Saturday 1 February 2014

A New Kind of River Dolphin?

Amazonian botos (probably)

I could, perhaps, have titled this post "I Told You So".

Back in August, at the end of my post on the newly discovered olinguito, I said: "Should we expect more new species of mammal to be found, perhaps even large and interesting ones? You can count on it." I also suggested in that post that remote jungles were exactly the sort of place you'd expect to find one. Well, since I wrote that, we have discovered at least one, and possibly two, new species of large mammal that fit exactly that description.

The first was the kaboman tapir (Tapirus kabomani), which lives in the Amazon jungle, and which was described a couple of months ago. I'll refer you to Darren Naish's blog if you want to know about that, since I have nothing useful to add to what he's already said. There is, so far as I can tell, no dispute or argument about the reality of this tapir as a new species - the first new tapir to be described since 1865. The acceptance of other one, published just last week in the online journal PLoS One, appears to not quite be so clear cut.

The animal in question is a new species of river dolphin, and it, too, was discovered in the Amazon jungle. But, before we look at the details, it's probably best if I start by describing what a river dolphin actually is.

Dolphins, on the whole, live in the sea; this is surely not a great surprise. Yes, they sometimes travel a short way up particularly large rivers, and some of them even do it on purpose. But, generally, the sea is where you expect to find them. Now, there is an exception, and I discussed it back in 2012: it's the tucuxi (Sotalia fluviatilis). This spends its entire life in freshwater, and, as I said at the time, it's the only true dolphin that does this.

With the emphasis on the "true" part of that sentence. I've talked quite a bit about dolphins over the years, and I've also mentioned how they aren't quite the same thing as porpoises. Well, there's another set of dolphin-like animals that are neither porpoises, nor members of the dolphin family. And these are the river dolphins.

Heading back in time, to before we had a good understanding of molecular and genetic evolutionary relationships, there were usually said to be three families of small cetacean: the dolphins, the porpoises, and the river dolphins. The river dolphins were distinguished from other other two, not just by being mostly freshwater, but also by a number of anatomical features. (And these aren't shared by the tucuxi, which is why we'd always known that really was a dolphin). For example, spending all their lives in muddy river water, they have lousy eyesight, since there really isn't anything much for them to see. They also tend to be rather small, and not very numerous, presumably because there's less room for them in a river than there would be in the open sea.

We now know that river dolphins aren't particularly closely related to one another. In retrospect, and armed with more modern systems of classification, it's easy to see why this is the case. Dolphins in the Indian Ocean can, if they swim far enough, meet dolphins in the Pacific or the Atlantic. But, if you can only live in fresh water, there is absolutely no way to swim from the Amazon, to, say, the Ganges. Which means that the river dolphins in each given river system have presumably been evolving in isolation from one another for a very long time.

There is general agreement on the existence of just four species of river dolphin, two in South America and two in Asia. Under at least one scheme, they're sufficiently different from one another that each individual species has a family all to itself (excluding, of course, any fossil species). That's not the only scheme, and it's not uncommon, for example, to put the two South American species, the boto and the franciscana, into the same family. Nor is it by any means certain that there are only four species; there may be five, or perhaps six.

And it's into that uncertainty that we have to throw the possibility that there might be six or seven. To understand what's going on here, let's ignore the Asian species, and look at the ones in South America. There's no real doubt that the franciscana (Pontoporia blainvillei) is a species. Just as the tucuxi is the only true dolphin to spend its life in freshwater, so the franciscana is the only "river dolphin" that regularly lives in salt water. Granted, it only travels very close to the coast, and many individuals spend much of their lives in the brackish estuary of the La Plata River, between Argentina and Uruguay. But, still.

At any rate, we're pretty sure what the franciscana is. The status of the boto is less clear. According to many, it's just one species (Inia geoffrensis), found in broad rivers throughout the Amazon jungle, and first described by Henri de Blainville way back in 1817. But in 1834, another French zoologist, Alcide d'Orbigny, thought he'd found another one. This, the Bolivian boto, inhabits the upper reaches of the Madeira River, and many of its larger tributaries. Just a few years later, having had a chance to examine de Blainville's specimen, d'Orbigny changed his mind, and decided that they were the same animal after all.

In the 1970s, new studies showed that the populations on either side of the Teotonio Rapids were a little different. The ones to the south - d'Orbigny's Bolivian boto - were thus identified as a distinct subspecies. Most modern texts still use this system. (Indeed, there is a third subspecies in the Orinoco and its tributaries, besides the originally named one in the Amazon proper).

So things stood until 2012 when the Society for Marine Mammalogy decided that, no, the Bolivian boto really is a separate species (Inia boliviensis). Textbooks and the like have yet to catch up, and not everyone agrees that the decision was justified anyway, but that's where things stood until last week.

The authors of that study, led by Tomas Hrbek, realised that if the Teotonio Rapids were enough to keep the Bolivian and Amazonian species of boto separate, something similar might also be true of the Tucuruí Rapids on the Tocantins River. These rapids mark the point where the Tocantins drops over the edge of a vast sandstone plateau into the lowlands. They're largely not there any more, because of a dam, but before that, they probably existed for about two million years. Furthermore, the Tocantins isn't even a tributary of the Amazon, and the mouths of the two rivers are about a hundred miles apart. Granted, they are separated in part by lengthy freshwater channels south of Marajó Island, so it's at least possible that something could swim from one to the other... but not certain that they did so in any numbers.

We already knew that there were botos living upriver of the rapids, on the Tocantins and its major tributary, the Araguaia, so the question was, just how different were they from those elsewhere? To find out, the researchers looked at some dead specimens, and also took small tissue samples from the tails of living animals, and compared them with those from elsewhere.

Numbers show estimated time since divergence,
in millions of years
The results showed that it was possible to tell the two populations apart by genetics alone. Furthermore, examination of the dead specimens showed that the animals from the Tocantins and Aragiaia had larger skulls and significantly fewer teeth than their Amazonian and Bolivian counterparts. They had both the genetic and physical differences needed to identify a new species, and named it the Araguaian boto (Inia araguaiaensis).

Of course, just because you have something you can identify doesn't prove it's a new species. The new Araguaian animal is much closer to the Amazonian one than the Bolivian boto is, so if you think the latter is only a subspecies, then the new one can't possibly be any more than that. This gets to the heart of where we draw the line between the two.

Realistically, we can't just try and cross-breed the two to see what happens, and even if we did it wouldn't necessarily prove much. Instead, the researchers statement that what they have is a new species rests in part on how long the two populations have been separated without interbreeding. Taking a number of species, and the age of the earliest known fossils of various kinds of dolphin and whale, they have estimated how rapidly these animals tend to evolve, and therefore how long the Amazonian and Araguaian forms must have been living apart to be as different as they are.

From this, they estimate that the last common ancestor of the two animals must have lived around two million years ago - about the same time that the rapids formed and the two river systems became clearly separated. By the same reckoning, the Bolivian species (if it is one) diverged from the common ancestor of the other two almost three million years ago. In all that time, they don't appear to have bred with one another for any significant degree.

This is quite a bit more than the estimates for how long ago some of the definitively recognised species of true dolphin have been apart, and most of them don't have obvious physical barriers between them. (And, indeed, do cross-breed on occasion). That's not proof, and it's worth noting that the two forms are almost impossible to tell apart visually, but, if confirmed, it is certainly suggestive. It depends a lot on what sort of evidence you're looking for, and how firm you'd like it to be, but to me at least, it looks plausible, if not definitive.

As a final note, whether or not the botos of the Tocantins and Araguaia are a distinct species does actually matter. That's because species tend to get more effort put into their conservation than mere subspecies. Whichever it is, the Araguaian boto is threatened, and may even be worthy of 'endangered' status.

Consider that the last time a new species of river dolphin was unambiguously identified was in 1918. That was the baiji (Lipotes vexillifer), which lives only in the Yangtze River. Officially, it's a critically endangered species, on the verge of extinction. "Officially", because, in practice, despite extensive attempts to find one, nobody has seen a living baiji since 2002. Almost certainly, one of the four species of river dolphin that nobody denies really are species... is already extinct.

[Photo by Stefanie Triltsch, from Wikimedia Commons. Chronogram adapted from Hrbek et al 2014].

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