Chewed Orca Fins on Russian Beach Hint at Cannibalism, Potentially Explaining Tight-Knit Pods

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Chewed Orca Fins on Russian Beach Hint at Cannibalism, Potentially Explaining Tight-Knit Pods

A series of striking discoveries along the Russian coastline has presented marine biologists with compelling evidence suggesting that cannibalism may occur among killer whales (Orcinus orca). Detached orca fins, found washed ashore and bearing distinctive tooth marks, indicate that these apex predators might occasionally prey on their own species. This potential behavior could offer a novel explanation for the complex and tightly-knit social structures observed in some orca populations, a phenomenon that has long intrigued scientists.

The findings, published in the journal Marine Mammal Science, stem from the work of Olga Filatova, a whale researcher at the University of Southern Denmark, and her colleagues. The investigation began in August 2022 when Sergey Fomin, a researcher at the Pacific Institute of Geography in Russia, discovered an orca fin on Bering Island. The fin was bloodied and bore clear tooth marks, characteristic of predation. Initially, Fomin suspected that the marks were inflicted by Bigg's orcas, a well-known ecotype of killer whale in the North Pacific that specializes in hunting marine mammals, including other whales.

However, genetic analysis later revealed that the fin belonged to a southern resident orca, a distinct population known for its fish-eating habits and complex social behaviors, such as elaborate greetings and cooperative foraging. This identification was surprising, as it suggested an orca had been preyed upon by another orca, rather than by a different species or ecotype. The initial discovery gained further significance when, in July 2024, Fomin found a second detached orca dorsal fin on the same beach. This fin, slightly larger and from a young male, also displayed the same telltale killer whale tooth marks.

The consistent nature of these findings led Filatova and her team to hypothesize that cannibalism is indeed occurring. "At that moment, I started thinking that this is a pattern," Filatova stated. She explained that orca fins are tough and not particularly nutritious, often discarded by predators after consuming the more valuable muscle and blubber. The fact that these fins were found suggests a specific predatory event.

In the North Pacific, two main types of orcas inhabit overlapping regions: Resident orcas, which live in large, stable family groups and primarily feed on fish, and Bigg's orcas, which are more transient, hunt marine mammals, and typically live in smaller groups. It was generally assumed that these ecotypes avoided direct conflict. However, the evidence suggests that Bigg's orcas may sometimes target resident orcas, particularly the southern residents, which are known for their distinct cultural practices like adorning themselves with kelp and performing social massages.

The researchers propose that this occasional predation by Bigg's orcas could be a significant factor driving the formation of large, cohesive social groups among resident orcas. In the animal kingdom, forming large groups is a common strategy to deter predators. While orcas are typically considered apex predators with no natural enemies, intraspecific aggression and predation have been observed. For instance, Bigg's orcas have been documented killing young orcas, although not necessarily for consumption.

Filatova suggests that the threat of predation from Bigg's orcas might compel resident orcas to stay together in larger numbers for protection. This could explain observations of large resident pods actively driving away smaller groups of Bigg's orcas. Filatova herself has noted instances where Bigg's orcas seem to avoid areas occupied by resident orcas, waiting for them to move on. "So, it looks like this defense strategy is really working," she commented.

However, the theory is not universally accepted. Luke Rendell, a biologist at the University of St Andrews who was not involved in the study, acknowledged the intrigue of the findings but urged caution. "I think the observations of tooth marks on fish-eating whale carcasses are interesting and the idea is worthy of further investigation, but there's not yet enough evidence to build a solid account of the social evolution of fish-eating orcas," Rendell stated via email. He suggested that other factors, such as the benefits of cooperative foraging and knowledge sharing about habitats and prey, could also be key drivers for the formation of large, localized groups.

Other marine mammals also exhibit social structures that may be influenced by predation pressure from orcas. Pods of long-finned pilot whales, for example, are known to confront and deter killer whales, a behavior attributed to their strong social bonds. Orcas have even been observed to retreat from the calls of pilot whales.

Michael Weiss, research director at the Center for Whale Research in Washington, echoed some of Rendell's points. "I certainly think it's possible that Bigg's killer whales predated on these two whales," Weiss said. However, he also raised the possibility that the rake marks could result from scavenging on carcasses already deceased from other causes, or from aggressive interactions between resident orcas themselves. "Therefore, it doesn't definitively show cannibalism or predation," Weiss concluded.

Filatova conceded that scavenging cannot be entirely ruled out, as orcas are known to scavenge from whale carcasses, particularly those resulting from whaling activities. She argued, however, that fresh orca carcasses typically sink rapidly, making them inaccessible for scavenging. They usually begin to float only after several days, by which point decomposition has advanced significantly. "You need to be really hungry to eat this," she remarked, implying that predation on live or recently deceased individuals is more likely.

Furthermore, Filatova differentiates the observed marks from those typically seen during fights between resident orcas, which tend to occur on the animals' flanks rather than on fins. She posits that predation pressure, possibly dating back 100,000 years when distinct Pacific and Atlantic orca lineages began interacting, drove the evolution of cohesive social structures in resident populations. These structures, once established for efficiency, persisted.

Interestingly, Filatova also raised the point that the concept of "cannibalism" might be perceived differently by these marine mammals, especially given the significant divergence between ecotypes. "They never socialize; they never spend time together. For them, it's just another whale. So why not eat it?" she pondered, hinting at the potential for distinct ecotypes to be viewed as separate entities by each other, blurring the lines of what constitutes cannibalism.

Ekhbary News Agency