Sunday 25 September 2011

A New Species of Dolphin

Common bottlenose dolphin
On September 14th, the discovery a new species of dolphin, the burrunan dolphin (Tursiops australis) was officially announced. There has already been a fair bit of coverage of this in the media (see, for example, the BBC story), but I want to focus here on how this all came about. How exactly do you go about naming a new species?

In theory, it's a fairly straightforward, if somewhat laborious, process. You find your new species, write up a description of what it looks like, and how to tell it apart from similar species, designate a holotype (more on this later), think up a name, and get it published. Leaving aside the difficulty of the first part of that - "first, find your new species" - that's often all there is to it. But, with dolphins, the story has been rather more complicated than that.

First, let's get our bearings. The sort of dolphin we're talking about here is a bottlenose dolphin, a particularly well known type, and commonly seen in sea mammal parks, where they have been trained to perform a number of tricks. They live in every ocean, avoiding only the very coldest of polar seas and are therefore extremely widespread.

The common bottlenose dolphin was first formally described by George Montagu in 1821, and given the scientific name Delphinus truncatus, which means something like "shortened dolphin", and refers to the relatively short snout. A second species, the Indo-Pacific bottlenose dolphin, was identified in 1832. Paul Gervais, in 1855, in the process of naming a third species, decided they were different enough to be given their own genus: Tursiops - a name borrowed from the Latin for "porpoise".

Over the next eighty years, more and more species of bottlenose dolphin were named, largely on the basis of "well, they look a bit different". In the days before genetic analysis, this was all fairly reasonable. It is, after all, difficult to see exactly which groups of dolphins are inter-breeding, since they're out at sea when they're doing it. At least four of the named species had scientific descriptions that were too vague for us to know what it was the author was trying to describe, but that still left at least eight, and possibly as many as twenty, species.

This all looked to be cleared up when it was decided that there was really only one species, that just happened to have a rather variable appearance. Because it was the first, the 1821 description and name stood, and all of the others were just newer names for the same thing - subspecies, at best. But this, it turned out, was very far from being the final word: in 1998, it was determined that the Indo-Pacific bottlenose dolphin really was a separate species from all the others, and so its species was resurrected, as Tursiops aduncus.

So, we have two species of bottlenose dolphin, the common and the Indo-Pacific. But were some of those nineteenth and early twentieth century naturalists right? Were at least some of the species they named real all along? Into this muddied picture swims the newly discovered species.

As you may have guessed, this is not an animal that nobody had ever seen before. We had seen plenty of them, off the coasts of southern and eastern Australia, but had assumed that they were just members of one of the other two species - though it wasn't very clear which. Indeed, it was one of the many groups previously suggested to form its own species, named the "southern Australian bottlenose dolphin" (Tursiops maugeanus).

When you name a new species, one of the things you have to do is identify a holotype; a particular specimen of your new animal. More than that, it is the crucial, defining specimen of the animal. It is the one specimen that you can, if need be, point to and say "this is what I'm talking about". If it later turns out that your species is really two or more different species, the name you chose stays with the species that the holotype belongs to, because that's the one you were really describing. On the other hand, if it turns out that your holotype isn't really a new species, then your chosen name is worthless, and cannot be used from them on, at least as any more than a subspecies.

So, when the authors of this paper wanted to prove that the southern Australian bottlenose really was its own species, one of the things they did was to take a look at the holotype specimen originally designated back in 1934. Problem is, there wasn't one. In those days, the rules had been less strict, and the scientists who had originally described the species noted that one of the interesting things about it was that the males and females looked quite different, something not so unusual among mammals. So they decided to designate not one, but two, type specimens, one male and one female. This, they hoped, would avoid all confusion.

But they were wrong.

Because, when the modern scientists analysed the DNA of the two specimens, they discovered that there was a very good reason that the male and female looked different: they weren't the same species. Which is precisely why, of course, you aren't supposed to designate more than one type specimen these days - fine though it was in the 1930s. The male, so far as they could tell, was just a common bottlenose dolphin, but the female seemed to be something else entirely. From genetic samples taken from a number of living and recently dead dolphins in the general area, they identified that she was closely related to a group of dolphins found off the coasts of Victoria, South Australia, and Tasmania - and nowhere else.

But that wasn't really enough to prove that she, and her living relatives, belonged to a new species. They were something distinct, certainly, but were they distinct enough? There are many different ways to define a species, and the authors of this new paper decided to cover their bases by describing their species in as many different ways as they could.

They delved into the genetics to demonstrate that the dolphins were so different from other bottlenoses that they couldn't be anything but a new species. But genetics wasn't enough. There has been so much confusion over how many species of dolphin there really are that there's an informal agreement among scientists dealing with these animals that, before they'll agree you've found something new, you need both genetics and the ability to prove you can tell your animal from others just by looking at it. Either one on its own just isn't enough; it's a stricter rule than is applied with most other mammals, but it does make things rather more certain.

The new species is much smaller than the common bottlenose dolphin, and somewhat larger, on average, than the Indo-Pacific species. Like the common species, but unlike the Indo-Pacific, it has a short, stubby snout, and a curving, rather than triangular, fin on its back. Unlike either form, it has a bluish-grey upper body, with a distinct pale grey stripe along its flanks, and a whitish underside and face, without any of the spots sometimes seen on other dolphins. There are also some noticeable differences in the skull, especially in the shape of the bones of the upper jaw, that are described as being quite unlike those of either of the other known species.

Examining the DNA to determine how the new dolphin relates to other species reveals some of the complexity that has made the classification of the dolphin family so difficult:

  Burrunan   Spinner   
 Bottlenose  Dolphin   
  Dolphin       |               Common
     |          |             Bottlenose
     |          |   Fraser's    Dolphin
     ------------   Dolphin,      |   
          |           etc.        |    Indo-Pacific
          |            ^          |     Bottlenose    Clymene
          |            |          |      Dolphin      Dolphin,
          |            ?          |         |          etc.
          -------------------------         |           ^
                        |                   |           |
                        |                   |           |
                        |                   -------------
                        |                         |
                        |                         |

Text in blue shows members of the genus Tursiops. Text in green shows the genus Stenella.

Notice that there's something very wrong about this chart. Not only does it show that the new species isn't particularly close to the common bottlenose, being closer to the Spinner dolphin (Stenella longirostris), but the two species of bottlenose dolphin that we already knew about aren't related to each other, either. This actually wasn't a great surprise; we'd already guessed as much, and this new discovery only makes things a little bit worse.

However, what you can't tell from this simplified version of the original is just how tiny some of the genetic differences are. Its not that the species aren't clearly different from one another - they are - its that most of them are different by almost exactly the same amount, which means that its very hard to tell in what order they would have branched apart. Quite where Fraser's dolphin goes isn't clear at all, for instance, and some of the others are nearly as uncertain.

Somebody, at some point, is going to have sort this mess out, but it hasn't happened yet, and the authors decided to leave their new species in Tursiops for lack of anything more obvious to do with it. They don't expect it to stay there, and went as far as to suggest a name for the new genus they would like it placed in when somebody finally works out what is going on with dolphin relationships.

So, having proven that we've found a new species, what do we call it? If it was completely new to science, the authors could just make their own name up, but, remember this one was first identified back in the 1930s. You might think therefore, that they'd just use the old name, which is exactly what happened when the Indo-Pacific bottlenose was resurrected from taxonomic oblivion. I get the impression that the authors argued quite a lot about this, since it depends on what you're doing with the holotype - and there isn't one.

Their eventual decision, which is probably fairly arguable, was that the original scientists had been describing the male specimen more than the female. Since the male isn't a member of their new species, they got to think up a new name, and, being Australian, chose Tursiops australis. The female, which died in the North Esk River in 1914, and currently resides in a museum in Launceston, was designated as the holotype of the new species.

If it's not quite the same thing as was described back in 1934, they don't have to continue calling it the 'southern Australian bottlenose dolphin', either. Instead, they chose 'burrunan dolphin' as the common name, borrowing the local aboriginal word for 'dolphin'. This common name has no particular scientific standing, and is just a suggestion, but it's likely the name that will stick.

The final step in naming a new species is to have the description officially published. Yet even that presented a bigger hurdle than it should have done, for they chose to publish in the online electronic-only journal PLoS ONE. But the ICZN, the international body that handles the naming of new animal species, has yet to enter the twenty-first century, and refuses to accept that anything electronic can really have been published. So they had to go to the trouble of producing a separate print-on-demand version and posting it to key libraries before anyone was allowed to pay attention to what they'd written.

The end result of this complex, messy, story is that we have a new species of dolphin, living off the coast of south-eastern Australia that we kind of knew about, but had never been formally described before. Given all the confusion that has reigned since the first bottlenose dolphin was described almost two hundred years ago, it won't be the last. There are more species of dolphin out there to be found, and it probably won't be too long until we hear about them.

[Picture from Wikimedia Commons. Cladogram adapted from Charlton-Robb, et al. 2011]

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