Delphinids: Spotted, Striped, and Spinning Dolphins

Pantropical spotted dolphin

The closest relatives of the common and bottlenose dolphins, which are perhaps the most familiar species to the western public, are those in the genus Stenella. Or at least, that might be true, because there has been a lot of confusion about the genus over the years.

It was first created in 1866 as a subgenus within Steno, the "narrow-beaked" dolphins, and contained just one species. It became a full genus in 1934,and by the end of the 20th century was agreed to contain five species - all of which had, in fact, been named before 1866. Since then, our understanding of genetics has greatly improved, and it has become clear that these various species cannot be so neatly arranged on a family tree as we might like.

That's largely because they, at least on rare occasions, interbreed with one another, mixing their genes in a way that ideally shouldn't be the case for a "species" but often is in reality. This makes it very hard to tell, for example, whether the common dolphin's closest living relative is the Indo-Pacific bottlenose dolphin or one of the Stenella species. (Indeed, one could plausibly argue about what "closest" really means in this context). Worse, genetic studies in 2006 appeared to show that at least two of the species don't belong in the same part of the wider family tree, which would mean that Stenella (as currently defined) doesn't even exist as a biological entity. 

So far, nobody is convinced enough about this to warrant changing anything from the last time we cleared the mess up in 1987. It's entirely plausible, but we don't have enough data on other closely related species to be confident as to what's really going on. It may yet be that some of the five species currently assigned to Stenella end up being moved elsewhere, but, as of 2025, those are the ones we recognise.

Even if everything else gets changed, the one species that will remain where we originally put it is the Pantropical spotted dolphin (Stenella attenuata), since that's the one that John Edward Gray created the genus for in the first place. This had first been described under an entirely different scientific name that roughly translates as "fast-swimming dolphin" by Cuvier in 1829 but Grey, presumably unaware he was describing the same animal, had renamed it in 1845 and it was his name that stuck. Technically, under the rules of scientific naming, Cuvier's name should be the correct one, but it's one of those rare cases where it had been forgotten for so long that an official exception had to be made.

This is a bit of a pity, since Cuvier's name is more appropriate than Grey's, which roughly translates as "little dolphin with a short, narrow beak". Which would be fine if it had a short beak, but it doesn't. 180 years on, though, it's a bit late to change things now. This isn't Brontosaurus...

The Pantropical spotted dolphin lives in tropical waters worldwide, generally preferring the temperature to be above 25°C (75°F). This, for the most part, corresponds to seas within 40° of the equator although, obviously that's not precise. Along the east coast of the Americas, they are found from Connecticut and Rhode Island in the north down to northern Argentina in the south, while they seem to be more restricted on the west coast, ranging only from Baja California to Peru. We would expect them to be found all the way around Africa, but records are incomplete so that's not entirely certain, and we know that they don't enter the Mediterranean. They are absent from the south coast of Australia, but are found throughout essentially the whole of the Indian Ocean and as far north in the western Pacific as the Sea of Japan.

Where they are found, they are amongst the most common of all dolphin species, with particular hotspots being the Caribbean and the Gulf of Mexico, Hawaii, the Philippines and Taiwan. While they are most common in coastal waters and the sorts of warm shallow seas that the most of those locations would imply, they are also found in much deeper oceans, allowing the species to spread worldwide.

Pantropical spotted dolphins are similar in size to common dolphins, but are comparatively slender, weighing up to 120kg (265 lbs). Like most dolphins, they have a dark back and paler underside, and they also have some stripes on the face. They are born without spots, and some individuals never develop any at all. Most have at least some, however, although these are generally faint and more visible on the paler underside. The spots are pale on the back and dark on the belly, contrasting with the background shading. The beak is notable for being longer and narrower than that of most other dolphin species.

They primarily feed on small surface-dwelling fish, with lanternfish and flying fish being particularly common prey; squid, especially "flying" squid, are also frequently eaten. Dives are relatively short compared with those of some larger dolphins, lasting no more than three or four minutes. They apparently stick to well-lit surface waters during the day, diving no deeper than 13 metres (42 feet), but they can dive as deep at 57 metres (190 feet) at night, when they do most of their feeding. While chasing food being dragged by a boat, they have been recorded as swimming up to 40 kph (25 mph).

Pantropical spotted dolphins gather in large pods, which may have over a hundred members. There is some evidence that they segegrate themselves by sex and age, with juveniles, adult males, and older females with their young all travelling in groups away from others. Pods are often found together with shoals of yellowfin tuna, which are too large for the dolphins to eat but which may, perhaps, have similar habitat requirements. This, of course, has not escaped the notice of fishermen, and it is only in the last few decades that "dolphin-safe" methods of tuna fishing have started to become popular... although, especially in the UK, the relevant food label often means that you're eating the species of tuna that doesn't associate with dolphins anyway and, where it isn't, dolphins can still be disturbed by fishing (or other) boats even when they aren't being killed by them.

Atlantic spotted dolphin

The same year that he described the Pantropical spotted dolphin, Cuvier also described what we now call the Atlantic spotted dolphin (Stenella frontalis). This has been described as being partway between its Pantropical cousin and a bottlenose dolphin in appearance, although it doesn't have the comparatively large size of the latter. At birth, the skin colours of the two species are very similar, with a bland grey body, darker on the back than the belly and only a few faint markings. The spots begin to develop from the age of three, and by the time it reaches adulthood there are small white blotches across the whole of the body, making it far more visibly spotted than its pantropical relative. The spots get larger and even begin to merge as the animal ages.

The Atlantic spotted dolphin is found only in its namesake ocean, but in a zone that, while it has a considerable overlap, is, on average, further to the north. In the south, it reaches only around 25°S and perhaps not even that far in the east, while in the north, it can reach 50°N in the mid-Atlantic, although it only reaches the coasts of Connecticut and Morocco on either side - like its cousin, it is not found in the Mediterranean. Reports indicate that they are particularly common off the coasts of Brazil, the Bahamas, and the Azores.

Close to Africa, they are typically found within 300km (190 miles) or so of the shore, and they are also found in very shallow waters off the Bahamas, often over sandbanks just 10 metres (33 feet) below the waves. Most, however, seem to prefer deeper waters, even beyond the continental shelf; it's possible that the deepwater and coastal variants are distinct subspecies, although this has yet to be confirmed.

They feed, like their relatives, on a variety of small to medium fish and squid, probably focussing on whatever is most plentiful where they live. In Brazil, for example, cutlassfish are the most common prey - these are about 75 cm long (2' 4"), but they are slender and eel-like, not anything especially muscular. Feeding dives last up to six minutes and reach depths of up to 60 metres (200 feet). 

Pods typically include 50 or more dolphins and, as in the pantropical species, there is some evidence that, within the larger groupings, they prefer to associate with others of their own sex and age. Such associations are, as with other dolphins, not long-lasting, the social membership of any group being, well... fluid.

The calls of Atlantic spotted dolphins are very similar to those of bottlenose dolphins, further supporting the idea that they may be more closely related than their taxonomy (and detailed anatomy) might suggest. Indeed, the two species are often found travelling together, and interactions between them run the full gamut from mild aggression to apparent companionship to sexual intercourse. Aggression is mainly between the males, something that may be influenced by the smaller spotted dolphins being more likely to gang up in single-sex groups than the bottlenose dolphins. Mating seems to be equally likely regardless of the species of the female, although male-male sex always seems to be initiated by the more physically powerful bottlenose dolphins.

There doesn't appear to be any specific mating season and young remain with the mother for many years, only being weaned at the age of four or five. At least some of this time is apparently spent with the mother teaching her calf how to forage. Females are sexually mature almost as soon as they leave their mother, but males seem to take as long as 18 years before being able to father children. Since the total lifespan is only around 23 years, this is probably due to competition with older, stronger, males, rather than any inherent lack of physical capability.

Spinner dolphin

Despite the ambiguity of the genetic evidence, what we have at least points towards the pantropical and Atlantic spotted dolphins being the result of parallel evolution within a group whose species were all fairly similar to start with. If so, it may be that the closest relative of the pantropical species is not the Atlantic spotted dolphin, but the spinner dolphin (Stenella longirostris).

This lives in almost exactly the same seas as the pantropical spotted species - tropical waters between 40°N and 40°S right around the world. Indeed, the two are often found together, such that the spinner dolphin is another one at risk from tuna fishing. While it has a similar shape, with the same long, narrow beak and a similar shape to the fins and flippers, the most obvious difference is that it isn't spotted, having a bottlenose-like three-tone pattern of dark grey back, median grey flanks, and pale grey underside. It is, however, also quite a bit smaller, weighing no more than 80 kg (180 lbs). 

They prefer shallow waters close to the coast, with one subspecies native to the Gulf of Thailand preferring coral reefs. They feed at night, eating fish smaller than 20cm (8 inches) and apparently avoiding the squid that many other dolphins also eat. In the Sulu Sea, this mainly means lanternfish, and the dolphins dive to at least 200 metres (650 feet) to reach them. Unusually, they are known to hunt in packs of 16 to 18 individuals, cooperating to herd the fish together into a tight clump so that they can feed on them. Many of these fish swim in deep waters during the day, avoiding the sunlight, but they rise towards the surface at night, bringing them into reach of the dolphins; during the day the dolphins rest above, but close to, the 100 metre (325 feet) depth line, keeping them in easy reach of their nighttime foraging grounds.

Pods range from a few dozen to over a hundred individuals. While their social structure seems to be based on the same constantly changing membership pattern as in their relatives, in at least some areas, long-lasting associations between individuals apparently form - although whether these are due to familial relationships or shared feeding grounds that merely happen to be convenient is unclear. Much as in many herd animals, the dolphins in a pod share the task of looking out for predators (such as killer whales or sharks) with at least some individuals taking on scouting duties from time to time while others feed or rest. In 2023, signature whistles - the calls used to identify specific individuals and that effectively function as their personal "name" - were identified in spinner dolphins for the first time, although they were well-known in some better-studied dolphin species.

Perhaps the best-known feature of the spinner dolphin is, however, the one from which it takes its name. Spinner dolphins regularly leap into the air while travelling, performing rotating barrel rolls that would be the envy of any human gymnast. (Olympic level divers regularly perform up to four twists during a dive; a spinner dolphin can manage seven in a one-second jump). The reasons for this are unclear. They may be displays intended to impress sexual partners or to help bind a pod together, but another possibility is that they dislodge blood-sucking fish.

Striped dolphin

The most widespread species within the genus is the striped dolphin (Stenella coeruleooalba), which has the same southern boundary at 40°S as its tropical relatives but also lives further north. In North America, it can reach Vancouver Island and Nova Scotia, while in Europe, it not only inhabits the Mediterranean but even reaches as far north as the north coast of Scotland - hardly what anyone would describe as tropical. They are often found where major sea currents result in the upwelling of deep water, keeping food plentiful.

The stripes for which they are named are far less obvious than they might be on, say, a tiger. For the most part, they have the same three-tone colouring as their relatives, with the only real change being that single dark stripe, which runs from the beak, around the eyes, and down to the front of the flipper. Another key difference from their relatives is that they are physically larger, with a heavier build. The largest reach about 160 kg (350 lbs), with, as in their relatives, the males being slightly larger than the females.

Striped dolphins are not as thoroughly studied as their close relatives, and this is probably because they tend to stay further out at sea, on the margins of continental shelves or even beyond them. Their heavier build probably allows them to dive deeper, and they take advantage of this, feeding on prey that are most often found at depths of 200 to 700 metres (650 to 2,300 feet) - although it doesn't necessarily follow that the dolphins go down to the furthest reaches of that range. Favoured prey include squid, cod, whiting, and pilchards, although, as usual, this varies by geographical region, with almost anything smaller than about 30 cm (12 inches) being fair game.

Striped dolphin pods can vary in size from 20 to over 100 individuals, and there is some evidence of sex and age segregation among them, as in their relatives. So far as we can tell, males mate with multiple females, and the lifespan may be as much as 58 years.

Clymene dolphin

The remaining species in the genus is the Clymene dolphin (Stenella clymene) which looks remarkably like a spinner dolphin and can only be visibly distinguished from it by a slightly different shape to the lower edge of the dark shading on the back and the precise shape of the dorsal fin. They are also slightly smaller, with a shorter beak, but not always enough to be really noticeable. They even spin in the air while jumping, just as spinners do. Indeed, while it was first identified as a separate species in 1856 (probably, since even that isn't entirely clear) it was regarded as just another name for the spinner dolphin in the early 20th century, only being restored in 1981.

For that matter, nobody really knows what "Clymene" is supposed to mean, since Grey never bothered to say when he named it. There is an ocean nymph of that name in Greek mythology, so that's generally regarded as the best bet (thus, it's assumed to be a proper noun, and gets a capital letter)... but other possibilities do exist.

This is another species that lives only in the Atlantic, but it seems far more widely distributed in the west than the east. In the Americas, it is found from New York to southern Brazil, but in Africa only in the tropics, from Mauritania to the Congo - although, given their similarity to spinner dolphins, it's at least possible that this is due to a lack of studies in the relevant areas. 

Like the striped dolphin, it doesn't help that it's a deep water species that does not often come in close to land. Analysis of stomach contents from stranded individuals suggests that they mainly feed on fish found at about the limits that sunlight can reach - although we'd guess that they probably do so at night, when they come closer to the surface. They probably live in pods of around 50 individuals or so, but estimates of pod sizes have ranged from just three to about 1,000. 

They are often found together with spinner dolphins and there may be a good reason for this beyond the cooperation we often see among related dolphin species. This is because it is now thought likely that, as I discussed in detail a few years back, the Clymene species arose after female striped dolphins mated with male spinner dolphins and gave birth to fertile offspring that then mated (mostly) with each other, creating a self-sustaining hybrid. 

This may go some way to explaining quite why the relationships between species in Stenella are so muddled, and why it may not even be a "real" genus. However, as the cladogram above shows, it isn't just the common and bottlenose dolphins that are intermixed with the genus - there's another group, too. Next time, I will turn to take a look at those.

[Photos by Joseph Ferris III, NOAA, "Wanax01", and the National Marine Sanctuaries, from Wikimedia Commons. Cladogram adapted from Cunha et al. 2011.]