Delphinids: Newest and Largest Dolphins

Fraser's dolphin

One of the largest features at the British Natural History Museum is a full-scale model of a blue whale, occupying a large chunk of one of the mammal halls. This was installed in 1938 by Francis Fraser, a Scottish zoologist with a lifelong interest in cetaceans. Eighteen years later, still working at the museum, he was put in charge of reorganising their collection of cetacean skeletons and came across one that hadn't been closely examined since it had arrived in 1895.

It had been donated by Charles Hose, a colonial administrator and amateur naturalist who had found the skeleton on a beach near a river mouth in Sarawak (then a British Protectorate). Hose hadn't been quite sure what it was, and simply labelled it "white porpoise ? Lagenorhynchus sp." before sending it on. When Fraser examined it, however, he soon realised that it couldn't possibly be what Hose had guessed and that it was, instead, an animal previously unknown to science.

In fact, while it did have some similarities with species in the Lagenorhynchus genus (most of which have since been moved elsewhere), in other respects, it resembled Delphinus dolphins. Fraser consequently gave his new discovery its own genus; it is now called Fraser's dolphin (Lagenodelphis hosei). 

What's significant about this is that it happened as late as 1956. There are certainly species of dolphin that have been named more recently, but generally because we have reanalysed something we already knew about and raised a subspecies, or even a population, to species level. But here was a relatively large mammal, completely unknown to science until the second half of the 20th century.

In fact, for a long time, all we had was that one skeleton. It wasn't until 1971 that anybody found a carcass and discovered what the animal actually looked like. Once we knew what to look for, however, they started turning up everywhere. It turns out that Fraser's dolphins live in tropical waters worldwide, with Malaysia being just a tiny fraction of their range.

Roughly speaking, they live within 30 degrees of the equator, in both hemispheres. This encompasses almost the whole of the Indian Ocean, both the east and west coasts of Africa, and across the Pacific from Australia and Japan on one side to Mexico and Peru on the other; in the western Atlantic, they range from about South Carolina to Uruguay. 

There are. however, a few reasons why they took so long to notice. For one thing, they may be widespread, but they aren't very common in any given place, at least compared with other dolphins. They are also rather shy. Many dolphins will bow-ride in the wake of ships, or even approach them closely, but Fraser's dolphins are much more wary, making them harder to spot. Perhaps most significantly, however, they prefer waters over 1000 metres (3,300 feet) deep, so they rarely approach close to coasts where most people are likely to see them.

This is one of those species where a high proportion of the scientific studies about them are just somebody saying that they've seen one for the first time in a particular place, such as some part of the Caribbean, Cape Verde, Angola, or (in the case of stray individuals that may have become lost), Korea or Scotland. 

Fraser's dolphins are comparatively large for dolphins, although, at up to 270 cm (8'10") in length, no more so than a bottlenose dolphin. They have a stocky build with a short but distinct beak and a triangular dorsal fin. Their upper bodies are grey, and the undersides white, with a black stripe running down the sides and from the head to flippers. This stripe develops as they age and is more prominent in males than females. Adult males also have a hump on the underside of the tail behind the genitalia and anus, although the purpose of this is unclear. Younger individuals have a grey stripe rather than a black one, and undersides that are more pink than white.

As one might expect, given their preference for deep waters, Fraser's dolphins dive for a lot of their food, being thought to reach depths of up to 600 metres (2,000 feet). Lanternfish and viperfish, both of which are bioluminescent, have been reported as common prey items, but there is likely a wide range, including a high proportion of squid and shrimp. Fraser's dolphins are also perfectly capable of feeding at the surface, especially when they venture into shallower waters, so they are probably quite adaptable animals. Notably, it has been reported that where they live in the same areas as pantropical spotted dolphins, they let them feed at the surface, while concentrating more on deeper prey themselves - thus ensuring there is enough fish for everyone.

While they may not be common, where they are found, it is in large groups of hundreds of individuals, sometimes over a thousand, and often travelling together with other dolphin species. They are fast swimmers, making long, low, leaps out of the water. They whistle to communicate, along with their echolocation clicks, although these whistles have been reported to be simpler in structure than those of many other dolphins. They are assumed to be sexually promiscuous, with pregnancy lasting just over twelve months, although their lifespan is thought to be shorter than for other similar species, at perhaps no more than 18 years.

Risso's dolphin

In addition to pantropical spotted dolphins, Fraser's dolphins often also overlap in their range with Risso's dolphins (Grampus griseus), generally ceding them regions of shallower water. While Fraser's dolphins belong to the 'delphinine' subfamily, along with common and bottlenose dolphins, Risso's are now known to be more related to the rough-toothed and snubfin dolphins, which belong in a separate branch of the family tree.

Risso's dolphin has been known for far longer than Fraser's, first being named by Cuvier in 1812, based on a specimen collected and analysed by French naturalist Antoine Risso. In 1826, John Edward Gray gave it its own genus, causing some confusion and even annoyance over the following years, because "grampus" is an old word meaning "killer whale", something it manifestly isn't. 

Nonetheless, Risso's dolphin is the largest species to be commonly referred to as a 'dolphin'. Adults are typically about 400 cm (13 feet) in length, and weigh around 450 kg (1,000 lbs), with the two sexes showing little distinction. They have stocky bodies with a curved dorsal fin and a very short, almost invisible, beak. They are born grey in colour, slightly paler on the underside, and this changes little throughout life. Adults, however, are covered in a patchwork of persistent and visible scars, growing denser as they age, gleaned through a lifetime of battling squid - and, to a lesser extent, each other.

Risso's dolphins are found worldwide. Superficially, they stay within 60 degrees of the equator, meaning that they are absent only from the Arctic and Antarctic Oceans and some neighbouring waters. In Europe, for example, they reach northern Scotland and southern Norway, while in North America, they reach Labrador, while in the south, they avoid only Antarctica and the Drake Passage. They even inhabit the Mediterranean, although not other partially enclosed seas such as the Black Sea, Persian Gulf, or the Yellow Sea. It's more accurate, however, to say that they avoid waters cooler than about 10°C (50°F), which means that, at the northern and southern edges of their range, they migrate towards the equator in the winter.

Although it's not obvious when spying one at sea, there is a key difference between Risso's dolphins and most other dolphin species. Typically, dolphins have plenty of identical-looking conical teeth. Fraser's dolphins, for instance, has 38 to 44 pairs of teeth in each jaw, while bottlenose dolphins have between 18 and 27, and the common dolphin can have anything up to 52 pairs of teeth in each jaw, giving it a total of over 200 teeth. This is far more than you would expect in land mammals (or, indeed, seals) and is part of the extreme adaptation of the animals to eating fish.

Risso's dolphin, however, has no more than fourteen teeth in total, sometimes as few as four, and they are all in the lower jaw. While the upper jaw does sometimes technically have one or two pairs of teeth, these never erupt, remaining vestigial structures inside the bone, and they are often missing altogether.

This strange, almost toothless, mouth is likely because Risso's dolphins eat almost nothing but cephalopods. This also explains their preferred habitat, over deep canyons and the steep slopes of the continental margins, in water between 200 and 1000 metres (650 and 3,300 feet) deep. Here, there are deep upwellings of nutrient-rich water, and squid regularly rise towards the surface at night to feed. Although they can probably dive as deep as Fraser's dolphins, Risso's dolphins rarely go below 50 metres (165 feet), and they are also unusual in being nocturnal, feeding only at night when the squid are closest.

At least in the Mediterranean, jewel squid seem to be particularly popular, comprising over half the diet, but this likely varies across the world. At 19 cm (7½ inches) not counting the tentacles, jewel squid are moderately sized for squid, but the dolphins also snack on much smaller species, as well as octopuses. 

Risso's dolphins typically travel in collections of 10 to 50 individuals, although much larger groups, numbering in the hundreds, have occasionally been seen, often mixed together with other species. However, these are variable associations, with the basic unit of their society consisting of clusters of three to twelve individuals, who seem to stay together over extended periods. This is more stable than the usual fission-fusion relationships of most other dolphins, and seems to be based on social stratification of a sort, with most members of a group being of a similar age. 

Although they are sexually promiscuous, males apparently have preferred female partners, although the latter do wander off on their own when they have calves to nurse. Younger, subadult, dolphins have less long-lasting relationships, arguably going through some sort of 'teenage' adjustment period before they choose long-term friends and partners to settle down with. Some of these groups return to specific places every year, or just stay in the same neighbourhood, but many move around and there is social mixing between groups, especially among females.

The echolocation clicks of Risso's dolphins are quieter and deeper in tone than those of most other dolphins, perhaps partly because of their larger size, but they are just as effective at their intended purpose. Their hearing is most acute for sounds coming from beneath them, and it may be that, in nature, they commonly direct their echolocation clicks in a downward beam. An unusual feature of their vocalisation is a combination of a whistle and a rapid burst of clicks; this is typically used at the surface, and so is presumably social in nature, although the details are unknown. There is some evidence that this is learned, rather than innate, since young Risso's dolphins raised with bottlenose dolphins imitate their foster parents.

Mating occurs during the summer, with pregnancy lasting 13 or 14 months. Females have preferred birthing grounds, where they gather away from the males in areas closer to the shore than where they would normally feed. Estimates based on their teeth and the accumulation of scars as they age suggest that they may live for as long as 45 years, which is quite impressive for a dolphin.

At close to half a ton, Risso's dolphin is, as I noted above, the largest animal to be commonly referred to as a 'dolphin'. It is not, however, the largest species in the dolphin family, coming in only at fifth place. There are, however, still two species of dolphin referred to by that name that I have yet to look at... and those will be the subject of my next post.

[Photos by Laurent Bouveret and "Daddy t3", from Wikimedia Commons.]