Delphinids: Pilot Whales

Long-finned pilot whale

The term "dolphin" is not, strictly speaking, a scientific one. It refers, in common parlance, to any small cetacean, often even including porpoises. Even ignoring the porpoises, however, not all dolphins are members of the dolphin family, technically referred to as "delphinids". This is because some freshwater animals are not closely related to the dolphins proper (or, indeed, to the porpoises). We call them "dolphins" because they're about the right size, a similar shape, and... well, we don't have a better word, at least for them all collectively.

But it works the other way, too. Not all members of the dolphin family are commonly called "dolphins". With the exception of the melon-headed whale, which it's hard to think of as anything other than a dolphin, this is because they're too big. We call them "whales" - another term that doesn't map to anything scientifically - since that's what we call any large cetacean.

Thus, in the scientific sense, pilot whales are dolphins. Big dolphins to be sure, but they're members of the same family, and are therefore dolphins in the same way that lions are cats. 

There are two species of pilot whale, of which the more thoroughly studied is the long-finned pilot whale (Globicephala melas), first identified as a species in 1809 from an individual found off the Orkney Islands. At the time, it was placed with all other dolphins in the genus Delphis, so its identity was clear even that far back, not being placed into its own genus until 1828.

The most familiar dolphin to most lay people is the common bottlenose, which is of above-average size for dolphins. The very largest reach 380 cm (12' 6") and 635 kg (1,400 lbs), which is not insignificant. In comparison, however, even an average adult male long-finned pilot whale is 670 cm (22 feet) in length and weighs around 2,300 kg (2½ tons), although females are noticeably smaller. That's a big dolphin.

In other respects, however, the similarities are evident. Pilot whales have a similar shape to dolphins, with, for example, a short beak, but not one as short as some of the smaller species. The long-finned species has, as you might expect, relatively long flippers, up to a quarter of the total body length, and these are curved, giving the impression of an elbow (although it's really the wrist). They are dark in colour when fully grown, sometimes black, more often a dark grey or occasionally brown. There are paler patches behind the dorsal fin, above the "eyebrows", and on the chest and genitals, with the latter two patches often being joined by a narrow pale streak.

Long-finned pilot whales live in temperate waters but, unusually, do so in both hemispheres. The two populations never come into contact, and are regarded as distinct subspecies; there has historically been some suggestion that they should be considered separate species, but this does not currently seem a popular idea. In the north, they live only in the Atlantic, reaching far north of the Arctic Circle off the coasts of Lapland and Greenland, and as far south as the Carolinas and Mauritania in West Africa. This includes the western Mediterranean, although the populations there are declining and suffering from genetic isolation. Skulls are known from the North Pacific, too, but the population appears to have gone extinct there no later than the 10th century.

In the Southern Hemisphere, however, the lack of continental barriers means that they can easily have a distribution right around the globe. Here, they can be found as far north as southern Brazil, Angola, South Africa, New Zealand, and southern Australia, and as far south as the edge of the circumpolar Antarctic Current. A rough guideline is that, in both hemispheres, they inhabit waters between 0° and 25°C (32° to 77°F) and that they therefore avoid both the tropics and the frozen polar seas. It is thought likely that they crossed the equator during the Ice Ages, and were then unable to return, leading to the two separated populations we have today.

Pilot whales feed almost entirely on squid, with little variation across their wide distribution range. They do, however, also eat octopuses, and, on rare occasions, even fish such as cod. Although they can be found in all but the shallowest of seas, they prefer waters over continental slopes, where the squid rise from the depths at night. Even so, they don't reach the surface, and pilot whales do most of their feeding at night, diving for up to 18 minutes at a time and reaching 300 to 1800 metres (1,000 to 6,000 feet) depending on the local terrain. Dive speeds reach 2.3 m/s (5 mph), but most, especially during the day when they aren't feeding much, are slower, shorter, and very much shallower.

Food is generally slurped down using the tongue and throat muscles rather than the teeth, sucking the squid down at short range. Like other dolphins, the skin of pilot whales can accumulate microorganisms, resulting in biofouling not unlike that which affects underwater technology. Analysis of the skin shows nanoscale ridges that, together with rapid swimming and jumping out of the water, may help it self-clean, in the absence of the sort of grooming seen in other animals. 

Pilot whales are highly social animals, living in pods of 10 to 20 individuals that in turn can accumulate into much larger assemblages of over a hundred. The pods are matriarchal, consisting of a dominant female and all of her younger relatives; unlike many land-based mammals, both males and females tend to stay with the same pod for all of their lives. This would reduce mating opportunities if the pods did not gather together into the larger groups, giving potential partners from different families the chance to hook up, if only temporarily.

Since they feed at night, breaking up into smaller groups to dive, socialisation mainly takes place during the day. At these times, dives are much shallower, typically no more than 7 metres (23 feet) and the whales often mill about at the surface. Individual pods often travel 100 km (60 miles) a day, and can do twice that, in some places migrating through the year to follow the squid schools.

Pilot whales produce the usual range of vocalisations that other dolphins do, including clicks, whistles, and pulsed calls. The whistles, with a typical pitch of 4480 Hz (C♯ 4 octaves above middle C), are mainly used while relaxing, but pulsed calls, rapid bursts of high-pitched tones, have a far wider range of uses. These are thought to be social calls and have been shown to be very complex, perhaps conveying information about levels of excitement or stress. One study was able to identify 40 different pulsed call types, the meanings of which remain unknown.

Breeding takes place in the spring and summer, with males likely taking several partners. There is, however, an unusually long gap between pregnancies for pilot whales, varying from about three to five years. This is likely to be because lactation also lasts much longer than in most other mammals, with young not being weaned for at least 22 months, and often more. Calves are often cared for, at least temporarily, by other members of the pod, something that's thought to be due to their highly social lifestyle rather than in expectation of any specific reward. 

Short-finned pilot whale

The other species of pilot whale is called, as you might expect, the short-finned pilot whale (Globicephala macrorhynchus). The two species look almost identical, although the short-finned species is, on average, about 5% larger and has shorter flippers, less than one-fifth its total body length. They also have fewer teeth, with up to nine pairs in each jaw, rather than thirteen. Despite not being split apart by obvious oceanographic barriers, there is some physical and genetic evidence that there may be two subspecies, meeting up off the south coast of Japan.

Unlike its close cousin, the short-finned pilot whale inhabits warm temperate and tropical waters around the world, including, thanks to the warm Davidson Current, the entire length of both coasts of the US (except Alaska). Other than this and where the cold Humboldt Current prevents access to Chilean waters, the distribution is roughly from 50°N to 40°S, which means that there is considerable overlap where both species are found - between Spain and Mauritania, in the case of Europe and Nova Scotia to North Carolina on the North American east coast. In such places, we have evidence that the two can interbreed, although they do not do so regularly enough to be considered the same species.

The two species are similar in more respects than their physical resemblance. They both feed almost entirely on squid, although again, fish are sometimes eaten. In at least some areas, they may be feeding on deeper water squid than their cousins, implying that they are swimming further from the coast. This may, however, not be a universal trait, as evidence is lacking from other parts of the world. Either way, their hunting behaviours are otherwise similar, occurring only occasionally during the day, but reaching depths of as much as 1,500 metres (4,900 feet) at night, sometimes staying below the waves for over 40 minutes.

The social structure of the two species is also similar, with matriarchal family units being grouped into pods that are then gathered into clans that remain stable over the long term. There is some evidence that individual clans have distinct "accents" that develop from their social cohesion and enable mutual identification. Despite their similar size, short-finned pilot whales produce calls almost an octave higher than their long-finned cousins.

In both species, their strong social cohesion does come with a downside. Mass strandings of mostly healthy whales are common because if the matriarch becomes ill or disoriented, her family will follow her regardless, unwittingly putting themselves at risk. This, in fact, is where the name "pilot whale" comes from, since, even in prescientific times, it was thought that one individual piloted the herd, with the others tagging along behind.

Pilot whales are not, however, the only unusually large delphinids. Next time, I will look at the others, including the largest dolphin of them all...

[Photos from "Mmo iwdg" and "espinya", from Wikimedia Commons. Cladogram adapted from Cunha et al. 2011.]