Delphinids: Dolphins of the Deep Seas

Rough-toothed dolphin

To our human eyes, it's easy to distinguish the major habitat types on land. There are pine forests, tropical jungles, open prairies, deserts, mountains, and so on. When it comes to the sea, however, it's less obvious. Most maps show the sea as a solid mass of blue which is, of course, what most of it looks like from the surface. But there are different environments and habitats within it, even if they aren't necessarily arranged in quite the same way.

While for some dolphin species the only real limitation is pack ice preventing them from surfacing to breathe, most have more specific requirements. Temperature is the most obvious, with some species preferring tropical or subarctic seas, but the depth of the underlying water is also significant. Species such as common and bottlenose dolphins are most comfortable over the continental shelves, where nutrients well up from the sea bed to feed the fish and squid on which they prey. Rather more species prefer shallow waters, close to the coast. Here, the water is shallow enough for light to reach the bottom, allowing seaweed or coral to grow, which benefits a different kind of fish than those further out.

Some dolphin species, however, prefer genuinely deep water, where an environment with little, if any, nearby land favours different fish species again. Although they may come closer to the coast, especially if they become lost, these are often less familiar, since you have to take a longer trip to maximise your chance of spotting one. One such species is the rough-toothed dolphin (Steno bredanensis). 

This was first identified as a distinct species by French naturalist Anselme Desmarest in 1817, but, unfortunately for him, the name he picked for it had already been taken for a different species, so the citation you'll see for the current name only dates to 1828 and is credited to a later author. Originally, like all dolphins, placed in Delphinus, it was given its own genus in 1846, and it's sufficiently distinct that it remains the only species in that genus today.

It's a fairly typical size for a dolphin, with males around 270 cm (8' 10") in length and females around 250 cm (8' 2"). With a relatively slender build, they are lighter than the similarly sized common dolphins, at around 155 kg (340 lbs). Their colour is also unremarkable, dark grey over most of the body and fading gradually to a pinkish white on the underside, often with a paler grey stripe running along each side of the back.

 A more distinctive feature is the absence of a crease on the front of the head, with the forehead sloping down smoothly to a long and narrow beak. No other long-beaked dolphin has this feature, although it's common enough on short-beaked species (and porpoises). The flippers are also unusually large, reaching around one-sixth of the total body length. Finally, although this is obviously not something you can tell at a distance, the teeth have a roughened, finely wrinkled surface - a unique feature, the significance of which is uncertain.

They live in tropical and subtropical waters around the world. The original specimen described by Desmarest was off the coast of Brittany, which is about as far north as they ever get in the eastern Atlantic. In the US, they reach North Carolina and a little north of San Francisco, while in South America they reach roughly level with the southern borders of Brazil and Peru. On the other side of the world they reach northern Japan, but not the waters south of Australia; they do, however, swim south of the Cape in Africa, allowing the Atlantic and Indo-Pacific populations to mix there. Despite which, one recent study found evidence for the two being separate subspecies that last shared a common ancestor 890,000 years ago during the Ice Ages.

Rough-toothed dolphins prefer waters away from the continental shelves, primarily those over 1,000 to 1,500 metres (3,300 to 4,900 feet) deep. However, this isn't to say that they never approach the coast, especially of oceanic islands like Hawaii, the Azores, and Tahiti. Furthermore, while they are less common there, they do sometimes live in shallow seas such as the Caribbean, the Gulf of Thailand, and the Red Sea. Reports from the Mediterranean may be due to individuals that accidentally strayed in from the Atlantic, although there is some evidence for an isolated but potentially stable population in the eastern half of the sea.

While we know that rough-toothed dolphins feed on fish, squid, and octopuses, most of our more detailed knowledge comes from examining the stomachs of fatal strandings, which presumably aren't of healthy individuals. Nonetheless, the species has been observed chasing and eating mullets, snipefish, and smelts and also some much larger species such as houndfish and mahi-mahi. Despite preferring deep water and being physically capable of deeper dives if required, they do not normally dive any deeper than about 25 metres (80 feet), feeding on surface-dwelling fish rather than those further down. 

They often hunt fish cooperatively, having been seen herding fish up into shallower waters to trap them, blowing bubbles from underneath to disorient them, tossing dead fish over to one another, and so on. Scavenging sea birds are often seen nearby when the dolphins hunt, feeding on their leftovers.

As might be expected from this they are, like many other dolphins, sociable animals. Pod sizes are usually reported as being ten and twenty individuals, although these are generally in shallower seas, and they may be larger in their preferred deep-sea habitats. What's more unusual is that some reports indicate that the groups are unusually stable, with members remaining long-term rather than the more fluid fission-fusion society seen in other dolphins.

Although they can put on bursts of speed, rough-toothed dolphins are normally slow swimmers, and, while they will bow-ride boats and occasionally leap, when they do, they mostly skim along the surface, splashing in the water. One group of radio-tagged dolphins swam 55 to 100 km per day while heading out to deep waters after surviving a stranding event, although it's hard to know how typical this might be under more normal circumstances. Their echolocation clicks are short and have a deeper tone than those of most other species, peaking at around 25 kHz, while their communication whistles are typically between 2 and 14 kHz. (For context, the highest note on a piano is around 4 kHz, and humans cannot hear beyond 20).

Melon-headed whale

Despite the name, the melon-headed whale (Peponocephala electra) is manifestly a dolphin. Indeed, while it's quite a bit heavier at up to 275 kg (600 lbs), it's almost exactly the same length as a rough-toothed dolphin - and thereby smaller than the familiar bottlenose dolphins of aquaria. It has a short beak and an unremarkable grey body, with a darker 'cape' across the back and only a narrow region of whitish skin along the underside. It's sometimes referred to as the "electra dolphin" to avoid this confusion, while many non-English languages use a variant of "melon-headed dolphin" or otherwise ditch the "whale" part. Since 1966, it, like the rough-toothed dolphin, has been considered the only species in its genus.

Although they are not closely related to rough-toothed dolphins, melon-headed whales have similar habitat preferences, only more so. They, too, live in tropical and subtropical waters, but do not stray so far from the equator, and are thereby split into two populations by the presence of southern Africa, albeit there is no clear evidence of any subspecies. In the eastern Atlantic, they range from Angola to just south of Morocco, while in the west they reach only as far north as South Carolina. In the Pacific, their northern limit runs from southern Japan to about San Diego, and their southern from Sydney to Peru; they inhabit much of the Indian Ocean, but apparently not off the west coast of Australia.

They are also more reluctant to swim over continental shelves, so that they only reach close to land around oceanic islands where the seafloor drops off rapidly, such as Hawaii. It's not quite true that they never reach shallow waters, and they do occasionally strand when suffering from infections or deafened by sea traffic noise, but it's notable that the first confirmed report of any in the Gulf of Mexico dates from as recently as 1993.

Although they will also eat fish, over 75% of their diet consists of squid. They forage mostly at night, when the squid rise towards the surface, but even so, dive to depths between 600 and 3300 metres (2000 feet to 2 miles) to do so, depending on the local environment. Analysis of their muscles shows high levels of myoglobin, something that would enhance dicing ability, although it is speculated that, in their case, this may be more to do with long-distance endurance swimming across stretched of empty ocean.

Since they rarely travel close to the shore unless unwell, we know relatively little about the details of their lives. They travel in large pods, typically with over a hundred individuals, and often in association with other similarly-sized dolphins - including the rough-toothed species. When mass strandings occur, around two-thirds of the victims usually turn out to be female, which may imply some sexual segregation in their communities, with many closely related females banding together with a smaller number of males. Their whistles are short, lasting around half a second, and may be highly repetitious, perhaps to ensure they are heard amongst the cacophony of large groups.

Pygmy killer whale

The closest living relative of the melon-headed whale is thought to be the pygmy killer whale (Feresa attenuata), which is equally obviously a dolphin, and is not closely related to the actual killer whale. In fact, it's physically almost identical to the melon-headed whale, with the same size, general shape, and colour pattern. A slightly more rounded shape to the head and flippers are about the only distinguishing features. Although first identified as a distinct species in 1827, the current name dates only to 1874, due to a similar mix-up as occurred with the rough-toothed dolphin. 

Pygmy killer whales are another tropical and subtropical species, reaching as far as North Carolina and the north coast of Spain when they can follow warm currents, but otherwise staying closer to the equator - on the east coast of North America, they barely reach the Gulf of California in Mexico. Not only do they rarely travel over continental shelves, but they aren't common even far out at sea. Add to this the fact that they seem wary of boats, and they're one of the least likely dolphin species to be seen.

Consequently, we know little about them. The bulk of the scientific literature on the species amounts to "ooh, we saw one!" followed by some locality where they hadn't been seen before. Nonetheless, we do have some data from stranded individuals which confirm, for example, that (at least when they're ill) they eat plenty of squid, but a wider range of fish than the melon-headed whale seems to. Unlike their relative, they live in small groups, typically with no more than ten individuals, although the fact that they strand together implies a strong degree of social cohesion within their pods. 

A couple of individuals that were rescued from a stranding and successfully released back into the wild with radio tags gave us some information on their behaviour. They dive mostly at night, for up to nine minutes at a time, reaching a maximum depth of 368 metres (1,207 feet). During the 88 days before the last tag gave out, they travelled about 300 km (230 miles) in waters up to 1,200 metres (3,900 feet) deep, although it's likely that most prefer much deeper seas than this. Their echolocation clicks are similar to those of related species.

As with the other two species described in this post, we know essentially nothing about their mating habits and reproduction.

Relatively few species of dolphin specialise in deep waters as these three do. That's probably partly due to the lack of barriers, making it easy for the species that do exist to mix and limiting the drivers that would split them into new ones. Coastal waters are a different matter, due to the vagaries of coastlines and it's at least possible that this is the main reason that there are more identifiable and distinct species in such waters than further out. I've already looked at some, but next time I'll turn to another two, including the most shallow-water adapted of all members of the dolphin family...

[Photos by Laurent Bouveret, Oliver Dugornay, and "Adam U" of NOAA.]