The Toughest in the Ocean: A review of killer whale predation of large whales.

Over millions of years, the ocean had been dominated by a select group of apex predators. After the rise and fall of Mesozoic marine reptiles, megatooth sharks, and macroraptorial sperm whales, the power vacuum is currently filled by one species, the killer whale (Orcinus orca). It truly is the apex predator of extant marine vertebrates, but the extent of their prowess is a matter of debate. This specifically concerns their predation on the largest extant animals, the great whales species. This controversy warrants a review of the documented records of killer whale’s hunting large whales, their behavior and techniques, and the assessed vulnerability of each species to killer whale predation.

Introducing the ”Whale Killer”

The killer whale’s reputation as a fierce animal is rooted in historical record across many cultures. Its genus name, ”Orcinus” pertains to a Roman god of the Underworld, Orcus. The English common name is likely derived from the name ”ballena asesina”, which meant ”whale killer” to Basque whalers. Across other European languages killer whales are called some variation of ”murder whale” or ”sword whale”. In Indigenous American populations like the Haida, ”Skana” means ”killer demon”. One of the words the Aleut have for this species, polossatikmeans ”the feared one”. Clearly, the killer whale’s predatorial behavior has left an impact on everyone across the world. But is this reputation deserved?

It is true that the killer whale’s perception caused even experts to exaggerate their bloodthirsty nature. The most profound example was Dale Eschricht’s 1862 report on a killer whale, where he found pieces of 14 porpoises and 13 harbor seals (1). This report is often misquoted to suggest each specimen was present in its entirety! Nonetheless, the facts still speak for themselves, killer whales are the top predator with no equal in the ocean. In addition to fish, killer whales effectively predate on any pinniped or small cetacean  species that overlaps with its distribution, even other dolphins as large as the false killer whale (Pseudorca crassidens) and beaked whales (2,3).

Sharks aren’t safe either.  Killer whales have been documented directly attacking and killing great whites (Carcharodon carcharias). In fact, the threat of killer whales seems to be a selective pressure on the distribution of great white sharks (4). They also prey on even larger species like the basking shark (Cetorhinus maximus), attacking full-grown individuals exceeding 9.2 meters/30 feet in length (1,5).

Most impressively, killer whales are the sole predators for many calves and juveniles of large whale species (6). While rare instances have been also been observed with sharks, they often include entangled individuals who have already been brought to a debilitated state (7). On the contrary, killer whales themselves have no known predators. Not even a calf or juvenile has been known to fall victim to a large shark.

It should be noted that attacks of this scale are restricted to the mammal-specializing ecotypes such as  the transients of the North Pacific, Type 2’s of the North Atlantic, and the Type A and B killer whales of the Southern Hemisphere. As a species, killer whales originally evolved from a piscivorous ancestor that was more like a conventional dolphin and evolved adaptations towards a macropredator niche (7). Over time, these ecotypes evolved into mammal-specialists independently.

While mammal-eating killer whales don’t form a clade of their own, they are very similar in their size parameters. The males in each of the ecotypes average 7.3-7.8 meters/ 24-26 feet in length at physical maturity while weighing 5.7-6.8 tonnes (9,10). The largest individuals from each ecotype have been confirmed to exceed 9 meters; a record-holding individual of 9.45-9.80 meters would weigh around 11 tonnes  (11,12). By comparison, males of the fish-eating populations are comparatively smaller, with maximum lengths only slightly above 7 meters in length. They are even outclassed by the female mammal-eaters, who are similar in average length (6.4-6.9m) with a maximum length of 8.2 meters (10,11,12).

Killer whale size comparison
Size comparison between mammal eating and fish-eating killer whales.

Past this point is where this species’ abilities are called into question. While it was long accepted that killer whales are a danger to small cetacean species and young/compromised individuals of the larger species, experts disagreed on whether they consistently posed a threat to the healthy adults (6). Attacks have been witnessed over time for adults of all species, but successful kills aren’t always verified. Furthermore, beyond simply isolated events, there’s still the question of if these successful kills are frequent enough to have any major ecological impact.

Is anything safe from the orca? Time to find out.

To Kill a  Leviathan

For a long time, it was assumed that adults of the great whale species were immune to predation from killer whales, but that’s being called into question. Interestingly, whales are categorized as either a ”fight” species or ”flight” species (13). It’s as simple as it sounds; fight species like sperm whales, balaenids, humpbacks, and gray whales are known to directly combat killer whales. Flight species , which encompasses each member of the Balaenoptera  genus, avoid danger by running away with high bursts of speed. However, regardless of their size, they make no attempts to fight if the killer whales manage to slow them down. If they fail to outpace their pursuers, they’re done for.

To begin this review, I will start with the flight species. Specifically, the smaller rorquals such as the minke whale (Balaenoptera acutorostrata & B.bonariensis), bryde’s whale (B.edeni) and sei whale (B.borealis).

Adult minke whales are about 7-8m on average and reach maximum lengths of around 10-12m. Given their relatively small size, it should be no surprise that minkes are easily the most vulnerable amongst adult baleen whales. They are occasional prey to transients and Type B’s and are the primary prey item of Type A’s (14,15). However, even for this species, success can be hit or miss. While killer whales achieve higher top-speeds than minke whales, minke whales have better stamina, holding their speed over longer distances. For transient killer whales, successful hunts for minke whales is comparably far less frequent than for small toothed-whales and pinnipeds (14).

Successful attacks are rarer for the Bryde’s and sei whales than minkes, but have been verified. In 1988, a 12m Bryde’s whale was successfully hunted down by a group of killer whales in the Gulf of Mexico (16). Adult sei whales were killed after being chased into shallow waters near a salmon farm in Chile (17,18).

In all of these attacks, and in those to follow, killer whales display consistent strategies in taking on larger whales (6,13,14,16,17,18,19). They roughly follow this typical sequence:

  • Lowering vocalizations to avoid detection
  • Biting down on their flukes and pectoral fins to impede their movements
  • Leaping and slamming their bodies onto the whale to drown them
  • Ramming to inflict serious injury, often towards the jaw.
  • Will occasionally strip off and consume flesh off of whale while alive.

Now let’s cover some of the fight species: the Balaenids and the gray whale.

The gray whale has a notable reputation for being a very violent and aggressive species when cornered, being dubbed the ”Devil Fish”.  Before fighting, gray whales may try to utilized kelp beds to mask their presence from the orca’s echolocation (13). Despite these efforts , killer whales have been confirmed to kill  adult gray whales as long as 12m.  This suggests that calves aren’t the only vulnerable members of their species. However, most were juveniles less than 2 years old and 9 meters or less in length (20).

As for right whales, evidence is more scant when it comes to verified successful kills of large adults. Southern right whales (Eubalaena australis) with calves stick to shallow waters as the low depth limits the potential directions an orca can approach from to a 2D- plane. Futhermore, southern right whales were observed to form a ”rosette” formation in response to attacks . Here, the whales surround a calf or sick member with their heads facing inward and their tails out towards the killer whales. When not in a group, right whales will violently roll around and thrash their tails (13). In parts of the Southern Hemisphere, records of attacks on right whales shows no preference towards calves, with lone adults composing the majority of attempted attacks (21). This implies that adult right whales may be vulnerable to predation by killer whales.  Nonetheless, adults have been seen to hold their own, as their tails manage to strike orcas clean out of the water.

Bowheads (Balaena mysticetus) often rely on retreating towards sea ice to protect themselves from killer whales (6,13). If caught, they violently thrash around in a similar manner to right whales. Eschricht once reported an instance where an orca was likely killed by the fluke of a panicked bowhead. According to Inuit hunters, adult bowheads were witnessed to be killed by larger groups of orcas (22). Among 18 bowheads described to have been killed by killer whales, only 8 were described as calf/yearling size (23). However, no precise size was described for the other 10. Since there is no report for a successful killing of adult bowheads outside of the accounts provided by the Inuit hunters, it’s likely these instances are rare. Studies of rake mark frequencies on bowheads in the Eastern Canada-West Greenland and the Bering-Chukchi-Beaufort Seas populations show higher rake mark percentages on larger individuals (24,25,26). Since these rake marks are being taken from whales that have survived, this means that hunting success is far lower when targeting older bowheads. Overall it seems that while bowheads are often targets, older ones are rarely taken.

Now we move on to the largest of the flight species: the fin and blue whales:

When it comes to fin whales, the only confirmed kill I could find was from a report with footage showing transient killer whales successfully hunting down a fin whale described to be ”at least 50 feet” (27). This would roughly correspond to a subadult if below 55 feet or maybe one just on the cusp of sexual maturity if closer to 60 feet.  Another attack was reported for a group of 20 fin whales estimated to be 18-20 meters in length, but this was confirmed to be a failure (28).

As for blue whales, this hit the news pretty big this year. While there’s been a documented history of killer whales attacking blue whales, the outcomes were either failures or uncertain. Over the past few years, we’ve gained three confirmed accounts of killer whales successfully killing and eating blue whales (29). Two of these were calves ranging from 10-14 meters. The largest was estimated to be between 18-22 meters.  For reference, a whale of this length would be roughly 40-60 tonnes. This is well within the adult range of the local pygmy blue whale population. From what the researchers could tell, this individual wasn’t emaciated either, it appeared to be healthy. Among the other reported attacks on blue whales, the largest ones were roughly 20m. This likely hints that larger blue whales may be more resistant to attacks, as a 23+ meter individual would be a lot harder to take down. Still, it appears that even the largest species on Earth isn’t immune to killer whale predation. However, there are still 2 more of the fight species to cover.

Sperm whales (Physeter macrocephalus) were long thought to be invincible to killer whale predation, however we now have many documented accounts showing that isn’t the case. Specifically, females and immature individuals have been killed many times by killer whales (6,30). Just like right whales, sperm whales deploy a rosette maneuver in combination with threat displays to defend against killer whales (19).  While females and immature whales are known to be vulnerable, direct kills on bull sperm whales remains unconfirmed. In attacks where bull sperm whales were present, the killer whales were successfully driven off (31). Killer whales have only ever attacked large males who interfere in their pursuit of females or calves. In contrast to adult balaenids, bull sperm whales themselves are never the targets.

The sex-segregated distributions of sperm whales also hints that killer whales are significantly less of a threat to bulls. Upon reaching maturity, bulls move to live in high-latitude waters that are more densely-populated by killer whales. Females and juveniles stay in the lower latitudes where killer whales are less common. In consistency with the documented accounts, it appears that the mortality rate of bull sperm whales to killer whale attacks is exceptionally low compared to every other whale we’ve covered so far.

However, it still seems bulls are put off when they detect a killer whale’s presence. When exposed to killer whale sounds, male sperm whales immediately cease their solitary foraging and engage in social-antipredator defense (32). They oddly choose not to flee but prepare to stand their ground in response. This is likely the mechanism behind a group of males in Sri Lankan waters who broke off from their main pod to engage killer whales attacking a maternal group (31). So it seems we’ve covered our first creature that killer whales can’t seem to kill reliably, but they’re not the last.

While sperm whales are tough, humpbacks are the real mavericks! Not only has there never been a confirmed kill of an adult humpback by killer whales, but there are more documented instances of humpbacks attacking killer whales than the other way around (33). They remain the only species of large whales to actively mob killer whales and even go as far as to interfere when heterospecifics are targeted, like gray whales and seals.  It’s speculated that this may be a form of animal altruism, though I wouldn’t entertain that. I think it’s more likely that the humpback interferes to the detriment of the killer whale rather than for the sake of the other species.  If a humpback can help starve or injure a local group of orcas, that’s less to worry about during migration season. This behavior is consistent as humpbacks are very aggressive in general, where males are in heavy competition for mate access. It’s even speculated that their disproportionately large pectoral fins had evolved as weapons against killer whales (33).

So at the end of the day, it appears that amongst all the great whales: bull sperm whales and humpbacks are the best at holding their own. However, that’s just among the large whale species…

An unexpected challenger

Aside from an aggressive bull sperm whale or humpback, there’s another creature that seems to consistently intimidate killer whales, the long-finned pilot whale (Globicephala melas).

That’s right, long-finned pilot whales seem to have a rampant history of mobbing killer whales in the North Atlantic. Despite their smaller size, they seem to overwhelm killer whales with their raw numbers. Studies looking into this behavior have uncovered that the killer whales that are being chased off do not predate on pilot whales, nor do they compete for the same prey (34). This led to the hypothesis that pilot whales are confusing these harmless killer whales with a separate mammal-eating population that had gone extinct. Assuming this behavior is learned and is most likely passed down matrilineally, I amusingly refer to this explanation as the ”Racist Grandma” hypothesis.

Outside of the North Atlantic, this behavior is observed in other regions, as larger groups of pilot whales immediately rushed to the scene in 2 out of 3 recorded blue whale killings (29). There were no direct interactions, but it is likely that they were attracted to the vocalizations of killer whales.

So there you have it, whales that killer whales cannot confidently defeat: bull sperm whales, humpback whales, and long-finned pilot whales.

Remaining Mysteries

Now an interesting topic regarding killer whale hunting behavior is the role of adult males in attacks on large whales. It was once assumed males were necessary for successful kills (28), but that has been debunked many times (6,13,29). Very often males would remain in the periphery during hunts and may occasionally participate near the end (6,14,16,29). Meanwhile, females and subadults are performing most of work. It’s proposed that females are given priority in these hunts because they have more young to feed (29). Alternatively, it may be because mature males experience extra drag from their proportionately larger pectoral and dorsal fins. Couple this with being much larger than females, their stamina would deplete in chases against flight species and would lack the maneuverability for pursuits leading to shallow water (14). I think this hypothesis works out as males were far more common and active in attacks on the slower fight species and minke whales in offshore waters (6, 21,31,32,33).

Another big question is what influence mammal-eating killer whales have on the population dynamics of mature individuals. It has been long-believed that calf mortalities to killer whales was a huge selection pressure in driving the distributions and migration routes of large whale species (6,13).  By contrast, I don’t think the direct hunting of mature baleen whales has a strong ecological impact. I believe that the killer whale’s effect on populations of large whales is primarily on the immature cohort as most sources show that juveniles and calves make up the majority of documented kills  for species larger than minkes (6,13,23,24,25,26). These findings are also supported by studies where the vast majority of individuals within observed baleen whale species receive their rake marks as calves and only a minority receive new ones over time (35,36).  Even the adults that are killed are smaller than average, like the 20-meter blue whale or the 15-16 meter fin whale. It is very likely that Antarctic blue whales, where the average adult is 25-meters long and twice as heavy as the recently-documented pygmy, is virtually immune to killer whales.

Nonetheless, the current research still shows we’ve been underestimating killer whales and the impact that they do have on immature baleen whales. Which may be a concern for bowhead whales, as loss of sea ice in the Arctic is increasing the frequency of attacks on calves and juveniles. Without the ice, killer whales are able to remain in Arctic waters for longer periods of time (23,24,25). It’s also not outside the realm of possibility that other baleen whales will become increasingly vulnerable from poor health conditions due to climate change or net entanglements, echoing this previously unseen great white shark attack on a juvenile humpback (7). Killer whale predation can be a potentially large threat to recovering whale populations when compounded with anthropogenic influences.

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Author: CallmeJoe

A 23-year-old College Graduate in Biology who's primarily a fan of Godzilla and other properties.

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