility were observed, indicating a high degree of behavioral tolerance between the two species.

Although the precise energetic benefits and reciprocity of this relationship remain to be quantified, the findings suggest that such cooperative foraging behavior may offer ecological advantages during the seasonal migrations of Chinook salmon. Future research involving more extensive tagging and behavioral tracking will be necessary to determine whether this relationship represents a stable hunting strategy or a temporary interaction that emerges only when high-value prey are available.

（Author: Bai Leng）

Reference:

Fortune, S. M. E., Cheng, X. Holmes, K., Trites, A. W. (2025.). Cooperative foraging between dolphins and fish-eating killer whales. Scientific Reports.