Delphinids: Killer Whales

Orca / killer whale

The dolphin family is scientifically defined as including all the species more closely related to the common dolphin than they are to porpoises. The great majority are what we'd normally think of as "dolphins" but four species are so much larger that, instead, we tend to call them "whales". Three, including the two species of pilot whale, are of roughly similar size to each other, but the fourth is noticeably larger still.

It's the biggest "dolphin" of all: the orca or killer whale (Orcinus orca).

That it is a dolphin has never been seriously doubted from a scientific perspective. It is one of just three species of dolphin to be listed as such in the first catalogue of scientific names in 1758 - and one of the other two is a porpoise, and so has since been moved elsewhere. On the other hand, it has been recognised as belonging to a distinct subfamily with the dolphins since 1846, and modern genetic studies confirm that its ancestors diverged from those of most other dolphins unusually early. 

It is, of course, also an exceptionally large dolphin, dwarfing even the pilot whales. Even the females can reach 770 cm (25 feet) in length when fully grown, weighing up to 4,700 kg (5.2 tons), while the males are much larger at 980 cm (32 feet) and 6,600 kg (7.25 tons), similar to an African elephant. For that matter, at around 250 cm (8 feet) and 170 kg (375 lbs), even a newborn baby killer whale is about the size of an adult common dolphin.

Killer whales also have a very distinct appearance, that it's hard to mistake for anything else. They are mostly black, with white patches on the chin, above the eyes, and in a U-shape on their undersides and flanks. Most also have a grey streak on the back behind the flippers. In other respects, other than the size, they resemble short-beaked dolphins with a large and prominent dorsal fin; the latter, along with the flippers and tail flukes, is proportionally larger in males.

Officially, there are no subspecies of killer whale and a 2002 genetic study was unable to find any consistent pattern of variation across the world. Just the following year, however, a study of whales off Antarctica revealed the existence of three "ecotypes" - forms that have distinct physical and behavioural features to a level that we'd normally consider enough to call subspecies. Since then, genetic analysis of the ecotypes has shown that they last shared a common ancestor between 150,000 and 700,000 years ago.

If so, there's a good argument that they may be full species, never mind subspecies. This has yet to be widely accepted, leading, among other things, to the IUCN Red List (one of the main international bodies for listing endangered species) refusing to make a ruling on whether the species is threatened or not because they aren't entirely sure what it is. If there are multiple species, there are plenty of existing names available for them, given that many 19th-century taxonomists thought that they had found new species, with two more named in the 1980s. If any of these happened to be described based on a specimen that we subsequently find belongs to a new species, the old name can be resurrected, as was suggested for a couple of putative species just last year.

If they are all one species, killer whales are among the most widely distributed mammals on Earth, behind only humans and those commensal and agricultural species we bring with us. They live in almost every body of salt water you can think of, excluded only from those that are isolated from the oceans (such as the Caspian Sea) or where thick permanent ice shelves prevent them from surfacing to breathe. Otherwise, tropical, Antarctic, open ocean, the enclosed waters of the Mediterranean... they live everywhere.

Having said which, they do have preferences. Killer whales are more common in cooler waters, and closer to the coast, possibly because this is where food is most common. If there are multiple species, not all of them are worldwide, with at least two of the proposed ecotypes being unique to the Antarctic region, although the one that would keep the original scientific name does seem to be especially wide-ranging. There is also some evidence that populations focusing on particular food sources have distinct habitat preferences, likely determined by prey behaviour, even if the whale populations don't represent distinct species or subspecies.

Killer whales are apex predators, feeding on a wide variety of animals. As one might expect from their wide distribution, not to mention the possibility of multiple species, their diets can vary significantly across their range. For example, in the Salish Sea between Vancouver Island and the US/Canadian mainland, the whales feed on almost nothing but salmon, preferring the large, fat-rich Chinook species to the smaller but more abundant coho salmon. Off Iceland, they have a broader diet, with an apparent preference for herring.

But move just a little further out, even in the Pacific right next to the Salish Sea, and some distinct populations feed, not on fish, but on other mammals - making them the only species of whale to do this on a regular basis. Harbour seals are common prey, but sea lions, dolphins, porpoises and sea otters are all eaten in places where they can be found. Move to the Antarctic, and we find the three ecotypes there feed respectively on fish, seals, and minke whales, with the whale-eating form living further offshore than the other two. 

Click to enlarge

This variation in diet has led to a variety of different feeding techniques, some of which may be passed on from parents to young by teaching rather than pure genetics. Killer whales off Norway cooperate to steer herring shoals into a ball to feed on them, while those near Gibraltar feed on tuna by chasing them to exhaustion. Those that feed on marine mammals near the coast often gather into groups to chase their prey into bays where the options for escape are limited, then slap and batter them into unconsciousness before eating them.

Other options are, perhaps, even more dramatic. In the Antarctic, killer whales feed on seals by splashing in the water to raise waves that knock them off ice floes. If that fails, they may even slap the edge of the floe, hoping to break it or tip it over so that the seal slides off. Perhaps the most extreme example is in Patagonia, where killer whales deliberately strand themselves to catch seals lounging about on the shore, dragging them back into the water to drown.

As you might expect from all this, killer whales are highly social animals, regularly cooperating to obtain food or meet their other objectives. In fact, they have among the most complex societies known for non-human mammals, with up to four different social tiers.

At the bottom are the matrilines, individual families composed of a matriarch and up to four generations of her descendants. Killer whales rarely, if ever, leave their matrilines, which remain tightly bonded for the whole of their lives. The second tier is that of the pod, which can consist of a single matriline, but more often inludes two or three, for a total of up to around 55 individuals. In many respects, the pods are the most important social group, travelling together and interacting over extended periods, although matrilines may leave or join pods over the course of several years.

Pods are arranged into clans, which are defined by each possessing a distinct dialect of social calls. Genetic testing shows that each clan descends from a founding pod, developing over time as mothers teach the dialect to their children. While the clan does not move and travel together in the way that a pod does, the individual pods within it regularly interact with one another, maintaining a degree of social continuity.

The highest social level is that of the community, which consists of one to three clans. These are defined by the way that the pods within them interact, as they consist of multiple different clans that therefore have different dialects and are not necessarily related by blood kinship in the way that the smaller units are. Specifically, pods from different clans will interact, sometimes meeting up to form temporary mixed-clan groups, but pods and clans from different communities avoid one another. Why they do this, or how the communities get started, remains a mystery; perhaps their social and feeding habits are just too different. Communities are often large; there are just two known from New Zealand, while the Northeast Pacific has three, containing six clans and a total of 30 pods.

Killer whales are vocal animals, producing much the same types of calls as other members of the dolphin family: whistles, rapid pulses, and echolocation clicks. Analysis of their hearing capabilities suggest that these are similar too, although, surprisingly, their ultrasonic whistles, which are used mainly for short-range communication, have a higher pitch than those of smaller dolphins. Those that feed on mammalian prey, such as seals or other dolphins, are much less vocal than fish-eaters, calling only when necessary - presumably because their prey would otherwise hear them coming.

With respect to this, it's worth noting that the killer whale has one of the largest brains of any animal in terms of absolute volume; larger, for example, than that of a blue whale. The large brains of cetaceans in general are likely due to a combination of their overall body size and the demands of aquatic living, but here, it may be that the need to pursue mobile and often relatively intelligent prey may have driven evolution to make it larger still. Indeed, not only is it large, but the brain of orcas is unusually complex in form, with a particular enlargement of the limbic system - the part of the brain responsible for processing emotions in humans.

Mating takes place when a male briefly leaves its matriline to meet up with females from other pods, usually from the same clan, and is followed by an 18-month pregnancy. Even though a female will only give birth about once every five years or so, she can still raise up to five offspring throughout her life, because killer whales can live for up to 80 years. They are also among the few non-human mammals to experience a clear menopause, often living for decades after they have ceased to be fertile. Although other factors may also be at play, the primary reason for this is thought to be the ability to pass on complex ecological and behavioural knowledge to children and grandchildren without competing for reproductive resources with the whale's own daughters.

False killer whale

The false killer whale (Pseudorca crassidens) is much less studied than its partial namesake and wasn't identified until the 19th century. The species was originally named, as a type of porpoise, by Richard Owen in 1846, but he based his description on a fossil skull found in Lincolnshire and assumed that the animal must be long extinct. Fifteen years later, in 1861, some carcasses washed up on a shore in Denmark, and when zoologist Johannes Reinhardt examined them, he was able to confirm that they belonged to the supposedly fossil species, which was very much still alive.

It's Reinhardt who gave them the name "false killer whale", and he did so because of similarities in the shape of the skull, not on the basis that they were otherwise likely to be confused. For one thing, they are much smaller, and slightly more so than the typical pilot whale, reaching 510 to 610 cm (16' 9" to 20') in length and 1,200 to 2,200 kg (1.3 to 2.4 tons) in weight. As in true killer whales, males are noticeably larger than females.

They are slimmer than orcas, albeit with a similarly blunt snout, and also have much smaller dorsal fins, as well as S-shaped flippers and a pronounced bulge on the forehead that is more prominent in males. They are almost entirely black, with only a grey stripe along the underside; since they must surely have been seen before 1861, they were likely confused for pilot whales at the time, and certainly not for killer whales.

They live in temperate and tropical waters within 50° of the equator worldwide, although they are rare in the Mediterranean, and not thought to live in the Baltic or the Black Sea at all. They prefer warmer waters, following the Gulf Stream to the seas around Britain, but not being found at similar latitudes on the east coast of the US, where they reach only as far as North Carolina. Nonetheless, they can tolerate cooler climates when they must, with some having been reported at the Falkland Islands in 2019. Depth of the water seems less of a concern for them, and there is, for example, a population around Hawaii with distinct subgroups close to the islands and in the deeper seas nearby.

They feed on squid and larger fish, notably including salmon, tuna, and sea bass. They are also known to attack smaller dolphins and porpoises, although they themselves are not large enough to entirely avoid predation by true killer whales. They dive mostly at night, reaching depths of 600 metres (2,000 feet) and staying underwater for up to 14 minutes at a time. 

Their echolocation signals are similar to those of bottlenose dolphins, but their pulsed calls and whistles are more distinctive. The whistles, presumably used in short-range communication, can have a complex structure, and there is some limited evidence that they may be unique to individuals, allowing them to be identified by their fellows.

They are highly social animals, travelling in groups of 20 to 100 individuals. Little else is known of their social structure, but their ties are clearly strong. For one thing, individuals have been spotted staying in together over a 15-year period, suggesting that social groups may remain stable for at least that long. Secondly, and more tragically, much of what we know of the species comes from mass strandings (including the second-ever sighting of the species, in 1862). That they do this with more frequency, relative to their presumed population, than most other cetacean species suggests that they are willing to follow their fellows into danger even if those leading the pod are ill or disoriented.

Their breeding habits remain largely a mystery although we do know that they can live for decades, and there is some equivocal evidence that, like true killer whales and short-finned pilot whales, females can experience menopause.

That brings me to the end of my survey of the dolphin family, although such is the taxonomic debate that new species may well be added in the coming years. Normally, I'd spread each group of mammals out for an entire year, but, for various reasons, I've moved through these ones more quickly than usual. So it will be for the next group I will be looking at, which are very different from the aquatic mammals I've been looking at so far this year. Which ones they are will be revealed in about a month's time...

[Photos by Jim Cotton of NOAA, in the public domain, and Christopher Michel, from Wikimedia Commons. Cladogram adapted from Cunha et al. 2011 and Pichler et al. 2002.]