Delphinids: Common and Bottlenose Dolphins

Common dolphin

When the genus Delphis was first named in 1758 at the dawn of taxonomy, it included three species of small to medium-sized toothed cetacean. By the time the dolphin family, Delphinidae, was named in 1821, one of those species (the porpoise) had been moved elsewhere, but five new ones had been added. Many more followed, but, from the mid-18th century onwards, naturalists began to notice subtle differences between animals that were probably more mysterious to them than land-dwelling mammals, and many species of dolphin began to be separated out into newly created genera.

So much so in fact, that by the time we reach the 21st century, only one species remains in the genus originally created to contain all dolphins and dolphin-like animals. That species is, of course, the one that we believe Linnaeus happened to be thinking of when he named the genus, and therefore is the defining (or 'type') species not only for its genus but for the dolphin family as a whole. This is the aptly named common dolphin (Delphinus delphis).

The common dolphin is sufficiently varied in form that, in the latter half of the 20th century, it was itself thought to constitute two or even three species, and it took until 2015 to confirm that they really aren't. Much of the distinction between the putative species had been based on the length of the beak, so many sources prior to 2015 (and a few since that date) make reference to the "long-beaked common dolphin" as a separate species. But it isn't, with the longer beak being a variable feature within the species that pops up time and again even within relatively closely related populations. 

To further complicate matters, this might not be true of the long-beaked dolphins in the northwest Pacific, since, unlike the others, they apparently don't interbreed with their neighbours. A study as recent as April last year suggested maintaining those ones as a separate species, and that may well be accepted more widely... but as it stands, they're usually recognised as a subspecies, and I won't consider them separately here.

As its name suggests, the common dolphin is, indeed, common. While there is no detailed worldwide census, the total population is thought to number over 6 million. They are found in temperate and tropical seas worldwide, being absent, so far as is known, only from the Caribbean and the Gulf of Mexico. Linnaeus originally described the animal as living in the Mediterranean but today that's sadly an area where the local populations are now endangered, due to a mixture of bycatch, overfishing of their food supply, and pollution. Elsewhere, however, the species as a whole is doing well.

Once we sweep away all those differences that had led to the recognition of supposed different species in the late 20th century, common dolphins are very distinctive animals. This isn't due to the shape or the size, both of which are typical for dolphins. Adults weigh between 80 and 235 kg (175 to 520 lbs), with the males slightly larger than females, and the body length is typically in the range of 1.9 to 2.3 metres (6'3" to 7'6"). The subspecies living in the Black Sea, which is doing slightly better than its Mediterranean counterpart, is noticeably smaller, with an average body length of just 1.6 metres (5'3").

Instead, the distinctive appearance comes from the colouration of the species. The back of the animals is black, and the underside near-white, but these two regions are separated by an "hourglass" pattern along the flanks. The forward part of this pattern is yellowish, and the hind part grey, with the two meeting at the neck of the hourglass positioned below the dorsal fin. Black stripes run along the chin and around the eyes.

Common dolphins prefer to stay offshore, rarely swimming in waters less than 180 metres (600 feet) deep. On the other hand, they rarely travel out into the truly deep oceans, preferring to stay above the continental shelf, where upwelling of nutrients can provide food for the fish they themselves feed on.

Like most cetaceans, common dolphins are top predators - there are relatively few larger animals that will eat them - with about 90% of their diet consisting of fish. Given how widespread they are, the details can vary depending what fish are found in their local area, but favoured prey appear to be mackerel and herrings (the latter being a broad category that includes sardines and pilchards), with salmon, lanternfish, and mullet also being common foods in some regions. The remaining 10% of the diet consists of squid, although this proportion may be higher for dolphins living further offshore where such animals would be more common.

Common dolphins catch their prey cooperatively, herding schools of fish to the surface, primarily in the early parts of the night when they tend to swim upwards anyway. Since the fish are doing this to follow the plankton which in turn follows the light, there is some evidence that the dolphins are more active when the moon is new, and the fish are coming closest to the surface.

They are highly social animals, living in groups that can number up to several thousand, although these are made up of smaller subgroups of a few dozen or so. Membership of these groups is constantly changing, with new members coming and going, although individuals will tend to stick to the same geographic locale over the years. Tests of genetic relationships show that these pods are not comprised of link families, with membership essentially random beyond mothers tending to their offspring. Despite this fact, dolphins in a pod tend to help one another, and there are reports of dolphins trying to rescue group members who have become injured or died. On a larger scale, pods tend to migrate as their food supply does, and often travel together with other dolphin species living in the same area.

Common dolphins spend much of their time travelling between different places where they expect to find food. During this time, they regularly call to one another, maintaining a degree of group cohesion that is apparently less necessary when they are engaged in more dispersed feeding between journies; some of these calls may include information about the individual identity of the caller, although this is less certain than for some other dolphin species. When travelling, common dolphins commonly make acrobatic jumps out of the water, and they may take advantage of the bow wave of larger cetaceans or - in more modern times - of boats.

Mating occurs primarily in the spring and summer, at least in the North Atlantic, and probably also elsewhere. Although males are slightly larger than females, this isn't by very much, which suggests that they may not compete much for mates. Furthermore, while the need for streamlining means that dolphins do not have scrotums, the testicles of common dolphins are unusually large for their size, with a maximum weight of 5 kg (11 lbs). When this is seen in land-dwelling mammals it is usually a sign of sexual promiscuity; females mate with whomever they please whenever they feel like it, so the best chance a male has of fathering a child is to produce more sperm than his rivals.

Pregnancy lasts for 10 to 12 months, with adult females typically giving birth to a calf every two or three years, although they sometimes get pregnant while still nursing their last one. Females reach sexual maturity at around six to eight years, while males take a little longer. Both sexes live around 30 years in the wild.

Common bottlenose dolphin

The common dolphin is not, however, the dolphin species with which most people are probably familiar. That honour likely goes to the common bottlenose dolphin (Tursiops truncatus) due to its widespread presence in aquaria and (for Americans of a certain age) the TV show Flipper. First identified in 1821 from an individual that had wandered into a Scottish river, they are also by far the most widely studied dolphin species.

Bottlenose dolphins are found in temperate and tropical waters across the world, preferring to stay over continental shelves, but more likely than the common dolphin to venture out into the deep ocean far from land. In the north, they are found from the Faroe Islands to Nova Scotia and from California to Japan,. while in the south, they reach the Falkland Islands and New Zealand with a continuous distribution south of both Africa and Australia.

Physically, they are larger than common dolphins, between 190 and 380 cm (6'2" to 12' 6") long and with a maximum weight as high as 635 kg (1,400 lbs). They have a relatively short beak, a distinct crease between that and the forehead, and a smooth grey colour with only a few faint markings. 

Given their larger size, it should be no surprise that bottlenose dolphins tend to eat larger fish than their common relatives. Cod, pollock, hake, and conger eels are all common prey items and, where they are found, croaker or drum fish from a significant part of the diet, probably because the dolphins can hunt them by the sounds they make. In general, however, they are opportunistic, eating whatever is suitable that they come across, and they do eat squid as well as fish. As with common dolphins, most hunting occurs at night.

Bottlenose dolphins commonly live in groups of up to fifteen individuals, although much larger associations have sometimes been reported. As with common dolphins, these have a fission-fusion structure, where the membership of groups changes regularly. Apart from mothers and their offspring, individual pairs of dolphins rarely stay together for long, so that the majority of dolphins even within a given group can be best described as 'casual acquaintances' rather than long-term friends. 

Nonetheless, there is clearly more complexity to bottlenose dolphin society than this might imply. Males and females are more likely to aggregate with members of their own sex, and some males do seem to form long-term partnerships, especially if they are closely related. Furthermore, membership of pods is not entirely random, with some neighbouring pods being more likely to swap members than others - suggesting a hierarchical structure where the pods are grouped into larger communities. Indviduals with a more bold, outgoing personality (as measured by their interest in, rather than fear of, new and unusual things) play a central role in maintaining these networks and probably also in disseminating information about their wider environment.

Bottlenose dolphins associate with other cetacean species, sometimes travelling together in mixed-species groups. There are, however, apparently limits to this, and they don't seem to get on well with common dolphins, often being aggressive towards them, using their greater physical strength to harass or even kill their perceived rivals.

Bottlenose dolphins, like many cetaceans, have larger brains than we humans do. While brain size does not directly correlate with intelligence this fact, taken together with their complex social lives, is certainly suggestive. There has been considerable research on the topic, which is well beyond the scope of this post, but we can say that they have some concept of number, can identify specific objects by vision as well as echolocation, and, perhaps most significantly, they can pass the "mirror test" that is often used as evidence of self-awareness - for example, being able to tell the difference between a live reflection and an image of themselves on a TV screen.

Like common dolphins, bottlenose dolphins use signature whistles to identify each other individually - effectively giving themselves 'names'. It has even been noted that when mothers speak to their calves, they do so in a higher-pitched voice than they use in normal speech - much as human mothers do when speaking to babies, or many people to their pets.

Bottlenose dolphins mate throughout the year, but births are most common in late spring to early summer, something that suggests that, as in humans, sexual activity is not solely associated with reproduction. Pregnancy lasts a full twelve months, with the timing of birth (and thus, also conception) varying among different populations, presumably to best fit in with the local climate. Weaning can take up to two years, longer than for most terrestrial mammals, with the young dolphin reaching sexual maturity at six years of age or older. Bottlenose dolphins are estimated to live for up to 50 years in the wild.

Indo-Pacific bottlenose dolphin

Until 1998, bottlenose dolphins were considered to form a single species. In that year, one of the subspecies was raised to full species status as the Indo-Pacific bottlenose dolphin (Tursiops aduncus). This lives along the coasts of the Indian Ocean and the southwest Pacific, in many cases alongside its close relative. The Indo-Pacific species is slightly smaller than the common one, reaching no more than 270 cm (8' 10") in length, with a slender body, broader fins, and often black spots on the belly. The echolocation clicks of the two species are also similar, albeit with a stronger emphasis on the higher frequencies in the Indo-Pacific dolphin.

So far as we can tell, the two species are otherwise similar in most respects, although it may be relevant that we have only identified the Indo-Pacific bottlenose as a distinct species for a couple of decades or so, and detailed observation of cetaceans can be tricky. A notable difference is that, while Indo-Pacific dolphins can dive as deep as 200 metres (660 feet), they prefer to stick close to shore, almost never being found where they can't reach the bottom, and usually in seas shallower than 100 metres (300 feet). This may be at least partly related to their diet, which, at least off the coast of Zanzibar, consists mainly of eels and bream.

It is, however, no longer clear that the Indo-Pacific and common bottlenose dolphins are each other's closest relatives. If this is true, the Indo-Pacific species would have to be moved to a different genus, although such is the difficulty of sorting through the relevant genetics that it's also not clear which one that would be. What we can say is that bottlenose dolphins and common dolphins share a relatively recent common ancestor with another collection of species... and it is to those that I will turn next time.

[Photos by Ed Dunens, Safari Travel Plus, and "טל שמע" from Wikimedia Commons.]